Abstract: Two boreholes of G2 and G3 were drilled at Tanggu and Dagang in Tianjin, and undisturbed samples were extracted at various depths. Through laboratory test of all soil samples, this study measured the physical and mechanical properties and conducted consolidation test and repeated loading and unloading tests for clayey soils. By analyzing the test results, this study delineated the characteristics and influence factors of elastic-plastic deformation. Study results indicate that clayey soils in depth less than 100 m are under-consolidated soft soil. Clayey soils in depth from 100 m to 400 m are in the states of over-consolidated or slightly over-consolidated, which causes by long-term over-exploitation of groundwater in the past. Deep clayey soils in depth more than 400 m show the state of normal consolidated. Based on the repeated loading and unloading tests of clayey soils in boreholes G2 and G3, it is found that plastic deformation decreases gradually and elastic deformation remains unchanged, independent on repeating times. The test results also suggest that clayey soils will gradually become an elastic body in the process of groundwater fluctuation, and deformation will rebound when groundwater recovers. By analyzing correlation between elastic-plastic deformation and depth, natural moisture content and clay content of clayey soil, it is found that elastic deformation shows positive correlation with depth, natural moisture content and clay content, and plastic deformation shows negative correlation with natural moisture content and clay content and no obvious correlation with depth.

Key words: Tianjin coastal area, clayey soil, elastic-plastic deformation, land subsidence