Research On The Creep Characteristics Of The Reservoirs Deposited Sand-silt Mixture In The Loess Plateau Of Southern Ningxia

The construction method of damming on the silt face in front of dam is mostly applied in the reinforcement of dangerous reservoirs in the mountainous region ofsouth Ningxia.The engineering characteristics of silt in front of dam are directly related to the dam’s stability and the operation safety of the reservoir after construction.The sand-silt mixtures are used as the object to investigate the mechanical properties and creep properties of silt in front of the reservoir dam under one-dimensional and three-dimensional conditions,considering the actual situation that the main component of silt in front of the reservoir dam in the mountainous region of south Ningxia are the sand-silt mixtures.Combined mercury intrusion test with isothermal adsorption experiment,the paper qualitatively analyzes the influencing mechanism of sand content on the mechanical and creep properties of the sand-silt mixtures from the micro and micro perspective.On this basis,the nonlinear stress-strain model and one-dimension and three-dimension creep models of the sand-silt mixtures are proposed.The models were validated by comparing values calculated with the models and the experiment data of sand-silt mixtures and related examples.In the end,the rheological consolidation differential equation which is used in the analyses of the dissipation of pore water pressure for sand-silt mixtures and the post construction settlement of Yashigou Reservoir,considering the temperature and secondary consolidation effect,is derived.The research results provide the theoretical support and calculation basis for the engineering practice of the construction method of damming on the silt face in front of dam.The main research results obtained are as follows:(1)The drilled sampling is conducted based on the silt layer in front of the reservoir dam through some typical reservoirs(silt-trap dams)in the middle-south of Ningxia survey.The results show that silt and sand make up the largest percentage of the particle size composition of silt soil in front of silt-trap dams.Thus,silt in front of the reservoir dam can be generalized as sand-silt mixtures foundation with different sand contents for the construction method of damming on the silt face in front of dam.(2)The results of triaxial consolidated drained shear tests of sand-silt mixtures with different sand contents show that as the confining pressure increases,the strain softening behavior of sand-silt mixtures are gradually obvious.When the sand content is less than 28.57%,the peak deviator stress,internal friction angle,and cohesion gradually increase with the change of sand content,and then decrease after exceeding 28.57%,which is mainly caused by changes in the microstructure of the sand silt mixture.Considering sand content and confining pressure play an essential role in the mechanical properties of sand-silt mixtures.Based on the Duncan-Chang model,a new constitutive model considering both two factors was established.Following this,its reliability was verified through complex tests.(3)The creep test results of sand-silt mixtures show that with the increase of stress level and the decrease of sand content,the secondary consolidation coefficient and creep rate increase gradually,and the creep variable increases continuously;Isothermal adsorption test and mercury injection test are carried out for sand-silt mixtures with different sand content,and it is found that the creep characteristics of sand-silt mixtures with weakly bonded water are greatly affected by the sand content.With the increase of external load,the large and middle pores are gradually transformed into micro and small pores,free water and weak bound water are gradually discharged,the bound water film is continuously thinner,and the creep characteristics of soil are changed accordingly.The larger the sand content is,the smaller the weak bonding water content of sand silt mixture is,and the less obvious the difference of creep characteristics is.(4)On the basis of equal timeline theory,a one-dimensional equal creep line elastic viscoplastic model of sand-silt mixtures considering the influence of sand content is established.In addition,by introducing the fractional calculus theory,a three-dimensional fractional creep constitutive model considering sand content was established.Compared with test values and model calculated values,the rationality of the creep constitutive model was verified.(5)The rheological consolidation tests of the sand-silt mixtures showed that the Mandel-Cryer effect appeared more and more obvious in the early stage of the test with the enlargement of sand content.On the basis of Terzaghi consolidation theory and isotropic viscoelastic model,a consolidation-rheological model which integrates the sand content and sub-consolidation effect is established,which can better reflect the dissipation law of pore water pressure at the bottom of the sand-silt mixture specimen.Comparison between the measured data of post construction settlement of silted soil in Yashigou Reservoir and the values calculated with the consolidation-rheological model shows that the rheological consolidation model can be applicable for the prediction of the post-construction settlement of silted soil foundation in front of dam.