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 10. Why is bentonite slurry commonly used in diaphragm wall construction? (DW3) Bentonite slurry is one of the most common excavation fluid used in constructing diaphragm wall. Bentonite clay (in powder form) and water are combined in a colloidal mixer and clay particles bond to each other and set to form a gel when left to stand for a period of time. When the bentonite is set in motion, it reverts back to the fluid state rapidly. Bentonite slurry shores the trench to stabilize the excavation and forms a filter cake on the slurry trench walls that reduces the slurry wall’s final soil permeability and to reduce ground water flow. The gel strength and viscosity properties of the bentonite clay allow for cutting suspension and removal. 11. Should high density or low density bentonite slurry be used in diaphragm wall construction? (DW3) The use of high density bentonite slurry could improve trench stability. It helps to retain cuttings and particles in suspension and reduce the loss of bentonite slurry into soils of high permeability such as sand. However, the use of low density bentonite slurry is desirable from operation point of view. Low density bentonite slurry tends to be more neatly displaced from soils and reinforcement. Moreover, the pumping of low density bentonite slurry is more easily to be carried out. 12. What is the difference in mechanism in resisting clay and normal soils by bentonite slurry in diaphragm wall construction? (DW3) For normal soils, water in bentonite slurry penetrates into the sandy walls and leaves behind a layer of bentonite particles on the surface of the soils. The bentonite particles form the filter cake of low permeability on the excavated faces. The filter would be formed only when slurry pressure is greater than the pore water pressure in excavated soils. The filter cake serves as impermeable layer and allows the application of full hydrostatic pressure of bentonite slurry on the excavated surface of soils. For soils with low permeability such as clay, there is little water passage from slurry to excavated clay surface so that filter cake would hardly be formed. As a result, slurry pressure simply applies on clay surface. 306
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU Level One (Core FAQs) Part III: Earthwork and Tests 1. Do soil compaction test results over 100% mean over-compaction? (C1) Soils can experience over-compaction if the compactor makes too many passes over it. In fact, relative soil compaction test results over 100% do not necessarily mean over-compaction because the relative compaction is based on the maximum dry density of the soil obtained by the Proctor test and this does not necessarily refer to absolute maximum dry density. Over-compaction is considered undesirable because it may eventually create cracks in the underlying compacted material so that it results in a decrease in density. Moreover, it causes waste of machine power and manpower which is undesirable. 2. In soil compaction test, if a test result exceeds 100%, should engineers accept the result? (C1) Soil compaction is the process of increasing the soil density by reducing the volume of air within the soil mass. Soil compaction depends mainly on the degree of compaction and the amount of water present for lubrication. Normally 2.5kg rammers and 4.5kg rammers are available for compaction in laboratories and the maximum dry densities produced by these rammers cover the range of dry density obtained by in-situ compaction plant. Regarding the second factor of water content, it affects the compaction in the following ways. In low water content, the soils are difficult to be compacted. When water content is increased gradually, water will lubricate the soils and this facilitates the compaction operation. However, at high water content, as an increasing proportion of soils is occupied by water, the dry density decreases with an increase in water content. For soil compaction tests, the dry density obtained from compaction carried out in-situ by vibrating roller/vibrating plate is compared with the maximum dry density conducted in laboratories using 2.5kg rammer of compaction with similar soils. In essence, the in-situ compaction is compared with the 307
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A Self-Learning Manual - Institution of Engineers Mauritius

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