Civil Engineering Reference
In-Depth Information
6.3 Site exploration methods
6.3.1 Trial pits
A trial pit is simply a hole excavated in the ground that is large enough if necessary for a ladder to be
inserted, thus permitting a close examination of the exposed sides. The pit is created by removing suc-
cessive layers of soil using a hydraulic excavator until the required depth is reached. Progression by cuts
of depth about 400 mm is quite common. The excavated soil is usually placed beside the pit to enable
easy backfilling once the pit is ready to be closed up again. The sides of the trial pit are never assumed
to be stable and, if personnel are to enter the pit to perform close inspection of the soil, to take samples
or to perform
in situ
testing, the sides of the pit must be fully supported.
Groundwater conditions can be accurately established from a trial pit and undisturbed block soil samples
are obtainable relatively easily. In addition, undisturbed samples can be obtained using cylindrical steel
sampling tubes gently pushed in to the soil by the excavator bucket.
Below a depth of about 4 m, the challenges of side support and the removal of excavated material
become increasingly important and the cost of trial pits increases rapidly. In excavations below ground-
water level the expense may be prohibitive. Trial pits should not be made at locations where pad founda-
tions might be cast later in the project.
6.3.2 Hand excavated boreholes
A hand auger can be used in soft and loose soils for creating a borehole of up to about 6 m (using exten-
sion rods) and is useful for site exploration work in connection with roads. A choice of auger types exist,
each of which is used for a specific type of soil. In clay soils a clay auger as shown in Fig.
6.2a
is used,
whereas in sands and gravels, the gravel auger (Fig.
6.2b
) is used. The auger is connected to drill rods
and to a cross bar at the top to enable the auger to be turned by hand and advanced into the soil. A
rotary engine arrangement nowadays is more commonplace than the cross bar.
Hand excavated boreholes are useful for cheap, rapid sampling and assessment of ground conditions
where only 1 or 2 locations on a site are of interest. For larger scale investigations, the boreholes will
invariably be created using full scale drilling equipment. All samples of soil retrieved from hand excavated
boreholes are classified as
disturbed
. However, 38 mm diameter
undisturbed
samples can be obtained
from the undisturbed soil below the bottom of the borehole (see Section
6.4.1)
using the sampling tube
shown in Fig.
6.2c
.
200 mm
38 mm
(a)
(b)
(c)
Fig. 6.2
Hand augers: (a) clay auger; (b) gravel auger; (c) 38 mm undisturbed sample tube.
