Experimental investigation of face instability for tunnels in sandy cobble strata

In order to investigate the influence of face instability for tunnels with different burial depths in sandy cobble strata on earth pressure and the instability region,geomechanical model tests and numerical simulations were performed.The continuous excavation method was adopted to reduce the pressure of the soil bin and restore the real engineering situation.Earth pressure in three directions of the obser-vation section in front of the tunnel face was monitored during the tunneling of the shield.Evolutions of the lateral stress ratios at dif-ferent stages were also investigated.The instability area of the shield tunnel face in sandy cobble strata with different burial depth ratios during the instability stage was obtained based on the change ratio of earth pressure and compared with existing researches.The earth pressure began to change when the excavation was one shield diameter away from the observation section,and when the excavation reached the observation section,the earth pressure decreased significantly.The burial depth of shield tunnel in the sandy cobble strata has a significant impact on the evolution of soil arch and the size of the failure area.The numerical simulation of the continuum medium cannot reflect the stress redistribution characteristics of the granular body like sandy cobble strata,and the failure area or stress distur-bance area obtained by the model test is larger than the numerical simulation result.Existing methods have deviations in analyzing the failure area of shield tunnel face in sandy cobble strata.It provides not only guidance for shield tunnel excavation engineering in sandy cobble strata,but also a reference for the theoretical research on failure areas.

Analytical solutions of limit support pressure and vertical earth pressure on cutting face for tunnels

This paper focuses on theoretical analytical models to calculate the limit support pressure and vertical earth pressure on the cutting face for tunnels.The failure zone is divided into two parts:a sliding failure zone and an upper loosen zone,and the limit support pressure calculation equation is derived.To verify the rationality of the theoretical model,it was compared with the existing theory,numerical simulation,and centrifugal test,and then the parameter analysis was carried out.The results show that the results of this paper agree well with the existing theory,numerical simulation,and centrifugal test.The inclination angle of the proposed mechanism is determined based on the results of the existing centrifuge test,and the recommended inclination angle is between 52°+φ/2 and 54°+φ/2.The method is proven to be safe and accurate.It can provide a theoretical basis for similar projects.

Calibration method of mesoscopic parameter in sandy cobble soil triaxial test based on PFC3D

This paper presents a rapid and effective calibration method of mesoscopic parameters of a threedimensional particle flow code(PFC3D)model for sandy cobble soil.The method is based on a series of numerical tests and takes into account the significant influence of mesoscopic parameters on macroscopic parameters.First,numerical simulations are conducted,with five implementation steps.Then,the multi-factor analysis of variance method is used to analyze the experimental results,the mesoscopic parameters with significant influence on the macroscopic response are singled out,and their linear relations to macroscopic responses are estimated by multiple linear regression.Finally,the parameter calibration problem is transformed into a multi-objective function optimization problem.Numerical simulation results are in good agreement with laboratory results both qualitatively and quantitatively.The results of this study can provide a basis for the calibration of microscopic parameters for the investigation of sandy cobble soil mechanical behavior.