**6. Gotvand dam**

fractures classified as fracture swarms [40] is also recognized here. Fracture swarms are nearly large-scale features, which dissect significant parts of the stratigraphic sequence. In the Khaviz anticline, fracture swarms are represented by faults with displacements of a few meters up to 150 m, and are associated with relative narrow damage zones with locally very high fracture frequency [40]. Three large thrust faults cut the region with a NW-SE general trend. These are named as Behbahan, Arajan, and Tashan faults [14]. A hidden fault cuts the core of Khaviz anticline parallel to Behbahan fault as well. While Karun-3 dam site lies in the northern part of Izeh fault zone, the Marun dam site is located at the southernmost part of it. The dam site is also affected by active faulting as indicated by its seismic activity especially for Tashan Fault. The first and prominent problem encountered in Marun dam site was leakage through a diversion tunnel named as second diversion tunnel during the first impounding. Immediately after impoundment, considerable leakage was observed in the pressure tunnel (**Figure 16**) and efforts to open the stop logs failed. At the same time, an embankment was constructed and subsequent grouting controlled the leakage in the pressure tunnel. Old karst channels along a vuggy zone cut by the second diversion tunnel were reactivated and leakage occurred [39]. The total amount

/s. The unlined second

of water leakage through the left bank of Marun dam was about 10–15 m3

**Figure 16.** A general layout of Marun dam and appurtenant structures.

of up to 7 m3

86 Dam Engineering

mately 4.5 m3

diversion tunnel had a key role in connecting reservoir with karst conduit system.

The embankment was overtopped with increasing water elevation, and considerable leakage

/s occurred from weak zones upstream of the plug. The major flow of approxi-

/s was from two large solution channels and leakage around the concrete plug.

The Gotvand or Upper Gotvand dam as the highest rock fill dam in Iran is located in the north of Khuzestan Province (**Figures 1** and **17**). It was constructed across the Karun River in the north of Shushtar city. It is a 178 m high rock fill dam with central clay core and a crest length of 760 m. The normal reservoir level is situated at 232 m above sea level at which the reservoir reaches a capacity of 4.5 billion m3 and around 90 km in length. The Gotvand dam has the hydropower plant with the highest energy production capacity in the country. The main purpose of the project is the production of (4250 GWh) hydroelectric energy. The impoundment of the reservoir started in July 2011 and by 2014, the reservoir water level had reached 223 m a.s.l. The main problems in the dam site and reservoir area are instability of the abutments [42] and with a minority, probable seepage potential through the foundation and abutments [43]. The dam foundation and part of the right abutment is underlain by Agha-Jari Formation of Mio-Pliocene Age (**Figures 17** and **18**). Agha-Jari lithology consists of gray, calcareous sandstone with gypsum veins and red marlstone and siltstone. Its rocks contain veins of gypsum usually associated with claystone beds. They are naturally soluble and can lead to excessive seepage. The left abutment of the dam is composed of Bakhtyari

**Figure 17.** An upstream (eastward) view of Gotvand dam site and the outcropped formations; Bakhtyari (Bk), Agha-Jari (Aj), and Gachsaran (Gs). DRM is displaced rock mass.

them are composed of Bakhtyari formation. An elongated asymmetrical structure named Gach-e Mun or Gach-e Moh anticline occupies the north side of the dam site in a general NW-SE direction. Its south flank is obscured by the presence of Pir-Ahmad fault [44]. Kuh-e Reshteh is the southern part of the Bakhtyari outcrop on which the dam left abutment is placed. The Lahbari active fault passes along the contact between this outcrop and southern plain (**Figure 18**) and defines the mountain-plain boundary in the Dezful Embayment

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**Figure 19.** A close view of Pir-Ahmad Fault in the northern side of Kuh-e Reshteh anticline.

In fact, it forms the northern boundary of the Dezful Embayment (foreland basin of the Neogene molasse of the Agha-Jari-Bakhtyari formations). The river bed is occupied by a small tight anticline composed of Agha-Jari layers with an E-W trend. The anticlinal fold axis is parallel to a fault having the same E-W trend. Agha-Jari formation that composed the dam foundation contains three main joint sets. One set is parallel to the bedding plane. Severe change of joint dips is most probably due to tight folding of the layers and faulting along the river course. The joint surfaces are mostly smooth, polished, and slicken-sided with varying

As mentioned above, one of the challenges at Gotvand dam is instability of the dam abutments [42] and its reservoir banks [44]. The dam abutments include considerable volumes of dislocated rock mass of Bakhtyari formation. This is more critical in the right abutment as the rock mass is highly disturbed and deformed (DRM) and apparently illustrates a rock topple (**Figures 17** and **20**). On the other hand, the rock mass in the left abutment shows evidence of a rock slide. The DRM area is specified by extensive development of joints and cracks and its

structural unit [14].

dip angles.

Conglomerates of Pliocene Age that displays almost horizontal bedding (**Figure 17**). The Bakhtyari Formation is wholly composed of terrigenous, clastic sediments ranging from silt to conglomeratic boulders. Fractures within this formation are usually vertical and have relatively large openings up to several meters. It is also composed of part of the right abutment as a dislocated and ruptured block (DRM). Along the northern margin of the river, Gachsaran layers are thrust over the Agha-Jari layers by Pir-Ahmad fault (**Figures 18** and **19**). The dam site area is dominated by an anticlinal structure comprising Kuh-e Reshteh and Kuh-e Charkhineha in the north and south of Karun River, respectively [44]. Both of

**Figure 18.** Geological section across the Gotvand dam region.

**Figure 19.** A close view of Pir-Ahmad Fault in the northern side of Kuh-e Reshteh anticline.

Conglomerates of Pliocene Age that displays almost horizontal bedding (**Figure 17**). The Bakhtyari Formation is wholly composed of terrigenous, clastic sediments ranging from silt to conglomeratic boulders. Fractures within this formation are usually vertical and have relatively large openings up to several meters. It is also composed of part of the right abutment as a dislocated and ruptured block (DRM). Along the northern margin of the river, Gachsaran layers are thrust over the Agha-Jari layers by Pir-Ahmad fault (**Figures 18** and **19**). The dam site area is dominated by an anticlinal structure comprising Kuh-e Reshteh and Kuh-e Charkhineha in the north and south of Karun River, respectively [44]. Both of

**Figure 17.** An upstream (eastward) view of Gotvand dam site and the outcropped formations; Bakhtyari (Bk), Agha-Jari

**Figure 18.** Geological section across the Gotvand dam region.

(Aj), and Gachsaran (Gs). DRM is displaced rock mass.

88 Dam Engineering

them are composed of Bakhtyari formation. An elongated asymmetrical structure named Gach-e Mun or Gach-e Moh anticline occupies the north side of the dam site in a general NW-SE direction. Its south flank is obscured by the presence of Pir-Ahmad fault [44]. Kuh-e Reshteh is the southern part of the Bakhtyari outcrop on which the dam left abutment is placed. The Lahbari active fault passes along the contact between this outcrop and southern plain (**Figure 18**) and defines the mountain-plain boundary in the Dezful Embayment structural unit [14].

In fact, it forms the northern boundary of the Dezful Embayment (foreland basin of the Neogene molasse of the Agha-Jari-Bakhtyari formations). The river bed is occupied by a small tight anticline composed of Agha-Jari layers with an E-W trend. The anticlinal fold axis is parallel to a fault having the same E-W trend. Agha-Jari formation that composed the dam foundation contains three main joint sets. One set is parallel to the bedding plane. Severe change of joint dips is most probably due to tight folding of the layers and faulting along the river course. The joint surfaces are mostly smooth, polished, and slicken-sided with varying dip angles.

As mentioned above, one of the challenges at Gotvand dam is instability of the dam abutments [42] and its reservoir banks [44]. The dam abutments include considerable volumes of dislocated rock mass of Bakhtyari formation. This is more critical in the right abutment as the rock mass is highly disturbed and deformed (DRM) and apparently illustrates a rock topple (**Figures 17** and **20**). On the other hand, the rock mass in the left abutment shows evidence of a rock slide. The DRM area is specified by extensive development of joints and cracks and its

south and north of the dam site, respectively, caused severe compression resulted in tight folding of the Agha-Jari layers (**Figure 21**). Subsequently, simultaneous compression of the competent sandstone layers and incompetent siltstone and mudstone layers in the Agha-Jari formation resulted in flexural-slip along the bedding planes, accompanied by shearing and

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Continual development of forced-folding widens joint apertures that make them favorable for gypsum precipitation. Results of Lugeon test for Agha-Jari rocks at the dam foundation show an average value of 6 up to a maximum of 30 LU. Laboratory experiments estimated the solution rate for the core-drilled gypsum of the dam foundation as 2.49 cm/year. As the number of fractures in gypsum layers increases, solution processes progress. It is calculated that the dissolution of gypsum veins increase the mass equivalent permeability up to 75–300 times depending on aperture width and spacing. The availability of fractures and fissures are primary and main factors for solution progress. In this regard, gypsum veins observed in Agha-Jari formation at the dam foundation could be threats to safety and proper performance

The knowledge of geological structures gives a reasonable in-sight in to dam construction studies. Site geology and availability of various geologic data obtained from site investigation is a key point in dam construction. It is also clear that the geological nature of different sites is not the same and depends on local and regional geology. Existing experience in dam construction projects shows that geological structure plays an important role in dam site geology and imposes major limitations on dam behavior during and after construction stages. In Karun-1 dam, for example, seepage through two springs downstream the right abutment are related most probably to existing shear zone at that abutment. Besides, rock falling and sliding on the dam abutments due to the downslope dip of bedding is another structure-related problem at the dam site. In Karun-3 dam, almost similar problems are observed but here, the presence of two sub-parallel thrust faults with opposite dip direction resulted in high stresses in the rock that intensified fracturing and subsequent permeability. Variability and complexity of geological structures regarding their tectonic situation result in different scenarios regarding dam's behavior. In this regard, the intersection of longitudinal and transverse faults in the vicinity of Marun dam caused the development of well and highly densed joint systems in the dam abutments that facilitated karstification. This in turn increased permeability of the rocks that was followed by extensive seepage during the dam construction. The role of geologic structure on geotechnical properties of dam sites is clearly seen in cases such as Gotvand Dam. Here, high fracturing and instability in the right abutment caused costly treatments including various stabilizing works such as grouting, geomembrane, and shotcrete. Finally, all the abovementioned examples indicate the impact of geological structure on various procedures either during constructing a dam or after its completion and close relation between structural geology

jointing of rocks.

of the dam.

**7. Conclusions**

and rock behavior.

**Figure 20.** An upstream view of the displaced rock mass (DRM) on the right abutment [42].

extent along the dam axis was estimated between 150 and 200 m [45]. Consequently, water seepage was considered as an extra problem in the right abutment. Lugeon tests in this zone indicated values more than 60 due to the extent of fracturing. The prevailing hydromechanical behavior of the rocks at this abutment was dilatation and washout based on geotechnical investigations [44, 45].

The second problem expected for the Gotvand dam site, is leakage through its foundation via Agha-Jari layers [46]. These layers contain veins of gypsum usually associated with claystone beds. They appear as thin films on the beddings and along joint surfaces with a maximum thickness of 2 cm. The action of Lahbari and Pir-Ahmad thrust faults in the

**Figure 21.** Simplified geological cross section along the dam axis [44].

south and north of the dam site, respectively, caused severe compression resulted in tight folding of the Agha-Jari layers (**Figure 21**). Subsequently, simultaneous compression of the competent sandstone layers and incompetent siltstone and mudstone layers in the Agha-Jari formation resulted in flexural-slip along the bedding planes, accompanied by shearing and jointing of rocks.

Continual development of forced-folding widens joint apertures that make them favorable for gypsum precipitation. Results of Lugeon test for Agha-Jari rocks at the dam foundation show an average value of 6 up to a maximum of 30 LU. Laboratory experiments estimated the solution rate for the core-drilled gypsum of the dam foundation as 2.49 cm/year. As the number of fractures in gypsum layers increases, solution processes progress. It is calculated that the dissolution of gypsum veins increase the mass equivalent permeability up to 75–300 times depending on aperture width and spacing. The availability of fractures and fissures are primary and main factors for solution progress. In this regard, gypsum veins observed in Agha-Jari formation at the dam foundation could be threats to safety and proper performance of the dam.
