Slope stability modelling and landslide hazard zonation at the Seymareh dam and power plant project, west of Iran
- First Online:
- 1.8k Downloads
The Zagros Mountains Range is an important structural unit in south western Iran and accommodates a significant portion of the 2.5 cm/year convergence between Arabia and Eurasia. This structural unit includes folds, thrusts, strike-slip faults and salt diapirs. There is evidence of past failures in the area, probably some 11,000 years ago. In the order of 30 million cubic metres of rock debris was moved from the Kabir Kuh Anticline as well as rock failures from the Ravandi Anticline. Investigations indicated 4.5 m of landslide/rock fall debris underlying about 28 m of lake deposits and 5.5 m of recent river alluvium upstream of the Seymareh Dam area. The direction of the Seymareh River bed was displaced by about 1,000 m to the northeast, forming a sharp river meander near the entrance to the gorge of the Ravandi Anticline where the Seymareh Dam is being constructed. The paper reports the rock slope instability modelling, kinematic analysis of slope faces and landslide hazard zoning undertaken.
KeywordsSeymareh dam Rock slope Kinematic analysis Instability modelling Landslide hazard zonation
La chaîne du Zagros est une unité structurale importante du sud-ouest de l’Iran. Elle enregistre une partie importante des 2,5 cm/an de convergence entre l’Arabie et l’Eurasie. Cette unité structurale présente des plis, des chevauchements, des failles de décrochement et des diapirs de sel. Il existe des preuves de jeux de failles dans la région, il y a 11 000 ans probablement. Environ 30 millions de mètres cubes de matériaux rocheux se sont détachés de l’anticlinal de Kabir Kuh, de même que des glissements rocheux ont eu lieu à partir de l’anticlinal de Ravandi. Des reconnaissances de terrain ont montré que 4,5 m de débris issus de glissements de terrain et de chutes de blocs se trouvaient sous environ 28 m de dépôts lacustres et 5,5 m d’alluvions récentes en amont de la zone du barrage de Seymareh. Le lit de la rivière Seymareh a été déplacé d’environ 1000 m vers le nord-est, formant un méandre serré à l’entrée de la gorge de l’anticlinal de Ravandi où le barrage de Seymareh est en train d’être construit. L’article présente la modélisation des instabilités des pentes rocheuses, l’analyse des conditions de stabilité des versants et le zonage des risques de glissements de terrain.
Mots clésBarrage de Seymareh Pente rocheuse Analyse cinématique Modélisation Zonage des risques de glissement
The river valley is U shaped with relatively steep slopes which overhang in some places. The slopes are nearly vertical up to 760 m, above which the angle decreases to 25° where weak marlstone interbeds are present. In the site area, the river is between 35 and 40 m wide. The main lithology at the dam foundation is the Asmari Formation (limestones). The limestones are typically strong, supporting the high near-vertical slopes.
Due to this localised mass movement, the direction of the Seymareh River was changed locally from northwest–southeast to northeast–southwest adjacent to the entrance gorge of the Ravandi Anticline (Fig. 3b, zone B). Stratigraphic evidence from the exploratory boreholes indicates that this event most probably occurred contemporaneously with the Kabir Kuh anticline about 10 km south west of the Seymareh Dam. In addition field investigations indicate some slope instabilities on the left bank towards the future reservoir area (zone A) and also at two abutments in the downstream area (zone C).
Upper Asmari (As.3): 150 m, medium to thinly bedded, crystalline, bioclastic limestone and marly limestone.
Middle Asmari (As.2): 238 m, massive to thickly bedded and karstified microcrystalline, dolomitic limestone and marly limestone.
Lower Asmari (As.1): 188 m, medium to thickly bedded fossiliferous marly limestone and microcrystalline limestone.
Most rock instabilities are related to the middle and upper parts of the Asmari Formation.
Joint study and rock slope kinematic analysis
For the kinematic analysis, the lower hemisphere stereographical projection method, described by Hoek and Bray (1981), Goodman (1989) and Maurenbrecher and Hack (2007) was used. Planar and wedge failure modes were kinematically studied. Planar failure occurs if the discontinuity plane daylights into the slope face and the difference between the strike of the discontinuity plane and that of the slope face is 20° or less. Wedge failure occurs if the intersection of the main discontinuities is located in the area between the slope face and the great circles representing the angle of internal friction (Φ). The toppling failure is based on a two dimensional relationship (90-δ + Φ < α where δ = dip of discontinuity, Φ = friction angle of the discontinuity and α = slope angle.
Considering the anticline axis (azimuth 281) in addition to the field stress directions (derived from all axial planar orientation) joint sets Js.1 and Js.2, with azimuths parallel and perpendicular to the anticline axis, respectively, can be assumed to be extensional joints while Js.3 may be classified as a shear joint.
Major discontinuity sets
Figure 9 shows the kinematic analysis and geometrical characteristics of slope instabilities at the northern flank of the Ravandi Anticline, just upstream and downstream of the dam. The arrows represent the direction of rock failures.
the northern flank of the Ravandi Anticline is potentially unstable, particularly for planar failure through the bedding planes (Js.4) toward the northeast (green arrow),
the downstream area of the right bank has the potential for wedge failure due to the intersection of Js.1 and Js.3 toward the southeast (red arrow),
the downstream left bank is most likely to experience toppling instability due to Js.3, toward the northwest (blue arrow).
Slope stability analysis
Zone A (left bank, upstream area)
One of the important geotechnical problems is related to the stability of detached blocks from the Upper Asmari unit on the northern flank of the Ravandi Anticline. At an elevation of 620–800 m, there is a 250 m × 300 m area (Figs. 3, 4) which is covered by unstable blocks caused by the intersection of Js.1, Js.2 and the bedding planes on the left bank (about 200 m east of the dam axis). Based on stereographic projection of the joint sets (Fig. 8), the bedding planes will be the main rock sliding surface for most rock blocks, if instabilities occur (planar failure). The thickness of the unstable zone is estimated to be 20–25 m.
Zone B (right bank, upstream area (zone B)
Similar blocks can be observed on the right bank on the northern flank of the Ravandi Anticline. The slopes cut during access road construction induced rock mass instability (Fig. 4). Slope stabilization was carried out in some places, such as at diversion tunnels and hydropower tunnel headwalls, but according to the field investigations most of the road cuts are either not supported or insufficiently supported. In addition, the increase of water pore pressure after impounding of the reservoir or heavy rainfall may reduce the shear strength of discontinuities, especially on bedding planes, and hence facilitate rock sliding.
The evaporites of the Gachsaran Formation imply rotational sliding could take place, due to their high plasticity and flexibility (marl, gypsum and salt).
The cluster of all possible axis points above the slope can be seen in Fig. 15. For a non-circular slip analysis, these points are automatically generated by SLIDE©, and are the axis points used for moment equilibrium calculations. Block search is used to randomly generate the locations of the slip surface, such that the lower safety factor surface can be determined. As seen in Fig. 15, a F.S. of 1.2 was generated for the Gachsaran Formation evaporates after reservoir impoundment; the radius of the slip surface being 323 m. Figure 16 indicates an increase in pore pressure when the reservoir is impounded could reduce the shear strength along the slip surface.
Zone C (downstream area)
Landslide hazard zonation (LHZ)
Typical values of LHZ parameters (Gupta and Anbalagan 1995)
(a) Kind of rock
Quartzite and Limestone
Granite and Gabbro
Ferrous sedimentary rocks, well cemented, Sandstone with thin interbeds of clay stone
Ferrous sedimentary rocks, weakly cemented, Sandstone with thin interbeds of shale and clay
Slate and phyllite
Shale with interbeds of clay stone or non clay materials
Shale, phyllite and schist, highly weathered
(b) Kind of soil
Old alluvium deposits, well cemented
Clayey soils with alluvium
Sandy soils with alluvium
Rock debris with sandy and clayey soils (slope wash);
Old and well cemented
Young loose materials
(a) Parallelism between joints and slope face
Planar (αj- αs)
Wedge (αi- αs)
(b) Joint dip in planar mode of
Failure (βj- βs)
Wedge (βi- βs)
IV. 0 to (−10)
(c) Discontinuity dip
(a) Escarpments, cliff
(b) High angle
(c) Moderate angle
(d) Low angle
(e) Very low angle
Land use and land cover:
(c) Thick vegetated land cover
(d) Moderately vegetated
(e) Sparsely vegetated
(f) Barren land
(g) Soil cover depth
The maximum rating of LHEF landslides (Gupta and Anbalagan 1995)
Max. rate (LHEF)
Land use and cover
relief relative to the free face,
land use and land cover,
ground water condition.
The classification of landslide hazard zonation (LHZ) according to Gupta and Anbalagan (1995)
Very low hazard
Local zones, susceptible to instability
Very high hazard
Total estimated hazard zoning at Seymareh dam site according to the Gupta and Anbalagan method
Zone A, B (planar failure)
Zone C-right bank (wedge failure)
Zone C-left bank (toppling failure)
Gypsum and Marl
0.2 × 3
0.2 × 3
0.2 × 3
0.5 × 0.7 × 0.3
0.2 × 0.7 × 0.5
0.3 × 0.8 × 0.5
Land use and land cover
Total estimated hazard (TEHR)
Value (from 10)
Description of zone
Low to moderate hazard (LHZ–MHZ)
Moderate to high hazard (MHZ–HHZ)
Low to moderate hazard (LHZ–MHZ)
Low to moderate hazard (LHZ–MHZ)
The Asmari Formation limestones and Gachsaran Formation evaporites constitute the main rock foundations for the Seymareh Dam project. The Upper Asmari limestone with intercalations of marls and the Gachsaran Formation composed of gypsum, marl and salt, are susceptible to water absorption and dissolution which may result in a reduction in the shear strength of the rock mass when the reservoir is impounded and/or as a consequence of heavy rainfall.
The study has shown that the failure surface in the Asmari Formation limestone is likely to be planar towards the reservoir with wedge/toppling in the downstream area, while the flexibility of the Gachsaran evaporites implies failure will be rotational or along a composite surface.
The unstable areas adjacent to the dam site and through the reservoir area are composed of old mass failures, high angled rock slopes and crushed zones in the limestones. However, although the limestones with intercalations of marl and clay dip toward the reservoir, the Gachsaran Formation evaporites present the greatest risk of slope failure following impoundment of the reservoir.
In addition, the previous failures in zone B indicate the potential for landsliding at the northern flank of the Ravandi Anticline where the inlet of diversion and hydropower tunnel portals are situated and where access roads have been created, while more recently wedge and toppling failures downstream (zone C) also give cause for concern.
This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.