تعيين منحني تنش محصوركننده - بيرونزدگي جبهه كار تونل با استفاده از مدلهاي عددي مطالعه موردي: متروي خط 7 تهران

Determination of Tunnel’s confinement stress-Extrusion Curve by means of Numerical Models Case Study: Tehran Metro-line 7

كاهش تدريجي تنش از روي سينهكار تونل به هنگام حفاري، منجر به ايجاد تغييرشكلهايي در زمين دربردارندهي آن مي‌شود. بيرونزدگي سينهكار اولين مولفه پاسخ تغييرشكلي محيط به حفاري است. اين پديده به مقاومت و خواص تغييرشكلي هستهي پيشروي و همچنين ميدان تنش اصلي كه در معرض آن قرار گرفته بستگي دارد و روي سطح سينهكار و در امتداد محور طولي تونل نمود پيدا ميكند. در پژوهش پيشرو به كمك مدلسازي سه بعدي قسمتي از تونل متروي خط 7 تهران، رابطهي بين تنش محصوركننده سينهكار تونل و مقادير بيرونزدگي آن بررسي شده و نتايج در قالب منحني تنش محصور كننده-بيرون زدگي يا منحني مشخصهي تونل ترسيم گرديده است. در ادامه منحني مشخصه تحت محورهايي به شكل بدون بعد ترسيم شده و مشاهده گرديده است كه اين منحنيهاي بدون بعد، با رعايت شرايط خاصي براي تونلهاي مختلف برروي هم قرار ميگيرند. شرايط لازم براي اين منظور نيز در انتها آورده شده است.

The Geological hazard and the lack of appropriate underground investigation have always made the design and construction of underground works a risky affair. As a consequence a larger safety factor should always be considered for underground works compared to similar surface constructions. During excavation of a tunnel and by the opening a cavity the flow lines of stress field in the soil or rock mass are deviated and are channeled around it to create a zone of increased stress around the walls of the excavation. This channeling of the flow stresses or arch effect ensures that the cavity is stable and will last over time. Depending on the size of stresses created and the strength and deformation properties of the ground, arch effect can be produced in three states. In first state arching is close to the profile of excavation, when the ground around the cavity withstands the deviated stress flow around the cavity well, responding elastically in terms of strength and deformation. For second state, arching is far from the profile of the excavation, when the ground around the cavity is unable to withstand the deviated stress flow and responds an elastically-plasticizing and arch effect forms far from walls of the excavation. In the last state arching is not produced at all, when the ground around the cavity is unable to withstand the deviated stress flow and responds in the failure range producing the collapse of the cavity. The stability of tunnel can be determined and optimized through numerical modeling by finding stresses around tunnel and face movement. The gradual reduction of horizontal stress from the tunnel face during the excavation process leads to the formation of deformation on the ground surrounded tunnel. Face Extrusion is the first response of environment to the excavation process. Extrusion appears on tunnel face and along the longitudinal axis of the tunnel and its geometry could be symmetric or asymmetric. This phenomenon depends on the resistance and properties of the advance core, arch effect and the main stress area exposed to the tunnel. In the present study a part of the Tehran Metro Line 7 tunnel in analyzed. The selected part for modeling locates in 0+900 to 0+1000 meters from the beginning of the southeast of line 7. Using geotechnical reports, the soil properties are selected and an elasto-plastic stress-strain behavior is considered for soil. All numerical modellings are done by finite difference method using in Flac 3D software. Based on results of numerical modelling the relationship between the stresses of the tunnel face and its extrusions are investigated and the results are presented in the form of confinement stress-extrusion curve or “tunnel characteristic curve”. The characteristic curve is plotted under dimensionless axes and it is observed that these non-dimensional curves are placed on each other under special conditions for different tunnels i.e. there is only one characteristic curve for tunnels with different geometry. Conditions for unique characteristic curve are also presented here.