Experimental Study On The Non-coaxiality On The Soft Clay

The traditional plasticity theory assumes that the principal axes of stress and plastic strain increment are coaxial, but lots of researchers found the non-coaxiality behavior or non-coincidence of the principal stress and principal plastic strain increment directions of soil by experimental tests and numerical simulations, and the non-coaxiality behavior is quite remarkable under the complex stress path as principal stress rotation. The importance of non-coaxiality has been widely recognized in the geomechnical engineering, without considering the non-coaxiality may lead to an unsafe soil structure. However, most researchers carried out their research using granular materials, few non-coaxiality studies were made for soft clay. Giving this, a series of tests have been carried out on soft clay to study the stress-strain relationship, the non-coaxiality behavior and the influence factors using ZJU-HCA. The major tasks and research results are as follows:(1) In the previous work of non-coaxiality, lots of researchers paid their attention to the deviation of principal plastic strain increment direction to the principal stress direction. However, the non-coaxiality refers to the non-coincidence of principal plastic strain increment and the principal stress directions of soil. So, this paper further defines the non-coaxiality on the basics of the past researches.(2) To explore the principal plastic strain increment direction, the plastic strain should be separated from the total strain. And the elastic modulus must be given to figure out the elastic strain. As a result, a series of tests with monotonic loading along fixed principal stress directions and cyclic rotation of principal stress are carried out. It shows that the results of elastic modulus and shear stiffness are almost the same even they are got from different types of stress passes.(3) The angle of non-coaxiality is relatively small with monotonic loading along fixed principal stress directions. When a=0°and90°, the angle of non-coaxiality stays around0°, while a=30°,45°and60°, the angle of non-coaxiality is relative large at first; then it decreases toward0°with the increase of a; at last, there is a sudden increase, and the angle of non-coaxiality is not equal to0°when the specimen failed. The principal plastic strain increment axes deviate slightly from current principal stress axes to the direction of α=45°. It will underestimate the non-coaxiality of soft clay without considering the elastic strains, and angle of non-coaxiality of reconstituted soft clay is smaller than that of intact clay under monotonic loading along fixed principal stress directions.(4) There exist remarkable stress-strain hysteretic behavior in the intact soft clay, and the angle of non-coaxiality is quite large with the rotation of principal stress. The angle of non-coaxiality cyclically fluctuates with the increase of a, of which the cycle period is90°. It will overestimate the non-coaxiality of soft clay if not considering the elastic strains, and the elastic strains have a significant effect on the angle of non-coaxiality at the beginning of loading. The larger of the intermediate principal stress parameter b is, the larger the angle of non-coaxiality is. Though the deviator stress doesn’t affect the wave characteristic, it does affect the development tendency of non-coaxiality.(5) The non-coaxiality of reconstituted soft clay is also quite pronounced, even though not so remarkable, and the angle of non-coaxiality fluctuates just as that of intact soft clay. The wave characteristic of non-coaxiality has a close relationship with the induced anisotropy, while the inherent anisotropy can lead to greater non-coaxiality. It shows that the non-coaxiality is not only entirely determined by the inherent anisotropy, the induced anisotropy also has a great impact. By establishing the relationship between the plastic strain increment direction and stress increment direction and stress direction, a new flow theory of plasticity capable of representing the plastic strain increment direction under principal stress rotation is given based on the present studies. It has proved that using this theory can give a good forecast of the angle of non-coaxiality.