Water: Surface and Subsurface - Geotechnical Engineering Investigation - page 644

Because seepage pressure is directly proportional to the hydraulic gradient, the most dan-
gerous areas for liquefaction to occur are those where the upward gradient is large and the
counterbalancing weight is small. The analysis of flow-through soils and seepage pres-
sures may be performed with flow nets (see Section 8.3.3) .
Rock Masses
Water pressures that develop in rock-mass fractures are often termed cleft-water pressures .
At very high levels, they can result in the instability of rock foundations for concrete dams,
and they are the common causes of slope failures. Seepage can result in the softening of
joint fillings, and the development of high pore pressures in the filling material can reduce
strength.
Stress changes can significantly affect seepage and permeability in rock masses.
Compressive stresses cause closure of joints even under relatively low stress levels, reduc-
ing seepage flow, although sufficient closure of other voids to reduce permeability occurs
in most rocks only under relatively high stress levels. Tensile stresses can increase perme-
ability and flow, with the increase commonly occurring as a rock slope begins to deflect.
The failure of the Malpasset Dam (see Section 8.3.4) is considered to be the result of ten-
sile stresses increasing under the toe of the concrete arch dam in the foundation gneiss. It
has been estimated that the gneiss had a permeability 1000 times greater under the tensile
stresses than when in compression. The greater permeability permitted an increase in
uplift pressures beneath the foundations, resulting in excessive deflections of the dam.
Leakage
Leakage occurs through natural slopes; through the embankment, foundation, or abut-
ments of dams; and beneath sheeted excavations. Sloughing of the downstream face of a
dam embankment or a natural slope is a fairly common phenomenon. It usually occurs
where the phreatic level intersects the slope. Seepage forces in the zone of emergence
cause a loosening of the surface materials and raveling, and local failures occur.
Leakage through dam foundation materials is more common than through the
embankment, since foundation soils are generally less dense and more erratic than the
structure that results from manufacturing an embankment. Uncontrolled seepage beneath
an embankment manifests itself as springs near the toe; and, as fine soil particles are car-
ried along (i.e., piping occurs) they are deposited on the surface around the springs as
“sand boils.” These can also be found at the toe of a cut slope, on the surface after earth-
quakes in areas of fine-grained cohesionless soils, or behind levees during flood stages.
Underseepage can also cause the development of excess pore pressures under the
embankment toe. The loss of stability of the foundation materials can result in a deep
downstream slide. Since failure does not relieve the pore pressures, sliding will continue,
and if not immediately corrected, failure of the dam may occur.
Piping
Piping is the progressive erosion of soil particles along flow paths. Fine soil particles near
the point of emergence can be removed by flow, and as they wash away, flow and erosion
increase in the soil mass, in time developing channels which result in greater flows and
erosion, and finally catastrophic failure (see Section 8.3.4, discussion of dams). The term
piping is also used to refer to the phenomenon of boiling described previously.
Piping through an embankment occurs in finer soils along layers of free-draining coarse
materials, through cracks in embankment soils, or adjacent to rock masses where fractures
are in contact with fine-grained embankment soils. Embankment cracks can result either