Environmental Engineering Reference
In-Depth Information
erodibility, and a weathering grade alone (e.g. slightly weathered rock) may not be ade-
quate or may be too conservative. If weathering grade is used as one of the require-
ments, it should be clearly defined in the documents what the different weathering
grades are.
- Planning and selection of cutoff foundation requirements should use all available infor-
mation. This information includes borehole log data - rock type, degree of weathering
and fracture log, the Lugeon water pressure test results, presence of limonite and clay
infill joints, seismic refraction data and bulldozer trench information.
It will often be found that there is some correlation between these data which will allow
logical decision making, e.g.:
- In developing general excavation requirements the soil/extremely weathered rock will
have low seismic velocity (300-1000 m/sec) and will commonly coincide with blade
refusal in the dozer trench. Hence the low seismic layer boundary may form a useful
means of extrapolating between “hard data” in the trenches and boreholes.
- The base of highly fractured rock, high-moderate Lugeon value, and medium seismic
velocity (e.g. 1400-2000 m/sec) will sometimes coincide and indicate a reasonable cut-
off level. Again the seismic data allows interpolation around the site.
- The base of limonite staining in joints often coincides with the base of high Lugeon
rock. If weathering grade is related to this staining there may also be a correlation
between weathering grade and Lugeon value.
Selection of planned cutoff criteria is sometimes done by laying out the drill core and
the design engineer and engineering geologist observing the core, logs and other data and
selecting the cutoff level or levels and the associated criteria there and then.
Often there will be more than one choice, with a “better” cutoff being available at
depth and an “adequate” cutoff closer to the surface. The “adequate” cutoff may require
a wider cutoff trench, more grouting, a grout cap, downstream filter protection etc., the
need for which has to be offset against the shallower depth.
Similarly, for dams on a soil foundation, the cutoff depth and criteria can often be best
established during the logging of test pits.
If this approach is used, the geological features which were used to select the cutoff can
be described and readily identified in the construction process. The engineering geologist
and design engineer, who make the design decision, should be present during construction to
define and confirm those conditions in the cutoff trench.
17.6
SLOPE MODIFICATION AND SEAM TREATMENT FOR
EMBANKMENT DAMS
17.6.1
Slope modification
Excavation of near vertical or overhanging surfaces and/or backfilling with concrete is
required.
For general foundation:
-To allow compaction of earthfill and avoid cavities under rockfill due to overhangs in
rock in the abutment.
For cutoff foundation:
-To allow compaction of earthfull to give a low permeability contact between the earth-
fill and the foundation;
