Experimental Study On The Maximum Shear Modulus Of Coarse-grained Soils

Coarse-grained soils have many advantages,such as good compaction performance,strong water permeability,convenient material collection,etc.,and have been widely used in the construction of civil engineering,water conservancy,transportation and other projects.Under earthquake or other dynamic loads,there is a close relationship between the safety of earth-rock dam,high soil slope,breakwater,bank protection,expressway subgrade and high-speed railway foundation which take coarse-grained soils as the main building materials and the dynamic properties of coarse-grained soils.Therefore,it is of great engineering significance to accurately grasp the dynamic properties of coarse-grained soils.In the study of dynamic properties of coarse-grained soils,dynamic shear modulus and its variation with shear strain are essential basic data in seismic design,analysis and calculation of major engineering structures such as geo-structures,and also one of the main factors affecting seismic response analysis of soil layers.When the soil is in the elastic stage at very small strain level,there is an initial value of the dynamic shear modulus,namely,the maximum shear modulus Gmax,which is an important index to comprehensively reflect the stress state,compactness and structural characteristics of the soil,and has become a hot topic in academic research.According to the definition in the standard for engineering classification of soil,coarse-grained soils can be further divided into sands and gravels.At present,the testing techniques for the maximum shear modulus of sands are relatively mature and the research findings are abundant.In laboratory tests,the resonant column and the bender element methods have been widely used in the measurement of the maximum shear modulus of sands.Scholars have obtained the main influencing factors of Gmax through extensive experimental studies,and established the corresponding models for the prediction of Gmax.Compared with sands,the particle size of gravels has increased,and the specimen preparation method and test conditions have changed to a certain extent,which put forward new requirements for the volume of the instrument and test technology.In addition,the model parameters and prediction results of the existing Gmax models for sands are different to varying degrees.Whether the prediction models suitable for sands can be directly applied to gravels is not yet known,and the intrinsic mechanism affecting the maximum shear modulus remains to be further studied.In this dissertation,the shear wave velocity testing system for coarse-grained soils was developed based on the large triaxial instrument test platform of the State Key Laboratory of Coastal and Offshore Engineering,Dalian University of Technology.The developed system overcomes the technical problem that the traditional resonant column and bender element can not meet the requirement of measuring the maximum shear modulus of gravels.On this basis,the influence of particle shape,particle gradation and particle size on the maximum shear modulus of coarse-grained soils was investigated in detail,and the prediction model for the maximum shear modulus of coarse-grained soils considering the influence of particle characteristics was proposed and verified.The main research contents and conclusions of this study are as follows:(1)Based on the principle of torsional vibration and piezoelectric effect,the piezoelectric stack actuator was embedded in the top cap of the large triaxial instrument,and the integrated testing system of shear wave velocity and triaxial test without disturbance is developed successfully by integrating accelerometer,data acquisition equipment,sealed switching element and so on.Then,dynamic tests were carried out on the polymethyl methacrylate specimen(continuum media),glass bead and Fujian sand specimens(granular media)respectively.The system’s functionality and reliability were verified from the aspects of test accuracy and test stability,which provided an advanced technical support for the study of dynamic properties of coarse-grained soils.(2)Based on the Fourier morphological analysis method,the particle shape factor SF of coarse-grained soils was quantified.The influence of particle shape on the maximum shear modulus Gmax was studied under the same particle gradation.The results show that,the Gmax increases with the increase of SF,that is,the more the particle contour deviates from the circle,the greater the maximum shear modulus is.Based on Hardin and Jamiolkowski void ratio functions,the Gmax prediction models considering the influence of particle shape were established.The results show that the influence of particle shape on the maximum shear modulus can be reflected by the modulus coefficient A,which increases with the increase of SF,and the stress exponent n is nearly independent of particle shape.(3)The effect of uniformity coefficient Cu on the maximum shear modulus Gmax was systematically investigated for coarse-grained soils of the same lithology.The results show that the maximum shear modulus is sensitive to the change of particle gradation curve.The Gmax decreases with the increase of Cu,and the decreasing trend gradually flattens.In addition,the influence of Cu on the Gmax can be reflected by the modulus coefficient A and stress exponent n,where A decreases with the increase of Cu and n increases with the increase of Cu.Therefore,the Gmax model which only considers the influence of void ratio and effective confining pressure cannot reasonably predict the maximum shear modulus of coarse-grained soils with different particle gradations.(4)The intrinsic mechanism of macroscopic elastic properties of coarse-grained soils was investigated by the discrete element method.The results show that the uniformity coefficient Cu has a significant effect on the coordination number distribution,and there is no one-to-one correspondence between the apparent void ratio and the average coordination number.Effectively stable void ratio eeff-4 is better than apparent void ratio e to characterize the maximum shear modulus of granular materials with different particle gradations.For the specimens with the same eeff-4 increases with the increase of Cu,which leads to a decrease in the maximum shear modulus.Therefore,a more ideal prediction effect can be obtained by introducing Cu into the classical Hardin model.(5)The effect of particle size on the maximum shear modulus of unifrom and non-uniform coarse-grained soils was systematically investigated by the parallel gradation method.The results show that the maximum shear modulus of unifrom soil increases slightly with the increase of particle size.For non-uniform soil,the maximum shear modulus increases with the increase of particle size,and the increasing trend is more obvious than that of uniform soil.The influence of particle size on the maximum shear modulus can be reflected by the modulus coefficient A,which increases with the increase of particle size,and the stress exponent n is nearly independent of particle size.Furthermore,the relationship between the maximum shear modulus and particle size may be affected by the particle shape,which is more obvious in uniform coarse-grained soils.(6)In view of 8 common Gmax prediction models in the academic world,the prediction error,prediction trend and application range of the models were compared and analyzed in detail.The results show that the prediction results of existing models vary to varying degrees,and can only satisfy part of the test data.According to the experimental results,the Gmax prediction model considering the influence of particle characteristics was proposed,and its applicability was verified by the test data in this study and in the existing literature.For coarse-grained soils that have not been or are difficult to obtain the test data,the maximum shear moduli can be predicted according to the model suggested in this study to provide a reliable basis for seismic design,analysis and calculation of engineering.