Experimental Study On Mechanical Parameters Of Pile-soil In Volcanic Soil Layer

With today’s high-quality social and economic development,a large amount of infrastructure development is on the rise.Improving the structure of the railway network in the Northeast is a key task in the 14 th Five-Year Plan.The new Shenbai high-speed railway crossing the volcanic soil accumulation area of Changbai Mountain will inevitably use the volcanic soil as the foundation for major projects.Due to the special material composition and structural characteristics of volcanic soils,their mechanical properties are different from other types of soils,and there is a lack of sufficient research on the engineering geological properties of volcanic soils,so there is no basis for the determination of the bearing capacity of volcanic soil foundations and pile-soil mechanical parameters.Based on the summary of the existing research results,this paper takes the volcanic soil in the volcanic pile region of Changbai Mountain as the research object,conducts physical and mechanical tests on the volcanic soil,shear tests on the volcanic soil-concrete interface,indoor small-scale pile foundation model tests,and combines the finite difference numerical simulation approaches to investigate the pile-soil interaction parameters in the volcanic soil layer,and analyses the relationship between the pile diameter and the disturbance range of the soil around the pile.The main research work and results are as follows:(1)Particle analysis tests are used to obtain particle gradation curves,inhomogeneity coefficients and curvature coefficients and to determine the gradation state of the soil;compaction tests are taken to determine the optimum moisture content and maximum dry density of the test soil,which provides a basis for the preparation of specimens.(2)By conducting large scale direct shear tests on volcanic soils,it was obtained that the variation of shear strength with shear displacement at optimum moisture content exhibited strain softening characteristics.The tests showed that the cohesion of the volcanic soil increased by 26.96% and the angle of internal friction increased by 7.35% with increasing compaction.(3)Shear tests at the volcanic soil-concrete interface were carried out by means of an improved test method.The shear strength parameters of the shear interface were available from the maximum shear stress-normal stress fitting curves of each group of tests.The tests show that the interface shear process is mainly divided into elastic,plastic and damage phases;the increase of concrete curing age mainly increases the interface shear strength by increasing the internal friction angle of the interface.The increase in the angle of internal friction for different compaction levels ranges from15.1% to 52.8%.The increase in compaction of volcanic soils will cause different degrees of increase in internal friction angle and cohesion,with a more significant effect on cohesion.(4)Load settlement curves were obtained from indoor static load model tests on single piles.The results show that the pile lateral frictional resistance is fully developed at small displacements and mainly carries the total pile top load.The pile end resistance then gradually increases to carry the total pile top load.(5)It is clear that pile diameter,pile length and volcanic soil compaction have an effect on the ultimate pile load capacity,end resistance load sharing ratio and pile lateral frictional resistance.The increase in pile diameter will lead to an increase in the ultimate pile capacity and end resistance load sharing ratio.The increase in pile length increases the ultimate pile load capacity by 8.23% and 6.37% for different test groups,while the end resistance load sharing ratio decreases by 20.77% and 11.02%respectively.Therefore,the influence of pile length on the end resistance load sharing ratio is more significant than that of pile diameter;increasing the compaction of volcanic soil will also increase the ultimate load carrying capacity and reduce the end resistance load sharing ratio.As the pile diameter,pile length and the compaction of the volcanic soil increase,the pile lateral frictional resistance also increases.The pile length has the most significant effect on the pile lateral resistance,while the pile diameter has the lowest effect.(6)Numerical simulations were executed utilizing finite difference software and compared with indoor model test findings to verify the reasonableness of the indoor model size setting.The effect of pile diameter on the disturbance range of the soil around the pile was analysed.The results show that an increase in pile diameter will cause an increase in the perimeter soil disturbance range.The proposed quadratic polynomial and logarithmic models can better reflect the variation of the perimeter soil disturbance range with the pile diameter.