Stability of Bottom of Excavation.

a. Piping in sand.  The base of an excavation in sand is usually stable unless an unbalanced
hydrostatic head creates a "quick" condition.  Among the methods to eliminate instability are dewatering,
application of a surcharge load at the bottom of the excavation, and deeper penetration of the piling.

b. Heaving in clays.   The stability against heave of the bottom of an excavation in soft clay may be
evaluated from figure 14-12.  If the factor of safety is less than 1.5, the piling should be extended below the base of the excavation.  Heave may also occur because of unrelieved hydrostatic pressures in a permeable layer located below the clay.

c. Care of seepage.   Small amounts of seepage into the excavation can be controlled by pumping from sumps.  Such seepage can be expected if the excavation extends below the water table into permeable soils.  If the soils consist of fine sands and silts, the sumps should be routinely monitored for evidence of fines being washed from the soil by seepage.  If large quantities of fine-grained materials are found in the sumps, precautionary steps should be taken to make the lagging or sheeting watertight to avoid excessive settlements adjacent to the excavation.

Stability of bottom of excavation in clay.
Figure 14-12.  Stability of bottom of excavation in clay.


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