Environmental Engineering Reference
In-Depth Information
Figure 3.24.
Landslides which can occur in a slope where sandstone beds are covered with colluvium
(Based on Figure 13c of Deere and Patton, 1971).
slope than the sandstones. Joint-water pressures during extreme rainfall periods and
earthquake forces may also contribute to the slight movements of the sandstone blocks.
Opening up of the joints in the sandstones causes the permeability of layers near the
surface to be greatly increased.
Where whole hillsides are underlain entirely by rocks of the sandstone group, these
rocks commonly form steep slopes or cliffs. Slope failures are rare and usually occur by
rockfalls or toppling from the cliff portions.
More commonly, sandstones occur together with shales or siltstones as shown on
Figures
3.23
and 3.24. In these situations weathering extends deeper into the mass and landsliding
is more prevalent.
Figure 3.23, taken from Figure 13b of Deere and Patton (1971), shows a slope underlain
by horizontally interbedded sandstones and shales. In high rainfall areas or during the wet
season in other areas, the water table on Figure 3.23 will be high and of the form shown,
due to the relatively free-draining nature of the sandstone beds. Springs occur at the bases
of sandstone outcrops and the shale below and between the sandstone beds is either con-
tinually wet or alternately wet and dry. Under these conditions chemical weathering of
both rock types occurs but is usually more pronounced in the shale. This rock is at least
partly weathered to clay and contains clay-coated joints or fissures, often slickensided.
The weathered shale either slumps or its bearing capacity is exceeded, allowing large
movements and eventual collapse of the outermost sandstone block. Continuation of
these processes leads to development of layers of scree and colluvium on the slope, and to
“cambering” of the near-surface part of the sandstone bed as shown on Figure 3.24.
Deere and Patton also describe landsliding observed commonly on interbedded sand-
stone/shale slopes where sandstone beds have become covered by colluvium, as shown on
Figure 3.24. The colluvium restricts drainage from the sandstone (Bed 2) which may
become a semi-confined aquifer with piezometric surface as indicated. Pore pressures so
developed cause sliding to occur, usually along the colluvium-weathered shale contact
(Slides A and B on Figure 3.24).
Slide B is a double slide in which the toe of the first slide (No.1) has overloaded the meta-
stable top of the slope below, causing Slide No.2 (Deere and Patton 1971).
Deere and Patton (1971) point out that deepseated slides, such as Slide C on Figure
3.24, can occur when a combination of unfavourable geological conditions exists. These
could include bedding surface crushed seams along the shale-sandstone boundaries, high
water pressures in sandstones Nos 2 and 3 and high water levels in the affected mass.
Deere and Patton point out that after the first small movements, the permeability of the
deeper part of the mass will be increased and softening and weathering of the near-surface
shale will be accelerated.
