Experimental Study On Dynamic Characteristics Of Soft Clay Considering Seepage Effect

With the proposal of the 13th Five-Year Plan for the development of modern comprehensive transportation system,the multi-level network scale of China's transportation infrastructure has been further expanded.Especially in the more developed coastal areas,the construction of transportation facilities covers the widest area and has the largest density.However,soft clay is widely distributed in coastal areas.The"three highs,two lows"bad engineering properties of soft clay with high void ratio,high compressibility,high water content,low permeability and low shear strength lead to a great reduction in the stability of the foundation of traffic facilities under cyclic loading,and even threaten the normal safe transportation of traffic facilities.In addition,the abundant precipitation and the developed surface water system in the coastal areas cause the active movement of groundwater and the significant seepage movement of pore water in the soil,which to a certain extent has an impact on the engineering characteristics of soft clay foundation.Therefore,it is necessary for engineering construction and daily maintenance to study the dynamic characteristics of soft clay considering the influence of seepage.It is of great significance to prevent and control the unstable development of foundation and provide disaster response strategies.In this paper,the saturated soft clay in Tianjin coastal area was taken as the research object.Through a series of GDS drainage triaxial tests,SEM scanning electron microscopy and mercury intrusion tests,the effects of osmotic pressure,consolidation confining pressure,dynamic stress ratio,vibration frequency and vibration times on the dynamic characteristics of soft clay were analyzed.The dynamic deformation characteristics,stiffness dynamic development characteristics and permeability change properties of soft clay under dynamic-seepage coupling field were obtained.And qualitatively and quantitatively reveals the inherent reasons for the macroscopic mechanical properties of soft clay considering seepage.The mainly results are follows:?1?There were two development modes of dynamic deformation of soft clay under seepage:progressively stable and developmental,and there were three development stages of plastic deformation in each development mode:instantaneous growth phase,deceleration growth phase and stable development phase?gradual stability?or linear growth phase?developmental?,which was related to osmotic pressure,dynamic stress ratio,and vibration frequency.The soft clay dynamic deformation considering the influence of seepage was larger than that in the undrained dynamic stress field,and the lower the vibration frequency,the more obvious the difference.In dynamic-seepage coupling field,the cumulative plastic strain increased exponentially with the increase of osmotic pressure and dynamic stress ratio,and decreases with the increase of vibration frequency.Based on the Monismith exponential model,a soft clay plastic deformation prediction model considering osmotic pressure,consolidation confining pressure,dynamic stress ratio and vibration frequency was established.The empirical model could predict dynamic deformation of soft clay under the influence of seepage.?2?The dynamic elastic modulus of the soft clay considering the seepage increased rapidly with the increase of the vibration times and then decreased slowly,but the degree of reduction was small.The stiffness of soft clay considering seepage was greater than that of undrained dynamic stress field,but as the vibration frequency increased,the effect of seepage on stiffness decreased.The dynamic elastic modulus of soft clay was related to the osmotic pressure,dynamic stress ratio and vibration frequency,and decreased with the increase of osmotic pressure and vibration frequency,and increased with the increase of the dynamic stress ratio.The dynamic elastic modulus estimation coefficient?was introduced.According to the magnitude of soil deformation under different osmotic pressures,there were two quantitative relationships between coefficient?and cumulative plastic strain:power function relationship(1%)and quadratic polynomial function relationship(>1%).Under different dynamic stress ratios,the coefficient?decreased linearly with the logarithm of cumulative plastic strain;At different vibration frequencies,the coefficient?decreased linearly with the cumulative plastic strain.The SEM images showed that the soft clay soil skeleton considering the influence of seepage was more structural,the joint effect between the soil aggregates was strong,and the pores had good coherence and large pores.?3?Under cyclic loading,there were two stages in the development of soft clay permeability coefficient:fast decay phase and stable development phase.The two stage boundary point was near 2000 vibration times,and the permeability coefficient attenuation could reach 90%in the first phase.After 10000 vibrations,the decrease of permeability coefficient decreased exponentially with the increase of osmotic pressure,and increased linearly with the increase of the dynamic stress ratio,and decreased with the increase of vibration frequency.The pore structure characteristics of soft clay obtained by quantitative analysis of mercury intrusion test indicated that the larger the total pore volume,the larger the peak pore size and its content,the larger the corresponding permeability coefficient,indicating that the soil had better permeability.