This paper presents a research which was carried out in order to analysis the stability of deviations binary tunnels at dam situated in the southwest of Iran. The excavation of underground spaces has been an important task for civil and mining engineers since the past decades for their wide range of applications. The diameter of each of these tunnels is 13.7 meters and they are around 1000 meters long. The tunnels are excavated in steps of consecutive cuts. They are passing through seven different geological zones with various specifications. In order to study the characteristics of these zones nine boreholes of total length of 904 meters have been drilled. Around 140 various laboratory tests have been conducted on the core samples. Testing and analyzing the cores taken from the boreholes have resulted in series of data required for the investigation. These data reveals physical and mechanical properties of the seven various rock zones, including RED, joint sets and joint properties. Based on the collected data the values of RM and Q and therefore the class of the rocks have been determined for all seven zones. Stability analysis has been conducted and appropriate supports have been offered for both tunnels by RM and Q methods. Based on the ratings ascribed to each zone by any of the two methods a relationship has been driven between RM and Q for this particular project.

1- Introduction

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. Moreover, they allow obtaining other important information like support needs, stand-up time, geotechnical parameters among others [1]. Different classification systems have some well known drawbacks and limitations due mainly to their empirical base [2]. However, they are still very useful in practice. Therefore, there is a need to improve their efficiency. Two of the most used classification systems are the RM Rock Mass Rating and the Q-system [1].The RM and Q systems have evolved over time to better reflect the perceived influence of various rock mass factors on excavation stability [3]. This paper discusses the evolution of these systems, as well as problems associated with estimating the Q, RM indexes for water deviation binary tunnels in the Bakhtiyari dam of Iran.

2- Site geology

Bakhtyari dam site is in the southwest of Iran, almost 70 km North-East of Andi mesh town (Khuzestan province) and almost 65 km South-West of Doored town (Lodestar province). The dam axis lays at 290725 E and 3648729 N points. Figure1 shows the Location of the project area [4]. This formation consists of a series of asymmetric folding and faults The project area is covered by the sedimentary bedrocks of Larval and Faro formations. The Larval Formation is divided into 7 units from Sv1 (oldest) to Sv7 (youngest). At project site the Faro Formation is younger than the Larval Formation and is divided into two units [5].

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