Environmental Engineering Reference
In-Depth Information
reduced the preexisting strengths to the extent that the original slope inclination was
unstable. Piezometers installed as part of an investigation revealed that the highest pore
pressures were in the fractured rock zone, under the colluvium.
Remedial measures:
To correct the slide, Dr. Karl Terzaghi had a number of horizontal drill
holes and galleries extended into the fractured rock as shown in
Figure 9.108.
The holes
drained at rates of 10 to 100 L/min, and the water level in the piezometers continued to
fall as work progressed. The slide was arrested and subsequent movements were reported
to be minor.
Case 3: Construction of a Large Cut Slope
(
D'Appolonia et al., 1967
)
Geologic conditions
: As illustrated on the section, (
Figure 9.109)
,
conditions were character-
ized by colluvium, overlying sandstones and shales, and granular alluvium. Explorations
were thorough and included test pits which revealed the overburden to be slickensided,
indicating relict failure surfaces and a high potential for instability.
Treatment:
Construction plans required a cut varying from 6 to 18 m in height in the col-
luvium along the slope toe. To prevent any movement, a system of trenches, drains, and
galleries was installed. A cutoff trench, vertical drain, and gallery were constructed ups-
lope, where the colluvium was relatively thin, to intercept surface water and water enter-
ing the colluvium from a pervious siltstone layer. A 2-m-diameter drainage gallery was
excavated in the colluvium at about midslope to intercept flow from a pervious sandstone
stratum and to drain the colluvium. Sand drains were installed downslope, near the pro-
posed excavation, to enable the colluvium to drain by gravity into the underlying sand
and gravel lying above the static water level, thereby reducing pore pressures in the col-
luvium. An anchored sheet-pile wall was constructed to retain the cut face; the other sys-
tems were installed to maintain the stability of the entire slope and reduce pressures on
the wall.
9.4.5
Side-Hill Fills
Failures
Construction of a side-hill embankment using slow-draining materials can be expected to
block natural drainage and evaporation. As seepage pressures increase, particularly at the
toe as shown in
Figure 9.110a,
the embankment strains and concentric tension cracks form.
Cutoff trench, vertical
drain of gallery
900
Siltstone
Sand drains
Excavation and
anchared wall
Colluvium
800
Sandstone
Flow
Potetial failure
Shale
Cut
Coal bed
Alluvium
700
Claystone
GWL
Sand and gravel
Lower drainage gallery,
diameter = 6 ft
600
Scale 1:1
FIGURE 9.109
Stabilization of a colluvial soil slope (Weirton, West Virginia) with vertical drains and galleries. (From
D'Appolonia, E.D. et al., 1967.)
