تاثير شرايط زهكشي در توزيع فشار آب حفره‌يي و تنش‌هاي پوشش در تونل‌هاي زير سطح آب زيرزميني

Effect of Drainage Conditions on Pore Water Pressure Distribution and Lining Stresses in Tunnels under Groundwater Table

حفاري تونل زير سطح آب زيرزميني در زمين‌هاي آبرفتي، باعث اندركنش بلندمدت بين پوشش تونل و زمين اطراف مي‌شود. در اين مطالعه به‌منظور ارزيابي آثار به‌كارگيري زهكش محيطي در تونل در توزيع فشار آب حفره‌يي و ميزان بار وارد بر پوشش، مدل‌سازي عددي تونل واقع در زير سطح آب زيرزميني با استفاده از نرم‌افزار به روش تفاضل محدود انجام شده است. شرايط مرزي مختلفي از قبيل: تونل زهكشي‌شده، تونل زهكشي‌نشده، و سيستم زهكشي با نفوذپذيري متغير مورد ارزيابي قرار گرفته است. اثر پارامترهاي هندسي تونل از قبيل: قطر و ارتفاع خاك روباره و همچنين تراز آب زيرزميني مورد بررسي و آثار مخرب بسته‌شدن سيستم زهكشي مورد مطالعه قرار گرفته است. نتايج نشان داده‌اند كه با به‌كارگيري زهكش محيطي در ديواره‌ي تونل، به چه ميزان فشار آب وارد بر پوشش كاهش مي‌يابد. همچنين با كاهش عملكرد سيستم زهكشي در بلندمدت، بار فشار آب حفره‌يي وارد بر پوشش افزايش مي‌يابد، لذا نرخ افزايش مورد بررسي قرار گرفته است.

Tunnelling in a saturated soil often produces a long-term interaction between the tunnel lining and the surrounding soil. Infiltration and external pore-water pressures are often important factors that must be considered in the lining design. Development of pore water pressure may accelerate leakage and cause deterioration of the lining. This can be particularly troublesome to structural and functional components of the tunnel and can often lead to structural failure. In this paper, an axi-symmetric circular tunnel under the groundwater table is modeled using finite difference software, and different conditions, such as an impermeable tunnel, a permeable tunnel and a lining drainage system with varying permeability are investigated. The structural behaviour of the lining and flow through the tunnel lining were modelled using a combination of beam and solid elements. The effects of tunnel parameters, such as depth and diameter and variation of underground water levels, on the effectiveness of the lining drainage system are examined. Moreover, the deterioration of a drainage system caused by clogging is investigated and the importance of giving careful consideration to lining permeability and hydraulic boundary conditions is highlighted. As a result of coupled analysis, a lining drainage system in a shallow tunnel with small diameter is more efficient, and effective drainage conditions reduce pore water pressure loads, which decreases the bending moments and tensile stresses of the tunnel lining. By performing a parametric study for a tunnel with different lining permeabilities, a design curve to evaluate pore water pressure loads on the lining has been proposed. It is shown that the hydraulic deterioration of the drainage system significantly develops a magnitude of pore water pressure load on the tunnel lining