Environmental Engineering Reference
In-Depth Information
exploratory adits should be incorporated in the design of the dam, e.g. as drainage or
grouting galleries.
Shafts using 1000 mm or larger diameter augers or clam-shell excavators are sometimes
used to explore alluvial and glacial deposits.
5.9
DRILL HOLES
5.9.1
Drilling objectives
The main objectives of drilling are to extend the knowledge obtained from surface map-
ping, test pits and trenches below the depth limitations of these methods and to:
-provide control for the interpretation of any geophysical investigations;
-provide samples from these greater depths;
-provide access for test equipment e.g. for measurement of water levels, pore pressures
and permeability etc.
The major advantage of drilling is that, subject to the choice of a suitable rig, there is
little restriction to the depth to which the investigation can be taken. In the assessment of
the subsurface profile the properties of the lowest rock or soil unit which could affect the
structure usually have to be determined. In many cases drilling provides the most practi-
cal method.
Drilling also has little effect on the environment. Maximum hole size is usually less than
200mmand holes can be easily covered, backfilled or neatly preserved. Surface disruption
is commonly restricted to the preparation of a drilling pad on sloping ground.
In site investigation drilling it is imperative to recover as much information as possible
on the subsurface profile. The drilling and sampling method should be selected to max-
imise the recovery of low strength material. Zones of crushed rock, gouge or clay seams are
particularly important but some core losses can hardly be avoided. The recording of
drilling parameters e.g. the drill penetration rate, drill fluid pressure, can assist in the
interpretation of subsurface conditions.
5.9.2
Drilling techniques and their application
Table 5.3
summarises the different drilling techniques available, their applicability, advan-
tages and disadvantages.
In the selection of the drilling method care should be taken not to disturb the existing site
conditions. In the investigation of landslides the use of drilling fluid could lead to increased
pore pressures, softening and hydraulic fracture. For the same reason the investigation of the
clay core of existing embankment dams should always use dry drilling techniques, e.g. hol-
low flight auger, auger, dry cable tool.
Drilling has the disadvantage that information obtained is almost always indirect -
either from the observation of resistance to rig penetration, by the measurement of
in situ
properties with equipment lowered down the hole or by the logging of samples recovered
by the drilling (e.g. Thin Wall Tube samples). Direct observation of the ground is restricted
to the use of mirrors, down-the-hole camera, television or fiberscope. Large diameter holes
drilled by augers or the calyx method can, in some cases, be entered and logged.
Drilling rates depend on the machine and the material type and typically are 4 to 5
auger holes to 10 m in soil per 8-hour shift or 15 m of core drilling and water pressure
testing per 8-hour shift. Rigs are often truck mounted - which makes them mobile but
restricts access in sloping ground. Skid mounted rigs require considerably more time for
establishment at each site.
