Environmental Engineering Reference
In-Depth Information
Figure 10.17.
Slurry trench excavation by dragline and clamshell (Xanthakos, 1979, reproduced with
permission of McGraw-Hill).
Table 10.2.
Typical gradation limits for slurry trench backfills
(adapted from Xanthakos, 1979).
Percentage passing
Sieve size (mm)
UK and Australian cutoffs
USA cutoffs
75
80-100
80-100
19
40-100
40-100
4.75
30-70
30-80
0.6
20-50
20-60
0.075
10-25
10-30
in lumps. Table 10.2 gives particle size distributions for the combined backfill material
quoted by Xanthakos. Xanthakos indicates that at least 10% silt and clay fines passing
0.05 mm is needed to give adequately low permeability.
The backfill may be mixed by withrawing, dozing and blading the excavated soil to
remove lumps of clay and silt or pockets of gravel. The soil is then mixed with the ben-
tonite slurry, either by using the same earthmoving equipment or, preferably, by using a
concrete batch plant type mixer to ensure a uniform product.
The backfill is placed into the trench by gradually pushing it in by bulldozer, so that the
backfill slope is between 6 H to 1 V and 8 H to 1 V. To start backfilling, the initial slope
should be formed by lowering the backfill using (say) a clamshell bucket, as dropping the
backfill through the slurry would cause segregation. A gap of 15 m to 45 m is left between
the excavation face and toe of backfill to ensure proper cleanup of the trench base can be
carried out.
No attempt is made to densify or compact the backfill.
Xanthakos (1979) indicates that in some cases 0.6-0.9 m of concrete may be placed at the
bottom of the trench to provide protection against erosion of the base under seepage flows.
The top of the trench must be protected from drying by placing earthfill over the top.
Settlement of the cutoff is quoted by Xanthakos (1979) as being between 25-150 mm for
trenches 15 m to 25 m deep and 25 m wide. Less settlement is observed in narrower trenches.
Settlement mostly takes place in the first 6 months.
