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 Coefficient of restitution refers to a value indicating the strain energy during collision regained after the bodies reverting back to their original shapes. If the coefficient of restitution is equal to unity, it means that the collision is elastic and all energy has been returned after the impact action. Hence, this is an index showing the degree the impact action in terms of elasticity. In mathematical forms, coefficient of restitution = -(v1-v2)/ (u1-u2) where u=initial velocity and v=final velocity after impact 14. In pile driving operation, would soils always exhibit an increase in pore water pressure? The change in pore water pressure varies in different soils. In loose sands and sandy silts, the pore water pressure increases during pile driving owing to soil densification. The increase in pore water pressure reduces the soil strength. However, after piling operation ceases for a certain period of time, upon dissipation of pore water pressure the soils would result in increased strength by soil “set-up”. For dense sands, the piling operation cause dilation and increases the pore volume. As such, water may not be fast enough to infiltrate to equalize the pore pressure and this results in a reduction of pore water pressure. Therefore, the apparent increased soil strength is temporary only and it would be reverted back when soil relaxation takes place soon. 15. Should thin or thick bitumen layer be used to reduce negative skin friction in driven piles? (DP8) When piles are driven through an upper layer of granular soils, thick (10mm) bitumen layer would be scrapped off during the driving process. A thin coat of 1mm to 2 mm thick is not likely to flow in storage and to peel off during pie driving. The bitumen can be applied by brushing or sprinkling after heated to a liquid state. In cold weather conditions, it may be difficult to handle hot bitumen. As such, the bitumen could be mixed with solvent to soften the bitumen which should be able to cure rapidly to ensure that the bitumen coat stays on the piles. For precast piles, a primer could be added to achieve better cohesion of bitumen to the piles. 214
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU 16. Should bitumen be applied to the whole section of driven piles? In a certain region of H-piles subjected to ground water table fluctuation, painting is sometimes applied on the surface of H-piles because the rise and fall of water table contribute to the corrosion of H-piles. On the other hand, to reduce the effect of additional loads brought about by negative skin friction, bitumen is applied on the pile surface corresponding to the region of soils that has negative skin friction. However, bitumen should not be applied to the whole section of H-piles because it will be unable to derive the designed frictional reaction from soils. Actually, some engineers have reservation of the effectiveness of bitumen slipcoat because the bitumen may get removed during pile driving. 17. What is the purpose of coating driven piles with bitumen? (DP8) Coating driven piles with bitumen serves the following purposes: (i) It acts as friction reducer and could effective reduce the effect of downdrag. (ii) It offers protection of steel piles against acid attack from soils. (iii) It helps to prevent pile corrosion. 18. What are the possible methods to reduce downdrag on piles? (DP8) (A) Coating piles with friction reducer such as bitumen. (B) Predrill a hole firstly, followed by putting in the pile and subsequent filling of annulus with bentonite slurry. (C) Drive the piles with an oversize shoe and fill the annulus with bentonite slurry. (D) Adopt double tube method in which the inner pile takes up the structural load and the outer pile carries the downdrag load. (E) Preload the soils to accelerate settlement. (F) Use electro-osmosis to enhance water content around the piles and reduce the friction between pile and soils. (G) Increase the pile capacity by using larger diameter or longer length so that the effect of downdrag is thereby reduced. 19. In press-in piling, how can driving shoes help to reduce jacking loads? Press-in piling involves the use of hydraulic rams to provide the force to 215
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A Self-Learning Manual - Institution of Engineers Mauritius

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