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Steel Designers' Manual - 6th Edition (2003)
Base heave
One form of base heave can arise as a result of higher pore water pressure in underlying
soil layers that is created by the excavation. The heave can occur in clay layers
overlying a permeable sand or gravel that has a sufficiently high pore water pressure
to force the clay up into the base of the excavation. A simple calculation comparing
the weight of the thin clay layer with the pore water pressure beneath it will
indicate the potential of failure in this manner. See Fig. 29.18.
Another form of base heave arises if the soil at the base is not strong enough to
support the overburden stress imposed by the soil adjacent to the excavation. In
this case the base of the excavation will fail and the soil will be forced up. See Fig.
29.19.
As this type of failure can occur during construction and before any base slab is
installed, analysis is usually performed using the undrained shear strength, cu.The
most commonly used method is the Bjerrum and Eide method. 35
Hydraulic instability in granular soils
Where groundwater exists above the base of the excavation, and where the toe of
the wall does not penetrate into an impermeable layer, flow will occur under the
Water pressure
Clay
Sand
Fig. 29.18 Base heave resulting from excessive water pressures
Fig. 29.19 Base heave due to weight of adjacent soil
Clay
Sheet piles 895