Strength and deformation properties of Norwegian clays from laboratory tests on high-quality block samples

Abstract

It has for a long time been a challenge for the geotechnical profession to determine the true behaviour of clays based on soil sampling and laboratory testing. The main difficulty has been to retrieve sufficiently undisturbed samples that maintain the clay structure and correctly depict the in situ stress–strain and strength behaviour of the clay under any kind of imposed drained or undrained loading conditions. The extent and impact of the disturbance caused by sampling depends on a number of factors, such as the specific sampling equipment and sampling procedures that are used, index properties of the clay (sensitivity and plasticity index in particular), and sampling depth. Experience with laboratory tests on conventional piston samples shows that sample disturbance commonly affects the test results. In the mid 1970s, the University of Sherbrooke developed a special block sampler that was shown to give samples of very high quality. Between 1982 and 2010, the Norwegian Geotechnical Institute (NGI) has used the Sherbrooke block sampler at 22 different sites in Norway and one site in the UK. Two to five block samples were collected at each site, on which oedometer, undrained triaxial, and direct simple shear (DSS) tests were carried out in the laboratory. Essentially all samples tested showed far superior quality as compared to conventional piston samples. It was therefore considered to be valuable to summarize test results obtained on such high-quality samples, as presented herein. Deformation and strength parameters from individual tests are summarized and compared against index data for the different clays tested. This has resulted in a series of correlation diagrams. The index parameters found of most relevance to use as correlation parameters are the natural water content and clay sensitivity. The latter is expressed in two categories: medium to low sensitivity (St< 15) and high sensitivity (St> 15). The overconsolidation ratio is another key parameter. The correlation diagrams presented include the following: (i) pre-consolidation pressure, compressibility, permeability, and permeability change index as derived from oedometer tests; (ii) normalized peak and post-peak shear strength, peak effective friction angle, stiffness, and strain at failure from undrained triaxial and DSS tests. When design analyses are to be based on the results of tests on high-quality block samples, as compared to conventional piston samples of poorer quality, it is important to ensure that the end result complies with past semi-empirical experiences.