Geoscience Reference
In-Depth Information
10 STABILITY OF SLOPES
Stability of soil masses is characterised by the stability of its slopes or sides.
They can be unsupported or supported by earth-retaining structures, such as
geotextile, soil nailing, retaining wall, gravity wall, or strutted or anchored wall
(sheetpiling), permanently or temporarily (building pit). Unsupported slopes can be
created by nature (erosion) or made by man by excavating (cuttings) or by building
(embankments). Cuttings cause a decrease of total stresses and embankments an
increase. Particularly for cuttings the local effect on groundwater flow and
corresponding pore pressures are to be considered, both during construction (pore
pressures decrease by temporary drainage) and after construction (pore pressures
increase again). The long-term stability of a cutting may be more critical than
during construction (vanishing suction pressures). For embankments on soft soils
the induced excess pore pressures in the subsoil during construction may induce
collapse. In the case of water retaining dams and dikes, a critical situation may
occur when the retaining water is quickly lowered, referred to as the rapid draw-
down instability. Landslides are usually triggered by rainshowers.
Gravity and loading may cause movement (instability) and collapse (failure). A
massive movement may take place as result of a shear failure along one or more
internal slip surfaces. The soil may develop plastic zones (limit effective stress) or
liquefaction (loss of effective stress). For failure one distinguishes falls (steep
slopes), flow (mud), translational slides (parallel to the surface), rotational slides
(short slopes), wedge failures (horizontal sliding) and collapse of retaining
structures. Due to weathering and degradation many natural slopes may exist in a
state of incipient failure, i.e. ultimate limit state (ULS). In this chapter the limit
state of slopes is discussed.
A
LIMIT ANALYSIS
The real mechanical three-dimensional state of a soil mass is usually so
complex, that only approximations can be adopted. The critical condition of a soil
mass, i.e. the limit of stability, is then obtained by considering an approximate limit
load. There are two situations possible.
- A lower-limit load, which the soil certainly can carry, i.e. equilibrium and
boundary conditions are satisfied everywhere, nowhere does the stress exceed
the failure condition, but whether failure. i.e. a possible kinematic pattern, really
occurs is unsure.
- An upper-limit load, under which the soil will certainly collapse according to a
plausible kinematic pattern, i.e. compatibility and boundary conditions are
satisfied everywhere and equilibrium conditions are satisfied for the chosen
displacement pattern, but whether stress does not exceed failure conditions
everywhere is unsure.
Each of these approaches has the advantage that a strict relation between stresses
and strains is not required, which makes the analysis relatively simple and
straightforward.
From plasticity theory it can be shown that for materials, which possess cohesion
but no internal friction, the real limit load, satisfying everywhere equilibrium and
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