تأثير بارگذاري ديناميكي بر نفوذپذيري توده سنگ درزه‌دار به روش شبكه‌ي درزه‌ي منفصل – روش المان مجزا

Dynamic loading effect on the permeability of fractured rock mass using DFN-DEM approach

بارگذاري ديناميكي پديده‌اي است كه ممكن است در طبيعت به توده سنگ اعمال شده و حتي باعث تغيير در برخي ويژگي‌هاي ژئومكانيكي آن شود. نفوذپذيري يكي از خواص مهم و حياتي توده سنگ در پروژه‌هاي معدني و عمراني است كه ممكن است تحت تأثير بارگذاري‌هاي ديناميكي مذكور، دچار تغيير شود. از طرفي طبيعت توده سنگ شامل شبكه‌ي شكستگي‌هاي مجزا است كه مي‌توان آن را با رويكرد تلفيقي شبكه‌ي درزه‌ي منفصل و روش المان مجزا (DFN-DEM) و نرم‌افزار UDEC مدل كرد. ازاين‌رو در اين مطالعه، مدلسازي هيدرومكانيكي عددي تحت شرايط استاتيكي و ديناميكي به‌منظور مشخص كردن تأثير بارگذاري ديناميكي بر نفوذپذيري انجام گرفته است. پس از بارگذاري‌هاي مذكور، جريان سيال در مدل‌ها شبيه‌سازي شده و مؤلفه‌هاي نفوذپذيري محاسبه و با يكديگر مقايسه شده‌اند. نتايج نشان مي‌دهد كه بر خلاف نظرات منعكس‌شده در مطالعات قبلي، بارگذاري ديناميكي قابليت انتقال درزه‌ها را تغيير داده و به‌تبع آن نفوذپذيري توده سنگ درزه‌دار را تحت تأثير قرار مي‌دهد. با وجود سختي نسبتاً بالاي درزه‌ها در اين مطالعه، نفوذپذيري محاسبه‌شده در شرايط بارگذاري ديناميكي به ميزان 26% بيشتر از بارگذاري استاتيكي است. علت عمده‌ي اين مسئله جابه‌جايي پي‌درپي بلوك‌ها طي بارگذاري ديناميكي و تا حدود اندكي تغيير در نحوه‌ي قرارگيري آنها نسبت به حالت قبل از عبور موج است.

SummaryThis study has been performed with the aim of investigation of dynamic loading effect on permeability of rock mass. The method is 2D numerical modeling that because of discontinuous nature of rock mass and also existence of fracture networks in it, the modeling has been carried out by Discrete Fracture Network-Discrete Element Method (DFN-DEM) conflation approach. Results show that dynamic loading changed the transmissivity of fractures and consequently increased the permeability of fractured rock mass.IntroductionDynamic loading is a phenomenon that may be applied to rock mass in nature and even leads to changes in some of its geo-mechanical properties such as permeability. The variation in the amount of fluid flow from which the predicted value in a sensitive project such as underground power stations, hydrocarbon fluid flow in its reservoirs and repositories of buried of nuclear waste can cause damages and demolitions. Hence, investigation of dynamic loading effects on the permeability of rock mass is important. In previous studies, some research has been accomplished in the field of rock mass hydromechanics and interaction between static stress and fluid flow in rock mass. However, the lack of evaluation of stress form (static or dynamic) on permeability of rock mass has been felt.Methodology and ApproachesIn this study hydro-mechanical numerical modeling has been carried out under static and dynamic stress conditions. All of geometrical and mechanical properties of the models have been for Sellafield site in Cambria, England. As previously mentioned, modeling has been performed by DFN-DEM conflation approach using UDEC. In order to realize results of the method, data from a real earthquake have been utilized as a dynamic boundary conditions. Modeling in this study has been conducted in two groups. Group 1 contains models which have been placed under fluid flow without dynamic loading (in static conditions). In the group 2, the same models have been put under dynamic loading and then under fluid flow conditions.Results and ConclusionsThe results show that in contrary with the previous consideration, at least dynamic loading changes the transmissivity of fractures and therefore violates the permeability of fractured rock masses. Despite the fact that in our case study, fracture stiffness is relatively high, calculated permeability of rock mass is greater by 26% at dynamic loading compared with the static loading condition. The major reason is that dynamic loading has caused successive moving the blocks and possible changes in their positions relative to the previous state.