تحليل تونل‌هاي زير سطح آب زيرزميني با درنظرگرفتن نيروهاي تراوش

ANALYSIS OF UNDER-WATER TUNNELS CONSIDERING SEEPAGE FORCES

در تونل‌هاي نفوذپذير زير سطح آب زيرزميني، جريان تراوش و تغييرات فشار آب منفذي شكل‌گرفته در عكس‌العمل پوشش و توده‌ي سنگ تاثير مي گذارند. بنابراين طرح تونل‌هاي زير سطح آب زيرزميني بايد بر مبناي تحليلي باشد كه جنبه هاي هيدروليكي و مكانيكي در نظر گرفته شوند. در اين نوشتار، يك روش تحليل كشسان‌خميري براي تونل‌هاي دايره‌يي زير سطح آب زيرزميني ارايه شده است. در روش ارايه‌شده، نيروي تراوش و نفوذپذيري وابسته به كرنش در توده‌ي سنگ در نظر گرفته شده است. شرايط اوليه‌، تنش هيدروستاتيك است؛ بنابراين از شرايط تقارن محوري استفاده مي‌شود. نتايج به‌دست‌آمده از روش ارايه‌شده با نتايج به‌دست‌آمده از برنامه‌ي تفاضل محدود مقايسه شده‌اند. نتايج به‌دست‌آمده از روش ارايه‌شده نشان مي دهند كه تراوش با افزايش تنش موثر پيرامون تونل مي تواند باعث كاهش پايداري تونل شود. همچنين لازم است كه اثر تغييرات نفوذپذيري در ناحيه‌ي شكسته‌ي پيرامون تونل در نظر گرفته شود.

Tunnels below a groundwater table can be either sealed or drained. Sealed tunnels do not influence the groundwater regime; however, the lining has to undertake the full water pressure. On the other hand, in a drained tunnel, a seepage flow will be developed. The seepage flow and pore water pressure developed around the tunnels affect the responses of the lining and of the rock mass, significantly. Thus, the design of underwater tunnels must be based on a powerful analysis, in which the hydro-mechanical aspects are taken into account. In a tunnel with permeable lining, the induced seepage forces can be the most important issue in the design and construction of the tunnel. In this condition, the applied inward seepage body forces are generated by pore-water pressure gradients, which are dependent on rock-mass permeability. On the other hand, the permeability of the fractured plastic zone around the tunnel depends upon deformations induced by excavation and applied seepage forces (strain-dependent permeability). A theoretical solution is proposed, in this paper, for analysis of underwater tunnels, excavated in an elasto-plastic، strain-softening and Hoek–Brown rock material, considering strain-dependant permeability. The problem is considered under axial symmetric conditions; thus, the initial stress state is assumed to be hydrostatic. For the plastic zone, as the derived analytical equations do not have closed form solutions, a computer program has been prepared for solving the corresponding equations numerically and examining the analysis. On the other hand, the governing equations for the elastic zone are solved, analytically. The results obtained with the analytical solution are compared with results obtained with the hydro-mechanical option in a commercial finite difference code. The proposed solution is also used to obtain results and explain the behavior of the tunnel under different hydro-mechanical conditions, when the ground is assumed to behave elasto-plastically. The results show that when a pervious lining is used or a drainage system is employed for the tunnel, the effects of seepage flow and pore water pressure must be taken into account. Application of the proposed method to several tunnel problems showed that in the case of drained tunnels, seepage flow causes the in situ effective stress to increase (i.e. a decrease in stability). Furthermore, it is shown that the state of stress and the seepage forces must be considered as dependent parameters, because of the remarkable permeability variations induced by the development of the plastic zone around the tunnel.