Environmental Engineering Reference
In-Depth Information
Examples
Case 1: Open-Pit Mines
(
Brawner, 1975
)
General
: Problems encountered in open-pit mines in soft rock (coal, uranium, copper, and
asbestos) during mining operations include both bottom heave of deep excavations (of the
order of several hundred meters in depth) and slides, often involving millions of tons.
Solutions:
Deep vertical wells that have relieved artesian pressures below mine floors
where heave was occurring have arrested both the heave and the associated slope insta-
bility. Horizontal drainage in the form of galleries and boreholes as long as 150 m installed
in the toe zone of slowly moving masses arrested movement even when large failures
were occurring. In some cases, vacuum pumps were installed to place the galleries under
negative pressures. Horizontal drains, consisting of slotted pipe installed in boreholes,
relieve cleft-water pressures in jointed rock masses.
Case 2: Failure of a Cut Slope
(
Fox, 1964
)
Geological conditions:
The slope in
Figure 9.108
consists of colluvial soils of boulders and
clay overlying schist interbedded with gneiss. Between the colluvium and the relatively
sound rock is a zone of highly decomposed rock.
Slide history:
An excavation was made to a depth of 40 m into a slope with an inclination
of about 28°. Upon its completion cracks opened, movement began, and springs appeared
on the surface. The excavation was backfilled and the ground surface was graded to a uni-
form slope and covered with pitch. Monuments were installed to permit observations of
movements. Even after the remedial measures were invoked, movement continued to
endanger nearby structures. The greatest movement was about 2.5 cm/day. Failure had
T
unnel M-30
A
Core
Crack visible
P l a n
Clay and boulders - overburden
}
Wea
thered and soft
deco
mposed schrist
May or may not be in place
Hard, dark-grey sericite
schist, quartz schist and quartzite
}
Bedrock
Former crack
Test pit 4 completed in first stage of drainage.
Tunnels and Ax-size drainholes from tunnels
completed in final stage.
Water from weathered
rock only
Small amount
of water from
20±m
Water from open joint
at 29.5 ±
Te s t pit 4
Tu n nel M-30
2
1
Schist
?
2
Clay and boulders
Large open joint
all rock above this joint
decompressed or badly weathered;
rock below joint probably
general unweathered.
Water from
open joint
?
Small spring
Soft, decomposed schist
2''
peformed
pipe
Ty p ical geologoc
section of Serra
slide, Santos, Brazil
By portland p. fox
6/16/56
?
?
S e c t i o n A - A
160
140
120
100
80
60
40
20 meters
FIGURE 9.108
Stabilization of a failure in a colluvial soil slope using lateral drains and galleries. (From Fox, P.P.,
Engineering
Geology Case Histories Numbers 1-5,
The Geological Society of America, Engineering Geology Division, 1964,
pp. 17-24. With permission.)
