Geology Reference
In-Depth Information
to lower water is just to let it happen naturally as the excavation
proceeds, to channel water in
ow to collection sumps and to then
pump this water away, although disposal may be an issue on
environmental grounds, and pumping from great depth will
require a series of pumps at different levels. Active dewatering is
generally conducted using well-point systems or submersible
pumps in wells. Details are given by Puller (2003). As noted
elsewhere, dewatering is often important to the stability of slopes
and semi-permanent solutions include drains, drainage caissons
and adits. Emergency pumping systems are sometimes set up to
be triggered if piezometric levels become dangerously high.
6.8.7 Geotextiles
Geotextiles are fabric or plastic sheets that have many different uses in
ground engineering. A few of these are discussed below.
To improve site access, sheets of plastic mesh may be laid on the
ground and then a layer of gravel placed and compacted on top. The
purpose of the geotextile is to prevent the gravel being pushed into and
mixing with the underlying soil that may be wet and soft. In this way,
temporary road access can be provided. In other circumstances, more
complex solutions might be designed involving elements such as stone
columns or piles, together with a geotextile grid draped across and
linking the structural elements.
Geotextile mats and strips are also used in the design of reinforced
earth structures (as are metal grids and strips), as illustrated in
Figure 6.21.
Basically, the frictional resistance between the soil and
grid or mats, placed horizontally and regularly within a
fill structure,
enhances the overall strength of the soil mass and prevents it failing.
Where facing walls are used or the geotextile is wrapped around at the
face to prevent soil erosion, the
finished structure can be very steep or
even vertical.
Plastic grid boxes, in
lled with rock cobbles, have been used to form
gabion walls as barriers. Care must be taken that the situation is not
one where the
re and that the
deterioration rate is acceptable given the proposed lifetime of the
structure.
finished structure can be destroyed by
6.8.7.2 Drainage and barriers
Geotextile sheets are available that are highly permeable but also
designed with a mesh size that restricts soil erosion, in the same way
as traditional soil
filter systems. Geotextiles are therefore used, for
