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 noise back to the site and increase the reverberation of the site. Instead for the face of noise screen facing the piling operation shall be lined with a layer of sound-absorbing material such as glass fibre. Moreover, openings on noise screen should be avoided because it can substantially reduce the performance of noise screen. 25. What is the difference between compaction grouting and fracture grouting? (FD13) Grouting can be implemented in two common modes, namely compaction grouting and fracture grouting. For compaction grouting, high viscosity grout is commonly used for injection into soils. Upon reaching the soils, the grout would not penetrate into soil spaces. Instead it forms a spherical bulb and remains as a homogeneous mass. The formation of bulb displaces the nearby soils. Fracture grouting involves the use of low viscosity grout. Upon injection, the grout would split open the ground by hydraulic fracturing and penetrate into the fractures. Similarly, soils are displaced during the process. 234
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, 235
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A Self-Learning Manual - Institution of Engineers Mauritius

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