Engineering Geological Assessment of Diversion Tunnel of Bakhtiari Damsite (Biggest Two-Arch Concrete Dam in Southern Iran)

Bakhtiari dam is located on the Bakhtiari river, 120 km away from the north of the Andimeshk city. Upper diversion tunnel of this dam with large cross section (13.7m excavation diameter) and more than 1 km length is a huge construction. The tunnel is placed in the Sarvak formation carbonate rocks of Bangestan group which passes through seven di ﬀ erent geological zones with various speciﬁcations (SV1, SV2, SV3, SV4, SV5, SV6, and SV7). Joint studies show two main discontinuit including bedding and a main group of joint (J1) together with random joints (faults and fractures). Most of discontinuities have been ﬁlled mainly by calcite or calcite and clay. Data deduced from testing and analysis shows good-to-excellent RQD classes with 75 to 90 values. Based on RMR and Q methods, generally rock masses have good to very good quality with 61 to 95 values for RMR and 10 to 35 values for Q . Based on conducted stability analysis, suitable supports were suggested for tunnel by RMR and Q methods. As a result, it can be concluded that all units have a good stability. Therefore, systematic rock bolting with 40–50 mm unreinforced shotcrete has been proposed for some special place. For rock support, according to RMR method, 3m rock bolts in crown, 2.5 m spacing and with 50mm shotcrete in crown has been proposed also 3m rock bolts, based on Q method, 2.3-2.4 m spacing with systematic Bolting without shotcrete or 40mm unreinforced shotcrete in some units, has been proposed. According to RMR method, for SV5 zone with very good and excellent quality, local 33 bolting without shotcrete and 3m rock bolts, 3 m spacing and spot bolting according to Q method has been proposed.


Introduction
In recent years, following the increasing need to create spaces underground with larger scale and in greater depth in poor areas (such as underwater), identifying more and more of the earth is evident. In relation to construction of dams, geological survey is the most important parts of studies which can be useful and valuable information about the design of underground spaces offer [2]. Feasibility of these constructions in natural materials, such as rock and soil, causes the geological conditions to play a major role in their stability [3]. Dams are considered as one of the most important civil structures. Arch dams with high stresses on their foundation highlight the role of rock mechanics studies. It should be noted that many geological data cannot be directly applied in the design of underground constructions, so in recent years; many efforts have been made for geotechnical classification [4]. In order to design dam and its appurtenant structures and assure about their stability, it is required to evaluate the engineering behavior of their surrounding rock masses. In this regard, physical and mechanical properties of the rock masses should be estimated based on engineering geological and rock mechanics field and laboratory investigations.
Rock mass characterization is normally carried out through the application of empirical classification systems, which use a set of geotechnical data and provide an overall description of the rock properties [6]. Moreover, they provide other important information like support needs, stand-up time, and geotechnical parameter among others    [7]. Two of the most used classification systems are the RMR-Rock Mass Rating and the Q-system [7]. RMR and Q systems have evolved over time to better reflect the perceived influence of various rock-mass factors on excavation stability [8]. Stability and support design for water diversion tunnel of Bakhtiari dam, based on five boreholes at the upstream and downstream cofferdams and four boreholes along the diversion tunnels path, has been investigated in previous studies [9]. This paper presents the results of rock mechanics studies of the upper diversion tunnels at Bakhtiari dam

Methodology
In this research, according to a detailed study during drilling, engineering geological properties and stability for upper water diversion tunnel of the Bakhtiari Dam has been investigated and rock support design has been proposed. For this purpose, lithological properties of rocks along the tunnel were evaluated during drilling with the manual sampling. During drilling, properties of discontinuity (such as dip/dip direction, roughness, Infilling, and spacing) in the rocks were studied. Series of data required for the investigation have resulted from testing and analysis of the excavated. These data indicate physical and mechanical properties of the seven various rock zones, including joint sets and discontinuities types with its properties. Based on these data, the values of RQD, RMR, and Q and class of all the seven zone rocks were determined. Finally, Stability analysis has been conducted, and appropriate supports were suggested for tunnel by RMR and Q methods.

Discussion
3.1. Geological Setting. Bakhtiari dam as the world's tallest concrete dam, is located in lower part of Bakhtiari river in Lurestan province and in southwest Iran, in southwestern of Zagros Mountains and in regional with 48 • , 46 , 50 east length and 41 • , 57 , 32 north latitude [11] (Figure 1). Deviation system of Bakhtiari dam includes two tunnels, namely upper and lower tunnels. The diameter of circular cross-section of the upper tunnel is 13.7 m, and the length of this tunnel is 1181 m [11].
According to the interpretation of surface geology and data from drilling and exploration boreholes, damsite and its surrounding consists of folded carbonate sedimentary rocks which belong to Sarvak formation from Bangestan group. Rock type in diversion dam system in the Bakhtiyari damsite is mainly composed of carbonate deposits from Sarvak formations [12].
The Sarvak formation is divided into 7 units from SV1 (oldest) to SV7 (youngest) [13]. In Figure 2, longitudinal geological section of upper diversion tunnel has been shown. Table 1 illustrates the above mentioned units in detail. It is necessary to mention that the SV1 unit is not exposed in this area and hidden under the SV2 unit.
(i) F1 fault caused the chevron fold zone and thus increased the amount of d discontinuities in the tunnel inlet portal.
(ii) F3 fault splits into two branches and crosses the middle part of the diversion tunnels.
(iii) F2 fault affects the end part of the tunnels.

Discontinuities
System. The framework of all rock mechanics analysis is based on geological data [14]. These data help to identify the types of rocks, rock-mass characteristics and structural discontinuity [15]. In order to identify and determine the quality and effectiveness of discontinuities on rock mass behavior requires that the discontinuities in the quality such as discontinuity system or rock structure are explained [16]. Joint study in all parts of the rock mass show that the main discontinuities in the Bakhtiari dam diversion section consists of two set of discontinuities, bedding, a major joint (J1), and also random joints (faults and fractures). Characteristics of discontinuities have been studied during the drilling of the tunnel (underground). The dip and dip directions of the discontinuities are presented in Table 2, and specifications of the bedding and J1 joint system are presented in Table 3.

Rock Mass
3.3.1. Classification. Engineering classification of rock masses is presented in various ways by different researchers and has been used for designing tunnel supports by many researchers [17][18][19][20]. The main aims of application of rockmass classification systems are to classify the rock masses existing at a project site, based on their main geotechnical feature and to estimate the geotechnical parameters of the rock masses. The role of classification is generally to get a better overview of a phenomenon or set of data in order to understand them or to take different actions concerning them [21]. In this regard, simple techniques are used for quantitative evaluation of a number of the main geotechnical features of the rock masses and then the rock masses are classified based on these classification systems.
In order to classify the rock masses in the Bakhtiari dam diversion tunnel, the rock-mass quality-index method (RQD), geomechanical rock-mass rating (RMR) and rock mass classification of tunnels containing the Q system are used.

Rock-Mass Quality Index (RQD).
Rock-mass quality index can be measured through direct core drilling or indirect, in cases where there is no possibility of the core, such as seismic methods or volumetric counting joints. In the project area, geological structures such as the F1, F2, and F3 faults, the kink-band zones, the anticline-axis zone, the      II  II  II  II  II  I  II  II  II  II joint sets, and in some cases the lithological bedding planes have a remarkable effect on the RQD value. According to this method, the numerical quality-index values corresponding to each part of the rock masses of the Bakhtiari dam tunnel has been done. In order to determine the numerical values of rock mass quality index from the volumetric joint count method in the surface outcrops and the tunnel space, that proposed by Palmstrom was used [22][23][24][25]. Under this method, small amounts of this parameter are measured in accordance with the following equation: In this regard, the JV is the total number of discontinuities in rock mass per unit volume. Based on RQD values five definite rock-mass classes are described (Table 4) [1]. RQD values in all zones have been calculated and their descriptions are presented in Table 5.

Geomechanical Rock-Mass Rating (RMR)
. RMR is one of the various methods in geomechanic rating [26]. Geomechanical rock mass rating (RMR) was introduced in scientific research and industrial (CSIR) in South Africa by Bieniawski [27]. It was based on his experiences in shallow tunnels in sedimentary rocks. All the rock units along the diversion tunnels have been classified using RMR system proposed by Bieniawski [5]. In Table 6, ranges of RMR values for the five definite rock-mass classes along with their description as suggested by Bieniawski [5] are shown. The final results of RMR classification of the rock masses along the upper diversion tunnel together with their description are presented in Table 8. As it is shown in Table 7, different rock units along the upper diversion tunnel are classified using RMR system, have good to very good Quality.

Rock-Mass Quality (Q).
Rock-mass quality (Q) system that was developed by Barton et al. [1], mainly for tunneling has been proposed. In this classification system, the Q values of the rock masses along the tunnels are evaluated based on six parameters, and the required support system is specified for each rock mass [10,17]. The Q system is developed as an empirical design method for estimating rock support [28]. The ranges of Q values are illustrated in Table 8. Accordingly, the Q values for the rock masses of different geological units along the di version tunnels are presented in Table 9. Based on Tables 8 and 9, all rock units have good quality in the Q classification.

Rock-Support Design Based on Empirical Methods.
Empirical methods have been developed based on the statistical analysis of the records on the stability and also instability of underground excavations performed in different types of rock masses in several countries. The two most widely used engineering rock-mass classification systems for estimating rock support system of underground openings are "Rock Mass Rating" (RMR) and "Rock-Mass Quality" (Q). These classification systems have been applied for categorizing the rock masses along the diversion tunnels.
As the first step of rock support design for the diversion tunnels at Bakhtiari project site, having the results of rockmass classifications by the two above mentioned systems, the rock-support measures relevant to each rock mass class have been estimated and proposed in the following sections.

Estimation of Rock-Support System Based on RMR.
The required rock-support systems for the upper diversion tunnel were estimated considering the RMR values attributed to the rock masses along these tunnels, as presented in Table 8. A guideline proposed by Bieniawski [5] for selection of the tunnel rock-support measures when they are excavated in one of the five main rock-mass classes of RMR system. The estimated rock-support systems for the diversion tunnels are presented in Table 10.

Estimation of Rock-Support System Based on Q.
In order to estimate the required rock support system for an underground opening based on the Q support chart, the diameter or height (m) of excavation span shall be converted to "Excavation equivalent dimension" (De). In this regard, real diameter should be corrected by dividing it to a parameter called as "Excavation Support Ratio" (ESR) which is related to the intended use of the excavation and degree of security which is demanded of the support system installed to maintain the stability of the excavation. The relevant ESR value for the diversion tunnel was taken as 1.6, according to Barton et al. [1]. Excavation diameter of the upper diversion tunnels is 13.7 m. Having De and Qvalues for different types of rock masses along the diversion tunnels, the required rock support measures for the different rock masses were estimated based on the Q-support chart proposed by Grimstad and Barton [10] as shown in Figure 3.
The results of estimation of the required rock support system for the diversion tunnel are presented in Table 11.

Summary and Conclusion
Based on studies conducted during the drilling upper di version tunnel of Bakhtiari dam, was found that: