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Experimental Study On The Properties Of Structural Soft Clay Under Cyclic Loading

Posted on:2008-02-05Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y P ChenFull Text:PDF
GTID:1102360212986314Subject:Geotechnical engineering
Abstract/Summary:PDF Full Text Request
Thick layers of structural soft clay are widely spread on the southeast China, which are constantly affected by traffic cyclic loading for more and more traffic construction had been established. To ensure the safety and stability of more and more large constructions, it is meaningful to investigate the undrained properties of soft clays under cyclic loading when their structure changes, and to pre-estimate these properties.On the base of summary of research on cyclic charateristics of soft clays and the shear wave velocity measurement with bender element, some subjects as follows were investigated through tests in this dissertation:1. The piezoceramic bender element system for shear wave velocity measuring was developed in consolidometer and the change of shear wave velocity of the structural soft clays during consolidation were measured by applying this improved apparatus. The tests indicate that the variation of shear wave velocity changed with stress level corresponds well with the deformation characteristics of natural soft clay, which could be divided into three stages. The preconsolidation pressure and structural yielding stress are the cut-off point of these three stages and they could be determined by shear wave velocity measurement easily.2. Undrained cyclic triaxial tests were performed on natural, remolded and cemented clays. Turning points are all obviously found on the strain curves with cycle number. When the strain is less than that at the turning point, it is at a small level. If not, the strain would increase very fastly. The strain at the turning points changed with different stress amplitudes, but they almost fall on a line in semi-log coordinate and could be fitted by a linear equation. The change of fitted parameters of natural soft clay could also be divided into three stages whichcorresponded well with consolidation tests. The fitted parameters of remolded soft clay almost did not change with stress level as the same as the consolidation tests, and those of cemented soft clay change linearly with mixing amount of cement.Soil's structure has a great effect on their cyclic characteristics. When the confining pressure exceeded the yielding stress, the characteristics of strain and cyclic strength of natural soft clay would be close to that of remolded soft clay.3. The analysis of the relationship between shear wave velocity V_s and cyclic stress ratio indicated that the V_s of natural soft clay varies with the coupling influence of soil's structure and void ratio e. As a whole, it increases with increasing consolidation pressure non-linearly. By normalizing the V_s of natural soft clay using F(e) = (2.973-e)~2/(1+e) suggested by Hardin, it could eliminate the influence of void ratio and the mormalized V_s presents a good linear relationship with dynamic strength.The shear wave velocity of remolded soft clay is independent of soil's structure and only infected by void ratio. So the shear wave velocity exhibits a good linear relationship with dynamic strength.If normalized by F(e)=(2.973-e)~2/(1+e), the shear wave velocity of cemented clay could also fall in well linear relationship with dynamic strength. This indicates that it is likely to get a linear relationship between normalized shear wave velocity and dynamic strength for the same soil. If only normalizing the shear wave velocity and dynamic strength of cemented clay using F(e)和 e separately, a well relationship between natural and cemented clays could be obtained.4. Based on the cyclic triaxial tests, a strain model was proposed to reflect the rule ofdeformation under deviatonic stress or not. It is shown that the fitted results agree well with those of tests. A traffic project of long-term settlements of subgrade soft soil ground was analysed by employing the suggested model and the 2.5 dimensional finite element method together with thin layer element method. The result indicates that the settlement will increase with time and the speed of train until to a stable state which would almost happen within a critical depth underground. Grouting method was proposed to reduce the settlement of soft clay ground.Finally, some conclusions were drawn and advices for further study were given.
Keywords/Search Tags:bender element, shear wave velocity, velocity measurement, undrained cyclic triaxial test, strain, pore water pressure, dynamic strength, constitutive model
PDF Full Text Request
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