slope stability analysis using sinmap (case study: havenan’s landslide zone, birjand, iran)

1. introduction generally, landslide studies include process identification, risk analysis and landslide risk prediction (taleby and ezaddoost, 2012). nowadays, there are different methods for landslide simulation such as statistical, descriptive and process-based approaches. however, most studies in iran are based on statistical and descriptive methods (taleby and ezaddoost, 2012). landslide zonation models are mostly based on grid analysis and landslide density per unit area. meanwhile, they need more data layers to obtain more accurate results. however, deterministic models like sinmap has been established based on numerical computations and embed precise physical parameters. the sinmap model has been applied with different applications around the world (acharia, 2003; waver & nowocien, 2003; deb & el-kadi, 2009; terhorst & kreja, 2009; memarian et al., 2013). this study is mainly aimed at landslide risk zonation using sinmap, while specifically analyzes the physical, tectonical and morphological parameters which affect the stability of slopes. 2. study area the havenan village has been located in the southwest of birjand, the north of bagheran mountain with a coordinate of 59° 10’ 12” e and 32° 48’ 11” n. the havenan landslide zone is not uniform. its failure length and width is 1500 m and 500 m, respectively. the height difference between the crest and toe of the failure is 350 m. the main cliff is 20 meters high and the movement extent is higher than 10 m. therefore, slope movement speed is low and already is in suspension condition (saman sadrood consulting engineers co., 2010). 3. materials and methods landslide risk zonation was performed on the land unit 1-8-1, which mainly involves phyllites, schist, and split formations with rocky outcrops and steep slopes. sinmap methodology is based upon the infinite-slope stability model that balances (with edge effects neglected) destabilizing components of gravity against stabilizing components of friction and cohesion on a failure plane parallel to the ground surface. based on the infinite-slope form of the mohr–coulomb failure law as expressed by the ratio of stabilizing forces (shear strength) to destabilizing forces (shear stress) on a failure plane parallel to the surface, the safety factor (sf) calculation in sinmap is: where cr is root cohesion (n m−2), cs is soil cohesion (n m−2), θ is slope angle (°), ρs is wet soil density (kg m−3), ρw is the density of water (kg m−3), g is gravitational acceleration (9.81 m s−2), d is the vertical soil depth (m), dw is the vertical height of the water table within the soil layer (m), and ø is the internal friction angle of the soil (°). θ is the arc tangent of the slope s, expressed as a decimal drop per unit horizontal distance. a variety of resources were used to determine the values of model calibration parameters (saman sadrood consulting engineers co., 2010; memarian & safdari, 2009; ab pooy consulting engineers co., 2008; morgan et al., 1998; nakhjavani, 1977). the estimated values of the parameters cr, cs, h, and ρs were equal to 200 (n.m-2), 2000 (n.m-2), 0/45 (m), and 2000 (kg.m-3). the quantity (t/r)sinθ [m] may be thought of as the length of hillslope (planar, not convergent) required to develop saturation in the critical wet period being considered. this concept may be useful for establishing field estimates of r/t through the field identification of the limits of surface saturation (memarian & safdari, 2009). therefore, in this work, this parameter was determined using field investigations and the havenan’s landslide profile (saman sadrood consulting engineers co., 2010). 4. results and discussion in order to a precise analysis of the havenan landslide zone, it was systematically overlaid with 100 points and they introduced as the landslide point to the sinmap. according to results, 71% of the landslide zone was grouped into the “quasi stable” and “stable” classes. the 24% of the studied zone was classified as the “lower threshold” slope zone and only 3% is grouped into the “upper threshold” zone. therefore, less than 30% of the havenan landslide zone showed a medium to high sensitivity to landslide risk. results showed that only a small area of the landslide zone was placed within the “saturated” region. thus, other external factors may affect the stability of the havenan landslide zone. the havenan region has been impacted by the three series of main faults, i.e. havenan, yusht and mazar. the intersection of these shear zones has created romboedric blocks. rock series has been cracked and transformed by these intersected joints. therefore, the volume and instability of the hillslope has been increased. consequently, the landslide event is expectable. dynamic loads and wash off of the heel’s materials play an important role in slip continuing. furthermore, the transformation of peridotite to serepentinite, brucite and talc increased the volume and instability of the hillslope. the hillslope instability has been amplified by the bagheran mountain tectonic upraise and dynamic stresses (gholami & khatib, 2000). 5. conclusion the sinmap simulation results with field investigations established that less than 30% of the havenan landslide zone has a sensitivity of medium to high to landslide risk. a small part of the landslide area was classified in the “saturated” zone, as well. hence, in addition to topography, hydrology and soil conditions, other external factors must be effective in landslide occurring. these factors are mainly including the placement of the split on the serepentinite and the existence of two series of parallel fractures with the faults havenan and mazar. the hillslope instability has been increased by the bagheran mountain tectonic upraise and cyclical seismic events.