Discussion of “Cyclic tests on high-quality undisturbed block samples of soft marine Norwegian clay”

Abstract

The article by Wichtmann et al. (2013) presents a comprehensive set of laboratory test data on the cyclic undrained shear strength of soft marine Norwegian Onsoy clay. The tests were performed on Specimen Quality Designation (SQD) B specimens (Andresen and Kolstad 1979; Terzaghi et al. 1996) cut from block samples taken by the Sherbrooke sampler (Lefebvre and Poulin 1979). This article is a valuable contribution because compared to data and interpretation for sands, undrained cyclic strength data for soft clays are rare (e.g., Andersen and Lauritzen 1988; Andersen et al. 1988). Furthermore, the previous Norwegian Geotechnical Institute (NGI) tests on soft marine Norwegian Drammen clay were performed on specimens consolidated to the compression range beyond the pre-consolidation pressure, p ′ , whereas the tests on Onsoy clay have been performed using high-quality specimens subjected to the in situ effective stress condition ( ho ′ , vo ′ ) before undrained shear. A meaningful impression of undrained shear strength behavior of soft clay deposits is obtained by normalizing undrained shear strength with respect to pre-consolidation pressure as both su and p ′ represent points on the yield envelope (Skempton 1948; Mesri 1975; Mesri and Huvaj 2007). The cyclic undrained shear strengths of sands have been commonly normalized with respect to effective overburden pressure, vo ′ , because it is not readily possible to determine pre-consolidation pressure of sand deposits. This discussion concerns cyclic triaxial shear data on specimens cut from samples taken in the depth range of 10.50 to 11.37 m, and consolidated before undrained shear to the in situ effective stress condition. The Onsoy clay samples had a pore-water salt content of 32.5 g/L, natural water content of 63% to 65%, liquid limit of 63%, and plasticity index of 33% to 34% (Lunne et al. 2006). At the depth range of the samples, the end-of-primary (EOP) pre-consolidation pressure, p ′ , corresponds to p ′ / vo ′ 1.4 (note that I reduced values of p ′ from the constant rate of strain (CRS) tests by a factor of 0.912 because the imposed strain rates, I, in the NGI tests were about 10 times the EOP strain rate, p (Mesri et al. 1994)). The in situ values of Ko = ho ′ / vo ′ can be determined according to the following empirical equations (Mesri and Hayat 1993),