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What are the reasons in observed settlements in rockfill foundation?

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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.

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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.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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