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?
Answer:
138.60 ft
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.
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