Experimental Study On Liquefaction Mechanism Of Saturated Sands Under Confining Condition

In recent years, earthquakes frequently occurred in the world. As a main form ofearthquake disasters sand liquefaction often caused uneven settlement of foundationand the damage of structure, which caused serious casualties and brought greatdisaster to humans. Especially, New Zealand’s6.3magnitude earthquake in February2011caused a rare liquefaction on a larger scale, and many new liquefaction problemsappeared in Japan3.11earthquake and China Wenchuan earthquake. These problemsmake people to realize the complexity of soil liquefaction and the necessity of furtherresearch. Conventional dynamic triaxial and dynamic torsional shear experiments aremain experimental methods of soil liquefaction research. But the conventionalexperiment results cannot explain actual liquefaction phenomenon. The main reason isthat the stress state of soil unit in the indoor experiment and the actual stress state ofsoil is not consistent.To simulate the characteristics of stress and deformation of soil in practical fieldreasonably, a confining test method is designed and developed by using geotechnicaldynamic true triacxial apparatus. Further, the mechanism of saturated sandliquefaction and the effects of consolidation conditions and soil properties areexamined in detail.Main research work and innovation of this paper is stated as follows:(1) The confining test method is designed and developed by using true triacxialapparatus. The actual soil confining constraint conditions is simulated successfully.The stresses change caused by stresses redistribution in the process of soilliquefaction are measured. That provides the reference for the reasonable descriptionof liquefaction mechanism.(2) Compared with conventional test, the liquefaction mechanism of saturatedsandy soil is analyzed under confining condition in this paper. The lateral deformationcaused by softening of soil under confining condition is restrained in the process ofdynamic loading which caused the redistribution in the stress of the surrounding soil.The pore water pressure and lateral pressure can increasing continuously, to continueto increase, until the lateral pressure increases to be equal to the axial stress, and porewater pressure increases to be equal to the lateral pressure. Finally, soil liquefactionoccurs under the condition that the axial pressure, side pressure and pore water pressure are equal.(3) Based on the above phenomenon, the concept of “continued liquefaction” isput forward in this paper.. Based on the analysis of the stress time history curves andeffective stress path,it can be found that in cyclic loading test pore water pressure andconfining pressure change with axial dynamic load synchronously and equivalentlyafter liquefaction under confined condition. Generalized shear stress and the effectivemean principal stress gradually decreases with increasing of cyclic number.Eventually, there is hardly any shear strength in soil, and always stays the liquefactionstress state.(4) To dense and medium dense sand liquefaction experiment, it can be foundthat "continued liquefaction" phenomenon still occurred in dense sand and coarsesand in confining condition. Further the liquefaction mechanism of dense and coarsesand is revealed.