Environmental Engineering Reference
In-Depth Information
when fresh are only a little stronger and more durable than hard clays. The Geological
Society (1995) provides a suggested approach for site-specific mass-type classification of
weathered profiles in these weaker mudrocks.
Weathered profiles in mudrocks are more uniform, gradational and generally not as
deep as those in other rocks. Deere and Patton (1971) describe these types of weathered
profiles in shale and point out that the lack of distinct boundaries within such profiles has
led to contractual disputes over the depth to acceptable foundations.
3.5.5
Stability of slopes underlain by mudrocks
Slopes underlain by mudrocks commonly show evidence of past instability, even when the
slope angles are small (e.g. 10° to 15°). This is not really surprising when we consider that
all of the characteristics described above in Sections 3.5.1 to 3.5.4 tend to lower the
strength of rock masses containing mudrocks.
Relatively shallow landsliding is common in the residual clay soils developed on the
weaker mudrocks. Taylor and Cripps (1987) provide a comprehensive review of slope
development and stability in weathered mudrocks and overconsolidated clays, mainly
relating to examples in the United Kingdom.
Deere and Patton (1971) give a useful review of experience with unstable slopes on
shales and on shales with interbedded sandstones. They point out that, in common with
most other rocks, weathering of shales usually produces a low-permeability zone near the
surface, underlain by jointed, less weathered shale, which is more permeable. Instability
can arise when groundwater transmitted through this lower zone or along sandstone beds
causes excessive pore pressures in the near-surface, more weathered shale. (See
Section
3.6.4
and
Figures 3.23
and
3.24
).
The most common situations in which larger scale landsliding occurs, or is likely to occur,
are where the bedding “daylights” on a valley slope. This occurred at both Sugarloaf and
Thomson dams in Victoria, Australia, as described in Chapter 2, Section 2.10.
3.5.6
Development of unusually high pore pressures
Stroman et al. (1984) and Beene (1967) describe a major slide which occurred during the
construction of a 30 m high section of the 5.5 km long Waco Dam in Texas. The dam was
located on near-horizontally bedded shales cut through by three steeply dipping normal
faults, which displaced the shale units by up to 30 m and caused them to be locally folded
(
Figure 3.20a
).
The originally designed embankment is shown in outline on Figure 3.20(b). The design
was based on the assumption that the weakest foundation material was a localized 12 m
thick layer with unconsolidated undrained strength of
144 kPa. No potential
for pore pressure development was expected (because of the high degree of overconsoli-
dation of the shales) and consequently no piezometers were installed in the original con-
struction.
The failure occurred in a 290 m long section of embankment constructed between two
of the normal faults. As can be seen on Figure 3.20c, the failure surface was mainly hori-
zontal within the Pepper Shale about 15 m below the main foundation level. This failure
surface broke out to the ground surface at an average distance of 235 m downstream from
the dam axis.
Figure 3.21
shows the pore pressure distributions before the sliding and at the end of
the reconstruction. Stroman et al. (1984) state that the unusually high pore pressures at
the Pepper Shale/del Rio Shale contact were the cause of the slide. However they also
report that direct shear tests on precut samples of Pepper Shale gave effective friction
angles between 7° and 9°, with zero cohesion. They state that “This is the laboratory test
5°, c
