Study On The Relationship Between Compaction State And Shear Wave Velocity Of Fine Soil Subgrade

As the rapid development of highway network construction in China,the requirements of construction efficiency are getting higher and higher.There is a trend toward using large tonnage roller for the filling of ultra-thick subgrade for the advantages of high compaction efficiency and low construction cost.In the highway subgrade construction,the compaction quality not only affects the quality of foundation and subgrade,but also has an important influence on the quality and durability of pavement.But for ultra-thick subgrade,traditional sand filling method has difficulty in compaction quality control,therefore,it is of great value to ensure the quality of subgrade compaction,improve subgrade construction efficiency and reduce the construction cost through developing a new method of compaction detection to implement effective control of compaction quality in the construction process and the rapid,convenient and nondestructive compaction testing.Based on the above problems,this paper proposes a method to evaluate the compaction quality by measuring the shear wave velocity of the compacted subgrade.Firstly,based on the pressure plate instrument test with bender element,the prediction formula of the shear wave velocity of silty clay in the Yellow River Flooded areas under one dimensional limit condition was obtained.Then the experimental results are extended to the three-dimensional unconfined condition,and the shear wave velocity fitting formula is modified.Finally,the subgrade compaction degree test system is developed,which can quickly measure the shear wave velocity and water content of the compacted subgrade,and calculate the subgrade compaction degree.The main results and conclusions are as follows:(1)The time-domain characteristic point method is used as the method of determining the travel time,which means defining the average of the time difference of starting points and the time difference of peak points as the travel time;in addition,the method of reverse wave is added during the test to produce symmetrical signals,and the starting point of the received waves can be determined accurately.(2)The variation of shear wave velocity under one dimensional limit condition with water content,compaction degree and axial stress was obtained.The results show that the shear wave velocity decreases linearly with the increase of water content and increases linearly with the increase of compaction degree,and the prediction formula of shear wave velocity is obtained by fitting.(3)The corresponding relationship between the shear wave velocity under the three-dimensional confined stress and the one-dimensional confined stress is obtained,and the fitting formula of the shear wave velocity is modified.The influence of soil depth on shear wave velocity and compaction degree is studied,and the variation law of shear wave velocity under dead load and additional load is obtained.(4)There is a great correlation between the measured compaction degree by cutting ring method and the shear wave velocity method by the fitting formula in this paper.When the test point is 20cm underground or deeper,the compaction degree differentials between the fitting values and the measured values are less than 1%,therefore the shear wave velocity method provided by this article can be used to detect the degree of compaction of subgrade compaction for filling subgrade,especially for the ultra-thick subgrade,and it has a good application prospect.(5)The test device and test method of subgrade compaction has been developed initially,which includes testing the shear wave velocity at different depths of subgrade through the field shear wave velocity test device and testing the soil moisture content through the electrical conductivity sensor,and the compaction degree of subgrade can be measured fast and non-destructive by adopting the fitting compactness calculation formula.