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
correlation to other parameters in some methods. For instance, Cone
Penetration Test can be employed to estimate the relative density of sands
as there is well established relation between cone tip resistance and
relative density.
On the other hand, shear wave velocity measurements using seismic CPT
and Spectral Analysis of Surface Waves can be used to obtain relative
density indirectly.
11. Why do compliance test be carried out some time after the
completion of vibrocompaction?(R7)
A process called “ageing” occurs after the operation of vibrocompaction. It
is observed that sand fill shows an increase in strength and stiffness. Upon
immediate completion of vibrocompaction, the apparent increase in
strength of sand fill is due to dissipation of air and water pressure. Then the
ageing process shall continue owing to creep and cementation.
Hence, sufficient time shall be established for ageing of sand fill to develop
prior to compliance testing (such as CPT test). Otherwise, it may result in
non-compliant strength and requires re-compaction.
12. For reclamation by hydraulic filling with sand, it is commonly
observed that the density of sand varies significant with depth. Why?
In reclamation works, the density of filled sand varies with the method of
placement. For example, bottom dumping is adopted for sand as the
method of placement and this results in higher density of filled sand. For
placement by pipeline discharge, the density of filled sand formed by this
method is lower than that by bottom dumping. Hence a weak zone is
formed for sands placed by pipeline discharge and hence these loose sand
fill (relative density about 30%) may cause settlement when subjected to
loading or vibrations.
In general, the filled sand above water table is found to be higher in density
as they are well compacted by bulldozers and traffic of constructional plant.
13. Why do prefabricated band drains become more popular than
sand drains?
There is a trend that sand drains are replaced by prefabricated band drains
due to:
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