Environmental Engineering Reference
In-Depth Information
Log from
augered
borehole
Log from
static cone
penetrometer
Depth
0m
Water
LEGEND
Sand
5m
Silty sand
Sandy silt
10 m
Clayey sand
Sandy clay
Clay
15 m
Figure 5.12.
Log of sandy and clayey soils below the water table from an augered borehole and from
static cone penetrometer.
a rapid method of establishing the depth and general properties of the material overlying
rock which will be investigated in more detail by some other method.
A major difficulty in auger drilling in cohesionless soils or soft clays is the stability of
the sides of the drill hole particularly below water. Figure 5.12 shows a log prepared from
an augered drill hole in sandy soils below the water table, compared with the log of a
nearby static cone penetrometer probe. Mixing and collapse of the hole have led to gross
inaccuracies in the auger hole log. Auger drilling in non-cohesive and weak soils is there-
fore restricted to above the water table.
Hollow flight augers have the advantage of providing support for the hole in these con-
ditions and allow sampling through the augers. However the action of removing the plug
at the end of the auger will often loosen and disturb the soil below the auger and hollow
flight augers are not recommended for drilling in cohesionless soils, particularly if they are
loose, or in weak clays.
The development of large diameter hollow auger drilling has enabled the recovery of con-
tinuous core with effectively undisturbed soil samples up to 300 mm in diameter (
Figure 5.11
and
Figure 5.13
). This equipment is used to drill up to 80 m deep in dam cores in the USA.
Hollow flight augers are an acceptable technique for drilling in the core of existing dams
because no drilling fluid is used which can cause hydraulic fracture. However the holes must
be carefully backfilled in stages with cement/bentonite grout to avoid hydraulic fracture.
5.9.4
Percussion drilling
There are three main types of percussion drilling as follows:
(a)
Cable tool (bit and bailer) drilling
(
Figure 5.14a
)
involves the successive dropping of
a heavy chisel type bit to the bottom of the hole. The fragments are recovered using a
