Geology Reference
In-Depth Information
purpose-built machine. At some sites consolidation and strengthening
is achieved by a technique termed vacuum preloading; references are
given in Charles (2002).
Stone columns can be used to enhance drainage and are installed to
depths of 10m and sometimes more. These are formed by using a
vibrating poker, pushed into the soil to form a void and then
lling
the void with gravel and sand, which is compacted in stages using the
same vibrating tool (McCabe
et al
., 2009). Stone columns have been
used to increase mass permeability and prevent liquefaction of loose
silty sand during an earthquake, although in such a usage settlement
will still occur but in a relatively uniform and non-catastrophic
manner. Stone columns are also used generally to improve the bearing
conditions at a site, the improvement depending upon the ratio of
cross-sectional area of stone columns to untreated ground. Groups
and lines of stone columns can be used as weak piles to provide support
to structures such as oil tanks.
Clay soils especially, can be improved by mixing with lime slag and
cement, either at the ground surface (to prevent erosion in slopes, for
example) or in columns or trenches, using hollow-stem augers and
similar equipment. The works will improve the bearing capacity of the
ground, although the improvement might be dif
cult to quantify.
Stronger columns can be formed by using jet-grouted columns
formed using high-pressure grout jets as a drilling string is rotated
and lifted from depth. The resulting column of mixed soil and grout
can be used to carry structures or to form cut-off barriers to restrict
water
flow, for example, beneath dams. Jet grouting is sometimes used
to form structural members during temporary works construction of
deep excavations (Puller, 2003; also see case study of Nicoll Highway
collapse in
Chapter 7).
6.8.6 Drainage
For deep excavations and tunnelling, it is commonly necessary to
lower the groundwater during construction, although there are
many factors that must be considered, not least associated settle-
ment of the ground due to increased effective stress and self-
weight compaction and consolidation and drying up of land in
adjacent properties (Preene & Brassington, 2003). New and steep
flow paths through the soil can lead to seepage piping and lique-
factioninthe
floor of excavations. The cheapest and simplest way
