Experimental Study On Effect Of Variation Of Principal Stress Orientation On Undisturbed Soft Clay

In order to suit the rapid economic development and meet the increasing demand for transportation and ocean resource development,it could be seen the construction of traffic and ocean engineering on an unprecedented scale in the south-east coastal areas of China in recent years. Meanwhile the soft clay of complex engineering characteristics is widespread in this region.So the research on the effect of vehicular or wave load on soft ground has important significance on rational design. Princial stress rotation is the typical stress path when vehicular or wave load is applied on the soil,which has quite different influence on soil from the fixed principal stress orientation shear. But the study in this aspect is less, and. need enrich eagerly. So based on the former researchers' achievement, the dissertation was focused on the experimental study on principal stress orientation variation on Hangzhou typical intact soft clay and the main aspects were studied as follows:1. The author participated in the research and development of Zhejiang University 5Hz hollow cylinder apparatus(ZJU—HCA)and the sampling & installation apparatus for intact soft clay.2.Based on the limitation that cell pressure of hollow cylinder apparatus should be constant during the dynamic loading,a series of practical loading programs which could generate cyclic principal stress rotation stress path were put forward.3. Static undrained shear experiments considering variation of principal stress orientation had been done,and it was found that(1) considerable pore pressure generated during the pure principal stress rotation,and Henkel pore pressure equation in the form of principal stress and principal stress increment could be used to seperately analyze the pore pressure generation during the fixed principal stress orientation shear and principal stress rotation.(2)in the total stress system,the peak value of shear stress at the failure point was mainly influenced by the principal stress orientation and medium principal stress,but not by principal stress rotation. And in the effective stress system, Lade-Ducan correction failure criterion could be used to analyze the influence of principal stress rotation on soils' strength.(3)samples showed obvious anisotropic during the fixed principal stress orientation shear and principal stress rotation.(4)principal stress rotation could cause the directions of principal stress increment and strain increment non-coaxial,but did less influence on the failure strain.4. Undrained cyclic principal stress rotation experiments had been done, and it was found that(1) the shear stress amplitude, frequency, principal stress rotation amplitude and overconsolidation ratio had the obvious influence on soils. There was a maximum shear stress, also called limit safe shear stress during the cyclic principal rotation which wouldn't cause samples fail. When shear stress was higher than limit safe shear stress,higher shear stress caused higher failure strain and less failure rotation cycles; while higher frequency caused lower failure strain and more failure rotation cycles. With the increase of principal stress rotation amplitude,the development of samples' strain componets changed drastically from the same magnitude to the different. Overconsolidated samples had much higher stiffness and dynamic strength ,but whether the negative pore pressure could generate during the rotation was decided by the feed-water condition when samples were being overconsolidated.(2)samples' safe limit shear stress and failure rotation cycles at the same stress were much higher in the one-way cyclic principal stress rotation than in two-way rotation.(3)samples' failure shear stress and failure rotation cycles were also decided by the maximum shear tress level during the cyclic principal stress rotation.(4)long-term cyclic principal stress rotation under limit safe shear stress had little inluence on the shear strength,pore pressure,but could cause stress-strain relationship harden and obvious non-coaxial between the directions of principal stress increment and strain increment during the subsequent static shear.