Cyclic Compression of Compacted Clayey Sand at Small Cyclic Strains

Ten drained cyclic strain-controlled direct simple shear tests were conducted on compacted low-plasticity clayey sand to measure its cyclic compression properties. The soil had 37 % fines, liquid limit of 28% and plasticity index 14. The relative compaction of specimens prior to consolidation and cyclic shearing was between 80 and 90 %. Cyclic compression is expressed as the accumulation of vertical strain with the number of cycles, N. Vertical strain recorded at the end of every cycle, ενc, increased with the cyclic shear strain amplitude, γc, and N. Such behavior is typical and has been obtained by others on other types of soils. Amplitude γc was relatively small, ranging between 0.008% and 0.24%. Such small cyclic strains are common in moderate and large earthquakes. The effects of the dry unit weight, γd, and corresponding void ratio, e, vertical consolidation stress, σνc, and certain aspects of the degree of saturation, S, on ενc are evaluated. The test results revealed that for the applied conditions ενc increases with σνc and e (decreases with γd) and is smaller if S is increased above approximately 90%. For this soil the cyclic threshold shear strain of about 0.02% was obtained. Simple mechanisms that most likely govern the cyclic compression of compacted soils are discussed

Damping at small strains in cyclic simple shear test

Cyclic tests were conducted to study damping properties of two reconstituted sands and three laboratory-made clays at small cyclic shear strain amplitudes of gamma(c) approximate to 0.001-0.04%, employing a recently developed constant-volume equivalent-undrained direct simple shear device for small-strain testing. The tests were strain-controlled with an approximately sinusoidal shape of cyclic straining. The effects of cyclic strain amplitude (gamma(c)), frequency of cyclic loading (f), plasticity index (PI), silt content, vertical effective consolidation stress (sigma(vc)') and overconsolidation ratio (OCR) on the equivalent viscous damping ratio, lambda, were investigated. The results show that, for a given gamma(c), lambda decreases with sigma(vc)' and OCR, but both of these effects become smaller if PI increases. The effect of f on lambda was not observed for f approximate to 0.01-0.1 Hz. The results also show that below gamma(c) approximate to 0.005%, lambda for clays is larger than lambda for sands, which is exactly the opposite of the trend above gamma(c) approximate to 0.005% established previously. Such a reversal of the trend of lambda with respect to the type of soil is explained by the relative contributions of soil nonlinearity and soil viscosity to the area of the hysteretic cyclic stress-strain loop at small versus large cyclic strains