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
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.
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