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?
Answer:
- 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 = Ï€ × a × b, ‘a’
and ‘b’ are the ½ major and ½ minor axis. If a = r = b the area is A = Ï€ × 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?
Answer:
- 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?
Answer: Thermometer
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?
Answer:
- 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.
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