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Soil Dynamics

Strength of Soil Under Dynamic Loading

Under the static loading conditions stress-strain curve that defines the modulus of elasticity assumes the monotonic loading that increases gradually allowing sufficient time to failure. This may simulate the real ground conditions like construction of buildings in which loading on soil build up gradually. However it should be noted that stress-strain relation is a function of the rate of loading. During dynamic loading conditions as in earthquake full value of load may apply within short span of time as in contrast to gradually increasing monotonic loading and stress-strain curve of soil in such case usually differs that from its corresponding static stress-strain curve.

Dynamic strength of soil during an earthquake generally differs from its static strength since the effect of “strain rate” and “cyclic loading” gets added.

During cyclic loading imposed by ground excitation, stress-strain relation is a function of the rate of loading. In most clays this effects are more pronounced while in sands there is hardly a noticeable influence of the loading rate in tests lasting between tenths of seconds and several minutes. In all cohesive soils strength increase markedly with strain rate. An increase of the order of forty percent is common for the usual strain rates of earthquakes above the strength of static loads. Also, cyclic loading decreases the strength of all cohesive soils. Whether the increase due to the strain rate exceeds the decrease due to the load repetition depends essentially on the number of repetitions, on the relative value of sustained and cyclic stresses and on the sensitivity of the soil. In very sensitive soils, sensitivity above 8, there is net decrease under a moderate number of cycles, whereas in relatively insensitive materials, the increase prevails even after sufficient numbers of cycles (Seed 1960).

Effects of the speed of loading are insignificant on the stress – strain relations of dry cohesion less soils within the range of speeds that is of interest in seismic problems, provided that conditions are such that the air in voids of the soil does not develop an appreciable change in pressure (Whitman and Healy, 1962).

In connection with saturated materials, repeated alternation of loading eventually causes volume decrease in all cohesion less soils. The matter is not particularly significant for dry or partially saturated materials.