Study On The Shear Wave Velocity Measurement From Bender Elements And Dynamic Properties Of Saturated Soft Marine Clay

The property of soft marine clay subjected to undrained cyclic loading such as earthquake and storm waves is an important consideration in the design of offshore installations and analysis of the stability of the seafloor. To ensure the safety of offshore facilities and stability of seafloor, it is meaningful to investigate the small-strain shear modulus and the undrained properties of soft marine clay under cyclic loading. Piezoceramic bender elements are versatile transducer and are widely installed in soil instruments to measure shear velocity and the small-strain shear modulus of soil. But reasonable installation and the method for precisely determining shear wave velocity of soil have not been completely solved yet and have to be further investigated.On the base of summarizing the important advances in the field of the methods of shear wave velocity measurement from bender elements and the properties of soft marine clay subjected to undrained cyclic loading, some subjects as follow were investigated through tests in this dissertation:(1) The piezoceramic bender element system for shear wave velocity was developed in the HX-100 static/dynamic triaxial apparatus. The method for precisely determining shear wave velocity of soil from bender element tests was studied, and the dispersion of shear wave velocity was investigated, and the value of shear wave velocity from bender element tests was compared with that of resonant column tests in Drnevich Long-Tor apparatus. It is concluded that the near-field effects and overshooting during the bender element tests can be eliminated by using suitable waveform and rational frequency of input signals, and the value of shear wave velocity and the very small-strain shear modulus can be precisely determined. It is also concluded that the value of shear wave velocity from the bender element tests does not vary with frequencies of input signals.(2) The small-strain shear moduli of soft marine clay were investigated by bender element tests and resonant column tests. The test results show that the mean effective stress ct0 , initial (or natural) void ratio e, and plasticity index IP of marine soil are main factors affecting G max of marine soils. For the marine clay in low plasticity index of this study, its Gmax can be estimated through the formula Gmax =(\437-234e-26Ip)cr0 from this study by initial void ratio e, plasticity index IP, mass density p and depth d of marine soil.(3) The dynamic modulus and the damping ratio for the soft marine clay were studied by using bender element and cyclic triaxial devices. The test results demonstrate that the modulus reduction curves versus cyclic shear strain of marine clay at Hangzhou Bay is below that of other marine clays, and Hardin-Drnevich model and Ramberg-Osgood model can fit them well, and its G/Gmax increases with the increase of plasticity index IP for the given shear strain. The test results also show that the damping curves versus cyclic shear strain of marine clay at Hangzhou Bay have a broad distribution, and Hardin-Drnevich model can fit them.(4) The small-strain shear moduli Gmax of saturated soils during undrained cyclic loading were investigated through cyclic triaxial tests and bender element tests. It is concluded that Gmax of saturated sand, and undisturbed silt and soft marine clayduring undrained cyclic loading have different properties. The Gmax of saturated sand during undrained cyclic loading is approximately equal to that of sand confined by same effective stress at static state. For the saturated silt and marine clay, the Gmax during undrained cyclic loading is similar to that of sand when its strain is less than its threshold strain, but the Gmax is less than that of saturated silt and marine clayconfined by same effective stress at static state when its strain is bigger than its threshold strain. So it is valid in dynamic effective-stress analysis of soil structure and construction site to use the Gmax of saturated sand determined by Hardin Equation during undrained cyclic loading; but it is unsafe to use the Gmax of saturated silt andmarine clay determined by Hardin Equation during undrained cyclic loading.(5) Based on a series of cyclic triaxial tests, the effect of cyclic frequency such as earthquake and ocean wave on the undrained behaviors of undisturbed marine clay was investigated. The test results indicate that, for a given dynamic stress ratio, theaccumulated pore water pressure and dynamic strain increase with the number of cycles. There exists a threshold value for both the accumulated pore water pressure and dynamic strain, below which the effect of cyclic frequency is very small, but above which the accumulated pore water pressure and dynamic strain increase intensely with the decrease of cyclic frequency for a given number of cycles. The dynamic strength increases with the increase of cyclic frequency, whereas the effect of cyclic frequency on it gradually diminishes to zero when the number of cycles is large enough, and the dynamic strengths at different frequencies tend to the same limiting minimum dynamic strength. The test results also manifest that the reasons for the frequency effect on the undrained soil behaviors are both the creep effect induced by loading rates and the decrease of sample effective confining pressure caused by the accumulated pore water pressure. Analysis of an ocean project indicates that wave frequency has effect not only on the magnitude of dynamic strength of seabed soil but on that of shear stress induced by wave also, and has significant influence on the stability and the liquefaction depth of seabed soils.Finally, some conclusions were drawn and advices for further study were given.