UFC 3-220-01N
15 AUGUST 2005
Table 12-5 Values of Constant K2 Used with Equation (12-24) 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 I.meters. Idriss, "Simplified Procedures
for Evaluating Liquefaction Potential" Journal Soil Mechanics and
Foundations Division, Vol 97, NoSM(, 1971, pp1249-1273. Reprinted
by permission of American Society of Civil Engineers, New York.)
12-1.5.6
Modulus and Damping at High Strain Levels. The effect of increasingly
higher strain levels is to reduce the modulus (Figure 12-10) and increase the damping
of the soil (Figure 12-11). Shear modulus and damping values at high strains are used
mainly in computer programs for analyzing the seismic response of soil under
earthquake loading conditions. The various empirical relations for modulus and
damping pertain to sands and soft, normally consolidated clays at low-to-medium
effective confining pressures, in the range of about 30 meters (100 feet) or overburden.
Stiff over-consolidated clays and all soils at high effective confining pressure exhibit
lower values of damping and higher values of modulus, especially at high strain levels.
As a maximum, the modulus and damping values for stiff or strong soils at very high
effective confining pressures correspond to values pertaining to crystalline or shale-type
rock.
12-1.6
Settlement and Liquefaction.
12-1.6.1
Settlement. Repeated shearing strains of cohesionless soils cause
particle rearrangements. When the particles move into a more compact position,
settlement occurs. The amount of settlement depends on the initial density of the soil,
the thickness of the stratum, and the intensity and number of repetitions of the shearing
strains. Generally, cohesionless soils with relative densities (Dr) greater than about 75
percent should not develop settlements. However, under 108 or 107 repetitions of
dynamic loading, even dense sands may develop settlements amounting to 1 to 2
percent of the layer thickness. To minimize settlements that might occur under
sustained dynamic loadings, the soil beneath and around the foundation may be pre-
compacted during the construction process by vibroflotation, multiple blasting, or
vibrating rollers acting at the surface. The idea is to subject the soil to a more severe
dynamic loading condition during construction than it will sustain throughout the design
operation.
12-22
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