**2. General geology and seismicity**

instability [1]. Water seepage and loss through dam foundation and abutments, especially those constructed in karstic regions lead to considerable costs, sometimes postpones dam construction targets. Although this is of less importance regarding dam instability, current experience shows that there could be a fair relationship between instability and water seepage, where fault and fracture systems are the main conduits for water flow. Thus, the study of geological relationships is of major importance, with main emphasis on the characteristics of the structural features and recent tectonic movements [2]. The structural geology has a great deal in contributing to the engineering projects, and knowledge of structural geology setting is essential for safe design of these projects [3]. A detailed study with focus on the analysis of fault and fracture systems, their activities, movement types, and position ought to be done. The evaluation of

74 Dam Engineering

stress relationships and possible temporal and spatial deformations are also necessary.

On a regional scale, structural relationships have an important role in forming the hydrogeological properties in karst regions. Although the possibility for karstification perpendicular to the geologic structures is considerably reduced, where an anticlinal structure exists between the reservoir and the lower erosion base level [4], in young orogens such as the Zagros belt, where transverse and antecedent drainage pattern is of particular importance [5], reservoir water tightness is highly risky due to dissolution fractures and conduits. One of the structural controls in the development of drainage in modern orogens is fault and fracture systems,

**Figure 1.** Locations of Karun-1 dam (1), Karun-3 dam (2), Marun dam (3), and Gotvand dam (4) considered for this study.

The Zagros fold-thrust belt (briefly named as Zagros mountains) being located to the northeast of the Persian Gulf extends for about 1800 km between northern Iraq and the Strait of Hormuz and comprises the deformed part of the Arabian plate following its continental collision with central Iran almost since the Miocene [9]. It is a branch of the Alpine-Himalayan Orogenic belt [10, 11]. The mountain range can be divided into two distinct zones say, the northeastern High Zagros (**Figure 2**) and the southwestern Simply Folded Belt (SFB) based on topography, geomorphology, exposed stratigraphy, and seismicity [12]. The Simply Folded Belt extends from the High Zagros Fault to the Persian Gulf [13]. Topographic highs are typical of the anticlines and synclines form the topographic lows. The existing landforms graphically reveal the geologic structure. Anticlines show remarkable regularity in relief over long distances and the rivers which cut them usually do so at sharp angles to the strike of the anticlines. The SFB is a fold belt characterized by NW-SE trending parallel anticlines and synclines and is composed of elongated whaleback or box-shaped anticlinal mountains. From a geomorphological view point, the anticlines can be divided into two sets; plain anticlines and mountain anticlines [12].

**Figure 2.** Seismotectonic features of the study area in the Khuzestan Province [14].

In fact, most part of the study area, that is, Khuzestan Province and the aforementioned dams are almost located in the Dezful Embayment, which is a structural unit of the SFB.

Some studies were later done regarding the construction of a new second powerhouse in the

Influence of Geological Structure on Dam Behavior and Case Studies

http://dx.doi.org/10.5772/intechopen.78742

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Karun-1 dam site is located on the southwestern limb of Kamarun anticline (**Figure 4**) with average bedding dip of 35° toward SW. The anticline is composed of Asmari Limestone of Olig-Miocene age. Asmari formation limestone is a suitable rock foundation for dams regarding its relatively exclusive characteristics such as rigidity and morphology. This formation constitutes a series of double plunging, asymmetrical folds with northwest-southeast trend having steeper southern flanks than the northern ones. The Asmari limestone forms the entire foundation of the Karun-1 dam. It is divided into three parts at the dam site namely, lower, middle, and upper Asmari. The dam is situated on low-karstified middle Asmari that consists of a relatively permeable zone, which in turn is overlain by an impervious shale layer. The upper Asmari limestone exposed just downstream the dam, is highly karstified [16, 18]. The anticline shows some axial plane rotation along its southeast plunge, however, its general

The region is seismotectonically very active regarding its location in the Zagros active foldthrust belt [14]. The main faults in the region are of thrust types of which Izeh fault zone cutting across east of the region is a very known feature due to its right lateral component of movement. The Andeka Fault is the main active fault close to the dam site that is characterized by very recent activity [24, 25]. Although no major fault exists at the dam site, recent investigations for excavation of a new powerhouse identified a fault with a general NW-SE trend [20], accompanied by high fracture density. This is most probably a hidden blind fault that is expected to cut the anticline core. The geologic structure of the dam site includes bedding, joint sets, and a spectacular shear zone in the right abutment. Recent study indicated a tectonic lineament with a NE-SW trend that passes through the right abutment, which could

left abutment of the dam [19–21] and recently, rock fall on the right abutment [22, 23].

trend is northwest-southeast as is common in the whole belt.

**Figure 4.** 1/100000 Geological map of the Karun-1 dam region [23].

Iran can be generally divided into five major seismotectonic zones (**Figure 2**) that are subjected to destructive earthquakes excluding one (Central Iran characterized by low seismicity). It is considered as a broad seismic zone over 1000 km width that extends from the Turanian platform (southern Eurasia) in the northeast to the Arabian plate in the southwest. The Iranian plateau is characterized by active faulting, recent volcanic activity, and high density of active and recent faults. Reverse faulting dominates the tectonic mechanism of the region. Southwest of Iran, where the studied large dams are located, belongs to the Zagros Active Fold Belt from the seismotectonic point of view [14]. Seismicity in this belt correlates well with topographic elevations greater than 1.5 km. Fault plane solutions for several earthquakes consistently show high angle (40°–50°) reverse faulting and the estimated depths range from 8 to 13 km with magnitudes that range from 4 to 6. The rate of seismicity in this zone is higher than the others, but the type of seismicity is mostly between small to moderate and seldom large. Due to its particular tectonic condition, the large earthquakes have rarely been accompanied by surface rupture in the SFB. Based on the available fault plane mechanisms of the regional events, the maximum principal stress, which is due to regional tectonic forces, strikes N30°± 5° (NE–SW) [15].
