بررسي عددي رفتار هيدرومكانيكي درزههاي سنگي منفرد تحت بارگذاري برش مستقيم

Numerical Modeling of Hydro-Mechanical Behavior of Rock Joints under Direct Shear Loading

نفوذپذيري توده سنگ يكي از مهمترين پارامترهاي كنترل كننده پروژههاي مختلف مهندسي سنگ اعم از مخازن دفن زبالههاي اتمي، پي سدها، فضاهاي زيرزميني همانند تونل‍‌ها و مغارها، پروژه هاي ژئوترمال و توليد نفت و گاز است. به طور كلي نفوذپذيري توده سنگ متشكل از نفوذپذيري سنگ بكر و ناپيوستگي هاي موجود در آن است اما در حوضههاي با سنگ ميزبان سخت و كريستالي نفوذپذيري سنگ بكر تقريباً ناچيز و قابل اغماض است و كنترل كننده اصلي جريان عبوري از توده سنگ، درزهها و ناپيوستگي ها هستند. در چنين شرايطي، علاوه بر پارامترهاي هندسي درزه لازم است تاثير عواملي همانند جابجايي برشي و تنش نرمال نيز بررسي گردد. در اين مقاله با روش عددي المان مجزاء يك كد عددي توسط نرم افزار UDEC براي بررسي كوپل هيدرومكانيكي درزه هاي منفرد تحت برش با مدل درزه داري بارتن- بانديس توسعه داده شده است. در كد عددي توسعه داده شده تاثير پارامترهايي همانند جابجايي برشي، تنش نرمال و سختي درزه بررسي شده است. براي راستي آزمايي مدل عددي نتايج آن با نتايج تجربي بدست آمده توسط السون- بارتن (2001) مقايسه شده است. بررسي هاي انجام شده، نشان ميدهد كه نتايج مدلسازي عددي هماهنگي خوبي با نتايج آزمايشگاهي السون ـ بارتن (2001) دارد. همچنين تحليل هاي عددي انجام شده حاكي از آنست كه متناسب با افزايش جابجايي برشي، مقدار بازشدگي هيدروليكي و نرخ جريان افزايش مييابد.

Summary This papers focuses on the permeability of rock joint under direct shear tests and a comprehensive study was made to numerically evaluate the hydro-mechanical behavior of rock joints. Introduction Rock mass permeability is a key parameter in rock engineering projects such as repositories for radioactive waste, dam foundations, excavation of tunnels and caverns, geothermal energy plants, oil and gas production, etc. Due to the stiffer rock matrix, most parts of rock mass permeability are related to the joints and discontinuities. Further, shear and normal stress, shear and normal deformation, joint roughness and etc. affect rock joint permeability. Therefore interaction between stress and permeability is a crucial factor during different stages of reservoir’s life such as: assessment, productivity and management. Methodology and Approaches In the present paper, it is intended to consider the permeability of rock discontinuity in direct shear test. For this purpose, a distinct element code is used to develop numerical modeling of the direct shear test. UDEC has the capability to perform the analysis of fluid flow through the fractures and voids of a system of impermeable blocks. Steady-state pore pressures can be assigned to zones within deformable blocks and boundary conditions may be applied in terms of fluid pressures or by defining an impervious boundary. The fluid-flow calculation can also be run either coupled or uncoupled with the mechanical stress calculation. A fully coupled mechanical-hydraulic analysis is performed in which fracture conductivity is dependent on deformation, and conversely, joint fluid pressures affect the mechanical computations. Two main experimental stress conditions under which the shear force of rock joints can be determined, are the Constant Normal Load condition, CNL, and the Constant Normal Stiffness condition, CNS. The numerical model was calibrated with experimental tests and it was tried to confirm the validity of the developed model. Finally, a sensitivity analysis was made to investigate the effect of mechanical rock joint parameters and properties of fluid on the flow rate of discontinuities. Results and Conclusions Variation of shear stress and rock joint transmissivity with shear displacement compared in the both numerical model and experimental test and it is shown that the results of numerical modeling have a good agreement with results of experimental models from literature. Numerical analyses revealed that with increasing the shear displacement, increases hydraulic aperture and flow rate of discontinuities. Also it is revealed that joint roughness coefficient has a direct relation with flow rate of discontinuity.