Experimental Study On The Force Model Of Composite Foundation With Plain Concrete Pile In Soft Soil On Steep Slope Section

In the construction of mountainous roads,steep slope sections built on soft ground are treated with composite foundations,and the stability evaluation of composite foundations reinforced with rigid piles follows the stability evaluation method of flexible piles,i.e.,the ultimate equilibrium method to calculate the shear strength of composite foundations.A large number of failure cases of composite foundations treated with rigid piles show that the ultimate balance method is problematic and overestimates the stability of composite foundations.In this paper,based on the research results of shear resistance of composite foundations with rigid piles at home and abroad,indoor tests and numerical simulations of composite foundations with plain concrete piles in soft soils were carried out,and 32 sets of indoor solid tests with different pile diameters,different pile spacing and different replacement rates were completed.The simulation software PFC was used to conduct virtual tests,and the results of the indoor solid tests were compared and analyzed with the virtual experimental results,and the following progress was made in the overview of the work content:a.The parameters of the shear strength of the reinforced soil and the shear strength of the reinforced pile,which constitute the shear strength of the composite foundation,were obtained by different indoor tests,specifically: physical and mechanical parameters of the weak soil and shear strength;compressive strength and shear strength of the plain concrete pile specimen.According to the results of this test,the unconfined compressive strength of the solid pile specimen is close to 18 MPa,and the code recommends that the shear strength of the solid pile be half or 1/3 of the unconfined compressive strength,but the measured shear strength is less than 5 MPa,and the recommended shear strength of the pile in the code is unsafe.b.Indoor tests and numerical simulations were conducted on the composite foundation model of plain concrete piles in soft soils,and according to the test results obtained,the composite shear strength of the composite foundation model in the range of4.52% to 28.27% of the replacement rate was less than the calculated value of the code formula.It was proved by a large amount of data that the shear strength of composite foundations calculated according to the current code for stability analysis would overestimate the stability of composite foundations.According to the data under different axial pressures,the larger the axial pressure,the larger the error in composite shear strength;the higher the replacement rate,the larger the error in composite shear strength.Side reflect in the deep or high replacement rate of the foundation,the compound shear strength formula is more inaccurate.c.The composite shear strength analysis in the stability evaluation of composite foundations in the existing code,combined with the results of indoor tests and numerical simulations,the pile damage form analysis of different pile group composite foundation models under 28.27% replacement rate after shear damage,combined with the pile displacement field analysis to reproduce its damage process.It is directly observed that the pile body is obviously fractured by bending and presents a monolithic overturning state,while the pile body subjected to shear damage exists after the pile body subjected to bending damage.Therefore,in the evaluation of the stability of composite foundations based on rigid foundations,the analysis of the bending resistance of the piles should be included or focused.d.The particle displacement map obtained from the numerical simulation is analyzed,and the displacement map analysis is carried out for the 4×150 pile group as an example.With the increase of shear displacement,the phenomenon of soil bypassing between piles exists,which is more obvious in the shear upper box.Therefore,in the analysis of composite foundation instability,the important factor of inter-pile soil bypass should not be ignored.e.According to the design of composite foundation model,the composite foundation model with different pile groups under the replacement rate of 28.27% in this paper was tested by controlling the pile spacing,pile arrangement,number of piles,pile diameter,and setting single pile and group pile arrangement.According to the test results,the composite foundation with group pile type has better performance than the single pile treatment in terms of composite shear capacity under deep or multiple loads.