A method for evaluating the unloading disturbance and sample quality of marine gas-bearing sediments based on shear wave velocity

Compared with the sampling on land, there are more disturbance factors on the sample in marine environment. Therefore, the quantitative evaluation of sample quality is of vital importance for marine geotechnical engineering. Based on the triaxial test of undrained unloading stress path and the bender element test, Hangzhou Bay marine fine sand was used to simulate the disturbance effect caused by stress release in the sampling process of marine water-soluble gas deposits. The change of shear wave velocity(V-s_lab/V-s_insitu) before and after unloading disturbance and its correlation with the rate of void ratio (Delta e/e(0)) were investigated, and the feasibility of using the shear wave velocity method to quantitatively evaluate the quality of marine sediment samples was also discussed. The results show that the shear wave velocity of sand decreased significantly due to the unloading disturbance; and the change in void ratio of sand Delta e presented a "S-shaped" curve with the increase of unloading amplitude. Moreover, the shear wave velocity ratio had a good relationship with the rate of void ratio before and after the unloading disturbance. The results of sample quality evaluation based on the shear wave velocity method are consistent with those from the method proposed by Lunne, which is based on the change rate of void ratio. This work provides a beneficial reference for the sample quality evaluation of sediments in the marine geotechnical engineering.