Experimental Study On The Undrained Shear Behavior Of Natural Sedimentary Structual Clays

With the development of sampling technique and further researches on natural sedimentary clays, geotechnical engineers have realized the distinct difference between natural and reconstituted clays, and these differences are attributed to the soil structure. Most natural clays are influenced by soil structure more or less. In practice, overlooking of soil structure causes unexpected failure to clays and the enlargement and complexity of engineering constructions makes the understanding of the mechanical properties of clays become an urgent demand, especially the accurate simulations of strength and the predictions of settlements. This paper investigate the effects of soil structure on the mechanical behavior of clays through contrast tests between natural and reconstituted samples collected in Mountain Gaogai and Mountain Luohan, Fuzhou. The main contents and conclusions are as follows:(1)A number of oedometer compression tests are performed on natural, reconstituted and preconsolidation reconstituted Fuzhou clays to investigate the difference in compression characteristics among them.The e-lg p compression curve of the reconstituted clay show an inflection point,similar to natural and preconsolidation reconstituted clays.The compression curves of both natural and preconsolidation reconstituted clays in the post-yield regime can be normalised well with the void index,consistent with the intrinsic compression line ICL for reconstituted clays.(2)A series of triaxial undrained shear tests are performed on natural clays and reconstituted clays with different intitial water content to evaluate their strength behavior.The stress-strain curves,pore-water pressure behavior,Mohr failure envelope and stress path of a clay depend on three parameters:the soil structure,the liquid limit and the initial water content.A new concept on the destructuration of soil structure is proposed based on the total stress strength envelope and the normalized stress-strain curves,pore-water pressure behavior in the post-yield regime of natural clays.The effects of soil structure do not change when the applied confining stress is lower than the triaxial consolidation yield stress, but disappear completely in the post-yield regime.(3)The strength envelope of the natural samples is a broken line,when the applied confining stress is larger than the triaxial consolidation yield stress,the strength envelopes of both the natural and the reconstituted Fuzhou clays are straight lines through the origin.When the data obtained from triaxial consolidation undrained shear tests of both the natural and the reconstituted Fuzhou clays are plotted in the plot of undrainded shear strength versus water content,it is found that the undrained shear strength decreases uniquely with the increase in water content.The liquid limit is a powerful parameter for normalizing undrained shear strength for natural clays and reconstituted clays with different intitial water content.(4)The factors of the intrinsic strength line ISUL is investigated.the natural clays in the post-yield regime characterised by the same change law in strength property as reconstituted clays with different intitial water content.The undrained shear strength has a good relationship with void index for a given value of undrained shear strength ratio,the plot of void index against undrained shear strength with a higher value of undrained shear strength ratio lies above that with a lower undrained shear strength ratio.When undrained shear strength ratio is equal to 0.33,the relationship between void index and undrained shear strength is identical to the the intrinsic strength line ISUL.