تحلیل پایداری شیروانی‌های خاک برداری شده غیراشباع در اثر نفوذ آب باران در مسیر احداث کانال‌های آبیاری-مطالعه موردی کانال اصلی خداآفرین

ناپایداری شیروانی‌های غیراشباع در اثر بارندگی، یکی از مهم‌ترین رویدادهای طبیعی است که معمولاً به هنگام بارندگی یا پس از آن روی می دهد. بسیاری از تپه های خاکی و دامنه های کوه‌ برای احداث زیرساخت‌ها و پروژه‌های بزرگ از جمله احداث کانال‌های آبیاری خاک برداری می‌شوند. در این تحقیق تاثیر نفوذ آب باران بر پایداری یکی از بحرانی‌ترین شیروانی های غیراشباع ایجاد شده برای احداث کانال اصلی شبکۀ آبیاری و زهکشی خدآآفرین در محدودۀ دشت مغان بررسی شد. مشخصات فیزیکی و مکانیکی مصالح تشکیل دهندۀ شیروانی، ضمن حفاری و اخذ نمونه های دست خورده و دست نخورده، با آزمایش‌های درجا و آزمایشگاهی تعیین گردید. برای ارزیابی تاثیر بارندگی در پایداری شیروانی‌، از مقادیر توزیع ماهانه بارش منطقه مغان استفاده شد. برای به‌دست آوردنپارامترهای منحنی مشخصه رطوبتی، از نرم افزار retcاستفاده گردید.پس از تهیۀ مدل، اثر نفوذ آب باران بر فشار آب منفذی، درجۀ اشباع خاک، تغییر شکل و تغییرات کرنش برشی در مدت بارندگی تحلیل شد. برای ارزیابی پایداری شیروانی نیز  از روش کاهش مقاومت برشی استفاده شد. نتایج بررسی ها نشان می دهد نفوذ آب باران در شیروانی های غیراشباع ایجاد شده در خاک های ریزدانه با نفوذپذیری کم، در کوتاه مدت تاثیر چندانی ندارد ولی با گذشت زمان و طولانی تر شدن مدت زمان بارندگی، ناپایداری شروع می شود و پس از حدود 80 روز از بارش های اعمال شده ضریب اطمینان به کمتر از یک می رسد و شیروانی به طور کامل گسیخته می شود. بنابراین می توان نتیجه گرفت که پایداری شیروانی های از جنس ریزدانه در برابر بارندگی ها، باید برای بارش های با مدت زمان طولانی بررسی شود.

Stability Analysis of Unsaturated Cut Slopes Due to Rainfall Infiltration(Case StudyKhoda Afarin Main Irrigation Canal)

IntroductionThe stability of unsaturated slopes during rainfall is an important natural hazard. Past studies have shown that slope failures generally occur during or after rainfall. A review of past studies shows that instability of natural or excavated slopes for infrastructure construction occurs mainly during or after rainfall. Due to rainfall infiltration into unsaturated soil, the apparent cohesion and shear strength provided by matrix suction reduces d, resulting in slope instability. Library studies for this research, revealed that in recent years, extensive studies have been conducted on the rainfall infiltration in unsaturated slopes and their effect on shear strength and stability. But much of the research has been on highintensity rainfall over a short period of time, however, some instability of slopes occurs during longterm rainfall or after rainfall. In addition to field research and physical modeling, numerical methods have also been developed to solve complex problems of unsaturated soils within the framework of various scientific theories in recent years. In this paper, in addition to considering the results of previous studies, the stability of the excavated soils with multiple berms has been investigated by considering transient fluidmechanical coupled analysis.Khodafarinn Irrigation network is one of the largest water resources development projects in northwestern Iran. The length of its main canal is 144 km. According to the topography of the project area, excavations were required to construct the main canal and its structures. The stability of the slopes created by excavation is of great importance for canal construction and operation. One of the critical slopes created along the main canal is in the trench created at the site of the Golmali cut and cover which was selected as a case study. MethodologyFLAC 2D is one of the most applicable geotechnical software based on the FDM and complex LAC 2D is one of the most applicable geotechnical software based on the FDM and complex geotechnical problems can be modeled by using FISH. The general frameworkof the study is that geometrical characteristics and materials parameters of the slope were first extracted using in situ and laboratory tests and then the average monthly rainfall was obtained from the meteorological station statistics. Then, transient fluidmechanical coupled analysis was prepared for rainfall infiltration and deformation analysis using TPFlow. As a result of the analysis, pore water pressure, degree of saturation and deformation were calculated and according to results, the stability of slope was evaluated by using the shear strength reduction method.Filter paper test and RETC code were used to extract the SWRC and HCF of unsaturated soil parameters by VanGenuchten model. Table.1 shows the parameters used in the analysis. Table 1Parameters of slope material for numerical analysisγsatGCKsϕθsθrαnKN/m3MPaPam/sDeg.19.110083401.42E0627.40.4530.051.2821.585 Based on the available meteorological data, the average annual rainfall and monthly values of rainfall were extracted for the study area. In order to reach initial conditions in terms of water content and degree of saturation, average annual rainfall (288 mm) was applied to the slope for six months. Then, the most critical time limit for performing the task in terms of rainfall was selected from monthly values of rainfall for the 4 critical months (MarchJune) Fig. 1. Results and DiscussionFig. 1 shows the changes in the factor of safety during the four months of rainfall. According to the results, the FOS was 1.72 at the end of the initial condition and after applying fourmonth rainfalls, it was reduced to less than 1 and as result failure occurs. By finding the trend of its changes, it can be concluded that, when the FOS was reached less than the unit after about 80 days and the slope became unstable. In other words, with the passage of time and the increased rainfall infiltration at higher depths, after about 80 days, the complete slope failure started from the slope claw.  Figure 16 Factor of safety and rainfall history for the 4 month Conclutions1)                  Due to very low permeability and high suction at a low saturation ratio, the behavior of unsaturated excavated slopes in finegrained soil is relatively complex compared to coarsegrained soil slopes.2)                  In finegrained soil slopes, pore water pressure and degree of saturation increase nonlinearly with increasing rainfall duration. However, the increase rate and the trend of increase  was different from studies for highintensity short duration rainfalls, The increase was faster at the slope surface but with increasing depth, water infiltration rate and consequently changes in pore water pressure will take longer time.3)                  The factor of safety decreases with the time of rainfall and after about 80 days, the instability of the slope begins and the reason for its prolongation can be attributed to the low permeability of the slope material. Therefore, for relatively low permeability soil slopes such as those in finegrained soils, longduration rainfall is more effective than highintensity but lowduration rainfall.4)                  According to the results of previous studies conducted on highintensity rainfall on slope stability in finegrained slopes, high intensity and low duration rainfalls cannot infiltrate due to low permeability of materials and moves in the form of runoff at the slope surface and their stability should be examined for moderate to longduration rainfall if necessary.