8
Principal geological factors affecting certain
engineering projects
8.1 Stability of slopes and cuttings
8.1.1 Geological factors affecting the stability of a new excavation
The static conditions that control the initial and also the later stability of a steep face cut
into soil or rock, and which may determine the need for support or remedial treatment,
are as follows.
The properties in bulk, particularly the shear strength, of the material forming the
cutting: the stability of a cutting in rock is usually dependent on the occurrence of joints
and other planes of weakness, and on the amount of cohesion and the friction across these
planes.
The structure of the rocks and soils, and specifically how any planes of weakness are
orientated relative to the newly exposed face: for example, horizontal bedding planes in
poorly jointed sandstone often give nearvertical faces which are stable, whereas faults or
joints striking parallel to the new face, and dipping steeply towards it, will probably be
planes of movement or potential instability.
The groundwater conditions: saturation significantly lowers the strength of most soils
compared with their values when dry. Certain soils weaken to a stage at which they run
like viscous liquids. High pore pressure of ground water in a layer, or in a plane of
weakness, lowers frictional resistance to movement. (The mechanism is the same as that
which allows a hovercraft to glide over water, or land on a high-pressure cushion of air.)
A dramatic and tragic example of instability triggered by high pore pressure within a
body of soils was the Aberfan disaster in 1966, when a spoil heap of mine waste slid
downwards on to a Welsh village enveloping and destroying a large part of the local
school.
Stresses produced by natural loads adjacent to the cutting: steep-sided valleys or
mounds affect the state of stress in the ground near the surf ace, not only below
themselves but also for some distance around. This lateral change in stress conditions
may be sufficient to cause instability of weak rocks and soils. For example, valley bulges
(Section 3.5.8) are produced in this way, and they may be accompanied by instability of
an adjacent slope.
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252
Figure 8.1 Minor ridges on a hillside produced by soil creep. (British
Geological Survey photograph, C1333, published by permission of
the Director; NERC copyright.)
Rotational slip affects clays or clay-rich rocks such as mudstones and shales. The
surface of movement is curved, and is such that shear resistance along it is assumed to be
given by the equation
where σ is the stress normal to the surface, u is the pore water pressure, C c is the
cohesion of the clay, Ф c is the angle of frictional resistance of the clay in terms of
effective stress, and S is the shearing resistance. Figure 8.2 shows rotational slip failure in
a slope in California.
Rotational slip is usually preceded by development of vertical tension cracks at the top
of the slope. These cracks may reach a maximum depth of 2C/γ, where C is the cohesion
of clay and γ is its density. Typical laboratory values for marl (a clay-rich rock) are
Principal geological factors affecting certain engineering projects 253
C=2.5 MN m–2 and Ф=42° (under
dry conditions)
C=2.0 MN m–2 and Φ=21° (under
saturated conditions)
Figure 8.2 Slope failure at Sonoma County, California. (Reproduced by kind
permission of California Division of Highways.)
8.1.4 Types of failure of natural rock slopes
Minor rock falls are produced by weathering acting on unstable rock slopes. The
susceptibility of a given rock to weathering processes can be estimated by determining its
saturation moisture content (i s) and swelling coefficient (E s). Igneous and high-grade
metamorphic rocks with is values of less than 1% are generally safe from weathering
effects. Sedimentary rocks and low-grade metamorphic rocks are considered to be safe on
slopes if their is values are less than 3%. If exposed rocks on a slope have high saturation
moisture contents, then tests should be carried out over a period of time, under both
freeze-thaw and wet-dry conditions, and their swelling behaviour noted. Ice action is
important if joints are present. If the rock mass has a low block volume, that is, less than
0.5 m3, minor rock falls may occur, even if the rock has a low saturation moisture
content.
Major rock falls usually result from collapse caused by undermining of rocks above a
weak layer (see Fig. 8.3). The agent may be weathering, erosion or mining. Common
weaknesses in a rock mass which can lead to collapse after weathering and etching out by
erosion are layers of clay rock, chlorite in joints, and carbonate rocks, including
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256
Figure 8.4 Housing tract, Daly City, California, located on unstable ground in
San Andreas Fault zone. Houses at end of tract (mid-right picture)
removed after destruction by slide. (Reproduced by kind permission
of US Geological Survey, Menlo Park, California.)
In rock masses with discontinuities that are not infilled, a slope will be stable only if
the angle of dip of any discontinuities orientated towards the slope is less than the angle
of friction, Ф, of the rock. Water pressure in the discontinuity may reduce the safe slope
angle, and must be taken into account. If the angle of dip of a discontinuity is greater than
the angle of friction, the slope will only be stable if there is some cohesion, that is, if C
has a value greater than zero. If the discontinuities are infilled with a weak material such
as clay, stability of a slope in such a rock mass will depend on its shear strength. Where a
rock mass consists of alternating layers of strong and weak rocks, tests in situ and in the
laboratory are necessary to determine whether failure is likely to occur because of shear
failure in the weak bands, or because of sliding along the bedding planes, if they dip
steeply.
Rotational failure occurs when the stresses generated by gravity on rocks near the
slope exceed the inherent strength of the rock mass. It may be accompanied by
translational failure. The extra load of a spoil heap may trigger failure of this type.
Creep failure takes place when certain rocks, such as clays, absorb water, swell,
slowly deform and move down hill. If they are overlain by a more stable, stronger layer,
translational failure may take place along the bedding plane between them. Cambering
(Section 3.5.8) is produced in this way.
Table 8.1 Angles of frictional resistance (Ф) and unconfined compressive strengths of
some common rock types. The data are from Hoek (1970), Hoek and Bray
(1974) and Attewell and Farmer (1976).
Φd (deg)
Φult
(deg)
Unconfined compressive strengths
(MN m–2)
Igneous rocks
basalt
dolerite
47
gabbro
similar to
basalt
andesite
31–35
porphyry
diorite
granite
<45
150–300
100–350
250–300
28–30
40
similar to basalt
30–34
similar to
granite
150–300
>35
31–33
100–250
Principal geological factors affecting certain engineering projects 261
mesh was hung on the face to minimise the risk from any rock fall.
Stability of the face in the western part of the dock was confused and reduced by the
presence of another important set of joints (the C joints in Fig. 8.6). The same
considerations ruled out a slope of 35° but the angle was reduced from 80° to 45° and the
face was hung with chain mesh. These solutions proved to be adequate, and during the
first two-year period after construction no rockfall occurred.
8.2 Impounded surf ace water: geology of reservoir and dam sites
8.2.1 Leakage and other considerations
A reservoir is meant to hold water, hence the principal geological criterion of the
suitability of a reservoir site is that the rocks and soils around and below it form an
impervious basin naturally, without need of excessive and expensive grouting of potential
leakage paths. Other geological factors to consider are the consequence of a change in the
position of the water table as the reservoir fills, and the rate at which sediment will
accumulate in the reservoir.
The important sources of leakage from a reservoir are (a) through permeable soils, (b)
through rock aquifers, and (c) along faults and master joints.
(a) In regions that have escaped glacial erosion, there may be a thick mantle of weathered
rock forming a permeable soil. In glaciated regions, however, soils (most notably
boulder clay) may form a useful impervious skin below the reservoir, but other
deposits (particularly sand and gravel) may offer paths for leakage. Buried channels
infilled with sand and gravel are a possible hazard in both types of region. Superficial
deposits can present other problems as well. Drift obscures the shape of the rock-head
surface. Peat may flavour and colour the water in the reservoir, and may need to be
removed before filling. A change in the water table as the reservoir fills may affect the
stability of screes and other soils on the hillsides and cause landslides.
(b) Leakage through rock aquifers is controlled by their structure, and its relationship to
the hydraulic gradients produced by the head of water in the new reservoir. For
example, leakage along the sandstone layer (Fig. 8.7) would take place in structure ‘b’
but not in ‘a’. The hydraulic gradient slopes down from the reservoir end of the aquifer
to its outcrop in an adjacent valley. If, however, the water table in an aquifer, which
forms one side of the reservoir (Fig. 8.7), were higher than the top water level of the
impounded water, then flow would be towards the reservoir, and there would be
replenishment rather than leakage. In general, dips of the aquifer towards the reservoir
inhibit leakage along it. Springs and seepages may be useful in indicating the position
of the water table and the movement of ground water around the reservoir. Ideally, a
reservoir should be sited on impervious rocks or, if only some layers are impervious,
on a syncline plunging upstream. Limestone, and to a lesser extent sandstone with a
calcareous cement, may present serious hazards. The calcite dissolves, especially if the
water has been in contact with peat and is slightly acidic, and leakage paths