LABORATORY TESTING RELATED TO UNLOADING MODULUS OF SOFT CLAY

Abstract: The unloading modulus, together with the permeability, are responsible for the swelling process of a soil that is subjected to unloading. Studies to examine the unloading modulus of clay has previously been performed and several relationships describing this parameter has hence been developed. The methods to study the unloading modulus has previously been both laboratory studies and field measurements. The laboratory studies has generally focused on using incremental loading (IL) oedometer tests. This master thesis has three aims: • Examine if the storage time of the samples will affect the obtained unloading modulus for soft clay. • To investigate if modified CRS-tests can be used in order to determine the unloading modulus of soft clay in the same extent as IL-tests. • Investigate if one set of model parameters, obtained from PLAXIS 2D and restricted to the Soft soil creep model, can describe the behaviour of the soil when exposed to both compression and unloading. Piston sampler of the type STI was used in order to retrieve undisturbed samples of clay to performed laboratory tests on. Samples from 3 levels; 17 m, 20 m, and 25 m were obtained from one location in central Gothenburg, Sweden. Standard CRS-tests were initially perform in order to develop an understanding of the stress history of the clay and serve as the basis for determining the unloading sequences of the samples from the borehole. Two rounds of unloading tests, using modified CRS-tests, were performed on samples with 8 weeks of storage time in between. There could not be found any tendency that suggested that the unloading modulus of soft clay is affected by storage time. The clay in these specific sample tubes however had a tendency to swell extensively which might have affected the initial tests. The result from the performed tests showed that modified CRS-test appears to work satisfactory in order to capture the unloading behaviour of soft clay in the same extent as IL-tests. The data showed that the unloading modulus, obtained with modified CRS-test, was larger for unloading sequences above the in-situ pre-consolidation pressure than below. If the set of parameters, that was obtained for compression of the clay in the software PLAXIS 2D and the material model Soft soil creep, was used to also describe the behaviour of the soil during unloading, the magnitude of the expansion of the soil was underestimated.