| The aeolian sands are widely distributed in China.However,due to the low cohesion and poor stability of aeolian sand particles,conventional construction measures,such as bolt anchoring and advanced support,basically do not play a role during tunnel construction.At the same time,it is difficult to form a collapsed arch due to properties of aeolian sands.During the excavation process,the working face is easy to collapse,and the risk of unsafety is very high.Therefore,it's important to systematically study the deformation and to reveal the failure mechanism of aeolian sand tunnel.It is also necessary to form the calculation method of the working face of tunnels built in the aeolian sand stratum.Therefore,this paper studies the construction of tunnels excavated in aeolian sand stratum in three aspects: the instability of deformation and failure of working face,the calculation method of the stability of working face and the design method of face surface pretreatment,relying on Project of the Yitongbulak No.1 Tunnel on Xinjiang Geku Railway.To determine the stability of tunnel face and pretreatment methods,the main conclusions and results of this paper are listed as following three aspects:1.A discrete element model is established to detect the deformation and failure of the working face of tunnel built in aeolian sand,and the whole process of instability and failure of working face of the aeolian sand formation tunnel is simulated,according to the calibration parameters including: particle coordination number,interparticle contact and interparticle gap.The meso-mechanical level indicators elaborated the variation trend of aeolian sand particles at various locations during the instability and deformation of working faces,from the macroscopic mechanical level of the support of working face and the deformation zones of surrounding rock.The instability mechanism,the change of support force and the analysis on the instability deformation mode of tunnel faces in sand stratum are discussed in detail,and the variation of the support force and the instability deformation mode are summarized,and the support of working face is obtained.The relationship between the ratio and the extrusion displacement is also derived.2.Through data investigation and theoretical analysis,the stability of tunnel faces in aeolian sand stratum is analyzed,and the theoretical calculation model of the instability of the tunnel faces in the aeolian sand stratum is determined by combining the instability and deformation mode of working face.The ellipsoid instability theory,under the limit equilibrium state,is improved to develop the traditional Terzaghi's theory for loose earth pressure calculation.The calculation formula of the lateral pressure coefficient with the slip angle is derived when the slip surface is elliptical.Further,the internal vertical of the sliding loose zone is improved.The stress is distributed in a trapezoidal manner along the horizontal direction,and the calculation formula of the loose earth pressure on the working face of aeolian-sand tunnel based on the DEM and ellipsoid instability theory is obtained.3.Through statistical investigation and analysis of the pre-treatment measures of tunnels excavated in completed aeolian sand stratum,the advanced support schemes which is most suitable for the aeolian sand stratum tunnel,that is,horizontal jet grouting piles and advanced core soil pre-reinforcement integrated system(type pre-reinforcement),are proposed.For simplification,the corresponding mechanical model is then established,and the theoretical calculation formula for calculating the horizontal jet grouting pile reinforcement and advanced core soil reinforcement parameters is deduced.Additionally,the pre-reinforcement mechanism of the working face of tunnel in an aeolian sand formation is quantitatively clarified and a set of design theories and methods are established.The rationality of the formula is verified by validations of the pre-reinforcement example of the working faces in the engineering site. |
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