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. Does liquefaction occur to sand only? (M1)
In liquefaction, the pore water pressure builds up steadily and eventually
approaches a value equal to the confining pressure. In an earthquake,
however, there is not enough time for the water in the pores of the soil to be
squeezed out. Instead, the water is trapped and this avoids the soil
particles from moving closer together. Consequently, this results in an
increase in water pressure which reduces the contact forces between the
individual soil particles, thereby softening and weakening the soil.
Eventually, soils particles lose contact with each other and behave like a
liquid.
Hence, the type of soils which is susceptible to liquefaction is the one like
sand whose resistance to deformation is mobilized by frictional forces
between particles under confining pressure. In case the soil is fine grained,
cohesive forces tends to develop between these fine particles and it is
difficult to separate them. Therefore, sand with increasing content of fines
tends to increase its resistance to liquefaction.
The consequence of liquefaction is that the subsequent settlements after
liquefaction may damage the overlying structures. Moreover, for sloping
ground lateral flow may result which is undesirable. Liquefaction only
occurs to saturated soils.
2. What are the measures to reduce the effects of soil liquefaction?
(M1)
To reduce the effect of soil liquefaction, it is intended to reduce the pore
water pressure induced during earthquake shaking. This can be achieved
by providing better drainage in soils (e.g. wick drains, sand drains etc. )
and densification of soils (e.g. vibroflotation, dynamic compaction etc.).
Liquefaction hazards can be reduced by improving the drainage ability of
the soil. If the pore water within the soil can drain freely, the build-up of
excess pore water pressure would be reduced accordingly.
321