A Self-Learning Manual - Institution of Engineers Mauritius

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A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU carry about 10% of normal load and this essentially increases the cost of dredging operation. If overflowing is allowed in the dredging operation, it shall work on unrestricted basis as the rate of overflow is nearly constant over the entire overflowing process. For dredging with overflowing, the particle size distribution of sand may differ from the in-situ grading because overflowing tends to remove the fine content of sand. The loss of sediment associated with overflowing poses environmental problems to the nearby water zone in eth following ways: (i) increased sedimentation (ii) decrease in dissolved oxygen (iii) increased turbidity (iv) increased amount of nutrients 20. Why can vacuum preloading be employed to accelerate the rate of consolidation? In vacuum preloading, the drainage boundary of clay is isolated from the atmosphere by a membrane. A partial vacuum (e.g. suction of 80kPa) is applied within the membrane to reduce the water pressure so as to speed up consolidation. The rate of consolidation can be increased by surcharge preloading in which the excess pore water pressure in clay is temporarily increased. Alternatively, the rate of consolidation can also be increased by vacuum preloading by a decrease in water pressure. Vacuum preloading is generally faster in operation than surcharge preloading which requires timely delivery of fill on top of clay. Moreover, it is unnecessary to consider the stability criterion which surcharge preloading should require. 21. Geotechnical Instrumentation is frequently employed for monitoring the condition of reclamation. Sometimes two piezometers are installed inside the same borehole. What is the reason behind this? For standpipes, they normally contain one plastic tube between its intention is to measure water level only. However, for piezometers, they are used for 184
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU measuring pore water pressure in a certain depth below ground. For instance, if there are two clayey layers below ground at different depths, a multiple piezometer including two separate piezometers may be sunk at the same borehole to determine the pore water pressure at these layers respectively. This arrangement has the advantage that it saves the cost of installation of separate boreholes for several piezometers. However, the installation of multiple piezometers within the same borehole is affected by occurrence of leakage along the pipes as suggested by Marius Tremblay (1989). 22. In case a road passes through a reclaimed land and an existing land, what is the main concern regarding the design of pavements? For an existing land, it is anticipated that there will be no major settlement within the design life of pavement structures. However, for a recently reclaimed land, even with surcharging and installation of vertical drains, some settlement will still occur after the construction. If a road pavement has to be constructed connecting these two areas, special design has to be made in this transition region. In order to avoid the occurrence of differential settlement which may damage the pavement structure, a transition slab may be designed to accommodate such movement (J. S. M. Kwong (1996)). 23. What are the functions of slip joints in blockwork seawalls? (R9) Slip joints are joints which are formed through a complete vertical plane from the cope level to the toe level of seawalls. These joints are designed in blockwork seawalls to cater for possible differential settlements between adjacent panels of seawalls. The aggregates inside the half-round channels in slip joints allow for the vertical movements induced by differential settlement and at the same time providing aggregate interlocking forces among adjacent panels of seawalls to link the panels in one unit against the lateral earth pressure exerted on seawall. Besides, slip joints provide a path for the relief of water pressure developed and allow the lateral movement (e.g. contraction) due to seasonal variations. Note: For details of slip joints, reference is made to CEDD Standard Drawing No. C3008C. 185
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A Self-Learning Manual - Institution of Engineers Mauritius

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