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INTERNATIONAL SOCIETY FOR SOIL MECHANICS AND GEOTECHNICAL ENGINEERING This paper was downloaded from the Online Library of the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE). The library is available here: https://www.issmge.org/publications/online-library This is an open-access database that archives thousands of papers published under the Auspices of the ISSMGE and maintained by the Innovation and Development Committee of ISSMGE. Neotectonics related landslide of Ambal salt piercement within the Upper Gotvand dam reservoir, Iran Arash Barjasteh, Ph.D, Noorali Damough, Mohammad Reza Hataf, Mehdi Badiefar Dam and Power Plant Development, Khuzestan Water and Power Authority (KWPA),Iran, [email protected] ABSTRACT: The Upper Gotvand dam is the highest rock-fill dam of Iran with a clay core and was constructed over the longest and biggest river of the country, Karun river, in Khuzestan Province. It is located in the Zagros Active Fold Belt with the unique exposure of salt piercement in its reservoir known as Ambal salt pillow or ridge. The occurrence of this salt piercement imposed some problems to the project including subsidence and land sliding of highly soluble layers within the dam reservoir. The piercement is composed of Gachsaran Formation of Oilgo-Miocene Age with dominant salt, gypsum and marly layers. Most of the formation exposures in the study area are fault related. The region is affected by neotectonic faulting in such a way that Ambal piercement is aligned with an N-S basement lineament. Based on a two year field observation and monitoring, a gradual and continuous sliding is occurring that is intensified by ground water circulation through evaporite karstic sinkholes and fracture systems. Subsidence and sliding of the Gachsaran evaporitic layers into the dam reservoir water increases the river salinity continuously and is considered as a critical geohazard for downstream areas. RÉSUMÉ: Le barrage Gotvand supérieur est le plus haut barrage en enrochements de l'Iran avec un noyau d'argile et a été construit sur le plus long et le plus grand fleuve du pays, Karun, dans la province du Khuzestan. Il est situé sur la ceinture de pli actif de Zagros avec l'exposition unique d’affleurement de sel dans son réservoir connu sous le nom du matelas Salbal Ambal ou de la crête. L'apparition de cet affleurement de sel a posé quelques problèmes au projet, notamment l'affaissement et le glissement de terrains de formations très solubles dans le réservoir du barrage. L’affleurement est composé de formations de Gachsaran de l'âge Oilgo-Miocene avec le sel dominant, le gypse et des couches marneuses. La plupart des expositions de formation dans la zone d'étude sont assujetties aux failles. La région est affectée par une faille néotectonique de telle sorte que l’affleurement d'Ambal soit aligné avec un alignement de sous-sol N-S. Sur la base d'une observation et d'un suivi sur deux ans, un glissement graduel et continu se produit, qui est intensifié par la circulation de l'eau souterraine à travers des puits karstiques d'évaporite et des systèmes de fracture. L'affaissement et le glissement des couches évaporitiques de Gachsaran dans l'eau du réservoir du barrage augmentent la salinité du fleuve en aval du barrage et sont considérés comme un risque géologique critique pour les zones avales. KEYWORDS: Neotectonics, landslide, Ambal piercement, Upper Gotvand dam. 1 INTRODUCTION projects and industrial factories along its about 380 km length downstream the reservoir of the Upper Gotvand Dam. Scientific advances lead to periodical revisions of the influence of neotectonic activity on engineering structures (Marín- Córdova et al. 2004). From a geological point of view, it is expected that the neotectonics associated with the Zagros Active Fold Belt (Berberian 1995) could affect such structures built in this region (Barjasteh 2012, Maleki and Behnam 2005, Tatar 2012). The activity of faults and related salt tectonics have affected enigeering structures in the Zagros fold belt including the Upper Gotvand Dam (Fig.1) in Khuzestan Province of southwest Iran (Barjasteh 2012, 2013). As a result, the dam and its reservoir have become important sources of possible future geohazard. Particularly, the safety of the dam reservoir becomes an important question because disasters due to existence of Ambal salt piercement could unfavorably damage high fertile downstream farms and agricultural land. Salt, because of its mechanical weakness and negligible yield strength, readily deforms by viscous flow. Salt bodies can be extremely mobile and flow laterally and vertically towards areas of lower load, both in the subsurface and at the surface. Most of the significant sub-aerial salt exposures are located in the arid–semiarid Zagros Mountains, Iran (Kent 1979, Bosak et al. 1998). Here, the role of neotectinics on subsicdence and landslide of Ambal piercement on the left bank of the Upper Gotvand Dam reservoir is presented and discussed based on a two year field observation and monitoring. Figure 1. Geographical location of the Upper Gotvand Dam (oval) and The Karun River is the most important stream in the country Ambal Ridge in Karun River basin (after Aghdam et al. 2012). originateng and recharging from high Zagros mountain Range. The River provides water for many cities, villages, agricultural - 1473 - Proceedings of the 19th International Conference on Soil Mechanics and Geotechnical Engineering, Seoul 2017 The Gachsaran Formation (Oligo-Miocene) occupying parts Salt piercement in the Ambal area consists of inter bedded salt, of the dam area consists of anhydrite, gypsum associated with anhydrite, marl and gypsum (Gutiérrez and Lizaga 2016, salt with total thickness of around 1,600 m in the area (Aghdam Aghdam et al. 2012). The bedding of the Gachsaran is steepest et al. 2012, Hassanvand et al. 2011).The more common facies directly above the Karun River where it dips east into the slope of the Gachsaran Formation consists of gypsum intercalated of about 50°. The margins of the Gachsaran overlap the Mishan with red and grey marls and fossiliferous limestones. It is a Formation on the eastern side of the Ambal section (Fig.2). tectonically incompetent unit and highly subjected to dissolution effects, being characterized by high mobility and Morphotectonic analysis based on satellite imagery and remote being subjected to differential pressures. Due to these sensing techniques (Ahmadi et al. 2012) has brought to light a characteristics, a complete stratigraphic sequence, as described probable N-S orientated tectonic lineament introduced as Lali- at the key outcrop (Stocklin and Setudehnia 1977) is quite Ahvaz lineament that cuts across Gachsaran Formation on the uncommon. Ambal section (Fig.2). The Karun River apparently follows length of this fault directly to the SW of the Ambal plug. The Salt tectonics indicates that salt may move into relatively fracture system pattern is fairly aligned with the thrust fault low pressure areas around developing folds and faults and trends (Barjasteh 1994, Haji Poor et al. 2010). The pattern of produce significant thicknesses of rock salt within a sequence of joint systems in the region is also exhibiting such trends. rocks. This is due both to the low density of salt, which does Additionally, widespread fracture system in the region and their not increase with burial, and its low strength (Mehrgini et al. continuation and extension towards south are observed which 2016). The Ambal ridge area seems to be an active salt structure are accompanied by land subsidence and sliding on the left (Barjasteh 2012) that tectonic pressures caused concentration of bank of the reservoir (Fig.3). salt units in the area. The salt layers ascend continuously and dissolution takes place in the same time. 1 .1 Dam reservoir geology The reservoir area is dominated by Lali Anticline which strikes NW-SE. The Ambal salt ridge is located on the Lali Anticline and is occupied by Gachsaran Formation outcrop (Fig. 3). This Formation was named after the Gachsaran Oilfield in Khuzestan Province, where it is best developed (James and Wynd 1965). It was divided into seven members based on a type section composed of intervals from the well logs in the mentioned oil field (Stocklin and Setudehnia 1977, James and Wynd 1965). Members two, three, four and six have extensive salt units. The non-salt part of Gachsaran Formation is divided Figure 3. Progressive subsidence of the sinkholes (on the top) and into three members that are seen in the west and the south sliding of the roads (in the bottom) on the left bank of the dam reservoir. central part of Iran. The tectonically incompetent units of the The photos (left to right) were taken in: March 2013, October 2014 and November 2016, respectively. formation are highly subjected to dissolution effects, being characterized by extreme mobility and being responsive to 1 .2 Sinkholes and landslides geometry differential stresses. Accordingly, one seldom finds a complete sequence as described at the type locality (Stocklin and Ambal salt ridge has a high density of sinkholes with a total of Setudehnia 1977). 693 sinkholes. They cover 6.7% of the salt dome area and with a density of 170 sinkholes/km² (Gutiérrez and Lizaga 2016). Collapse sinkholes in their initial stages are steep-walled hollows with sharp edges and cylindrical geometry. The alignment of them is mostly parallel to the existing faults. Especially in the southern part of the Ambal ridge (Fig.2), they are aligned with an E-W fault that is suggested to be a continuation of a fault trace that passes through the northern side of the right abutment of the dam. This trend is supported by a similar trending valley named as Shur valley and inverted bedding. Along the western margin of the Ambal plug, the sink holes are parallel to the Lali-Ahvaz Lineament.