The Liquefaction Susceptibility And Assessment Of Dynamical Resistance In High Clay Contant Sand

Earthquake liquefaction lead to huge property losses and casualties, and the evaluation in liquefied site has important scientific significance and engineering value. At present, the liquefaction study based on the clean sand and silt sand has been comparative comprehensive.However, the liquefaction behavior research in sand with high clay content is comparative deficient, and the internal mechanism in sand with high clay content is insufficient. In early years,people usually thought that earthquake liquefaction only happened in clean sand, while sand soil with some clay content and plasticity were not easy to liquefaction. But several major earthquakes,in where typically clayey silt and clayey sand liquefied sites come to light, occurred all over the world in recent years. The typical liquefied site in fine grained soil caused the attention of the related researcher to the problem of high clay content sand soil liquefaction. According to many liquefaction research of sand with fine content (FC) and some studies of sand with clay content(CC), the soil characteristics changes with the change of clay content. In low clay content, the dynamic characteristics of clayey sand is similar to the clean sand. With the increase of clay content, the clayey sand appear some clay properties. When the clay content near to the transform fine content (TFC), the clayey sand soil reflect sand properties and clay properties at the same time. Soil in this section also be defined as "transition zone". Based on the liquefaction study of sand with low clay content by research group, this paper focuses on the liquefaction behavior of sand with high clay content and carries out a series of laboratory tests to analyze the effect of clay on liquefaction behavior of clayey sand. Though a large number of lab dynamic triaxial tests and reasonable contrast analysis, the purposes of this paper are to understand the effects of clay content on the liquefaction characteristics of clayey sand and bring the liquefaction mechanism of clayey sand to light. Through the establishment of CRR-Vs1 model applied to clayey sand with different clay content, this paper prefer to put forward scientific and reasonable suggestions to the assessment of clayey sand soil liquefaction in practical engineering. In detail, this study includes topics as follows:1. To control the soil structural of sandy soil with low plastic, a test method of over consolidation ratio(OCR) is proposed: Liquefaction assessment by "point to point" method is a typical methods which reflect the characteristics of in-situ soil through lab experiment. And the key of this method is to design an effective test operation to make the soil structural level of lab soil sample comparable to the in-situ soil. Existing method which increase the soil structure of lab sample by pre-shake can effectively apply to clean sand lab sample but not suit for clayey sand with high clay content and some plasticity. This paper proposes a test operation of over consolidation ratio which can meet the requirements of the soil structure in clayey sand axial test.Based on the stress path of lab sample, stiffness theory by Hardin and the linear elastic theory, this paper illustrate the scientific rationality of restoring the soil structure by over consolidation ratio and give a suggested calculation to get the design over-consolidation ratio.2. The influence of the Clay content on soil stiffness characteristics of clayey sand: For specific clay content (CC30 and CC40), consolidation tests in different stress with shear wave velocity measured are carried out. Based on the analysis of the test results, the Hardin curve of clayey sand (CC30 and CC40) can be established. Making an integration analysis with existing Hardin curve results in clayey sand (CCO, CC10, CC15 and CC20) and get the fitting relation between clay content (CC) and the parameters (A and n) in Hardin formula. From the fitting analysis, a phenomenon is been found that the parameters (A and n) in Hardin formula in the lower and higher clay content show a different trend with clay content and the transform boundary of clay content is in the 15%?20%. Existed study has confirmed the transform fine content of clayey sand which mixed by Fujian sand and Xiaoshan clay is about 20%. This paper points that soil stiffness characteristic of clayey sand influenced by clay content show different trends in sand-like soil and clay-like soil. The equivalent skeleton void ratio (e*) which considers the contribution of sand particle and clay particle to the soil skeleton is selected as the standard for comparative analysis in this paper.3. The influence of the Clay content on soil dynamic strength of clayey sand: A series of dynamic axial liquefaction tests for specific clay content 30% soil and the specific equivalent skeleton void ratio clayey sand with low clay content (CC10 and CC20) are carried out. Making an integration analysis with existing results of dynamic axial liquefaction tests in clayey sand with low clay content (CCO, CC10 and CC20). Making a comparison on the dynamic strength curves of clayey sand with different clay content but same specific equivalent skeleton void ratio to summarize the influence of clay content on soil strength. According to the analysis, different trends of soil strength with clay content are found in different dense state of soil sample. Combined with the volume change rules of sand samples in different dense state, this paper summarize the trends of dynamic strength change with clay content in three different parts (dense, medium,loose).4. The CRR-Vs characterization model of clayey sand with considering the influence of clay content: Based on dynamic axial liquefaction test results of clayey sand (CC=30%) in different dense state, the CRR-Vs characterization model of clayey sand (CC=30%) is established.Combined with the analysis of the above results, this paper replace the parameters (kN,emin and n) in CRR-Vs model with clay content (CC) to propose the CRR-Vs characterization model applied to clayey sand with different clay content. The CRR-Vs characterization model, which can calculate the liquefaction strength of in-situ clayey sand soil though the basic information (CC, Vs and p) of soil, can guide the liquefaction evaluation in engineering practice.5. Case of liquefaction evaluation in engineering practice: Atypical "sand-clay-like" clayey silt which soil properties are in transform zone is found in an engineering project. Existing criterion method is not applicable to assessment liquefaction possibility of the clayey silt. This paper use three methods to evaluate the liquefaction possibility: 1) Carrying out lab triaxial test of "point to point" to ensure the liquefaction possibility of in-situ soil, 2) Based on the CRR-Vs characterization model and in-situ shear wave velocity values to ensure the liquefaction possibility of in-situ soil, 3) Based on the CRR-SPT boundary and in-situ SPT test to ensure the liquefaction possibility of in-situ soil. Comparing the evaluation results of different methods, the applicability of CRR-Vs characterization model in clayey silt are confirmed.