Laboratory Tests And Constitutive Modeling On The Mechanical Behavior Of Shanghai Clays

Due to the differences in geologic origin, chemical composition, and grain size distribution, the physical properties and mechanical behaviors of natural clays differs with regions. Shanghai clays, especially the 4th Shanghai soft clay, are well known for their particular engineering properties such as high natural water content and low strength. Their mechanical behaviors should be taken into consider in design and construction of underground structures, such as braced excarvation and tunnel. A careful research on the physical properties and mechanical behaviors of Shanghai clays is important both to numerical analysis and engineering practice. Laboratory tests on the general physical properties and mechanical behaviors of the 2nd~6th Shanghai clays are systematically studied in this research. Furthermore, the stress-strain relationships of untacted and remoulded Shanghai clays in normal and overconsolidated states are simulated by three different constitutive models. The conclusions can be obatained as follow:New apparatuses are fine enough to perform laboratory tests and resource saving. Using self-refromed testing apparatuses,(1) Existing laboratory testing apparatus is inconvenient to operate and the data is recorded manually. In order to solve these problems, triaxial and true triaxial testing apparatuses are reformed to be new ones with automated controlled and data acquisition. By these apparatuses, physical properties and mechanical behaviors of 2nd~6th Shanghai clays are studied systematiclly. A function relationship between the natural water content and the compression index of Shanghai clay is found for the first time. It will be usefull for the numerical simulation and routine design.(2) The influence of natural structure on compression index, shear strength, volume change and exceed pore water pressure of 4th Shanghai clay are studied by the laboratory tests. In oedometer tests, structured clay with different initial void ratio tends to reach the same void ratio under 1600 kPa vertical stress, but still not reaches the ICL of remoulded one. In drained triaxial tests, structuraled clay tends to produce more volumetric strain than the remoulded one. In undrained triaxial tests, structuraled clay shows the same pore water pressure index as remoulded one. But no obvious differences have been found in strength between structuraled and remoulded clay in both drained and undrained tests. In ture triaxial tests, strength envelopes of structuraled and remoulded clays are closed to the SMP failure criterion.(3) By applying test results to three constitutive models, it is found that modified Cam-Clay model (MCC) can simulate well the stress-strain relationships of the natural 2nd, 5th, and 6th Shanghai clays under normal consoliated state. By applying the subloading surface concept to the MCC model, the stress-strain relationship of natural 2nd, 5th, and 6th Shanghai clays under overconsoliated state can be simulate well. By applying the superloading surface concept to the MCC model , the stress-strain relationship of structured 3rd and 4th Shanghai clays can be simulated to a certain degree. And by adding SMP failure critiron in MCC model by TS method, this model perfrom well in simulating the mechanic behavior of Shanghai clay in 3D stress state.