Triaxial Text And Shear Strength Prediction Of Deep-sea Energy Soils Considering The Influence Of Multiple Factors

Natural gas hydrates have attracted worldwide attention due to their huge resource reserves,wide distribution range,high energy density and clean and non-polluting advantages.It is considered to be one of the most promising alternative energy sources for the 21st century.However,a number of engineering practices have shown that the extraction of gas hydrates still faces many challenges at this stage.This paper closely focuses on the specific engineering practice of hydrate test mining in the South China Sea in China.In response to the problems of single research factors and lack of comparability and reference in the current stage of energy soil research,a series of experimental studies were carried out in this paper.First,the authors performed triaxial shear tests on energy soils under the influence of different factors.Then the strength prediction of energy soils was developed based on the test data,and the strength prediction methods of energy soils under different criteria were compared.The main work carried out and the conclusions obtained in this paper are as follows:（1）Based on the physical indicators and particle gradation of natural gas hydrate-bearing sediments in the Shenhu sea area of the South China Sea.Using soft clay taken from the seabed of the South China Sea,clayey-silty samples were prepared manually in the chamber,and a triaxial shear test protocol for energy soils was designed.First,triaxial shear tests of energy soils in different specimen initial states were conducted.Among them,the influencing factors include hydrate saturation,clay content,fine particle content and over consolidation ratio,with a total of 36 sets of tests.After that,triaxial shear tests on energy soils under different triaxial shear conditions were carried out.The influencing factors include confining pressure,pore pressure,shear rate and temperature for a total of 30 sets of tests.（2）The results of the triaxial test study of energy soils in different specimen initial states showed that:The effects of hydrate saturation,fine particle content and over consolidation ratio on the strength and stiffness of energy soils as well as the Mohr parameter follow the same trend,all increasing with the value of the influencing factor.The clay content had no significant effect on the strength and Mohr parameters of the energy soils but was negatively correlated with the modulus of elasticity E₅₀.For the effect of energy soil deformation,all three factors,hydrate saturation,fine particle content and over consolidation ratio,promote the strain softening of energy soil and increase the brittleness of energy soil.However,the increase in clay content increases the plasticity of the energy soil and promotes strain hardening.（3）The results of triaxial experimental studies on energy soils under different triaxial shear conditions show that:The influence of different factors on the strength,stiffness and Mohr parameters of the energy soil is more complex,and there is a coupling effect between the effective confining pressure and each influencing factor,resulting in completely opposite rules of performance under different effective confining pressures.For the influence of energy soil deformation,the level of the confining pressure determines the form of energy soil damage.In contrast to low confining pressure levels,the increase in effective confining pressure at high confining pressure levels makes the energy soil more brittle and promotes strain softening.The increase of pore pressure also promotes the softening phenomenon of energy soils.With both the increase in shear rate and temperature,the tendency of hardening of energy soils increases.（4）This paper is based on the Mohr-Coulomb criterion,the Drucker-Prager criterion,and the Lade-Duncan criterion.By introducing parameters related to hydrate saturation,a shear strength prediction model for energy soils considering the effects of hydrate saturation and effective confining pressure was developed.The results of the study showed that the predicted model values of energy soil shear strength based on three different criteria had high agreement with the experimental values.They all reflect the relationship between the shear strength of energy soils and the hydrate saturation and effective confining pressure better.However,the accuracy and applicability of the three prediction models differed significantly.The comparison revealed that the prediction model based on the Drucker-Prager intensity criterion worked best and was more convenient and effective.Reasonable prediction and evaluation of the shear strength of energy soils is an important method to ensure sustainable extraction of gas hydrates,as well as to judge the stability of reservoirs.