TM 5-818-1 / AFM 88-3, Chap. 7
Table 17-5. Values of Constant K2 Used with Equation (17-24) to Estimate Cyclic Shear Modulus at Low Strains for Sands
e
K2
Dr(%)
0.4
70
90
0.5
60
75
0.6
51
60
0.7
45
45
0.8
39
40
0.9
33
30
(Courtesy of H. B. Seed and L M. Idriss, "Simplified Procedures for
Evaluating Liquefaction Potential," Journal, Soil Mechanics and
Foundations Division Vol 97, No. SM9. 1971, pp 1249-1273. Reprinted by
permission of American Society of Civil Engineers, New York.)
f.
Modulus and damping at high strain levels.
construction than it will sustain throughout the design
operation.
The effect of increasingly higher strain levels is to reduce
b. Liquefaction of sands.
The shearing
the modulus (fig 17-10) and increase the damping of the
strength of saturated cohesionless soils depends upon
soil (fig 17-11). Shear modulus and damping values at
the effective stress acting between particles. When
high strains are used mainly in computer programs for
external forces cause the pore volume of a cohesionless
analyzing the seismic response of soil under earthquake
soil to reduce the amount V, pore water pressures are
loading conditions. The various empirical relations for
increased during the time required to drain a volume V of
modulus and damping pertain to sands and soft,
water from the soil element.
Consequently, pore
normally consolidated clays at low-to-medium effective
pressure increases depend upon the time rate of change
confining pressures, in the range of about 100 feet or
in pore volume and the drainage conditions (permeability
overburden. Stiff overconsolidated clays and all soils at
and available drainage paths). When conditions permit
high effective confining pressure exhibit lower values of
the pore pressure, u, to build up to a value equal to the
damping and higher values of modulus, especially at
total stress, σn, on the failure plane, the shear strength is
high strain levels. As a maximum, the modulus and
damping values for stiff or strong soils at very high
reduced to near zero and the mixture of soil grains and
effective confining pressures correspond to values
water behaves as a liquid. This condition is true
pertaining to crystalline or shale-type rock.
liquefaction, in which the soil has little or no shearing
strength and will flow as a liquid. Liquefaction or flow
17-6.
Settlement and liquefaction.
failure of sands involves a substantial loss of shearing
a. Settlement. Repeated shearing strains of
strength for a sufficient length of time that large de-
cohesionless soils cause particle rearrangements.
formations of soil masses occur by flow as a heavy
When the particles move into a more compact position,
liquid.
settlement occurs. The amount of settlement depends
c. Liquefaction due to seismic activity. Soil
on the initial density of the soil, the thickness of the
deposits that have a history of serious liquefaction
stratum, and the intensity and number of repetitions of
problems during earthquakes include alluvial sand,
the shearing strains. Generally, cohesionless soils with
aeolian sands and silts, beach sands, reclaimed land,
relative densities (Dr) greater than about 75 percent
and hydraulic fills. During initial field investigations,
6
should not develop settlements. However, under 10 or
observations that suggest possible liquefaction problems
7
10 repetitions of dynamic loading, even dense sands
in seismic areas include low penetration resistance;
may develop settlements amounting to 1 to 2 percent of
artesian heads or excess pore pressures; persistent
the layer thickness. To minimize settlements that might
inability to retain granular soils in sampling tubes; and
occur under sustained dynamic loadings, the soil
any clean, fine, uniform sand below the groundwater
beneath and around the foundation may be
table.
The liquefaction potential of such soils for
precompacted during the construction process by
structures in seismic areas should be addressed unless
vibroflotation, multiple blasting, pile driving, or vibrating
they meet one of the criteria in table 17-6. In the event
rollers acting at the surface. The idea is to subject the
that
soil to a more severe dynamic loading condition during
17-14
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