Essential Civil Engineering Interview Questions For Freshers Part – 5
What are the reasons for geotechnical site investigations in Uganda?
To know the soil properties, in so doing be in position to determine whether the particular site is suitable for the purpose intended
To know the history of the site
To know what remedies need to be put in place before construction can start
Based on the soil properties, that can be determined on site and in the lab, design the appropriate foundation for the structure.
Did any of the Egyptian pyramids ever fall down?
Yes, many did. The great pyramid and others that have survived did so for a reason. Their shapes made them sturdier over time and were not the first design. Many steppe pyramids were built but were too steep and as the corners eroded, the whole pyramid fell under its own weight. They were built too steep and did not have the base structure as seen in the great pyramid.
In designing mini-piles, should the strength of grout be neglected during assessment of loading carrying capacity?
In designing min-piles, there are two approaches available:
In the first approach, the axial resistance provided by the grout is neglected and steel bars take up the design loads only. This approach is a conservative one which leads to the use of high strength bars e.g. Dywidag bar. One should note that bending moment is not designed to be taken up by min-piles because of its slender geometry.
In the second approach, it involves loads to be taken up by both grout and steel bars together. In this way, strain compatibility requirement of grout and steel has to be satisfied.
What are the functions of cap block, drive cap and pile cushion in driven piles?
Cap block is installed between the hammer end and the drive cap to control the hammer blow in order to protect both the hammer and the pile from damage. When the hammer hits the cap block, it compresses elastically and reduces the peak forces, thereby lengthening the time of hammer blow. Moreover, it should be capable of transmitting the hammer energy effectively to the piles.
Drive cap is inserted at hammer tip to enhance uniform distribution of hammer energy to the pile. Pile cushion is positioned between the drive cap and the pile top. It intends to protect the pile from driving stress induced during hammer blows. Moreover, it also serves to provide a uniform driving load on top of the pile.
What are the methods to tackle negative skin friction?
Use slender pile sections (e.g. H-pile or pre-cast pile) because smaller pile area when subject to the same working load would produce higher deformation, thus increasing the relative downward movement of piles.
In a certain region of H-piles for ground water table fluctuation, painting is applied on the surface of H-piles because the rise and fall of water table contribute to the corrosion of H-piles. On the other hand, to reduce the effect of additional loads brought about by negative skin friction, bitumen is applied on the pile surface corresponding to the region of soils that has negative skin friction.
However, bitumen should not be applied to the whole section of H-piles because it would be unable to derive the designed frictional reaction from soils.
Design the piles as end-bearing so that they can take up more loads.
How do you determine Specific gravity of cement?
Cement is usually purchased as a powdery substance that is mixed with sand, aggregate, gravel, and water to form concrete. Since the cement itself is usually a powder, it is hard to measure a standard value for its specific gravity. In addition, since cement is usually not used by itself, knowing its specific gravity is not particularly useful.
A more useful question is “What is the typical density of concrete?” A rule of thumb answer is that normal cured concrete has a density of about 150 pounds per cubic foot. This includes the weight of the cement, sand, aggregate, and that Part of the water that chemically binds with the cement to form the concrete. Since water weighs about 62.4 pounds per cubic feet, concrete is about 2.4 times as heavy.
Thus, the specific gravity of concrete is about 2.4. If you took cement and mixed it with water, you would eventually have a hard lump of useless cement and it would also have a specific gravity of between 2 and 2.4.
How and where are aqueducts built?
Aqueducts are built in areas where you have a bunch of motivated end users (like a town or group of farmers) at a low elevation in need of a more reliable source of water located somewhere fairly nearby at a higher elevation. The aqueduct builders construct a series of canals, elevated channels, and tunnels as required to get the water from the source to the end users.
Some good examples:
Roman engineers built aqueducts throughout Italy and France from mountain water sources to serve city dwellers
Water-needy Southern California cities and farms are served by an aqueduct that brings them water from sources in Northern California
New York City is supplied by an aqueduct and tunnel system from sources upstate.
Inca farmers in coastal valleys built irrigation aqueducts from sources higher up in the Rockies
Native American cultures in Phoenix area built irrigation canal systems that diverted water from sources at higher elevations to irrigate their crops.
What is the meaning of a blue land surveyor’s flag?
If the flag was placed by Utility personnel responding to a “One-call” locate request, the blue flag indicates a buried water line. You see these marked when a contractor calls the “Call before you dig number” a couple of days prior to excavating. This is required by law in each state to reduce the likelihood of damaging underground utilities when excavating.
The standard color code used by almost all utility companies for Painting & flags is:
White – Here is the area I plan on excavating!
Blue – water line
Red – electricity
Yellow – natural gas
Green – sewer
Orange – telephone and/or fiber optic line
If the blue flagging was a fuzzy blue marker nailed to the top of a wood surveyor’s stake, then it probably serves to indicate the top of the grade at which the engineer wants the earthmoving equipment to place fill dirt. These are called “blue-top” stakes.
What are advancements in civil engineering?
Unlike other fields of engineering, the major advancement of the field has been in the early years of the century before the last century where the use of concrete technology is advanced. The use of cement as a construction material is since the turn of the last century, improvement in the field increase by the use of steel elements in the construction of buildings and bridges of various types.
With the help of two, it was possible to do multi-story buildings in the world. Machineries were created to speed up the construction structures. The last century has also seen the advent of sophisticated design to withstand the effect of earthquake that was not possible before. With the use of computers, development of model and analysis of structures under the effect of loads was made possible. Before just two decades, it used to take months and months to design high-rise building and big bridges. Now it is a matter of hours.
How did the Romans get water up hills using aqua ducts?
Technically, the Romans were not able to get water to move uphill in a general sense. All aqueducts move water from an elevated source (spring-fed streams in the mountains) to end-users at a lower elevation. The water flows almost entirely from the source to the end user. If the water needed to cross a valley, the Romans would build an arched structure with an elevated channel to cross the valley, but even this channel would have a very slight downhill gradient that allowed water to flow towards the end user.
If a large hill was in their way, the Romans would either divert the channel around the hill, dig a trench through the hill, or dig a tunnel through the hill, all while maintaining a fairly constant, slight downhill gradient towards the end user.
The only exception to the rule of a generally constant downhill slope to the water channel is that specific tunnel segments, the Romans could build the tunnel as an inverted siphon (mentioned above) to cross a depression or valley and raise the water level on the downhill side almost to the level of the uphill side. To do this requires a well-sealed tunnel strong enough to withstand the increased water pressure within the siphon. Note, however, that except for gaining a little bit of elevation if you slow down fast-moving water, you normally cannot get water to flow out of the outlet at a higher elevation than the inlet. So technically, even the Romans were not able to get water to flow “up a hill”.
What is the difference between QA and QC?
Many people and organizations are confused about the difference between quality assurance (QA), quality control (QC), and testing. They are closely related, but they are different concepts. Since all three are necessary to manage the risks of developing and maintaining software, it is important for software managers to understand the differences.
They are defined below:
Quality Assurance: A set of activities designed to ensure that the development and/or maintenance process is adequate to ensure a system will meet its objectives.
Quality Control: A set of activities designed to evaluate a developed work product.
Testing is the process of executing a system with the intent of finding defects. (Note that the “process of executing a system” includes test planning prior to the execution of the test cases.)
What is horizon or horizontal mining?
Horizon or horizontal mining can be applied to extraction of material from seams of any stratified mineral such as limestone or ironstone, but it is more usually associated with coal particularly where there are several seams that are inclined or folded and/or faulted. Horizon mining involves long level roadways (horizons) being driven from the shafts to the extremity of the area to be mined.
The levels of the horizons are chosen to intersect the maximum number of seams the maximum number of times. As the seams are intersected, headings will be driven into the seam so that the desired material may be extracted. This method of mining requires a thorough understanding of the geological structure of the area to be mined so that the level of the horizons can be chosen for optimum results.
This method of mining is popular in modern coalmines with seams worked from several horizons. The considerable capital outlay of driving horizons before production can begin is recouped by the advantage of having long straight level roadways of generous dimensions unaffected by the crushing effect of nearby extraction of the mineral.
What is the difference between absorption & adsorption and sorption?
Absorption generally refers to two phenomena, which are largely unrelated. In one case, it refers to when atoms, molecules, or ions enter some bulk phase gas, liquid or solid material. For instance, a sponge absorbs water when it is dry.
Absorption also refers to the process by which the energy of a photon is taken up by another entity, for example, by an atom whose valence electrons make transition between two electronic energy levels. The photon is destroyed in the process. The absorbed energy may be reemitted as radiant energy or transformed into heat energy. The absorption of light during wave propagation is often called attenuation. The tools of spectroscopy in chemistry are based on the absorption of photons by atoms and molecules.
Adsorption is similar, but refers to a surface rather than a volume: adsorption is a process that occurs when a gas or liquid solute accumulates on the surface of a solid or, more rarely, a liquid (adsorbent), forming a molecular or atomic film (the adsorb-ate). It is different from absorption, in which a substance diffuses into a liquid or solid to form a solution.
Will Water damage concrete?
As far as only concrete is concerned i.e. plain concrete, the effect of water seepage is very little (depending upon the grade of concrete) whereas for RCC (reinforced cement concrete) water that seeps in corrodes the reinforcement and thus reduces the life of the structure.
The defects that water seepage induces in concrete are as follows:
Induces capillary formation (due to the detiorating characteristics of water)
With these capillaries the concrete starts spalling out; i.e. the places where capillaries are formed, with even slight amount of stress that portion comes out and exposes the steel to the atmosphere
Concrete has a pH of about 12 – 13. It also reduces the pH of the concrete when in salty water (or) when exposed to marshy areas.
Reduces the overall strength of concrete
Reduces durability
Reduces permeability to further water see Page
Results in ageing of structures
Should air test or water tests be selected to test the leakage of constructed gravity pipelines?
For gravity pipes, air tests or water tests are carried out after completion of laying and jointing of the pipes. These tests are conducted to check the water-tightness of joints and to ensure the pipelines are free from damage where leakage may occur.
Air test has the advantage that the test itself is simple and faster to be carried out. It does not require the disposal of significant quantities of water used in the test which is a mandatory requirement for water test. However, in case leakage exists in the constructed segment of gravity pipelines, the position of leakage can hardly be located in air test. Moreover, the rate of water leakage cannot be determined from air tests.
In addition, air test is readily affected by atmospheric condition because air has a relatively high coefficient of thermal expansion. The test is also influenced by the moisture condition of the test pipelines because it affects the passage of air through the pipelines.
For water test, though it is comparatively slow, it can detect the location of water leakage. However, the leakage rate results from water test may not truly reflect its actual leakage because pipeline materials like concrete and clay are porous and would absorb water during the test.
What is “preset” during installation of bridge bearings?
“Preset” is a method to reduce the size of upper plates of sliding bearings in order to save the material cost. The normal length of a upper bearing plate should be composed of the following components: length of bearing + (2 × irreversible movement) + (2 × reversible movement). Initially the bearing is placed at the mid-point of the upper bearing plate without considering the directional effect of irreversible movement.
However, as irreversible movement normally takes place at one direction only, the bearing is displaced/pre-setted a distance of (irreversible movement/2) from the mid-point of bearing in which the length of upper plate length is equal to the length of bearing + irreversible movement + (2 × reversible movement). In this arrangement, the size of upper plate is minimized in which irreversible movement takes place in one direction only and there is no need to include the component of two irreversible movements in the upper plate.
Note: “Preset” refers to the displacement of a certain distance of sliding bearings with respect to upper bearing plates during installation of bearings.
What is the function of water-stops in joints of box culverts and drainage channels?
The principal function of water-stops is to prevent liquids (e.g. water), water-borne materials and solids to pass through concrete joints. In essence, it aims at providing water-tightness to the drainage channel.
Besides, water-stops in drainage channels or box culverts can also serve two other purposes:
To avoid water contacting joints’ dowel bars and causing corrosion.
To avoid water seeping in from the underside of drainage channels or box culverts, thereby washing in soil particles and causing voids underneath these structures and finally leading to their failure.
To serve the second purpose, obviously only one water-stop is required at any depth location.
To serve the first purpose, a water-stop has to be installed on top of dowel bars to prevent water from drainage channels from leaking through. On the other hand, a water-stop has to be provided below dowel bars to avoid underground water from surging upwards.
In fact, the other way out to serve the first purpose is by using corrosion resistant bars.
Is stainless steel really stainless in construction application?
Stainless steel refers to alloy steels with more than 10.5% of chromium and consists of several groups like austenitic, ferritic, martenistic etc. Austenitic stainless steel is normally used in structural applications because of its high corrosion resistance. Austenitic and ferritic types of stainless steel cover about 95% of stainless steel applications. Stainless steel is not stainless although it is corrosion resistant under a wide range of conditions.
A passive layer of chromium oxide is formed on stainless steels surface which renders it corrosion resistant. This chromium oxide layer acts as a stiff physical barrier to guard against corrosion and makes it chemically stable. Moreover, when this layer is damaged, it can perform self repairing where there is a sufficient supply of oxygen. However, stainless steel will still corrode by pitting in marine environment where chloride attack occurs.
Therefore, appropriate grades and types of stainless steel have to be selected in polluted and marine environment to minimize the problem of corrosion. Reference is made to Euro Inox and the Steel Construction Institute (2002).
What is the function of drilling fluid in rotary drilling in site investigation?
Drilling fluid in rotary serves two main purposes:
Facilitate the rotation of drilling tube during rotary drilling;
Act as a cooling agent to cool down heat generated during drilling operation.
Traditionally, water is normally employed as drilling fluid. However, it suffers from the following drawbacks:
It affects the stability of nearby ground with the introduction of water into the borehole (borehole for soil; drill-hole for rock);
It affects the quality of sample by changing the water content of soil samples collected from the borehole/drill-hole.
Substitutes are available in market to replace water as drilling fluid (e.g. white foam).
Does the presence of rust have adverse impact to the bond performance of bar reinforcement?
In fact, the presence of rust in bars may not have adverse impact to the bond performance and it depends on the types of bar reinforcement under consideration.
For plain round bars, the rust on bars improves the bond performance by the formation of rough surfaces which increases the friction between steel and concrete.
However, for deformed bars, the same theory cannot apply. The presence of rust impairs the bond strength because corrosion occurs at the raised ribs and subsequently fills the gap between ribs, thus evening out the original deformed shape. In essence, the bond between concrete and deformed bars originates from the mechanical lock between the raised ribs and concrete. On the contrary, the bond between concrete and plain round bars derives from the adhesion and interface friction. With such differences in mechanism in bonding, the behaviour of bond between deformed bars and plain round bars in the presence of rust varies. Reference is made to CIRIA Report 147.
In the design of corbel beams in a pumping station, why are shear links designed in the top 2/3 of the section? What is the general advice on the design?
Corbel beams are defined as z/d < 0.6 where z is the distance of bearing load to the beams’ fixed end (or called shear span) and d is depth of beams. The design philosophy is based on strut and tie system. To establish the design model, it is firstly assumed the failure surface, i.e. shear cracks extending to 2/3 of depth of beam.
Experiment results verified that the failure cracks extended only to 2/3 of beam while the remaining 1/3 depth of concrete contributed as concrete strut to provide compressive strut force to the bearing loading. Horizontal links are normally provided to corbel beams because experimental results indicated that horizontal links were more effective than vertical links when shear span/depth is less than 0.6. For shear span/depth>0.6, it should be not considered as corbel beams but as cantilevers.
In designing corbel beams, care should be taken to avoid bearing load to extend beyond the straight portion of tie bars, otherwise the corners of corbel beams are likely to shear off. Reference is made to L. A. Clark (1983).
Should stiff or soft fenders be designed for berthing in piers?
The elasticity of fenders is related to the ability to release the stored energy during berthing of vessels. However, it has no effect on the reaction force and the deflection of fender system. The amount of energy that a fender can absorb is dependent on the reaction-deflection curve and is represented by the area under the curve. The higher is the reaction force, the higher amount of energy would be absorbed by the fender provided that the resistance of ships’ hull is sufficient to withstand the force without permanent deformations.
Although stiff and soft fender may have the same deflection under the same maximum reaction force acting on the berthing vessel, the amount of energy absorbed by stiff fenders is much higher than that of soft fenders. Consequently, stiff fenders should be employed for berthing purpose.
On the other hand, in mooring operations where vessels are constantly subject to wave action, it is desirable to keep the tension force on the rope to a low value. In this connection, it is recommended to use soft fenders.
In soil compaction test, if a test result exceeds 100%, should engineers accept the result?
Soil compaction is the process of increasing the soil density by reducing the volume of air within the soil mass.
Soil compaction depends mainly on the degree of compaction and the amount of water present for lubrication. Normally 2.5 kg rammers and 4.5 kg rammers are available for compaction in laboratories and the maximum dry densities produced by these rammers cover the range of dry density obtained by in-situ compaction plant.
Regarding the second factor of water content, it affects the compaction in the following ways. In low water content, the soils are difficult to be compacted. When water content is increased gradually, water will lubricate the soils and this facilitates the compaction operation. However, at high water content, as an increasing proportion of soils is occupied by water, the dry density decreases with an increase in water content.
For soil compaction tests, the dry density obtained from compaction carried out in-situ by vibrating roller/vibrating plate is compared with the maximum dry density conducted in laboratories using 2.5 kg rammer of compaction with similar soils. In essence, the in-situ compaction is compared with the compacting effort of using 2.5 kg (or 4.5 kg) rammer in laboratories.
In case the compaction test results indicate values exceeding 100%, it only means that the in-situ compaction is more than that being carried out in laboratories which is treated as the basic criterion for satisfactory degree of soil compaction. Therefore, the soil results are acceptable in case compaction test results are over 100%. However, excessive compaction poses a risk of fracturing granular soils resulting in the reduction of soil strength parameters.
What is the function of longitudinal joints in concrete road pavements?
A longitudinal joint consists of a tie bar placed at the mid-depth of a concrete pavement and it is not intended for joint lateral movement. Then one may doubt the reasons of placing longitudinal joints in concrete pavements. In fact, longitudinal joints are normally designed at a regular spacing e.g. 4.5 m to accommodate the effect of differential settlement of pavement foundation. When uneven settlement occurs, the tie bars in longitudinal joints perform as hinges (Ministry of Transport (1955)) which allow for the settlement of concrete carriageway.
Moreover, it also serves to cater for the effect of warping of concrete due to moisture and temperature gradients by permission of a small amount of angular movement to occur so that stresses induced by restrained warping can be avoided.
Dowel bars are provided in longitudinal joints for the following reasons:
In case of the occurrence of uneven settlement between adjacent panels, it helps to maintain a level surface by transfer of loads through dowel bars.
Keep the longitudinal joints close.
What is the problem in traditional marine piling system of steel tubular pile with concrete infill and what are the possible remedial measures?
In the design of marine piles of steel tubular piles with concrete infill, loads from pier deck are taken up by steel tubular piles before the occurrence of corrosion of steel piles above seabed. In fact, it is assumed that steel piles above seabed level will all be corroded after a certain year.
The load transfer mechanism after complete corrosion of steel pile above seabed is as follows: loads from pier deck are taken up by concrete infill above the seabed level. Below the seabed level, loads would be transferred to steel piles through frictional forces between concrete infill and steel casings.
However, substantial radial shrinkage and contraction occurs after concreting of concrete infill and this will hinder the load transfer from the concrete infill to steel piles because the bond may be ruptured by radial shrinkage. It is in doubt if frictional forces can be properly developed in this situation. To solve this problem, shear keys could be installed at regular spacing inside steel piles to ensure their rigid connection with concrete infill. Alternatively, expanding agents may be adopted in concrete mixes to ensure that there is no shrinkage after the concreting process.
For installation of silt curtains, why is it not desirable to design the curtain to touch the seabed?
Silt curtains are impermeable vertical barriers extending from the seawater surface to its designed depth. The curtains are held in a vertical position by the carrier float on their top and a curtain weight at their bottom. A tension cable is designed at the carrier float to resist stresses incurred by currents. Moreover, the silt curtains are anchored to the seabed to hold them in the designed configuration.
In essence, the depth of silt curtains should not be so long and touch the seabed because the bottom segment of the silt curtains would be trapped inside the newly accumulated sediment, thus resulting in sinking of the curtain. Consequently, it is difficult to remove these sunken curtains. Moreover, reversal tidal and current actions may cause the movement of bottom region of curtains which stir up the settled suspensions and induce additional turbidity.
What are the disadvantages of curing by ponding and polythene sheets?
The purpose of curing is to reduce the rate of heat loss of freshly placed concrete to the atmosphere and to minimize the temperature gradient across concrete cross section. Moreover, curing serves to reduce of the loss water from freshly placed concrete to the atmosphere.
Ponding: This method of thermal curing is readily affected by weather condition (cold wind). Moreover, a large amount of water used has to be disposed off the construction sites after curing.
Polythene sheet: This method of curing is based on the principle that there is no flow of air over the concrete surface and thereby no evaporation can take place on top of the freshly concreted surface by provision of polythene sheets. However, it suffers from the demerit that polythene sheets can be easily blown off in windy condition and the performance of curing would be affected. Moreover, for water lost due to self-desiccation, this method cannot replenish these losses.
What are different approaches for reclamation in deep water region and shallow water region?
To illustrate the different approaches adopted for reclamation in deep water and shallow water region, the following example is used:
In deepwater region, consider the seabed level is -8.5 mPD. After laying of geo-textiles and 1.5 m thick sand blanket, the top level of sand blanket is about -7 mPD. Split barges are deployed for dumping public fill to -2.5 mPD. Afterwards, end dipping of public fill by trucks will be carried out up to +2.5 mPD which is the designed reclamation level. Between level -2.5 mPD and +2.5 mPD, it is too shallow for split barges to enter the water, thus the method of end dipping is used instead.
For shallow water region, the seabed level is taken as -5.5 mPD in this example. With the laying of geo-textiles and 1.5 m sand blanket into position, the top level of sand blanket is about -4 mPD. In this case, split barges are also used for reclamation work between the level -4 mPD and -2.5 mPD. After that, if end dipping is used for reclamation work above -2.5 mPD, then in considering the relative thin layer of fill above seabed (1.5 m sand blanket + 1.5 m sand blanket), it stands a high chance that mud wave would occur in seabed. Therefore, half-loaded derrick barges are employed for reclamation up to level 0 mPD. With a thicker layer of public fill now, end dipping can then be used for reclamation between 0 mPD and +2.5 mPD.
This above reclamation sequence is just an example to demonstrate the different considerations for reclamation in deep water and shallow water region.
Given a 1 m high staircase resting on solid concrete, would it be adequate to design nominal reinforcement for the staircase?
For the design of staircase, there are three main scenarios:
Stairs spans longitudinally: This kind of stairs refers to stairs spanning between landings only without any side supports. In this case, the staircase should be designed as a beam between two end supports (i.e. landing) and the main reinforcement is provided at the bottom of staircase slabs.
Stairs spanning transversely: This kind of staircase is supported by sidewalls only and it may also be supported by stringer beams. For the case of sidewalls, it acts as a cantilever beam and the main reinforcement are provided the top surface of slab. For the case of staircase supported sideways by both sidewall and stringer beam, it should be designed transversely with end supports as sidewall and stringer beam and reinforcement is provided at the bottom of the staircase.
Stairs resting on solid support: For stairs resting on solid supports, only nominal steel reinforcement is provided to control thermal and shrinkage cracking.
What is the importance of air void content in bituminous pavements?
The air void content of bituminous materials is an important control parameter for the quality of bitumen being laid and compacted. If the air void content is too high, it allows for intrusion of air and water. Moreover, it also increases the rate of hardening of binders which produce premature embrittlement of pavements. In addition, too high a void content will also lead to differential compaction subject to traffic loads and result in formation of ruts and grooves along the wheel track.
However, a minimum amount of air void should be maintained to avoid instability during compaction process and to provide space for bitumen flow in long-term consolidation under traffic loads. A sufficient amount of air voids should be designed to make room for expansion of binder in summer and compaction by road traffic as suggested by National Association of Australian State Road Authorities (1968), otherwise bleeding and loss of stability may occur and the pavement will deform readily under severe loads.
What are the pros and cons of using timber fenders, plastic fenders and rubber fenders?
Timber fenders: They are low in strength and are subject to rotting and marine borer attack. Moreover, they have low energy absorption capacity and the berthing reaction depends on the point of contact. The contact pressure between fender and vessels are high. They are considered to be environmentally unfriendly because they consume tropical hardwoods in their production.
Plastic fenders: Their strength is similar to that of timber fenders but they have relatively high abrasive resistance. They are resistant to chemical and biological attack. Their energy absorption capacities are moderate and the berthing reactions are also dependent on the point of contact. The reaction is lower when compared with timber fenders for a given energy absorption. They are considered to be environmental friendly because they are manufactured from recycled material.
Rubber fenders: They possess high abrasive resistance and are also resistant to most biological and chemical attacks. They have moderate to high energy absorption capacity and the energy absorption performance is independent of the point of contact. Similar to plastic fenders, they are also environmental friendly products.
What are the functions of the following features observed in a typical manhole? (i) Groove near benching, (ii) R.S.J. (iii) double seal manhole cover and (iv) U-trap with rodding arm.
The groove is used to facilitate the maintenance of manholes and sewer/drain pipes. Shutoff boards are erected on the grooves during maintenance operation so that water flow coming from upstream is terminated in the manhole and backwater from downstream is also blocked. In addition, the groove also facilitates water flow diversion for routine maintenance operation.
R.S.J. is a small-scale size of universal beams and is used for resisting the high stresses incurred by heavy traffic loads acting directly on the upper narrow projected section of manholes.
Double seal terminal manhole covers are used for sealing off gases emitted inside sewer/drains and prevent them from releasing out of the manhole.
U-trap with rodding arms is also used for sealing off unpleasant gas smell by the trapped u-shaped water columns. Rodding arm is normally closed with rubber rings during normal operation. However, during maintenance operation, the rubber ring is removed and rodding can be carried out through the rodding arm.
Why are concrete profiles barriers designed with curved surface profiles?
fencings are designed to contain vehicles in the carriageway in which they are travelling and prevent them from rebounding into the road and causing hazards. For normal fencing design, when vehicles crash into safety fencings, it will give way so as to absorb as much energy as possible, thus reducing the impact forces on the vehicles. Moreover, it serves to realign the vehicles along the carriageway when vehicles hit on them.
However, for concrete profile barriers they are not designed to absorb energy during vehicle crashing, but to hold the vehicles hitting on them. In this connection, concrete profile barriers are designed with curved profiles so that vehicles can mount and go up partly on them, and yet they will not cause overturning of vehicles. Reference is made to Arthur Wignall, Peter S. Kendrick and Roy Ancil.
For shallow-angle crashing of cars, they would climb on the lower slope face of concrete profile barriers. On the other hand, when a car hits at a large angle to the barrier, the bumper collides with the upper sloping face of concrete profile barrier and the car rides upwards.
This provides the uplift of the car whose wheels move up the lower sloping face of the barrier. It is not intended to lift the car too high, otherwise it may result in rolling. Since the friction between the wheels and barriers provide extra lifting forces, it is undesirable to design rough finish on these faces. In essence, the kinetic energy of the car during collision is transformed to potential energy during its lifting up on profile barrier and finally converted back to kinetic energy when the car returns to the road.
Note: For details of concrete profile barriers, reference is made to HyD Standard Drawing No. H2101A.
What is the significance of direction of approaching velocities of ships during berthing operation?
One of the major effects of angle of approaching velocities of ships is its influence of the energy to be absorbed by the fender system. Consider several ships berth on the same pier at the same speed but with different angle of approach, though their kinetic energies are the same, the amount of energy absorbed by fender differs.
The amount of energy absorbed by fender is:
W = 0.5mv² (k² + r²cos²φ)/ (k² + r²)
Where W= energy absorbed by the fender
m = mass of the ship
v = velocity of the ship
k = radius of gyration of the ship
r = distance of centre of gravity of the ship to the point of contact of the fender
φ = direction of velocity
Hence, when the direction of approaching velocity of a ship is normal to the fender system
(i.e. φ = 90°), the amount of energy absorbed is smaller when compared with that of a ship whose velocity is tangential to the shoreline. Reference is made to F. Vasco Costa (1964).
When branch pipelines are connected to main pipelines, sometimes Y-junctions or fitting branched pipelines to main pipelines by formation of holes in main pipelines are used. Which one is a better choice?
By using standard precast units of Y-junction branch pipelines, it is beyond doubt that joints between branched pipelines and main pipelines are properly formed and the quality of joints is relatively less dependent on workmanship. However, it suffers from the problem that with fixed precast units of Y-junctions, sometimes it may be difficult for contractors to determine the precise orientation of specific angles of Y-junctions with respect to gullies. (e.g. gullies are connected through side branches to carrier drains)
By forming elliptical holes in main pipelines and fitting the side branches into them with cement mortar, the quality of pipe joints is highly dependent on workmanship. It is commonly found that in subsequent CCTV inspections side branches are projected inside main pipes. This is undesirable because the projected side branches reduce the cross sectional area of main pipes locally and affect their hydraulic performance. Moreover, the projected side pipes may trap rubbish and dirt in the vicinity. On the other hand, cement mortar may not be properly applied at connection joints because these areas are hidden from view and are difficult to be inspected by engineers. Therefore, in selecting between the two available methods, engineers should make their own judgments based on the above considerations.
What is the consideration in selecting the orientation of wing walls in the design of bridge abutments?
There are three common arrangements of wing walls in bridge abutments based on Dr.
Edmund C Hambly (1979):
Wing walls parallel to abutments: This is the simplest and shortest time to build but is not the most economical design. This design has the advantage that it has least disturbance to existing slope embankment.
Wing walls at an angle to abutments: This is the most economical design among the three options in terms of material cost.
Wing walls perpendicular to abutments: Though it is not the most economical design, the wing walls provide a continuous alignment with bridge decks which provide supports to parapets. However, they cause disturbances to adjacent structures and utility services during construction. Moreover, if the bridge is curved, the wing walls may hinder the road curvature.
One the other hand, when the wing walls are structurally connected to the abutment, then structural advantage can be taken by the stability of box structure.
What is the difference among cement plaster, cement render and cement screed?
Under what situations should each of the above be used?
The purpose of plastering, rendering and screeding is to create a smooth, flat surface to receive finishes like paint, wallpaper etc.
Plastering is the intermediately coating of building materials to be applied on the internal facade of concrete walls or block-walls.
Rendering is the intermediate coating for external walls only.
Screeding is the coating laid on floors to receive finishes like tiles, carpet, and marble.
Hence, these terms differ basically from the locations at which they are applied. Due to different locations of application of plasterwork, the proportion of material component for plaster and render is different.
For example:
Cement plaster
Undercoat- cement : lime : sand (by volume) = 1 : 4 : 16
Finishing coat – cement : lime : sand = 1 : 12 : 30
Cement render
Undercoat- cement : lime : sand (by volume) = 1 : 2 : 6
Finishing coat – cement : lime : sand = 1 : 3 : 6
How to determine the size of elastomeric bearings?
For elastomeric bearing, the vertical load is resisted by its compression while shear resistance of the bearing controls the horizontal movements. The design of elastomeric bearings is based on striking a balance between the provision of sufficient stiffness to resist high compressive force and the flexibility to allow for translation and rotation movement.
The cross sectional area is normally determined by the allowable pressure on the bearing support. Sometimes, the plan area of bearings is controlled by the maximum allowable compressive stress arising from the consideration of delamination of elastomer from steel plates.
In addition, the size of elastomeric bearings is also influenced by considering the separation between the structure and the edge of bearing which may occur in rotation because tensile stresses deriving from separation may cause delamination. The thickness of bearings is designed based on the limitation of its horizontal stiffness and is controlled by movement requirements. The shear strain should be less than a certain limit to avoid the occurrence of rolling over and fatigue damage. The vertical stiffness of bearings is obtained by inserting sufficient number of steel plates.
Why is the slump specified in concrete carriageway comparatively low (30 mm) when compared with normal concrete (75 mm)?
The slump of concrete carriageway is purposely specified to be a relatively low value, i.e. 30mm. For concrete carriageway, traffic loads directly act on concrete pavement surface and therefore the surface strength is detrimental to its future performance. In freshly placed concrete, segregation (may be in the form of bleeding) occurs within the mixture of cement paste and aggregates. The degree of resistance to segregation is related to workability of concrete.
If substantial segregation is allowed to take place, then the relatively porous and weak laitance layer will be formed on the carriageway surface and the aggregates will concentrate in the bottom. Hence, concrete which has insignificant bleed possesses a stronger surface layer and is more abrasion resistant. Consequently, a small slump value is specified to increase the wearing resistance of concrete and to achieve a suitable surface texture of concrete pavements.
Moreover, a low-slump concrete facilitates the use of slip-forms when constructing the concrete pavement. With concrete of a low slump value, it still remains its compacted shape and is not liable to deform when the paving machines go away. However, if a high slump concrete is used instead, the pavement surface would drop and the edges may deform readily.
Should emulsified asphalts or cutback asphalts be selected as tack coat in bituminous road-works?
Emulsified asphalt is a suspension of asphalt in water by using an emulsifying agent which imposes an electric charge on asphalt particles so that they would be join and cement together. On the other hand, cutback asphalt is simply asphalt dissolved in petroleum. The purpose of adding emulsifying agent in water or petroleum is to reduce viscosity of asphalt in low temperatures.
The colour of emulsion for tack coat is brown initially during the time of application. Later, the colour is changed to black when the asphalt starts to stick to the surrounding and it is described as “break”. Finally, when water has all evaporated, the emulsion is said to have “set”. Similarly, for cutback emulsion, it is described to “cure” when the solvent has evaporated.
However, there are several problems associated with cutback asphalts:
Emulsified asphalt can be diluted with water so that a low application rate could be achieved.
The evaporation of petroleum into atmosphere for cutback asphalt poses environmental problem.
The cost of production of petroleum is higher than that of emulsifying agent and water.
What is the difference between epoxy grout, cement grout and cement mortar?
Epoxy grout consists of epoxy resin, epoxy hardener and sand/aggregates. In fact, there are various types of resin used in construction industry like epoxy, polyester, polyurethane etc. Though epoxy grout appears to imply the presence of cement material by its name, it does not contain any cement at all. On the other hand, epoxy hardener serves to initiate the hardening process of epoxy grout. It is commonly used for repairing hairline cracks and cavities in concrete structures and can be adopted as primer or bonding agent.
Cement grout is formed by mixing cement powder with water in which the ratio of cement of water is more or less similar to that of concrete. Setting and hardening are the important processes which affect the performance of cement grout. Moreover, the presence of excessive voids would also affect the strength, stiffness and permeability of grout. It is versatile in application of filling voids and gaps in structures.
Cement mortar is normally a mixture of cement, water and sand. They are used as bedding for concrete kerbs in roadwork.
How can unreinforced concrete pavement function without mesh reinforcement?
For concrete carriageway, it is normally classified into two types: reinforced and unreinforced concrete pavement. The reinforcement in reinforced carriageway (in the form of mesh) is used for controlling cracking. Then one may query how unreinforced pavement can control cracking without the use of mesh reinforcement. To answer this question, one should pay attention to the features of unreinforced concrete pavement.
In accordance with Highways Standard Drawing No. H1109, an approximately 3mm wide groove with a depth of about one-third to one-fourth of slab thickness is designed with a regular spacing (normally 5m). The grooves are designed to be too narrow for stones to fall into when the cracks are open due to concrete contraction. The sectional area in which the groove is located is a plane of weakness and thus this groove acts a potential crack-inducing device in which any potential cracks due to shrinkage and thermal contraction may form.
Consequently, the cracks are formed at the base of the groove and thus it would not cause any unpleasant visual appearance on the exposed surface of unreinforced concrete pavement.
What are the differences in applications between pipe culverts and box culverts?
Basically, a culvert means a covered hydraulic structure which conveys fluid. Therefore in a broad sense, pipe culverts in a small scale represent normal pipes like precast concrete pipes.
In terms of hydraulic performance, circular section is the best geometrical sections among all. Therefore, for relative small discharge, precast concrete pipes and ductile iron pipes are normally used which are circular in shape. But for applications of very large flow, precast concrete pipes and ductile iron pipes may not be available in current market. In this connection, cast-in-situ construction has to be employed.
It is beyond doubt that the fabrication of formwork for circular shape is difficult when compared with normal box culvert structures. However, circular shape is the most hydraulic efficient structure which means for a given discharge, the area of flow is minimum. Therefore, it helps to save the cost of extra linings required for the choice of box culverts.
However, box culverts do possess some advantages. For example, they can cope with large flow situation where headroom is limited because the height of box culverts can be reduced while the size of pipe culverts is fixed. Secondly, for some difficult site conditions, e.g. excavation of structure in rock, for the same equivalent cross-sectional area, the width of box culverts can be designed to be smaller than that of pipe culverts and this enhances smaller amount of excavation and backfilling.
In General Specification for Civil Engineering Works (1992 Edition), the design of road-base material is based on recipe approach. Why?
The design of road-base material is based on recipe approach (David Croney and Paul Croney (1992) because Hong Kong government follows the traditional British practice by adopting recipe design in which the aggregate grading envelope, the quantity and grade of bitumen are specified in the bituminous mix. This recipe of bituminous mix is derived based on past experience and good workmanship during construction.
In fact, many countries nowadays adopt special design mix of road-base which proves to produce satisfactory bituminous mixes to suit different site and design conditions.
In fact, recipe specification of bituminous materials does suffer from several drawbacks. Firstly, the conditions of traffic and climate of newly constructed bituminous road may differ from the conditions on which the recipe design is based. In case adjustment has to be made to the recipe design, it is very difficult to determine and assess the modifications required. Secondly, it poses problem to site engineers to assess the effects of minor non-compliance if recipe specification is adopted. Finally, the recipe mix may not be the most economical design which is dependent on site conditions.
In incremental launching method of bridge construction, what are the measures adopted to enhance sufficient resistance of the superstructure during the launching process?
During the launching process the leading edge of the superstructure is subject to a large hogging moment. In this connection, steel launching nose typically about 0.6-0.65 times span length is provided at the leading edge to reduce the cantilever moment. Sometimes, instead of using launching nose a tower and stay system are designed which serves the same purpose.
The superstructure continually experiences alternative sagging and hogging moments during incremental launching. Normally, a central pre-stress is provided in which the compressive stress at all points of bridge cross section is equal. In this way, it caters for the possible occurrence of tensile stresses in upper and lower part of the cross section when subject to hogging and sagging moment respectively.
Later when the whole superstructure is completely launched, continuity pre-stressing is performed in which the location and design of continuity tendons are based on the bending moments in final completed bridge condition and its provision is supplementary to the central pre-stress.
For very long span bridge, temporary piers are provided to limit the cantilever moment.
Poly-tetra-fluoro-ethylene (PTFE) is commonly used in sliding bearings. Why?
The choice of sliding surface of bearings is of vital importance because the sliding surfaces generate frictional forces which are exerted on the bearings and substructure of the bridge. For instance, PTFE and lubricated bronze are commonly choices of sliding surfaces for bearings. PTFE is a flurocarbon polymer which possesses good chemical resistance and can function in a wide range of temperature.
The most important characteristic of this material is its low coefficient of friction. PTFE has the lowest coefficients of static and dynamic friction of any solid with absence of stick-slip movement (David J. Lee). The coefficient of friction is found to decrease with an increase in compressive stress. However, PTFE do have some demerits like high thermal expansion and low compressive strength.
In designing the complementary contact plate with PTFE sliding surface, stainless steel plates are normally selected where the plates should be larger than PTFE surface to allow movement without exposing the PTFE. Moreover, it is recommended that the stainless steel surface be positioned on top of the PTFE surface to avoid contamination of dirt and rubbish.
Lubricants are sometimes introduced to reduce the friction between the PTFE surface and the upper stainless steel plate. Hence, the PTFE may be designed with dimples to avoid the lubricant from squeezing out under repeated translation movements.
What is the difference between working stress approach and limit state approach?
For working stress approach, service loads are used in the whole design and the strength of material is not utilized in the full extent. In this method of design, stresses acting on structural members are calculated based on elastic method and they are designed not to exceed certain allowable values. In fact, the whole structure during the lifespan may only experience loading stresses far below the ultimate state and that is the reason why this method is called working stress approach.
Under such scenario, the most economical design can hardly be obtained by using working stress approach which is now commonly used in the design of temporary works.
For limit state approach, for each material and load, a partial safety factor is assigned individually depending on the material properties and load properties. Therefore, each element of load and material properties is accurately assessed resulting in a more refined and accurate analysis of the structure.
In this connection, the material strength can be utilized to its maximum value during its lifespan and loads can be assessed with reasonable probability of occurrence. Limit state approach is commonly used for the majority of reinforced concrete design because it ensures the utilization of material strength with the lowest construction cost input.
In the construction of a two-span bridge (span length = L) by using span-by-span construction, why is a length of about 1.25 L bridge segment is constructed in the first phase of construction?
Basically, there are mainly three reasons for this arrangement:
The permanent structure is a statically indeterminate structure. During construction by using span-by-span construction, if the first phase of construction consists of the first span length L only, then the sagging moment in the mid span of the partially completed bridge is larger than that of completed two-span permanent structure. To avoid such occurrence, 0.25 L of bridge segment is extended further from the second pier which provides a counteracting moment, thereby reducing the mid-span moment of the partially completed bridge.
The position of 1.25 L countering from the first pier is the approximate location of point of contraflexure (assume that the two-span bridge is uniformly loaded) in which the bridge moment is about zero in the event of future loaded bridge. Therefore, the design of construction joint in this particular location has the least adverse effect on the structural performance of the bridge.
In case of a pre-stressed bridge, pre-stressing work has to be carried out after the construction of first segment of the bridge. If the pre-stressing work is conducted at the first pier which is heavily reinforced with reinforcement, it is undesirable when compared with the pre-stressing location at 1.25 L from the first pier where there is relatively more space to accommodate pre-stressing works.
Note: Span-by-span construction means that a bridge is constructed from one bridge span to another until its completion.
Why should acetylene gas cylinders used for gas welding be erected in upright position?
Acetylene gas is commonly used for gas welding because of its simplicity of production and transportation and its ability to achieve high temperature in combustion (e.g. around 5,000°F). Acetylene is highly unstable and flammable and would explode in elevated pressure when reacting with oxygen in air. Storing acetylene gas in cylinders under pressure is very dangerous.
Hence, for welding purpose, gas acetylene is stored in cylinders of liquid acetone contained in porous material (like firebrick) to enhance there is no free space left for acetylene gas and for cooling purpose in the event of thermal decomposition. It also prevents the formation of high pressure air pockets inside the cylinder. Dissolved acetylene in acetone will no longer in contact with oxygen and is not subject to decomposition. On the other hand, acetone is used because it is capable of dissolving large amount of acetylene gas under pressure without changing the nature of the gas.
The cylinders for gas welding i.e. oxygen cylinders and acetylene cylinders, when not in use should be stored separately because any mixture of these gases resulting from accidental leakage can be highly explosive. When in use, acetylene cylinders should always be kept in upright position because acetone liquid will be drawn from the cylinders with the gas if they are kept horizontally. Consequently, significant leakage of acetone liquid will result.
Note: Oxygen and acetylene gas cylinders are commonly used in construction sites for gas welding.
If there is a delay of bituminous laying on top of sub-base, should tack coat be applied on the top surface of sub-base?
When there is a delay between bituminous laying of different bituminous layers (i.e. road-base, base course etc.), a tack coat is applied on top of the bituminous layers because it helps to enhance better bonding between bituminous materials. If there is insufficient bonding between adjacent bituminous layers, they behave as separate independent layers which can hardly resist the traffic loads.
When applying the tack coat, it should be sprayed uniformly on the bituminous surface and allowed for sufficient curing. The hot bituminous material laid on top of the coat would soften it, enabling the tack coat to partly fill voids in the bituminous materials. For emulsified asphalt type tack coats, they are normally diluted with water in order to achieve a more uniform application without excessive usage of asphalt.
After the subsequent compaction is carried out, the coat would be interlocked with the bituminous materials. On the other hand, care should be taken to ensure that excessive coat would not be laid, otherwise slippage or shear cracks in the bituminous material would occur due to the relative thick layer of the tack coat applied.
However, for sub-base surface, priming coat instead of tack coat may be applied in the event of a delay in laying of bituminous layer on top of the sub-base layer. The function of the primer serves to maintain the existing surface condition for a longer period and it also provides an impermeable surface to prevent ingress of water or water loss by evaporation. Moreover, it fills the surface voids and protects the sub-base from adverse weather conditions. In addition, it also helps to promote adhesion between adjacent road layers and to harden the surface.
Why are some manhole covers made of cast iron while some are made of ductile iron?
Traditionally, manholes covers are made of cast iron. However, in the viewpoint of pipe maintenance, frequent opening of manhole covers has to be carried out. Therefore, it poses potential safety hazard to the workers during the lifting-up process of manhole covers because cast iron manhole covers are very heavy to normal workers.
Consequently, research has been conducted and ductile iron is considered as a better choice than cast iron because it can resist the same traffic loads with lower self-weight. Moreover, as ductile iron is less brittle than cast iron, the traditional cast iron manhole covers are more susceptible to damage and thus require higher maintenance cost.
However, ductile iron manhole covers do suffer from some demerits. For instance, owing to their relative low self-weight, vehicles passing over these manhole covers would lead to the movement of covers and generate unpleasant noises. To solve this problem, instead of increasing the self-weight of ductile iron manhole covers which similarly causes safety problems to workers during regular maintenance, the covers can be designed to be attached to the manhole frames which hold them in firm position.
Under what situation shall engineers use jacking at one end only and from both ends in pre-stressing work?
During pre-stressing operation at one end, frictional losses will occur and the pre-stressing force decreases along the length of tendon until reaching the other end. These frictional losses include the friction induced due to a change of curvature of tendon duct and also the wobble effect due to deviation of duct alignment from the centerline. Therefore, the pre-stress force in the mid-span or at the other end will be greatly reduced in case the frictional loss is high.
Consequently, pre-stressing, from both ends for a single span i.e. pre-stressing one-half of total tendons at one end and the remaining half at the other end is carried out to enable a even distribution and to provide symmetry of pre-stress force along the structure.
In fact, stressing at one end only has the potential advantage of lower cost when compared with stressing from both ends. For multiple spans (e.g. two spans) with unequal span length, jacking is usually carried out at the end of the longer span so as to provide a higher pre-stress force at the location of maximum positive moment. On the contrary, jacking from the end of the shorter span would be conducted if the negative moment at the intermediate support controls the pre-stress force. However, if the total span length is sufficiently long, jacking from both ends should be considered.
Rational Method should not be used for large catchments in estimating peak runoff. Is it true?
Rational Method is suitable for small catchments only because the time of concentration of small catchments is small. In Rational Method the peak runoff is calculated based on the assumption that the time of concentration is equal to the rainfall duration. For small catchments, this assumption may hold true in most circumstances. One of the assumptions of Rational Method is that rainfall intensity over the entire catchment remains constant during the storm duration.
However, in case of a large catchment it stands a high probability that rainfall intensity varies in various part of the large catchment. In addition, for long duration of rainfall, it is rare that the rainfall intensity remains constant over the entire rainstorm and a shorter duration but a more intense rainfall could produce a higher peak runoff. Moreover, a reduction of peak runoff is also brought about by the temporary storage of storm-water like channels within the catchment.
In actual condition, the runoff rate within the catchment varies from place to place because of different soil properties and past conditions. As suggested by Bureau of Public Roads (1965), sometimes the peak discharge occurs before all of the drainage area is contributing.
For instance, when a significant portion of drainage area within the catchment has very small time of concentration so that a higher rainfall intensity can be used for this portion, the runoff coming solely from this portion is higher than that of the whole catchment in which a lower rainfall intensity is adopted because the remaining part of the catchment has comparatively large time of concentration. Therefore, this results in incorrect estimation of peak runoff of large catchments if Rational Method is adopted.
In bridge widening projects, the method of stitching is normally employed for connecting existing deck to the new deck. What are the problems associated with this method in terms of shrinkage of concrete?
In the method of stitching, it is a normal practice to construct the widening part of the bridge at first and let it stay undisturbed for several months. After that, concreting will then be carried out for the stitch between the existing deck and the new deck. In this way, the dead load of the widened part of bridge is supported by itself and loads arising from the newly constructed deck will not be transferred to the existing deck which is not designed to take up these extra loads.
One of the main concerns is the effect of stress induced by shrinkage of newly widened part of the bridge on the existing bridge. To address this problem, the widened part of the bridge is constructed a period of time (say 6-9 months) prior to stitching to the existing bridge so that shrinkage of the new bridge will take place within this period and the effect of shrinkage stress exerted on the new bridge is minimized.
Traffic vibration on the existing bridge causes adverse effect to the freshly placed stitches.
To solve this problem, rapid hardening cement is used for the stitching concrete so as to shorten the time of setting of concrete. Moreover, the stitching work is designed to be carried out at nights of least traffic (Saturday night) and the existing bridge may even be closed for several hours (e.g. 6 hours) to let the stitching works to left undisturbed.
Sometimes, longitudinal joints are used in connecting new bridge segments to existing bridges. The main problem associated with this design is the safety concern of vehicles. The change of frictional coefficients of bridge deck and longitudinal joints when vehicles change traffic lanes is very dangerous to the vehicles. Moreover, maintenance of longitudinal joints in bridges is quite difficult.
Note: Stitching refers to formation of a segment of bridge deck between an existing bridge and a new bridge.
What are the limitations of Rational Method in calculating runoff?
Computation of runoff is a complicated matter which depends on many factors like the ground permeability, rainfall duration, rainfall pattern, catchment area characteristics etc. Basically, Rational Method is a means to find out the maximum discharge suitable for design purpose.
In this method, it is assumed that the rainfall duration is the same as the time of concentration and the return period of rainfall intensity is the same as the peak run-off. Time of concentration refers to the time required for the most remote location of storm-water inside the catchment to flow to the outlet. When the time of concentration is equal to the rainfall period, the maximum discharge occurs and rainfall collected inside the catchment comes to the same outlet point.
Rational Method provides the peak discharge only and it cannot produce a hydrograph. If a more detailed pattern of runoff is required, unit hydrograph or other methods have to be used. The accuracy of rational method depends very much on our correct selection of run-off coefficient and delineation of catchment area.
Rational Method is a rather conservative method. One of the basic assumptions of the rational formula is that the rainfall intensity must be constant for an interval at least equal to the time of concentration. For long duration of rainfall, this assumption may not hold true.
Moreover, the runoff coefficient in Rational Method is difficult to be determined accurately and it depends on many factors like moisture condition of soils, rainfall intensity and duration, degree of soil compaction, vegetation etc. In addition, In Rational Method the run-off coefficient is independent of rainfall intensity and this does not reflect the actual situation.
Is it desirable to use concrete of very high strength i.e. exceeding 60 MPa? What are the potential problems associated with such high strength concrete?
To increase the strength of concrete, say from 40 MPa to 80 MPa, it definitely helps in improving the structural performance of the structure by producing a denser, more durable and higher load capacity concrete. The size of concrete members can be significantly reduced resulting in substantial cost savings. However, an increase of concrete strength is also accompanied by the occurrence of thermal cracking. With an increase in concrete strength, the cement content is increased and this leads to higher thermal strains.
Consequently, additional reinforcement has to be introduced to control these additional cracks caused by the increase in concrete strength. Moreover, the ductility of concrete decreases with an increase in concrete strength. Attention should be paid during the design of high strength concrete to increase the ductility of concrete. In addition, fire resistance of high strength concrete is found to be less than normal strength concrete as suggested by Odd E. Gjorv (1994).
Though the tensile strength of high strength concrete is higher than that of normal concrete, the rate of increase of tensile strength is not proportional to the increase of compressive strength. For normal concrete, tensile strength is about one-tenth of compressive strength. However, for high strength concrete, it may only drop to 5% of compressive strength.
Moreover, owing to a low aggregate content of high strength concrete, creep and shrinkage increases.
Why do BS8007 specify the allowable crack width of water retaining structure as 0.2 mm for severe or very severe exposure?
For crack width less than 0.2 mm, it is assumed that the mechanism of autogenous healing will take place in which the crack will automatically seal up and this would not cause the problem of leakage and reinforcement corrosion in water retaining structure.
When the cracks are in inactive state where no movement takes places, autogenous healing occurs in the presence of water. However, when there is a continuous flow of water through these cracks, autogenous healing would not take place because the flow removes the lime. One of the mechanisms of autogenous healing is that calcium hydroxide (generated from the hydration of tricalcium silicate and dicalcium silicate) in concrete cement reacts with carbon dioxide in the atmosphere, resulting in the formation of calcium carbonate crystals.
Gradually these crystals accumulate and grow in these tiny cracks and form bonding so that the cracks are sealed. Since the first documented discovery of autogenous healing by the French Academy of Science in 1836, there have been numerous previous proofs that cracks are sealed up naturally by autogenous healing. Because of its self-sealing property,
Why is it preferable to design storm-water drains to match soffit?
Storm-water drains collect storm-water in their corresponding catchment areas during rainstorm and convey the collected water through outlets to the sea. Therefore, in considering the hydraulic design of storm-water drains, other than normal drainage pipe capacity to be taken into consideration, one should check the backwater effect due to tidal condition at outlets if the drains are located quite close to the downstream end of outlets.
Storm-water drains are normally designed to match soffit to avoid surcharging by backwater effect or when the downstream pipes are running full. Normally pipe size increases from upstream to downstream. For the case of matching drain invert, when outlet pipes are fully surcharged by tidal effect of the sea or when the downstream pipes are fully filled with storm-water, pipe sections immediately upstream of the outlet are also surcharged too.
However, for the case of matching pipe soffit, the immediate upstream sections of outlet pipes are not totally surcharged even though downstream pipes are running full. However, it is not always practical to maintain soffit for all pipelines because it requires sufficient drop to achieve this.
Moreover, the flow of storm-water is mainly by gravity in the design of storm-water drains.
In case the drains are designed to match invert, then it stands a high probability that the flow in the upstream smaller pipes has to be discharged against a head.
Note: Matching soffit means that all pipelines are aligned continuously with respect to the pipelines’ crown level.
When designing a water storage tank, should movement joints be installed?
In designing water storage tanks, movement joints can be installed in parallel with steel reinforcement. To control the movement of concrete due to seasonal variation of temperature, hydration temperature drop and shrinkage etc. two principal methods in design are used: to design closely spaced steel reinforcement to shorten the spacing of cracks, thereby reducing the crack width of cracks; or to introduce movement joints to allow a portion of movement to occur in the joints.
Let’s take an example to illustrate this. For 30 m long tanks wall, for a seasonal variation of 35 degree plus the hydration temperature of 30°C, the amount of cracking is about 8.8 mm. It can either be reduced to 0.3 mm with close spacing or can be absorbed by movement joints. Anyway, the thermal movement associated with the seasonal variation of 35°C is commonly accounted for by movement joints.
For water-retaining structure like pumping stations, the crack width requirement is even more stringent in which 0.2 mm for severe and very severe exposure is specified in BS8007. It turns out to a difficult problem to designers who may choose to design a heavy reinforced structure. Obviously, a better choice other than provision of bulky reinforcement is to allow contraction movement by using the method of movement joints together with sufficient amount of reinforcement. For instance, service reservoirs in Water Supplies Department comprise grids of movement joints like expansion joints and contraction joints.
What is the function of shear keys in the design of retaining walls?
In determining the external stability of retaining walls, failure modes like bearing failure, sliding and overturning are normally considered in design. In considering the criterion of sliding, the sliding resistance of retaining walls is derived from the base friction between the wall base and the foundation soils. To increase the sliding resistance of retaining walls, other than providing a large self-weight or a large retained soil mass, shear keys are to be installed at the wall base. The principle of shear keys is as follows:
The main purpose of installation of shear keys is to increase the extra passive resistance developed by the height of shear keys. However, active pressure developed by shear keys also increases simultaneously. The success of shear keys lies in the fact that the increase of passive pressure exceeds the increase in active pressure, resulting in a net improvement of sliding resistance.
On the other hand, friction between the wall base and the foundation soils is normally about a fraction of the angle of internal resistance (i.e. about 0.8 ϕ) where ϕ is the angle of internal friction of foundation soil. When a shear key is installed at the base of the retaining wall, the failure surface is changed from the wall base/soil horizontal plane to a plane within foundation soil. Therefore, the friction angle mobilized in this case is ϕ instead of 0.8ϕ in the previous case and the sliding resistance can be enhanced.
In the design of watermains, how to decide the usage of double air valves and single air valves?
Single air valves allow squeezing air out of the pipeline in automatic mode in high-pressure condition and are normally designed in high points of watermain in which air voids are present. Double air valves basically serve the same purpose except that it has another important function: it can get air into/out of the pipeline during low-pressure condition.
In WSD practice, watermain are normally divided into sections by installation of sectional valves to facilitate maintenance. In a single isolated pipeline section bounded by two sectional valves, at least a double air valve should be installed. During normal maintenance operation like cleansing of watermain, water inside pipelines is drawn from washout valves. However, as normal watermain is subject to very high pressure like 1.5 MPa and the sudden withdrawn of water will cause a transient vacuum condition and will damage the watermain. Therefore at least one double air valve should be present to allow air to squeeze in to balance the pressure and this protects the pipeline from damaging.
In essence, for local high points single air valves should be installed. Within a section of pipeline, at least one double air valve should be installed.
In checking the quality of weld, what are the pros and cons of various non-destructive weld inspection methods i.e. ultrasonic test, radiographic inspection and magnetic particle flaw detection test?
Currently, there are three common non-destructive testing of weld, namely radiographic inspection, ultrasonic testing and magnetic flaw detection test.
The method of radiographic approach was used commonly in the past until the arrival of ultrasonic inspection technique. The major difference between the two is that ultrasonic testing detects very narrow flaws which can hardly be detected by radiographic method. Moreover, it is very sensitive to gross discontinuities. Tiny defects, which characterize welding problems, are normally not revealed by radiographic inspection.
Moreover, ultrasonic inspection possesses the advantages that it can accurately and precisely locate a defect as well as figure out its depth, location and angle of inclination.
In the past, it was expensive to adopt ultrasonic means for inspection. Nowadays, the rates for both inspection methods are comparable. Most importantly, the x-ray and gamma ray used in radiographs are radioactive and pose potential safety hazard to testing technicians on site. Reference is made to Paul G. Jonas and Dennis L. Scharosch.
Magnetic flaw detection test can only be used for checking flaws in any metallic objects.
This method is commonly used for inspecting surface cracks and slightly sub-surface cracks. However, surface and sub-surface cracks can be readily detected by radiographs and ultrasonic inspection.
If a contractor proposes to increase concrete cover beyond contractual specification (i.e. 40 mm to 70 mm), shall engineers accept the proposal?
In contractual aspect, based on the requirement of General Specification of Civil Engineering Works (1992 Edition), the tolerance of concrete cover is between +5 mm and -5 mm and engineers should not accept sub-standard work because they do not possess the authority to change the acceptance criteria. In case engineers consider contractor’s proposal acceptable in technical point of view, consent has to be sought from the employer regarding the changes in acceptance criteria.
From technical point of view, the effect on cracking due to an increase in concrete cover should be considered. In general, there are three main parameters which govern crack width, namely tensile strain at the point considered, the distance of longitudinal bar to the concerned point and the depth of tension zone.
For the second factor, i.e. proximity of longitudinal bars to point of section, the closer a bar is to this point; the smaller is the crack width. Therefore, closely spaced bars with smaller cover will give narrower cracks than widely spaced bars with larger cover. Therefore, with an increase of concrete cover, the crack width will increase which is undesirable.
Can a concrete structure be completely free of expansion joints and contraction joints?
Consider that the concrete structure is not subject to the problem of differential settlement.
For contraction joints, it may be possible to design a concrete structure without any contraction joints. By using sufficient steel reinforcement to spread evenly the crack width over the span length of the structure, it may achieve the requirement of minimum crack width and cause no adverse impact to the aesthetics of the structure. However, it follows that the amount of reinforcement required is higher than that when with sufficient contraction joints.
For expansion joints, the consequence of not providing such joints may be difficult to cater for. For example, a concrete structure has the coefficient of thermal expansion of 9 × 10-6 /°C and a Young’s modulus of 34.5kN/mm². With an increase of temperature of 20°C and it is restricted to free expansion, then the structure is subject to an axial stress of 6.21 MPa. If the structure is very slender (e.g. concrete carriageway), buckling may occur. Therefore, the structure has to be designed to take up these thermal stresses if expansion joints are not provided.
However, for water retaining structures, most of them are not affected by weather conditions because they are insulated from the water they contain internally and soil backfill that surround them. Therefore, it is expected that a smaller amount of thermal movement will occur when compared with normal exposed concrete structure. Consequently, expansion joints may be omitted in this case with the view that the compressive stress induced by thermal expansion toughens the structure to limit the development of tensile stress.
What is the difference between “hammer efficiency” and “coefficient of restitution” when using Hiley’s formula in pile driving?
Hammer efficiency refers to the ratio of kinetic energy of the hammer to the rate energy (or potential energy). In essence, there is undoubtedly certain energy loss induced by the hammer itself prior to the actual impact on the driven pile. For instance, these losses may include misalignment of the hammer, energy losses due to guiding friction, inaccurate dropping height etc.
Coefficient of restitution refers to a value indicating the strain energy during collision regained after the bodies reverted back to their original shapes. If the coefficient of restitution is equal to unity, it means that the collision is elastic and all energy has been returned after the impact action. Hence, this is a index showing the degree the impact action in terms of elasticity.
In mathematical forms,
Coefficient of restitution = -(v₁ – v₂)/ (u₁ – u₂)
Where u = initial velocity and v = final velocity after impact
What are the functions of different components of a typical expansion joint?
In a typical expansion joint, it normally contains the following components: joint sealant, joint filler, dowel bar, PVC dowel sleeve, bond breaker tape and cradle bent.
Joint sealant: it seals the joint width and prevents water and dirt from entering the joint and causing dowel bar corrosion and unexpected joint stress resulting from restrained movement.
Joint filler: it is compressible so that the joint can expand freely without constraint. Someone may doubt that even without its presence, the joint can still expand freely. In fact, its presence is necessary because it serves the purpose of space occupation such that even if dirt and rubbish are intruded in the joint, there is no space left for their accommodation.
Dowel bar: This is a major component of the joint. It serves to guide the direction of movement of concrete expansion. Therefore, incorrect direction of placement of dowel bar will induce stresses in the joint during thermal expansion. On the other hand, it links the two adjacent structures by transferring loads across the joints.
PVC dowel sleeve: It serves to facilitate the movement of dowel bar. On one side of the joint, the dowel bar is encased in concrete. On the other side, however, the PVC dowel sleeve is bonded directly to concrete so that movement of dowel bar can take place. One may notice that the detailing of normal expansion joints in Highways Standard Drawing is in such a way that part of PVC dowel sleeve is also extended to the other part of the joint where the dowel bar is directly adhered to concrete. In this case, it appears that this arrangement prevents the movement of joint.
If this is the case, why should designers purposely put up such arrangement? In fact, the rationale behind this is to avoid water from getting into contact with dowel bar in case the joint sealant fails. As PVC is a flexible material, it only minutely hinders the movement of joint only under this design.
Bond breaker tape: As the majority of joint sealant is applied in liquid form during construction, the bond breaker tape helps to prevent flowing of sealant liquid inside the joint.
Cradle bar: It helps to uphold the dowel bar in position during construction.
What are the considerations in determining whether casings should be left in for mini-piles?
Contrary to most of pile design, the design of min-piles is controlled by internal capacity instead of external carrying capacity due to their small cross-sectional area.
There are mainly two reasons to account for designing mini-piles as friction piles:
Due to its high slenderness ratio, a pile of 200 mm diameter with 5 m long has a shaft area of 100 times greater than cross-sectional area. Therefore, the shaft friction mobilized should be greater than end resistance.
Settlements of 10%-20% of pile diameter are necessary to mobilize full end bearing capacity, compared with 0.5%-1% of pile diameter to develop maximum shaft resistance.
Left-in casings for mini-piles have the following advantages:
Improve resistance to corrosion of main bars;
Provide additional restraint against lateral buckling;
Improve the grout quality by preventing intrusion of groundwater during concreting;
Prevent occurrence of necking during lifting up of casings during concreting.
If on-site slump test fails, should engineers allow the contractor to continue the concreting works?
This is a very classical question raised by many graduate engineers. In fact, there are two schools of thought regarding this issue.
The first school of thought is rather straightforward: the contractor fails to comply with contractual requirements and therefore as per G. C. C. Clause 54 (2)(c) the engineer could order suspension of the Works. Under the conditions of G. C. C. Clause 54(2)(a) – (d), the contractor is not entitled to any claims of cost which is the main concern for most engineers. This is the contractual power given to the Engineer in case of any failure in tests required by the contract; even though some engineers argue that slump tests are not as important as other tests like compression test.
The second school of thought is to let the contractor to continue their concreting works and later on request the contractor to prove that the finished works comply with other contractual requirements e.g. compression test. This is based upon the belief that workability is mainly required to achieve design concrete compression strength. In case the compression test also fails, the contractor should demolish and reconstruct the works accordingly. In fact, this is a rather passive way of treating construction works and is not recommended because of the following reasons:
Workability of freshly placed concrete is related not only to strength but also to durability of concrete. Even if the future compression test passes, failing in slump test indicates that it may have adverse impact to durability of completed concrete structures.
In case the compression test fails, the contractor has to deploy extra time and resources to remove the work and reconstruct them once again and this slows down the progress of works significantly. Hence, in view of such likely probability of occurrence, why shouldn’t the Engineer exercise his power to stop the contractor and save these extra time and cost?
In piling works, normally founding levels of bored piles are defined by using total core recovery or rock quality designation (RQD). Are there any problems with such specification?
The use of total core recovery to determine the founding level may not be able to indicate the quality of rock foundation for piles because it depends on the drilling technique and drilling equipment (GEO (1996)). For instance, if standard core barrels are used to collect samples, it may indicate sufficient core recovery for samples full of rock joints and weathered rock. On the other hand, if triple tube barrels are used for obtaining soil samples, samples with joints and weathered rock can also achieve the requirements of total core recovery.
In case RQD is adopted for determining founding levels, it may also result in incorrect results. For instance RQD does not indicate the joints and infilling materials. Moreover, as it only measures rock segments exceeding 100 mm, rock segments exceeding 100 mm is considered to be of good quality rock without due consideration of its strength and joint spacing.
What is the purpose of post-grouting for mini-piles?
Post-grouting is normally carried out some time when grout of the initial grouting work has set (e.g. within 24 hours of initial grouting). It helps to increase the bearing capacity of mini-piles by enhancing larger effective pile diameter. Moreover, it improves the behavior of soils adjacent to grouted piles and minimizes the effect of disturbance caused during construction. In essence, post-grouting helps to improve the bond between soils and grout, thereby enhancing better skin friction between them.
During the process of post-grouting, a tube with a hole at its bottom is lowered into the pile and grout is injected. The mechanism of post-grouting is as follows: the pressurized grout is initially confined by the hardened grout and can hardly get away. Then, it ruptures the grout cover and makes its way to the surrounding soils and into soft regions to develop an interlock with harder soil zones.
In order to enhance the pressurized grout to rupture the initial grout depth, a maximum time limit is normally imposed between the time of initial grouting and time of post-grouting to avoid the development of high strength of initial grout. Consequently, the effect of soil disturbance by installation of casings and subsequent lifting up of casings would be lessened significantly.
What is the purpose of conducting load test for piling works?
Pile load test provides information on ultimate bearing capacity but not settlement behavior. In essence, it can determine if the load is taken up by the stratum designed or if the centre of resistance is at the design location in piles as suggested by Robert D. Chellis (1961).
After conducting load tests, the curve of movement of pile head (Settlement against load) and the curve of plastic deformation can be plotted. By subtracting the curve of plastic deformation from the curve of pile head movement at each load, the curve of elastic deformation can be obtained. For piles of end-bearing type unrestrained by friction, the theoretical elastic deformation can be calculated from e = RL/AE where e is elastic deformation, ‘L’ is pile length, ‘A’ is area of pile, ‘E’ is Young’s Modulus of pile material and ‘R’ is the reaction load on pile. By substituting e in the formula, the elastic deformation read from the curve of elastic deformation, ‘L’ can be obtained which shows the location of the centre of resistance corresponding to that load.
In modeling a non-rigid mat foundation by using elastic springs, should a uniform modulus of sub-grade reaction be used along the whole base of mat?
By using a bed of springs to simulate the flexible behavior of mat subject to loads, care should be taken in selection of the modulus of sub-grade reaction. In fact, the modulus of sub-grade reaction depends on many factors like the width of the mat, the shape of the mat, the depth of founding level of the mat etc. In particular, the modulus of sub-grade reaction is smaller at the center while it is larger near the mat’s edges.
If a constant modulus of sub-grade reaction is adopted throughout the width of the mat, then a more or less uniform settlement will result when subject to a uniform load. However, the actual behavior is that settlement in the center is higher than that at side edges. Consequently, it leads to an underestimation of bending moment by 18% to 25% as suggested by Donald P. Coduto (1994).
In general, a constant value of modulus of sub-grade reaction is normally applied for structure with a rigid superstructure and the rigid foundation. However, a variable modulus of sub-grade reaction is adopted instead for non-rigid superstructure and non-dominance of foundation rigidity to account for the effect of pressure bulbs.