Evaluation of dynamic behavior of varved clays from the Warsaw ice-dammed lake, Poland

Soil stiffness degradation due to dynamic loading may result in damage to engineering structures. Therefore, dynamic soil properties gained importance in recent years in the areas where vibrations of different origins are transmitted to the ground. The aim of the study was to determine the stiffness properties of the Warsaw varved clay (central Poland), as a measure of its response to dynamic loading. A set of triaxial tests were conducted. Undisturbed samples were subjected to initial deviator stress of 100 kPa, reflecting vertical stress due to construction load, and then to three dynamic stages under axial displacement control conditions. Secant shear modulus G was adopted as a parameter of soil stiffness during dynamic loading. Main methodological result of the study is application of coefficient α (gradient of logarithmic change in stiffness with time) indicating type of soil behavior under dynamic loading. Negative α values (ranging from −0.48 to −0.91) indicated a weakening effect of the applied vibrations (decrease in stiffness), while positive α values (ranging from 0.11 to 0.21) indicated soil strengthening (increase in stiffness). Results of the study may be applied to compare an impact of several factors influencing soil stiffness during dynamic loading