Behavior Of Structured Clay And Its Application

Soft clays are widely distributed in the coastal area of the southeast China. These natural clays usually have high water content,extensive compressibility and low strength just as other soft clays all around the world.The most important is that the properties of natural clay differ from its reconstituted properties in a number of important ways due to the soil structure.Geotechnical designers commonly use test data of disturbed clays in their calculation and designing.Therefore,investigation of the effect of structure on properties of natural clays becomes an important issue for understanding the clay behavior and reliable design in practice.The compressibility and K₀ behaviour of natural structured clay is studied firstly in detail.Based on these basic charateristics,a non-linear elastic constitutive model considering the soil structure is developed in this paper.Finally the model is introduced to the finite element program to solve practical problems.In general, some developments for soft clays are involved in this paper as below.(1)The new predicting method of field compression curve is proposed.It is important to describe the one-dimensional compression of soils,one of the most popular problems in geotechnical engineering.Based on the multi-linear relationship of the void ratio and the consolidation pressure in semi-log scale,the FCC of natural clays is determined by the exact location of the two key stresses(yield stress and transition stress).For a case history,comparisons between the predictions and the observed results in situ verify the validity and applicability of this new procedure.(2)The mechanism of field disturbance and a new method for modifying the compression curve of disturbed soil are presented.Natural soil is prone to be disturbed.The field disturbance,distinct from the sample disturbance,usually results in the degradtion of bonding,reduction of structural strength and increase of compressibility.Then the evaluation of the field disturbance degree is discussed associated with the results of the vane test in situ.Furthermore,the additional settlements due to the field disturbance are analyzed.(3)The computation pattern to determine the value of tangent Poisson ratio is suggested.The faster formation of bonding inhibits the development of secondary compression in the process of soil deposition.K₀ value in situ,therefore,is still equal to that of the normal consolidated soil,i.e.the K_0NC.Besides,according to the laboratory test of natural clay in many regions,the behavior of tangent Poisson ratio is three-phase pattern with the turning point of the yield stress and transition stress. Then,the concept of phase transition curves is introduced and the value of tangent Poisson ratio is computed,which is applied to non-linear elastic model.(4)A new non-linear elastic constitutive model considering soil structure,NLS model,is developed.According to the theory of Duncan-chang model and basic elastoplastic theory,this model is developed based on the deformation mechanism for typical natural clays and.The seven parameters of the NLS model,including the yield stress ratio YSR,compression index after yielding C_c2,swelling index C_s,initial void ratio e₀,Poisson ratio v and strain at peakε_1p,are easier to obtained from the common labotary tests.Finally,this model is introduced into the finite element program.The computed results for a case history indicate that the NLS model is available in practice and the behaviour of structured clay can be simulated well.(5)Reseach in this paper can be used to the deformation and stability analysis of building and construction,such as the embankment or the dam on soft ground.