Question No. 91
After the construction of watermain, prior to hydrostatic pressure test, swabbing is carried out. What is the purpose of swabbing?
Answer: Pipelines should be tested before commissioning to check the strength of watermain and the absence of leak. Before carrying out hydrostatic pressure test, swabbing is conducted to clear out rubbish and dirt left inside the pipeline during construction. Swabbing is required for pipes less than 600mm diameter because for larger size of pipes, they can be inspected internally to ensure cleanliness.
After carrying out of hydrostatic pressure test, test for water sterilization is then conducted which involves collecting water sample from the pipeline. The purpose is to check the water quality like colour, turbidity, odour, pH value, conductivity etc. and is compared with the quality of water drawn from water supply point.
Question No. 92
What are the functions of grout inside tendon ducts?
Answer: Grout in pre-stressing works serves the following purposes:
- Protect the tendon against corrosion.
- Improve the ultimate capacity of tendon.
- Provide a bond between the structural member and the tendon.
- In case of failure, the anchorage is not subject to all strain energy.
Question No. 93
Is the procurement of third party insurance necessary to be incorporated in contract for construction works?
Answer: The purpose of third party insurance is to protect contractors from bankruptcy in case there are severe accidents happened to the third party due to the construction work. Therefore, in government contracts, contractors are requested contractually to procure third party insurance from the commencement of contract until the end of Maintenance Period. If contractors have the financial capability to handle the claims due to accidents to third party, the client is not bound to include this requirement in the contract.
Question No. 94
What is absolute pressure?
Answer: Absolute pressure is simply the addition of the observed gage pressure plus the value of the local atmospheric pressure.
Question No. 95
In the design of service reservoirs, horizontal reinforcement in walls of reservoirs is placed at the outer layer. Why?
Answer: Since service reservoirs are designed as water-retaining structures with stringent requirement of crack width control, the design of reinforcement of service reservoirs is under the control of serviceability limit state. For the walls of service reservoirs, contraction and expansion of concrete are more significant in the horizontal direction of walls because of their relatively long lengths when compared with heights. In this connection, in order to minimize the usage of reinforcement, horizontal bars are placed at the outmost layer so that the distance of reinforcement bars to concrete surface is reduced. Since the shorter is the distance to the point of concern, the smaller is the crack width and hence with such reinforcement arrangement advantages are taken if the reinforcement bars in the critical direction are placed closest to concrete surface.
Question No. 96
Can grout replace concrete in normal structure?
Answer: The mixture of cement and water alone cannot replace concrete (Longman Scientific and
Technical (1987)) because:
- Shrinkage of grout is several times that of concrete with the same mass.
- The effect of creep of grout is far more than that of concrete.
- Heat of hydration of cement with water is more than normal concrete and this leads to the problem of severe cracking.
Question No. 97
Under what situation should engineers use pot bearings instead of elastomeric bearings?
Answer: In the event of high vertical loads combined with large angle of rotations, rubber bearings are undesirable when compared with pot bearings. For instance, elastomeric bearings require large bearing surfaces so that compression can be maintained between the contact surfaces between the bearings and piers. Moreover, it also leads to uneven distribution of stress on the piers and some of these highly induced stresses may damage the piers. Consequently, pot bearings are better alternatives than elastomeric bearings in such a scenario as suggested by David J. Lee.
Question No. 98
How high should the water tank be to provide a 60 PSI at base of this water tower?
2.31 ft of water = 1 PSI
Question No. 99
What is sucker deck principle for variable depth bridge decks?
Answer: For a variable depth bridge deck, the depth of continuous multi-span bridge deck is increased in pier supports and this absorbs sagging moments in the mid-span with the consequent increase in hogging moments in pier supports. As a result, the mid-span depth can be significantly reduced due to the reduction in sagging moment. In essence, this sucker deck principle is applied in locations where headroom requirement is of great concern. Moreover, in terms of structural performance, sucker decks are effective in reducing dead loads than voided slab of equivalent uniform depth for span length between 20-40 m. In terms of aesthetics point of view, the public tends to appreciate the structural form of arches and curved soffit rather than boring uniform deck alignment. Reference is made to Brian Pritchard (1992).
Question No. 100
What is a projection line?
Answer: Projection line is the way, in which the earth is shown on a flat piece of Paper.
Question No. 101
In pre-cast concrete jacking pipes, sometimes grout holes are designed inside these pre-cast pipes. Why?
Answer: Grout holes are present in pre-cast jacking pipes for the following reasons:
- They serve as the locations for injection of bentonite or other lubricant. Lubricant is used for both granular soils and cohesive soils to trim down the frictional resistance. For cohesive soils, the soils cannot get onto the pipes by the presence of lubricant and the shearing plane lies within the lubricant as suggested by R. N. Craig (1983). On the other hand, for granular soils, the lubricant mixes with soils with a significantly reduced friction. With the use of lubricant, longer pipe lengths can be jacked without the use of intermediate jacking station.
- They provide the inlet locations for subsequent grouting works after completion of pipe jacking to fill completely the void space between the pipes and surrounding soils.
- They are used as lifting holes when placing the pre-cast jacking pipes into rails inside the jacking pits.
Question No. 102
In designing reservoirs, the indirect tensile strength of the concrete mix is specified to be less than a specific value (e.g. 2.8 N/mm²) for potable water. Why should engineers put an upper limit of indirect tensile strength?
Answer: The crack width formation is dependent on the early tensile strength of concrete. The principle of critical steel ratio also applies in this situation. The amount of reinforcement required to control early thermal and shrinkage movement is determined by the capability of reinforcement to induce cracks on concrete structures. If an upper limit is set on the early tensile strength of immature concrete, then a range of tiny cracks would be formed by failing in concrete tension. However, if the strength of reinforcement is lower than immature concrete, then the subsequent yielding of reinforcement will produce isolated and wide cracks which are undesirable for water-retaining structures. Therefore, in order to control the formation of such wide crack widths, the concrete mix is specified to have an indirect tensile strength at 7 days not exceeding a certain value (e.g. 2.8N/mm² for potable water). Reference is made to R. D. Anchor, A.W. Hill and B. P. Hughes (1979).
Question No. 103
How many kilo-Newtons equals 1000 kilogram?
Answer: When the 1000 kg are at rest, on Earth with gravitational acceleration 9.81 m/s2, 1000 kg equal 9.81 kN.
Question No. 104
What are the shortcomings of grillage analysis which is commonly used in structural analysis of bridges?
Answer: Grillage analysis suffers from the following shortcomings based on E. C. Hambly:
- For coarse mesh, torques may not be identical in orthogonal directions. Similarly, twists may differ in orthogonal directions.
- Moment in any beams is mainly proportional to its curvature only. However, moment in an element depends on the curvatures in the beam’s direction and its orthogonal direction.
Question No. 105
How many Pyramid in Egypt?
Answer: About 80
Question No. 106
What is the crack pattern induced by hydration due to internal restraint?
Answer: Let’s take a circular column as an example to illustrate this.
When the temperature is rising, the inner concrete’s temperature is higher than outer concrete’s temperature and the inner concrete is expanding. This induces pressure to the outside and the induced compressive stress will result in formation of radial cracks near the surface of concrete.
When the temperature drops, the concrete at the outside drops to surrounding temperature while the concrete at the central region continues to cool down. The contraction associated with inner concrete induces tensile strains and forms cracks tangential to the circular radius.
Question No. 107
Which type of bar reinforcement are more corrosion resistant, epoxy-coated bars, stainless steel bars or galvanized bars?
Answer: V-Based on the experiment conducted by the Building Research Establishment, it was shown that the corrosion resistance of galvanized steel was the worst among the three types of bar reinforcement. For galvanized steel bars, corrosion started to occur when certain chloride content in concrete (i.e. 0.4% by cement weight) was exceeded. However, for epoxy-coated bars, they extended the time taken for cracking to occur when compared with galvanized steel bars.
The best corrosion resistant reinforcement among all is stainless steel. In particular, austenitic stainless steel stayed un-corroded even there was chloride contamination in concrete in the experiment. Reference is made to K. W. J. Tread away (1988).
Question No. 108
How many feet are in 4 square yards?
Answer: There are 9 square feet in a square yard therefore there are 36 square feet in 4 square yards.
Question No. 109
In the design of a simply supported skew bridge, which direction of reinforcement should be provided?
Answer: In the conventional design of steel reinforcement for a simply supported skew bridge, a set of reinforcement is usually placed parallel to free edge while the other set is designed parallel to the fixed edge. However, this kind of arrangement is not the most efficient way of placing the reinforcement. The reason is that in some parts of the bridge, the moment of resistance is provided by an obtuse angle formed by the reinforcement bars which is ineffective in resisting flexure. In fact, the most efficient way of the arrangement of reinforcement under most loading conditions is to place one set of bars perpendicular to the fixed edge while placing the other set parallel to the fixed end as recommended by L. A. Clark (1970). In this way, considerable savings would be obtained from the orthogonal arrangement of reinforcement.
Question No. 110
What materials were used to build the Golden Gate Bridge?
Answer: Its materials were steel and concrete.
Question No. 111
What is the function of washers when using bolts?
Answer: The purpose of installing washers in a typical bolting system is to distribute the loads under bolt heads and nuts by providing a larger area under stress. Otherwise, the bearing stress of bolts may exceed the bearing strength of the connecting materials and this leads to loss of preload of bolts and creeping of materials. Alternatively, flanged fasteners instead of using washers could be adopted to achieve the same purpose.
Question No. 112
How much does the leaning tower of Pisa lean?
Answer: About 14 feet
Question No. 113
What is the significance of quality of bentonite slurry in the construction of diaphragm walls?
Answer: The quality of slurry plays an important role in the quality of diaphragm walls. Firstly, if a thick slurry cake is formed in the interface between slurry and in-situ soil, it has a tendency to fall off during concreting works and it mixes with freshly placed concrete. Moreover, large thickness of slurry cake would reduce the concrete cover and affect the future durability performance of diaphragm walls.
Question No. 114
What are the differences in design between normal pre-cast concrete pipes and pipes used for pipe jacking?
Answer: For pipes used for pipe jacking, they should possess the following characteristics:
- Pipes should have high concrete strength to withstand the stress induced during the jacking process.
- There is tight tolerance in pipe dimension and the pipe joints are specially designed to provide trouble-free joint details. Two commonly available joints are rebated joint and butt end joint.
- Pipes preferably should have smooth external concrete finishes to reduce the friction between the pipes and surrounding soil.
Question No. 115
Shall reversible moisture movement be taken into account in estimating movement for movement joints?
Answer: The size of concrete is affected by changes in atmospheric humidity: moisture causes expansion while drying causes shrinkage. Such moisture movement is reversible. This is totally different from drying shrinkage in which concrete slowly loses moisture during hardening, thus causing irreversible shrinkage.
In fact, the variation of humidity and the estimated reversible moisture movement is not significant (about 30%) and therefore, its contribution to movement does not justify for movement joints as suggested by MN Bussell & R Cather (1995).
Question No. 116
Which is stronger solid steel rod or hollow steel pipe?
Answer: On a per pound basis, steel pipe is stronger in bending and is less likely to buckle in axial compression.
Question No. 117
For long slender structures like beams, propping is required after removal of formwork. Why?
Answer: After concreting, the time at which striking of formworks should not be too long, otherwise it would affect the colour of concreted structures. For long span concrete structures, when they have attained sufficient strength to support their self-weight, creep deflection may occur in these structures if propping is not provided after the removal of formwork. Therefore, re-propping is carried out after removing formwork and these props should not be allowed to stand too long because creep loads may overstress them.
Note: Propping refers to provision of false-work to support slabs and beams during their gain in concrete strength after concreting.
Question No. 118
In joints of pre-cast concrete bridge segments, what are the functions of applying epoxy adhesive?
Answer: Epoxy adhesive is applied in these joints for the following purposes according to
International Road Federation (1977):
- It seals up the joints completely between pre-cast concrete segments to protect the pre-stressing tendons;
- By filling voids and irregularities along the segment joints, it helps to reduce stress concentrations otherwise it will be developed; and
- It helps in transferring of shear between the joints in case a large single shear key is used.
Question No. 119
What is aggregate?
Answer: Aggregate is the component of a composite material used to resist compressive stress.
Question No. 120
Can all utility detectors detect the depth of utilities?
Answer: In Hong Kong, underground utility detectors are normally divided into two types: electromagnetic detector and ground penetrating radar (GPR). For electromagnetic detector, it can detect the signals emitted by metallic utilities themselves by passive mode. While in active mode, the detector has to pick up the signals through a transmitter and sonda connected directly to the non-metallic utility. Both the alignment and depth can be found in active mode while only alignment can be found in passive mode. The electromagnetic detector available in market can detect utilities up to depth of 3 m.
For ground penetrating radar, it sends radio waves into the ground and receives signals from reflections from utilities. It has the advantage of locating both the depth and alignment of utilities. More importantly, it can detect both metallic and non-metallic utilities. However, it suffers from the disadvantage that it is quite expensive and interpretation of data is not simple. Reference is made to LD, DSD (2000).
Question No. 121
What are some structures that may be subjected to fatigue?
Answer: Bridges, hydraulic presses, burners trains
Question No. 122
In concrete pavement, why is the requirement of 95% by mass of quartz grains are specified in contract?
Answer: In General Specification of Civil Engineering Works (1992 Edition), in Clause 10.09 it states “Fine aggregate for concrete shall be natural river-deposited sand consisting of at least 95% by mass of quartz grains”. The aim of such specification is to control the quality of river sand. As quartz is a durable and hard material, using a high percentage of quartz in aggregates of concrete can enhance the strength and durability of the surface texture of concrete carriageway.
In addition, such specification requires a high percentage of quartz content, thereby reducing the probability of presence of impurities like shell.
Question No. 123
What are moment of inertia and its importance in civil engineering?
Answer: The moment of inertia measures the opposition any kind of body will have against a certain momentum (along that same axis) trying to rotate that body.
Question No. 124
What is the difference in application of surface-protecting fenders and energy-absorbing fenders?
Answer: Surface-protecting fenders are fenders that induce high reaction forces to berthing structures for the energy absorbed while energy-absorbing fenders are fenders which transmit low impact to berthing structures for the energy absorbed (Carl A. Thoresen (1988)). In fact, the principal function of fenders is to absorb the berthing energy and transmit a force to the structures without damaging them. Therefore, in open berth structures, it is desirable to use energy-absorbing fenders to reduce the loads acting on the relatively flexible structures. On the other hand, for solid berth structures the usage of surface-protecting fenders is adequate because they are capable of taking up large berthing loads.
Question No. 125
What is the absolute pressure scale?
Answer: Absolute pressure is calculated from a vacuum (0 PSI) and atmospheric pressure is 14.7 PSIa or 14.7 PSI above a vacuum 1 PSI on a tire pressure gauge is called 1 PSIg = 14.7 PSIa, 10PSIg = 24.7PSIa, 100PSIg = 114.7PSIa etc.
Question No. 126
In conducting triaxial test to determine shear strengths for soil samples, what consolidation pressures should be specified?
Answer: It appears that the selection of consolidation pressure is independent of in-situ soil stress theoretically. However, this may not be correct because the actual shear strength envelopes for soils are non-linear over a wide range of stresses. Therefore, consolidation pressure corresponding to the range of stresses relevant to site condition should be adopted.
Question No. 127
What is the difference between standpipe, standpipe piezometer and piezometer?
Answer: A standpipe normally contains plastic pipes with perforated holes at the base. The annular space between the perforated tube and casing is filled with gravel or sand backfill. Under such an arrangement, standpipe is used to measure water level of a certain region. A standpipe piezometer is a type of piezometer which measures pore water pressure at a certain level. It consists of plastic pipes without holes. The tip of the standpipe piezometer is perforated and the annular space between the tip of the piezometer and soil is filled with sand while the annular space between other parts of plastic tube and soil is filled with cement/bentonite grout to seal off water from entering the region of piezometer tip. This enables the pore water pressure in the region of piezometer tip to be measured. In essence, standpipe piezometers are installed to study the pore water pressure of a specified depth below ground. However, it suffers from the disadvantage that the response time is relatively slow in clayey soils. Reference is made to Marius Tremblay (1989).
Question No. 128
How does siphoning work?
Answer: Pretty good if the outside hose is a lot lower than the inside hose. The siphon works by a hose stuck in a fluid container higher than the receiving container. Suction is shortly placed on the lower side of the hose until the fluid is flowing down the down line. Gravity will cause a continued flow.
Question No. 129
In pumping stations one of the choices for the material of water tanks is fibre-reinforced plastic (FRP). What are the advantages associated with this kind of material?
Answer: There are two main advantages for FRP water tanks:
- It possesses high strength to weight ratio and this leads to the ease of site handling.
- It is highly resistant to corrosion and thus it is more durable than steel water tank.
Question No. 130
For rigid pavement, what are the advantages of using lean concrete sub-base instead of traditional granular sub-base?
Answer: There are several shortcomings of using granular sub-base in concrete carriageway:
- Since sub-base is permeable, water can seep through sub-base and soil particles will be pumped out through contraction/expansion joints when subject to traffic load. Consequently, voids are formed underneath the pavement structure and the concrete pavement may crack under severe traffic loading.
- Lean concrete increases the strength and renders the roads capable of carrying heavy traffic loads (David Croney and Paul Croney (1992).
- Due to workmanship problem, it may have uneven distribution of sub-base and this result in cracking of concrete carriageway when subject to severe traffic loading.
Question No. 131
For marine pile type of steel tubular piles with reinforced concrete infill, minimum toe level is often specified in contract drawings. What is its purpose?
Answer: The purpose of minimum toe level is two-fold:
- In detailed design stage, ground investigation should be conducted and the approximate level of rock-head is known. Therefore, to avoid the marine piles to be founded prematurely on boulders, minimum toe levels of marine driven piles are specified in contract.
- It provides sufficient length of soils for lateral and uplift resistance.
Note: Minimum toe level refers to the minimum level that a marine driven pile should be driven into seabed.
Question No. 132
How do you compute volume of airflow?
Answer: Orifice meter is used for measuring flow rate air.
Question No. 133
Soil nails are mainly designed for stabilization of major slips. How should designer cater for the stability of minor slips?
Answer: There are some methods to treat minor slips:
- Adoption of smaller diameter size bars at closer spacing;
- Installation of tie beams at the same horizontal levels;
- Provision of steel wire meshes in-between soil nails; and
- Provision of short soil nails in combination of long soil nails.
Question No. 134
In terms of pumping performance, how should engineers determine the use of radial flow pumps and axial flow pumps?
Answer: Specific speed is usually defined for a pump operating at its maximum efficiency. In order to minimize the cost of future operation, it is desirable to operate the pumps as close to the maximum efficiency point as possible. The specific speed for radial flow pumps is relatively small when compared with that of axial flow pumps. This implies that radial flow pumps tend to give higher head with lower discharge while axial flow pumps tend to give higher discharge with lower head.
Question No. 135
What is the formula for figuring out how many gallons of water are in a pipe by the foot?
Answer: 0.61 × d², where d is the diameter of the pipe in inches.