Geology Reference
In-Depth Information
repeated; it is often the pieces of core that are not recovered that are the
most important, because they are also the weakest. It is wise to include
a clause in speci
cations for the GI contractor, setting out a minimum
acceptable recovery, to encourage diligent work. A good driller can
generally achieve good recovery in almost any ground, providing he
has the right equipment and adjusts his method of working to suit the
ground conditions. If he does not have suitable equipment (or
flushing
medium), then that might be the fault of the engineer who speci
ed the
investigation, rather than the contractor, and this may need recti
cation
by issuing a variation order to the contract.
Once the preliminary geological model has been established ade-
quately at a site, then additional boreholes can be put down as neces-
sary to take samples for testing or to carry out in situ testing and to
install instruments for monitoring changes such as response of water
table to rainfall. The same approach (sample
first to prove the geolo-
gical model and to identify any geological hazards, followed by a
second phase for testing and instrumentation) should be used for any
investigation where geological features may be important. This can
only be judged by a competent engineering geologist aware of both the
local geological conditions and the factors that will control the success
or otherwise of the particular civil engineering project.
In practice, boreholes are often put down using a strategy of inter-
mittent sampling and in situ testing within a single borehole, which
means that the full ground pro
le is not seen. This can be cost-effective
for design when the site is underlain by relatively uniform deposits and
where the ground pro
le is already well-established from previous
investigations. The danger is that site-speci
c geological features
might be missed yet prove important for the project.
There are many different tools that can be used to investigate soils and
many of these are described by Clayton et al. (1995). In the UK, the
most commonly used machine for investigating soils is the shell and
auger, otherwise known as the cable-percussive rig, as illustrated in
Figure 4.22.
Such rigs are very manoeuvrable and can be towed behind
a
field vehicle or winched to the point where the hole is to be put down.
They can cope with a wide range of soils, which makes for their
popularity in the UK, where mixed glacial soils are common. The
hole is advanced by dropping a heavy shell (
Figure 4.23).
Material
between sampling points is usually discarded, although it should be
examined and recorded by the drilling contractor and disturbed bulk
samples are taken in bags, if speci
ed for the contract. All samples, of
course, should be sealed and labelled. If boulders are encountered in
the soil pro
le, these are broken up with a heavy chisel dropped down
the hole. Engineers usually specify alternate undisturbed samples for
