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
piles are considered as beams which are subjected to lateral loads such as
slope movement leading to pile failure.
Piles are slender columns with lateral support from foundation soils. When
the length of pile increases, the buckling loads decrease with the square of
pile length. For buckling failure, soils around the piles lose the confining
stress during earthquake and can hardly provide lateral support to piles. As
such, the pile serves as an unsupported column with axial instability. It will
buckle sideways in the direction of least bending stiffness under axial load.
5. What are the reasons in observed settlements in rockfill
foundation?
Compression of rockfill is normally caused by a reduction in dimension of
fill and by rearrangement of particles into closer packing.
When the rockfill are saturated, the strength of rock would be reduced
accordingly. In fact, wetting of rock surfaces does not reduce the coefficient
of sliding friction between rockfills. Considerable settlement may result not
from the lubricating effect of water but from a reduction of rock strength at
its point of contact. The contact points would then be crushed under
intergranular force and the contact area increases until contact pressure is
less than the strength of rockfill.
Rockfill with sharp corners proved to be more liable to settlement than
those of well-rounded.
To minimize settlement of rockfill, the intergranular force should be reduced
and this is achieved by grading the size of rock particles such that there is
minimum amount of voids and hence a maximum amount of particle
contacts. To avoid particle rearrangement under future loading, the rockfill
should be properly compacted with earth-moving machinery.
6. Should bentonite be added to improve the stability of grout? (G1)
For unstable grout, particles will come out of the grout suspension leading
to incomplete grouting and clogging of pipes. The stability of grout can be
improved by adding additives such as bentonite. However, bentonite
should not be used with very fine cements because its grain size is bigger
than that of fine cements. Tests conducted previously confirm that a grout
with bentonite is less stable under pressure.
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