Question No. 136
What is difference in purpose for conducting surface regularity test and sand patch test (test on texture depth)?
Answer: The purpose of surface regularity test is to measure the riding quality of pavements and the same requirement and standard is applied to both concrete and bituminous carriageway. On the other hand, sand patch test is used for checking the skid resistance of road pavements. Moreover, it is related to traffic noise because the intensity of noise generated from road traffic is related to texture depth of carriageway.
Question No. 137
For typical pile bents in marine piers, how is vertical loads related to horizontal capacity of the pile bents?
Answer: Let’s consider a pile bent with a top slab supported by two ranking piles, each inclining at an equal angle to the pier slab. In designing such a system, truss action is normally adopted to analyze the pile bent. When the reaction forces of these piles, horizontal forces (e.g. due to berthing and de-berthing of vessels) and vertical forces (e.g. superimposed deck loads) are analyzed by drawing a force polygon, it is noted that lateral resistance of the pile bent is dependent on the vertical load, i.e. lateral resistance is small when vertical loads are high.
Question No. 138
What is kn measurement?
Answer: kn (both letters lowercase) is knots, which is a speed nautical miles per hour.
Question No. 139
In determining the effective stress parameters of a soil sample, which test is preferable, consolidated un-drained test or consolidated drained test?
Answer: The effective stress parameters of a soil sample can be obtained from both consolidated un-drained test and consolidated drained test. However, consolidated un-drained test is normally selected because of the following reasons:
- Time taken for consolidated un-drained test is shorter than that of consolidated drained test. It is because consolidated drained test requires the full dissipation of excess pore water pressure of the soil during testing and it takes long time when soils of low permeability are tested.
- Useful information can be obtained from the stress path of consolidated un-drained test.
- Failure occurs in lower stress level when compared with consolidated drained test.
Question No. 140
What are the design considerations for dowel bars in joints of concrete carriageway?
Answer: The behavior of dowel bars resembles that of piles in soils subject to lateral loads. Failure of the joint occurs by yielding of steel under bending action or by crushing of concrete due to bearing stresses.
In general, the spacing of dowel bars should not be too close which may pose problems during construction. However, it should be not too wide to allow the occurrence of bending between the dowel bars. On the other hand, regarding the length of dowel bars, it should not be too long because the induced stress at the end of long dowel bars is insignificant and is not effective in transferring loads between adjacent concrete panels. However, if the dowel bars are too short, the stress at the face of joint is increased resulting in concrete crushing. Reference is made to Ministry of Transport (1955).
Question No. 141
What is the function of a sand layer underlying paving slab/interlocking blocks?
Answer: Normally after the laying of sub-base layer of the paving slab/interlocking blocks, a 30 mm thick sand bedding is screeded and tamped over the pavement area. Then paving slabs are laid horizontally with joints of 2-3 mm wide and are laid in uphill direction. After completing the laying of paving slabs, sand used for filling joints is spread over the surface of the units and brushed into the joints such that all joints are completely filled. The paving slabs are then bedded into final position by using plate vibrators.
The sand layer serves the same purpose of normal blinding layer under concrete structure:
- Provide a level and flat surface for the paving slab/interlocking blocks to lay on;
- Protect the foundation (i.e. underground sub-grade and sub-base layer) against adverse outside conditions (e.g. bad weather) during construction of the laying work of paving slab/interlocking blocks.
Question No. 142
How do we calculate absolute pressure?
Answer: Absolute is equal to gauge pressure plus atmospheric.
Question No. 143
What is the relation of bearing pressure on soil nail head to the ratio La /Lb, where
La is the length of soil nail before the potential slip circle while Lb is the length of soil nail beyond the potential slip circle?
Answer: The unstable soil mass before the potential circular slip is resisted by two components: soil nail head bearing pressure and friction of soil nail in the unstable soil mass. Therefore, the longer is the length of soil nail before potential slip circle La, the higher is the proportion of forces being resisted by frictional forces and hence the smaller amount is to be resisted by soil nail head. Hence, the smaller the ratio La/Lb, the greater is the resistance provided by soil nail head.
Question No. 144
In dredged reclamation, what are the considerations in selecting between trailer suction hopper dredgers and grab dredgers?
Answer: Trailer suction hopper dredgers are vessels which remove material off the seabed through hydraulic suction by using pumps. During the dredging operation, a mixture of soil and water is transported through suction pipe to storage hoppers. Significant turbulence inside the hoppers keeps the dredged mixture in suspension and this should be minimized to enhance the material to settle swiftly prior to the process of overflowing. Trailer suction hopper dredgers are mounted with drag-head or dragarm pumps which increases the dredging depth and trims down the occurrence of cavitation as suggested by John B. Herbioh (1992). This machine is limited to dredging relatively low-strength material. Moreover, they cannot be deployed in very shallow waters and instead grab dredgers should be used. However, its dredging capacity is higher than that of grab dredger and it can be mobilized in relatively deep-water region.
Trailer suction hopper dredgers are renowned for their mobility, versatility and capability to operate in unfavourable sea conditions.
Question No. 145
Can a sub-base layer serve as a drainage layer to remove water from pavement?
Answer: Besides providing load distribution in post-construction stage and working platform in construction stage, sub-base can also serve as a drainage layer to remove water coming from the pavement. For thick layers of road pavement, it is likely that the water leakage from pavement is insignificant and therefore the sub-base layer mainly serves to support the traffic stresses. However, for thin pavement layers, water penetration is quite substantial and therefore sub-base may also act as a drainage layer to remove this water.
Question No. 146
What is the origin of name Railway Sleepers?
Answer: When sleepers were added below the railway, lines the trains ran more smoothly and people fell asleep on their journeys.
Question No. 147
What is the mechanism of formation of soil plugs in marine driven steel piles with reinforced concrete infill?
Answer: During initial driving process, open-ended steel piles are driven through the soils at their bases. However, shaft friction will gradually develop between the steel piles and soils inside piles at some time after pile driving. The hitting action of driving hammers induces forces to the soil and later it comes to a stage when the inertial forces of inside soils, together with the internal frictional forces exceeding the bearing capacity of soils at pile toes. Consequently, the soil plug formed is brought down by the piles. Reference is made to M. J. Tomlinson (1977).
Note: A soil plug is a column of soil formed at the bottom portion of marine pile type of steel tubular piles with reinforced concrete infill.
Question No. 148
What is the design level of landings in piers?
Answer: Landings are designed as resting place for passengers during berthing and de-berthing of vessels. In general, landings are provided near mean high and mean low water levels to facilitate embarking and disembarking of passengers (BS6349: Part 2: 1988). Therefore, the level of landing steps should be different from place to place because of different mean high and mean low water levels in different locations.
Question No. 149
What are the advantages of assigning the central pier and the abutment as fixed piers?
- For abutment pier to be assigned as fixed pier while the bridge is quite long, the longitudinal loads due to earthquake are quite large. As the earthquake loads are resisted by fixed piers, the size of fixed piers will be large and massive. In this connection, for better aesthetic appearance, the selection of abutment as fixed piers could accommodate the large size and massiveness of piers. Normally abutments are relatively short in height and for the same horizontal force; the bending moment induced is smaller.
- For the central pier to be selected as the fixed pier, the bridge deck is allowed to move starting from the central pier to the end of the bridge. However, if the fixed pier is located at the abutment, the amount of movement to be incorporated in each bearing due to temperature variation, shrinkage, etc. is more than that when the fixed pier is located at central pier. Therefore, the size of movement joints can be reduced significantly.
Question No. 150
What are the major problems in using pumping for concreting works?
Answer: In pumping operation, the force exerted by pumps must overcome the friction between concrete and the pumping pipes, the weight of concrete and the pressure head when placing concrete above the pumps. In fact, as only water is pumpable, it is the water in the concrete that transfers the pressure.
The main problems associated with pumping are the effect of segregation and bleeding. To rectify these adverse effects, the proportion of cement is increased to enhance the cohesion in order to reduce segregation and bleeding. On the other hand, a proper selection of aggregate grading helps to improve the pumpability of concrete.
Question No. 151
How long does it take concrete to harden?
Answer: It takes 28 days to cure.
Question No. 152
For unreinforced concrete carriageway, what is the sequence of closing and opening of expansion joints and contraction joints?
Answer: Let’s take an example to illustrate the sequence of closing and opening of joints (Ministry of Transport (1955). For instance, an unreinforced concrete carriageway is constructed in winter. When temperature rises in the following summer, the section between expansion joints will expand as a whole single element resulting in the closure of expansion joints. This section of concrete pavement will move outwards from the mid-point between the expansion joints. In the next winter, each bay (i.e. concrete pavement between adjacent contraction joints) of concrete contracts about the midpoint of its length with opening of contraction joints.
Question No. 153
At what point does a circle start losing area when being flattened?
Answer: The area ratio decreases slowly at first then more rapidly.
The area of an ellipse is A = π × a × b, ‘a’ and ‘b’ are the ½ major and ½ minor axis. If a = r = b the area is A = π × r², which is a circle. Not hard to graph this but you will have to write the area of the ellipse in terms of the circumference.
Question No. 154
For underwater concreting, tremie pipes are normally used with the aid of hoppers. Sometimes tubes are inserted inside the hoppers. Why?
Answer: In placing concrete by tremie pipes, hoppers are connected to their top for receiving freshly placed concrete. However, air may be trapped inside the tremie pipes if there is rapid feeding of fresh concrete. To release the trapped air inside the tremie pipes, hoses (called ventilation tubes) are inserted and lowered down through the hoppers. Reference is made to Carl A. Thoresen (1988).
Question No. 155
What are the functions of bedding under storm-water drains?
Answer: Bedding, which is normally made of granular materials or concrete, serves four main functions as suggested by O. C. Young and J. J. Trott:
- To enhance a uniform support under pipes in order to reduce the bending moment longitudinally;
- To increase the load-supporting strength of the pipes;
- For pipes with spigot and socket joints, it enables pipes to be supported along pipe lengths instead of pipe sockets. Otherwise, uneven stress may be induced and it may damage the pipes;
- To provide a platform for achieving correct alignment and level during and after construction.
Question No. 156
What are the strongest shapes in building?
Answer: The pillars
Question No. 157
In a curved pre-stressed bridge, how should the guided bearings in piers of the curved region be oriented with respect to the fixed bearing in abutment?
Answer: To determine the orientation of guided bearings, one should understand the movement of curved region of a pre-stressed bridge. Movement of pre-stress and creep are tangential to the curvature of the bridge (or along longitudinal axis) while the movement due to temperature and shrinkage effects are in a direction towards the fixed pier. If the direction of guided bearings is aligned towards the fixed bearing in the abutment, the difference in direction of pre-tress and creep movement and the guided direction towards fixed bearing would generate a locked-in force in the bridge system. The magnitude of the lock-in force is dependent on the stiffness of deck and supports. If the force is small, it can be designed as additional force acting on the support and deck. However, if the force is large, temporary freedom of movement at the guided bearings has to be provided during construction.
Question No. 158
If concrete compression test fails, should Schmidt hammer test be adopted as an alternative test to prove the concrete strength?
Answer: The Schmidt hammer test is based on the elastic rebound of hammer which presses on concrete surface and it measures the surface hardness of concrete. Since the test is very sensitive to the presence of aggregates and voids at the concrete surface, it is necessary to take more than 10 readings over the area of test. However, it should be noted that Schmidt hammer test measures surface hardness only but not the strength of concrete. Therefore, it may not be considered a good substitute for concrete compression test.
Question No. 159
What is the advantage of sliding bearings over roller bearings?
Answer: In roller bearing for a given movement the roller bearing exhibit a change in pressure centre from its original position by one-half of its movement based on David J. Lee. However, with sliding bearing a sliding plate is attached to the upper superstructure and the moving part of bearing element is built in the substructure. It follows that there is no change in pressure center after the movement.
Question No. 160
How do you maintain water pressure?
Answer: If you have water pressure and wish to maintain it, do not cause flow in the line, which will reduce pressure due to friction.
To keep pressure up, reduce friction by increasing the line size or eliminating some other restriction.
Question No. 161
What is the difference in arranging pumps in series and in parallel?
Answer: For identical pumps with similar functions, if the pumps arranged in series, the total head is increased without a change to maximum discharge. On the other hand, for pumps arranged in parallel to one another, the discharge is increased without any changes to maximum head.
Question No. 162
What is the function of rebate in a typical construction joint?
Answer: Construction joints are created on sites to facilitate the construction process. However, if improperly constructed, the completed construction joints will leave an uneven scar on the concrete surface and affect significantly its appearance. To avoid this, a rebate is formed during the first pour of one side of construction joint. After the other pour is concreted, it will hide the uneven joint inside the rebate.
Question No. 163
Shall a layer of wearing course or additional thickness be designed on the surface of piers?
Answer: In the design of piers, consideration should be given to the effect of wearing action caused by passengers, other traffics and even sometimes vehicles. In maritime environment, the durability and integrity of concrete is detrimental to the servicing life of piers because it acts an essential barrier to chloride attack. However, in view of these gradual wear and tear generated by the loading traffic, some forms of surface protection should be provided on top of pier surface like wearing course or additional increase in concrete cover.
Question No. 164
The spacing of manholes in straight sections for different pipe sizes is stated in Storm-water Drainage Manual. How are these figures arrived at?
- For pipe size < 300 mm, rodding is usually adopted in which workers place about 1 m long rods through the pipes to the location of blockage and manually operate the rod to clear the blockage.
- For pipe size < 700 mm, water-jetting is normally employed in which water is supplied from nearby fire hydrants and pressurized water jet is used for clearing blockage.
- Winching method is adopted for all sizes of pipes.
For instance, for pipe size exceeding 1050 mm, it is stated in Storm-water Drainage Manual that maximum intervals between manholes along straight lengths should be 120 m. This is because for sizes over 1050 m, the main method of pipe maintenance is by winching whose maximum length of operation is 120 m. Similarly, the maximum intervals of manholes for other straight pipes are derived from their corresponding maintenance methods.
Question No. 165
What is unit weight of normal concrete?
Answer: 145 PCF
Question No. 166
General Specification for Civil Engineering Works (1992 Edition) Clause 15.09 specifies that tying wires for reinforcement adjacent to and above Class F4 and F5 finishes should be stainless steel wires. Why?
Answer: If plain steel tying wires are used for reinforcement adjacent to Class F4 and F5 finishes, it poses the problem of rust staining which may impair the appearance of exposed concrete surfaces. The rate of corrosion of plain steel tying wires is similar to normal steel reinforcement. However, for tying wires with very small diameter, upon long exposure it stands a high chance of rusting completely and this rust will stain the formwork and significantly affect the concrete finish. Therefore, stainless steel tying wires are specified for locations in the vicinity of high quality of finishes to avoid rust staining by corroded typing wires.
Note: Tying wires are wires used for fixing and connecting steel reinforcement bars.
Question No. 167
Which architectural movement is also known as Jazz Moderna?
Answer: Art nouveau
Question No. 168
Should raking piles of a bridge abutment be placed under an embankment?
Answer: For a bridge abutment to be supported on raking piles with different orientations, the movement between the ground and the pile group is difficult to predict. For instance, if some of the raking piles of the bridge abutment are extended beneath an embankment, then the settlement of embankment behind the abutment may cause the raking piles to experience severe bending moment and damage the piles as recommended by Dr. Edmund C Hambly (1979).
Question No. 169
In designing of access ramps for drainage channels, why should the direction of access ramps be sloping down towards downstream?
Answer: In the design of access ramps, the direction is normally specified to be sloping down towards downstream so as to avoid the occurrence of over-shooting of flowing water for supercritical flow in case of aligning the ramps in the reverse direction of channel flow.
Note: Access ramps refer to ramps used for maintenance vehicles during routine maintenance of channels.
Question No. 170
What is the difference in application between open stirrups and closed stirrups in concrete beams?
Answer: Open stirrups are provided principally to resist shear forces in concrete beams and they are applied in locations in which the effect of torsion is insignificant. U-shaped stirrups are placed in the tension side of concrete beams in which shear cracks would occur. However, when concrete beams are designed to resist a substantial amount of torsion, closed stirrups should be used instead.
Question No. 171
How do you convert wind speed into force?
Question No. 172
Why are voids filled with lightweight infilling material in raft foundation of pumping stations?
Answer: To reduce the dead load and hence to reduce the settlement of pumping stations, the voids inside the raft foundations are filled with light material. If instead concrete is placed inside these voids, it poses severe thermal cracking problem and drastically increases loads to pumping stations. The use of general fill is also not desirable because its self-weight is comparable to that of concrete. On the contrary, if these voids are left vacant, water may penetrate into these voids during future operation and increases the dead load of pumping stations during its normal operation. Therefore, lightweight infill material, which is non-water-absorbing and non-biodegradable, is designed inside these voids to avoid ingress of water and to reduce the dead load of the structure.
Question No. 173
Nowadays, most flap valves are made of HDPE. What are the advantages of using HDPE when compared with cast iron?
- It has no reaction with sewage and seawater and does not suffer from the corrosion problem associated with cast iron.
- No protective coating is required and it is almost maintenance-free.
- HDPE flap valves require very low opening pressure in operation (like 5mm water level difference). For cast iron flap valves, due to its own heavy self-weight, the required opening pressure of cast iron flap valves is higher than that of HDPE flap valves. This criterion is essential for dry weather flow conditions.
However, the pressure resistance of HDPE flap valves is not as good as cast iron flap valves. For instance, a typical 450 mm wide HDPE flap valve can only withstand about 5 m water column.
Question No. 174
Does brick grows bigger every year.
Answer: Usually bricks do not grow, but since stuff gets all over, it looks like it grows.
Question No. 175
Are diaphragms necessary in the design of concrete box girder bridges?
Answer: Diaphragms are adopted in concrete box girder bridges to transfer loads from bridge decks to bearings. Since the depth of diaphragms normally exceeds the width by two times, they are usually designed as deep beams. However, diaphragms may not be necessary in case bridge bearings are placed directly under the webs because loads in bridge decks can be directly transferred to the bearings based on Jorg Schlaich & Hartmut Scheef (1982). This arrangement suffers from the drawback that changing of bearings during future maintenance operation is more difficult.
In fact, diaphragms also contribute to the provision of torsional restraint to the bridge deck.
Question No. 176
How do engineer determine the number of cells for concrete box girder bridges?
Answer: If the depth of a box girder bridge exceeds 1/6 or 1/5 of the bridge width, then it is recommended to be designed as a single cell box girder bridge. However, if the bridge depth is smaller than 1/6 of the bridge width, then a twin-cell or multiple cell is a better choice as suggested by Jorg Schlaich & Hartmut Scheef (1982). However, one should note that even for wider bridges with small depths, the number of cells should be minimized because there is not much improvement in transverse load distribution when the number of cells of box girder is increased to three or more.
Question No. 177
What is the quantity of cement in Mix M20 concrete?
Answer: For nominal Mix Cement consumption for M25 will be 570 kg per cum. and for Design Mix it can be optimized to 510 kg per cum.
Question No. 178
When a drainage system (i.e. u-channels with catch-pits) is connected to a main drainage channel, a segment of short pipe is used. What is the reason of such arrangement?
Answer: There are three scenarios of such connection arrangement:
- A new drainage system is connected to an existing drainage channel
- An existing drainage system is connected to a new drainage channel
- A new drainage system is connected to a new drainage channel.
For all scenarios, what engineers consider is the total amount of differential settlement or lateral movement to be encountered between the drainage system and main drainage channel. For scenario (b) and (c), it is very likely that substantial differential settlement will occur and this will cause damage to the connecting concrete pipes. Therefore a segment of short pipes are designed so that they serve to provide flexibility to the pipes in case of uneven settlement occurring between drainage system and main drainage channels.
Question No. 179
What glue is least sticky?
Answer: Elmer’s glue is least sticky.
Question No. 180
For column reinforcements, why is helical reinforcement sometimes designed instead of normal links?
Answer: The use of links for column design in Britain is very popular. However, in U.S.A. engineers tend to use helical reinforcement instead of normal links because helical reinforcement has the potential advantage of protecting columns/piles against seismic loads. Moreover, when the columns reach the failure state, the concrete outside hoops cracks and falls off firstly, followed by the eventual failure of the whole columns. The peeling off of concrete outside helical reinforcement provides a warning signal before the sudden failure of columns as suggested by G. P. Manning (1924). In addition, it can take up a higher working load than normal link reinforcement.
For instance, helical reinforcement is adopted in the design of marine piles in Government piers.
Note: Helical reinforcement refers to shear reinforcement which is spiral in shapes.