Civil Engineering Reference
In-Depth Information
stress (if this is accurately known), and a reduced density can be used in the plane of
the neutral axis.
3.4.8 Excavation using a bentonite suspension
A method of borehole wall support developed from slurry trench techniques is the
use of a bentonite suspension. Where pile bores are required to penetrate considerable
depths of unstable soil, the installation of long lengths of temporary casing is a time-
consuming process.
By employing a bentonite suspension of around 6% by weight of bentonite, the
borehole walls may be effectively supported, provided there is a positive head of ben-
tonite slurry above the groundwater table of at least 1.5m. In permeable (granular)
soils, there is an initial flow of slurry into the borehole wall, where a 'filter cake'
rapidly builds up which then prevents further flow, and the excess hydrostatic pres-
sure in the borehole supports the ground. An additional benefit is the holding in
suspension of fine detritus from the boring operations. Before concreting, the ben-
tonite in the pile bore is checked for contamination and if necessary is exchanged
for fresh bentonite. Bentonite slurry is returned to the surface for settling and
de-sanding. Cleaned slurry may be re-used, with adjustment of bentonite content by
the addition of new bentonite or mains water as necessary. After some re-uses, the
bentonite slurry may become too contaminated for effective cleaning, and it is then
discarded.
Concreting is carried out by the tremie method, using concrete with a slump
in excess of 175mm. With clean bentonite slurry in the bore, the suspension is
effectively scoured and displaced by the fluid concrete. No decrease in pile adhe-
sion has been observed in properly constructed bentonite piles, whether the shafts
are in granular soils or cohesive clays. A full description of the constructional
method, and the results of many pile tests on bentonite piles, are given by Fleming
and Sliwinski (1977). The process is illustrated in Figure 3.25. To comply with
CEN EN 1536, longitudinal reinforcement for bores stabilized with bentonite should
be of ribbed bar.
3.4.9 Excavation using a polymer fluid
The use of a polymer fluid to stabilize a pile bore may be considered as an alternative to
the use of bentonite suspension although a polymer fluid is not as dense as a bentonite
suspension and therefore it is not suitable for pile bores with a water table less than
2m from the ground surface. Polymer support fluids are suitable in clays and granular
deposits except perhaps those that are the most permeable being poorly graded and/or
of coarse particle size but they will work in salt water. In salt water approximately twice
the quantity of polymer will however be needed which could be a deciding factor as
polymer fluids are more expensive than bentonite muds. The polymer consists of long
chain molecules that keep small particles of soil detritus in suspension and is generally
biodegradable. Polymer 'threads' can be seen in the spoil as it is spun off from an
auger (Figure 3.26). The action of the polymer fluid is not the same as bentonite mud,
as no filter cake is formed, but instead a high seepage gradient is maintained over the
bore/soil surround.
