The effects of sample disturbance on preconsolidation pressure for normally consolidated and overconsolidated clays

Abstract

Sample disturbance has always been a particularly challenging topic in Geotechnical Engineering exercise. The effect and importance of disturbance on stress-strain history and undrained shear strength of soft clays are well stated in the literature over the past decades. However, in practice, few qualitative methods are used even in the most complex engineering projects. Furthermore, there is not a widely accepted, quantitative method in order to assess sample disturbance and treat the lab engineering values to match the "in-situ" properties of the soil. This research consists of a systematic review of the methods available to estimate preconsolidation pressure and sample quality. It focuses on multiple disturbance-simulation triaxial tests on resedimented San Francisco Bay Mud (RSFBM) specimens. The effect of different ISA disturbance amplitudes (Baligh et al. 1987) on the compression curve and the preconsolidation pressure is quantified. After every disturbance simulation circle, the specimens are reconsolidated using SHANSEP technique, well into the Virgin Compression Line (VCL), to erase all the previous effects of disturbance. Then, every recompression curve is treated as an initial compression phase but, of known disturbance. A series of tests are performed in Normally Consolidated (NC) and Overconsolidated (OC) specimens over a wide range of stresses. The effect of disturbance on an element outside the centerline of the sampling tube is simulated. A new method to quantify disturbance based on a modified recompression and compression indices ratio is proposed. Results indicate that for medium sensitivity soils, disturbance does not have a significant effect on preconsolidation pressure. However, increasing disturbance decreases significantly the researcher's capability to define with accuracy the preconsolidation pressure value. The calculated value can be overestimated as well as underestimated. Santagata and Germaine's (1994) conceptual model is verified experimentally. The model characterizes the loss of effective stresses and the change of VCL slope as the two most important outputs of disturbance that play a key role in the value of preconsolidation pressure. Results also indicate that the effects of disturbance are much less apparent when the OCR increases and that accounting for swelling leads to an overestimation of the preconsolidation pressure.