Ground Subsidence, Collapse, and Heave - Geotechnical Engineering Investigation - page 864

10.5 Soil Subsidence and Collapse

10.5.1 General

Causes
Subsidence in soils results from two general categories of causes:

1. Compression refers to the volume reduction occurring under applied stress from

grain rearrangement in cohesionless soil or consolidation in a cohesive soil (see
Section 3.5.4). The phenomenon is very common and always occurs to some
degree under foundation loadings.

2. Collapse is the consequence of a sudden closure of voids, or a void, and is the

subject of this chapter. Collapsible or metastable soils undergo a sudden decrease
in volume when internal structural support is lost; piping soils are susceptible to
the formation of large cavities, which are subject to collapse.

The Hazard
Subsidence from compression or soil collapse is a relatively minor hazard, resulting in
structural distortions from differential settlements.
Piping erosion forms seepage channels in earth dams and slopes and in severe cases
results in the collapse of the piping tunnel, which can affect the stability of an earth dam
or a natural or cut slope.

10.5.2 Collapsible or Metastable Soils

Collapse Mechanisms
Temporary Internal Soil Support
Internal soil support, which is considered to provide temporary strength, is derived from a
number of sources. Included are capillary tension, which provides temporary strength in
partially saturated fine-grained cohesionless soils; cementing agents, which may include
iron oxide, calcium carbonate, or clay in the clay welding, of grains; and other agents,
which include silt bonds, clay bonds, and clay bridges, as illustrated in Figure 10.22.

Collapse Causes
Wetting destroys capillary bonds, leaches out cementing agents, or softens clay bonds and
bridges in an open structure. Local shallow wetting occurs from surface flooding or bro-
ken pipelines, and subsidence can be substantial and nonuniform. Intense, deep local wet-
ting from the discharge of industrial effluents or irrigation can also result in substantial
and nonuniform subsidence. A slow and relatively uniform rise in the groundwater level
usually results in uniform and gradual subsidence.
Increased saturation under an applied load can result in gradual settlement, or in a sud-
den collapse as the soil bonds are weakened.
Applied load of critical magnitude can cause a sudden collapse of the soil structure when
the bonds break in a brittle type of failure, even at natural moisture content.

Susceptible Soils
Loess
Loess is a fine-grained aeolian deposit. Distribution, characteristics, and engineering prop-
erties of loess are described in Section 7.5.3.