Civil Engineering Reference
In-Depth Information
(b) Preliminary investigations
Preliminary investigations are carried out at the site, rather than in the office, but
they do not yet involve major expenditure on drilling, sampling and testing. The
purposes are, firstly, to confirm or revise the findings of the desk study and, sec-
ondly, to add further information. This additional information will come from detailed
engineering geological mapping, and this is best done by engineers and geologists
working together or by experienced engineering geologists. Preliminary investiga-
tions may also involve some limited sub-surface exploration by trial pits, probing
or exploratory drilling and geophysical sensing using seismic, electrical resistivity and
other methods.
(c) Detailed investigations
Detailed investigations consist of drilling, sampling and laboratory and
in situ
testing.
They may also involve more detailed geological mapping, groundwater and chemical
studies and other appropriate investigations necessary for the works. This is where
the bulk of the expenditure is incurred and planning of the detailed investigations
should set out to discover the required facts in the most efficient way. This will
require some foreknowledge which can be gained from the desk study and preliminary
investigations.
17.4 Test pitting, drilling and sampling
The standardmethod of ground investigation is excavation and sampling supplemented
by
in situ
and laboratory testing. The excavations are usually done by drilling but also
by opening test pits.
(a) Test pitting
A test pit is an excavation that a geotechnical engineer or engineering geologist can
enter to examine the soil profile
in situ
. Pits can be excavated by large drilling machines
of the kind used for boring piles, by an excavator or by hand digging. Remember that
any excavation in soil with vertical or steep sides is basically unstable and must be
supported before anyone enters it.
(b) Drilling
Drill holes can be advanced into the ground using a number of different techniques;
the principal kinds are illustrated in Fig. 17.2. Augers may be drilled to shallow depths
by hand and large diameter augers can be drilled by machines used also for installation
of bored piles (see Chapter 23). Wash boring is used in sands and gravels and rotary
drilling is used mainly in rocks. Light percussion drilling is widely used in the United
Kingdom and you can very often see the typical tripod rigs at work.
In some soils, particularly stiff clays and in rocks, boreholes will remain open
unsupported, but in soft clays and particularly in coarse-grained soils the hole will
need to be cased to maintain stability. Boreholes should normally be kept full of water,
or bentonite mud, to prevent disturbance below the bottom of the hole.
