Study On Calculation Method Of Vertical Earth Pressure For High-Filled Load Reducing Cut-and-Cover Tunnels Based On Differential Settlement

Because the high-filled cut-and-cover tunnels（HFCCTs）can adapt to the special terrain factors in the northwest region,it is widely used in traffic construction.In order to reduce the huge overburden load caused by excessive backfill soil,laying a certain thickness of load reducing materials on the top of the tunnel has a positive effect on the reduction of vertical earth pressure on the top of the tunnel.This paper refers to Marston’s research on high-filled culvert structures,the establishment of a mechanical analysis model applicable to HFCCTs,deduces the calculation formula of vertical earth pressure on the top of the tunnel before and after load reduction,and analyzes the internal relationship between differential settlement and additional earth pressure in combination with cases.A method for calculating the vertical earth pressure of high-filled load reducing cut-and-cover tunnels（HFLRCCTs）based on differential settlement is proposed.Then,a discrete element numerical analysis simulation is established to study the influencing factors of earth pressure calculation,and the formula is corrected and analyzed.Expand its scope of application,combined with relevant papers at home and abroad to verify the correctness of the formula.The main research contents and results are as follows:（1）First,the particle flow numerical simulation software PFC2D is introduced,and its basic theory,calculation principle,and selection of contact constitutive calibration of mesoscopic parameters are described in detail.Through numerical uniaxial and biaxial compression test simulation,the mesoscopic parameters of loess and EPS are determined.Combined with the case,the generation method of backfill particles（improved layered compaction method）and the generation of EPS particles are introduced.（2）Aiming at the HFCCTs before and after using the EPS to load reduction,combined with the particle flow software,the transfer mechanism of the vertical earth pressure and the load reduction mechanism of the EPS are analyzed in detail,which provides a theoretical basis for the establishment of the physical and mechanical analysis model of the HFCCTs below.（3）The mechanical analysis model of HFCCTs is established,and the vertical earth pressure transfer mechanism of HFCCTs is analyzed.The vertical earth pressure formula of partial soil and soil settlement are derived.Combined with the case,the internal relationship between the differential settlement of the interior and exterior soil prism above the HFCCT and the vertical earth pressure on the top of the tunnel is analyzed,and the vertical earth pressure calculation formula based on the differential settlement is obtained.（4）The four influencing factors（slope angle,groove-width ratios,section type of HFCCT,backfill soil type）affecting the vertical load on the top of the tunnel are numerically simulated,and the influence of each influencing factor on theδ-P fitting curve function is analyzed.The results show that the change of influencing factors will not affect the function type ofδ-P fitting curve,the slope of theδ-P fitting curve under different influencing factors is compared,and the obtained formula is modified to expand its application range.Combined with the previous literature,the actual case of HFCCT is substituted into the obtained formula for calculation.The results show that compared with the actual results,the maximum error of the calculation results in this paper is 9.35%,which verifies the correctness of the formula.（5）The mechanical analysis model of HFLRCCTs is established,and the vertical earth pressure transfer mechanism of HFLRCCTs with a certain thickness of EPS is analyzed.The vertical earth pressure calculation formula of each part of the soil and load reducing material are deduced theoretically.Combined with the case,the vertical earth pressure on the top of the tunnel and the positive and negative differential settlement are calculated.The functional relationship between the negative differential settlement and the additional earth pressure generate is studied,and the calculation formula of the vertical earth pressure on the top of HFLRCCT based on the differential settlement is obtained.（6）The six influencing factors（slope angle,groove-width ratios,section type of the tunnel,backfill soil parameters,and thickness and density of EPS）affecting the vertical earth pressure on the top of the tunnel are numerically simulated.The influence of each influencing factor on theδ₀-p₂fitting curve function is analyzed,and the results show that the change of influencing factors will not affect the function type of theδ₀-p₂fitting curve.The slope of theδ₀-p₂curve under different influencing factors is compared and analyzed,and the modified formula is obtained.Combined with the previous research literature,the actual case of HFLRCCTs is substituted into the formula obtained in this paper to calculate and analyze the source of error.The results show that compared with the actual results,the maximum error of the calculation results in this paper is 4.58%,which can be used to calculate the vertical earth pressure on the top of the tunnel,and can provide some theoretical support for the design of the HFCCTs and the selection of the load reduction scheme.