UFC 3-220-01N
15 AUGUST 2005
12-1.6.2
Liquefaction of Sands. The shearing strength of saturated cohesionless
soils depends upon the effective stress acting between particles. When external forces
cause the pore volume of a cohesionless soil to reduce the amount V, pore water
pressures are increased during the time required to drain a volume V of water from the
soil element. Consequently, pore pressure increases depend upon the time rate of
change in pore volume and the drainage conditions (permeability and available
drainage paths). When conditions permit the pore pressure, u, to build up to a value
equal to the total stress, on, on the failure plane, the shear strength is reduced to near
zero and the mixture of soil grains and water behaves as a liquid. This condition is true
liquefaction, in which the soil has little or no shearing strength and will flow as a liquid.
Liquefaction or flow failure of sands involves a substantial loss of shearing strength for a
sufficient length of time that large deformations of soil masses occur by flow as a heavy
liquid.
12-1.6.3
Liquefaction Due to Seismic Activity. Soil deposits that have a history
of serious liquefaction problems during earthquakes include alluvial sand, Aeolian sands
and silts, beach sands, reclaimed land, and hydraulic fills. During initial field
investigations, observations that suggest possible liquefaction problems in seismic
areas include low penetration resistance; artesian heads or excess pore pressures;
persistent inability to retain granular soils in sampling tubes; and any clean, fine, uniform
sand below the groundwater table. The liquefaction potential of such soils for structures
in seismic areas should be addressed unless they meet one of the criteria in Table 12-6.
In the event that none of the criteria is met and a more favorable site cannot be located,
the material in question should be removed, remedial treatment applied, or a detailed
study and analysis should be conducted to determine if liquefaction will occur.
12-23
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