Environmental Engineering Reference
In-Depth Information
Table 13.5.
Minimum thicknesses of filters under rip-rap (from
Sherard et al., 1963).
Wave height (m)
Minimum filter thickness (mm)
0-1.2
150
1.2-2.4
225
2.4-3.0
300
After logging, photography and sampling for strength and durability testing, it is useful
to store the drill cores out in the open in a secure compound. Here they can be examined
and photographed at regular intervals to record any deterioration. It is also possible to
accelerate the “weathering” processes by a program of wetting and drying.
13.2.1.5
Design of filters under rip-rap
See Section 9.2.5.2 for a discussion on the particle size distribution requirements.
Sherard et al. (1963) recommend that the thickness be selected after consideration of:
- Size of wave;
- Gradation of the rip-rap. Rip-rap with less fines needs thicker filters;
- Plasticity and erodibility of the embankment earthfill. If the earthfill is well graded
non erodible granular soil, it needs less protection than a fine silty sand or dispersive
soil;
- The cost of the filter.
They recommend the minimum thicknesses shown in Table 13.5.
They indicate these should be absolute minimum thicknesses and that, if two layers of
filters are needed, each should be at least 150 mm thick.
US Corps of Engineers (1984a) recommend that filter layers should be at least three 50
percent size stones thick, but not less than 230 mm.
The authors feel that the thicknesses in Table 13.5 seem reasonable for filters which sat-
isfy no-erosion design rules, including limitations on maximum size particles. For filters
which are quarry or pit run with particles up to 100 mm size, the minimum thickness
would probably need to be at least 450 mm, so as to ensure that segregation of the larger
particles does not leave areas with no finer particles to protect the soil under the filter.
13.2.1.6
Use of soil cement and shotcrete for upstream slope protection
In some locations rock suitable for rip-rap may not be available within economic haul dis-
tance and consideration may be given to the use of soil-cement as an upstream facing.
Many dams in the USA have been constructed in this way.
ICOLD Bulletin 54 - Soil-Cement for Embankment Dams (ICOLD, 1986d) describes
the use of soil-cement in some detail. The important features of the method are:
- The soil-cement is usually placed in horizontal layers as shown in
Figure 13.6
. The
effective thickness normal to the slope has varied between 0.46 m and 0.76 m with
0.6 m being the most common. This gives a 2.4 m horizontal layer width for a 3H:1V
upstream slope, allowing placement by trucks. Conveyor placement can be used to
allow 0.46 m thickness, and on slopes 3H:1V or flatter, compaction up and down the
slope can allow use of thinner covers. This may be acceptable for smaller dams;
- Portland cement is mixed with the available soil, conditioned to around optimum water
content (standard compaction) and the soil-cement compacted in 150 mm to 300 mm
thick layers to a density ratio of 98%. The soil cement is then cured under water spray;
