Piecewise strength model for three types of ultra-soft fine-grained soils

Problems of ultra-soft soils, with the characteristics of high water content and very low strength, have been gradually distinguished from those of typical soft soils in recent years to address the new requirements of ocean geotechnical engineering, such as the improvement of dredged fills and hazard assessment of submarine landslides. Focusing on effects of high water content and different shear strain rates on the undrained shear strength of ultra-soft soils, a series of experimental investigations were conducted, using a combination test method, on three types of fine-grained soils with different grain size distributions: kaolin soil, Bohai Sea soil, and South China Sea soil. After preparing ultra-soft soil samples with high water content two to three times the liquid limit, the relationship between the strength and the water content of the soil was analyzed. Then, the changing trend of strength with shear strain rates in a broader range of 0.001-20 s(-1) was discussed. Based on the test results and theory of shear thinning behavior, a piecewise model for the ultra-soft soil strength was proposed. The piecewise strength model can adequately reflect the strength characteristics of ultra-soft soils at different shear strain rate ranges, and the parameters in the model can be measured, or simply estimated. (C) 2020 Production and hosting by Elsevier B.V. on behalf of The Japanese Geotechnical Society.