Research On The Stability Analysis And Engineering Countermeasures Of Tunnel Under The Joint Action Of Swelling And Softening Of Mudstone

Expansive rock is a type of rock mass that is mainly composed of strong hydrophilic minerals such as montmorillonite,illite,or montmorillonite illite.It has a high clay content,high plasticity,and can expand and contract when exposed to water.Due to the engineering properties of swelling rock,which absorbs water and expands rapidly,its strength decreases sharply and undergoes repeated deformation,it has a strong destructive effect on the stability of the tunnel.After tunnel operation,problems such as floor heave and lining cracking are prone to occur.In this thesis,taking the tunnel of Baotai Mountain mudstone section of Zhonglan Passenger Line as the engineering background,and uses research methods such as data research,theoretical analysis,indoor experiments,numerical calculations,and on-site tests to conduct indoor experimental research on the physical,mechanical,and expansion characteristics of mudstone.At the same time,conduct stability analysis of tunnel structures under different factors such as surrounding rock expansion levels,expansion depths,expansion locations,and cross-sectional forms,and provide reasonable suggestions for the anti expansion structural forms and deformation control measures at the bottom of expansive mudstone tunnels.The specific research results are as follows:(1)Research on the physical,mechanical,and swelling characteristics of mudstone.An indoor anhydrous core processing method suitable for easily disintegrating mudstone was proposed,and physical and mechanical properties tests such as water content,uniaxial compression,triaxial compression,and Brazilian splitting were conducted.The stress-strain curve of mudstone,as well as the changes in compressive strength,tensile strength,cohesion,and internal friction angle with water content were obtained.A modified Rocker strength criterion for expansive rock considering water content and confining pressure effects was proposed.At the same time,free expansion rate test,lateral constrained expansion rate test,and expansion pressure test were conducted on mudstone,and the variation law of mudstone expansion index during the entire expansion process was obtained.The functional relationship expressions between different expansion index and water content were derived.(2)Stability analysis of tunnel structure under the combined action of surrounding rock expansion and softening within the loosening zone.Based on the assumption that the change in water content is the internal cause of the expansion and softening of expansive rock,this article proposes that the expansive rock tunnel undergoes expansion and softening of the surrounding rock within the range of the loosening zone.Based on the secondary development technology of FLAC3 D numerical calculation,a modified Mohr-Coulomb yield criterion for expansive rocks considering the effects of expansion and softening was proposed.On the basis of inheriting the stress-strain relationship during the construction phase,a comparative analysis was conducted on the stress characteristics,deformation patterns,and overall stability of tunnel structures under different expansion levels.(3)Stability analysis of expansive rock tunnels under different influencing factors.Referring to the railway tunnel cross-section,various working conditions such as different levels of surrounding rock expansion,depth of surrounding rock expansion at the tunnel bottom,structural expansion positions,and different tunnel cross-section forms for single and double tracks were set up.The mechanical response behavior of the structure under different factors was compared and analyzed,and the most unfavorable influencing factors and regulations were clarified.(4)Research on the reasonable structural form and deformation control technology of expansive mudstone tunnels.Based on the characteristics of swelling and softening deformation of surrounding rock in expansive mudstone tunnels,a reasonable matching stiffness and rise span ratio based on deformation coordination for anti swelling structure was proposed,and a transverse and longitudinal partition drainage system suitable for expansive rock tunnels was proposed.Summarized the reasonable reinforcement depth of surrounding rock.(5)On site testing and verification.Based on the expansion test and preliminary exploration results on site,it was proposed to strengthen the support and lining structure design for the expansive mudstone section of the Baotai Mountain Tunnel.During construction,monitoring instruments were arranged to conduct long-term tracking and monitoring of the stress and strain in various parts of the initial support and secondary lining structure,verifying the rationality of the strengthened design for the expansive mudstone section of the Baotai Mountain Tunnel and ensuring the safety of the tunnel structure.(6)Design recommendations for expansive surrounding rock tunnels.Summarized the research content and achievements of this article,combined with the on-site test results of Baotai Mountain Tunnel,suggestions for the design of expansive rock tunnel were proposed from the aspects of exploration,testing,anti expansion lining structure design and construction of expansive rock tunnel.It can provide the reference for the preparation of relevant guidelines and specifications.