Geology Reference
In-Depth Information
construction equipment. Ground improvement might therefore be
carried out, often as an alternative to some engineering solution such
as piling, and the engineering geologist should be aware of the techni-
ques that might be employed to deal with a particular site condition
(Charles, 2002). Ground improvement might be used in temporary
works, such as freezing the ground to allow tunnelling through satu-
rated and potentially
flowing materials, or the construction of barriers
flow, or to restrict vibrations during construction. In other
situations, ground improvement might provide a permanent solution
such as densi
to water
cation or using chemical additives to provide additional
strength.
One of the simplest methods is dynamic compaction, which involves
dropping a large weight, up to about 30 tonnes, from a crane, over a
regular pattern and then back
lling the depressions with granular
material. Further drops are carried out at closer spacing. The depth
of improvement depends upon the weight dropped, size of pounder
and the height. Typically, a weight of about 15 tonnes dropped 20m
might be expected to improve ground to about 10m deep (e.g. Bo
et al
.,
2009). The method is most suitable for improving
fills and granular
soils generally, but sites underlain by clay have also been improved,
although consideration must be given to the pore pressures that might
be generated and how these dissipate. Generally, the improvement is
measured by tests before and after improvement, using techniques such
as the SPT, CPT or the Menard pressuremeter that was developed
speci
cally for this purpose (Menard & Broise, 1975). The technique
has been applied successfully for quite prestigious projects involving
large-scale reclamation, such as Nice Airport. In Hong Kong, it has
been used to densify the upper fewmetres in old
fill slopes in an attempt
to improve their stability.
If time allows, then an effective way to improve the consolidation
characteristics at a site is to preload it, often by placing an embank-
ment of
fill material that can be removed again later or re-graded at
site, compacted properly in thin layers. The process of consolidation is
generally accelerated by introducing a series of vertical drains to
increase the mass permeability and allow excess pore pressures to
dissipate, monitored using piezometers. The drains can be sand
wicks, which are sausages of geotextiles,
filled with sand and installed
in pre-drilled holes. Other systems include wick drains that are
geotextile-covered plastic elements pushed into the ground using a
