A Self-Learning Manual - Institution of Engineers Mauritius

A Self-Learning Manual
Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU
Level Two (Advanced FAQs)
Part II: Miscellaneous
1. During concreting of diaphragm walls, three tremie pipes are used
in one time. However, only one concrete truck is available. How
should the concreting works be carried out? (DW1)
The most ideal situation is to supply each tremie pipe with a single
concrete truck. However, if only one concrete truck is available, all the
fresh concrete in the truck should not be placed in one single tremie pipe.
With all fresh concrete placed in one single tremie pipe while the others left
void, then due to the huge supply of concrete to the tremie pipe, a small
concrete hump may form at the base of the tremie pipe and it is likely that it
may collapse and trap the slurry inside the diaphragm walls. Therefore, the
fresh concrete should be evenly shared among the tremie pipes to avoid
such occurrence.
2. What is the significance of quality of bentonite slurry in the
construction of diaphragm walls? (DW2)
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.
3. When are prestressed tiebacks used in sheet piling works? (SP1)
The use of prestressed tiebacks gets rid of the need of interior bracing.
Prestressed tiebacks are anchored into rock or granular soils and
excavation can be conducted by using powerful shovel instead of using
hand excavation or other small excavators. It provides less restraint and
allows free movement for excavation.
4. In seismic liquefaction, what is the difference of pile failures
mechanism between lateral spreading and buckling?
Most of design codes assume that pile fails during strong earthquake by
lateral spreading. Lateral spreading is based on bending mechanism
where the inertia and slope movement causes bending in piles. In essence,
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