Environmental Engineering Reference
In-Depth Information
The rotary drill bits can excavate into soil or soft rock. The cuttings are suspended
in the slurry and flushed from the trench, usually by reverse circulation, to improve
the lifting capacity. Screens and settling tanks are used to remove the cuttings.
(e) Percussion tools. Percussion tools and rock chisels are used where boulders, cobbles
or other hard materials are encountered. The broken pieces are removed by clamshell
bucket. Figure 10.27 shows examples of this equipment.
In some applications where boulders are present it may be economic to use a two
stage operation (
Figure 10.28
), where the first stage consists of excavating pilot holes
with percussion rotary drill and the second stage removing the material between the
holes with a clamshell bucket. The first stage holes are spaced to leave an intermedi-
ate piece 0.3 m to 0.6 m narrower than the clamshell bucket grab.
(f) Hydrofraise - which is a down the hole reverse circulation rig which excavates a 2.8 m
long panel. The use of this technique for Mud Mountain dam, is described in
Davidson et al. (1992). They also describe the use of grouting to control hydraulic
fracture in the dam core as the slurry wall was built.
Where bentonite mud is used to support an excavation, as required for virtually all the
methods described above, it should be noted that the chemical composition of the ground-
water will have an effect on the performance of the bentonite slurry.
In particular, groundwater with a high salt content (i.e. NaCl) or a high calcium ion
concentration will cause the bentonite to flocculate. This is caused by contraction of the
diffuse double layer around the clay particles as described in Chapter 8.
ICOLD (1985) indicate that:
-Fresh water should be used to make the bentonite slurry. Water with up to 5 g/litre of
salt may be used, but the bentonite content will have to be increased;
Figure 10.27.
Typical chisel details for breaking embedded boulders in site excavations (Xanthakos,
1979, reproduced with permission of McGraw-Hill).
