Laboratory study on undrained shear behaviour of overconsolidated sand-silt mixtures: effect of the fines content and stress state

The post-liquefaction stability analysis of silty sand soil deposits that are prone to liquefaction becomes a major challenge in Geotechnical Earthquake Engineering. The objective of this laboratory investigation is to study the combined effect of the confining pressure and the low plastic fines (I-p = 5%) on the liquefaction resistance (undrained shear strength) of medium dense (D-r = 52%) overconsolidated sand-silt mixtures under monotonic loading conditions. In this context, a series of undrained monotonic triaxial tests were carried out on reconstituted saturated silty sand samples with fines content ranging from 0 to 40%. The different samples were prepared with dry funnel pluviation technique and subjected to different overconsolidation ratios (OCR = 1, 2 and 4). The results of this laboratory investigation indicate that the influence of low plastic fines on the undrained shear strength (liquefaction resistance) is clearly observed for the lower overconsolidation ratios (OCR = 1 and 2). This effect becomes insignificant for higher overconsolidation ratio (OCR = 4) for both confining pressures under consideration. The obtained data confirm the existence of a linear relationship between the undrained peak shear strength of overconsolidated Chlef sand samples (q(peak_OC)) and undrained shear strength of normally consolidated Chlef sand (q(peak_NC)) for the two selected confining pressures (p'(c) = 100 and 200 kPa) and initial relative density (D-r = 52%) under consideration.