Question No. 01
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 unfavorable sea conditions.
Question No. 02
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. 03
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. 04
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. 05
What are the advantages of assigning the central pier and the abutment as fixed piers?
(i) 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.
(ii) 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. 06
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 pump able, 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 pump ability of concrete.
Question No. 07
How long does it take concrete to harden?
Answer: It takes 28 days to cure.
Question No. 08
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. 09
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. 10
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:
(i) To enhance a uniform support under pipes in order to reduce the bending moment longitudinally;
(ii) To increase the load-supporting strength of the pipes;
(iii) 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;
(iv) To provide a platform for achieving correct alignment and level during and after construction.