A Soil Structure Index to Predict Rate Dependence of Stress-Strain Behavior

Abstract

Soil structure depends on a number of influences, including geomorphology, groundwater fluctuations and geochemistry. In the case of soft clays, this structure is believed to play a significant role in the degree of rate dependence of the clay's stress-strain behavior. However, there has been no systematic method developed to predict what degree of structure will produce a particular degree of stress-strain rate dependence. This paper presents a methodology based on Burland's void index (I v ) framework to predict the type of rate dependence that could be expected for a soil's stress-strain behavior in one-dimensional (1-D) compression. Three categories of rate dependence in 1-D compression have been reported in the literature, characterized by the following: 1) rate dependence in both tbe preconsolidation stress (σ' p ) and virgin compression; 2) σ' p is not rate dependent, but the slope of the virgin compression line varies with strain rate; and 3) little or no rate dependence in any portion of the compression curve. Based on a series of 1-D tests on soils from Canada and the U.S., rate-dependent behavior types were compared to each soil's natural state in Burland's I v versus vertical effective stress space. A so-called structure number (SN) is introduced, indicating how far the soil's natural state is from its intrinsic or remolded compression curve (ICL; SN = 0) and its original sedimentation compression curve (SCL; SN = 1). The contour with SN=2 is parallel to these two, approximately parallel lines, but above the SCL at a distance equal to the SCL-ICL gap. Higher SN value contours are similarly obtained. The results indicate that SN correlates well to the type of rate dependence observed in the 1-D compression tests. These initial results appear to provide a rational link between soil structure and strain rate dependence.