ABSTRACT:

The arid Central Iran region suffers from water shortages and a limited water supply. Supplying sufficient water to this region has been a great challenge for the Iranian water supply industry in the last few decades. To satisfy these needs, one project will convey water from the adjacent Dez Highlands to a central Iran watershed known as Qomroud; it involves four tunnels and the longest section of this project is known as the Qomroud Long Tunnel (QLT) with a length of 36 km. This article describes rock mass lithomechanical characterization along nearly 10.4 km of the QLT based on extensive engineering geology and rock mechanics studies, including surface geology, joint-fabric study, borehole drilling and logging, and field and laboratory geotechnical testing. On-site rock mass characterization was based on classification systems such as rock mass rating (RMR), Qtunneling index, rock quality designation (RQD), geological strength index (GSI) and rock mass index (RMi). For defined geomechanical units, physical and mechanical properties were determined using ASTM standards and ISRM suggested methods. Results obtained from different rock mass classification systems for various rock units in different sections along the tunnel are presented, compared and interpreted in this article.

1. INTRODUCTION

Most water resources in Iran are situated in the west, south-west and northern parts of the country. Central Iran is, however, arid and suffers from water shortages and a very limited water supply. In the last few years water demands have increased because of the high rate of population growth together with the greater demand from industry in the country. Supplying sufficient water to Central Iran is a great challenge for the Iranian water supply industry. To address part of these needs, one project will convey water from the adjacent Dez Highlands to a central Iran watershed known as Qomroud; it involves four tunnels (total length of 54 km), three small dams and a large dam. The longest tunnel of this project is known as the Qomroud Long Tunnel (QLT) with a length of 36 km. From the initial geological investigations, the main tunnel-axis strata appeared to be schistose rocks such as pelitic schist, calcareous schist, and graphitic phyllite alternating with interlayers of meta-dolomite and metaquartzite. Construction using drill-and-blast techniques began in 2001; at 919 m of uneventful progress, a huge mass of saturated silty clay flowed into the tunnel, creating a sinkhole at the ground surface, 52 m above the crown. The presence of saturated alluvial materials along the axis had not been foreseen. To clarify the geology model and implement an appropriate construction approach, an additional engineering geological study was designed and conducted. Results from conducting a complementary engineering geological study dictated a substantial change in the geological mod-el of the tunnel compared with results from earlier exploration stages. Drill-and-blast techniques were deemed no longer suitable for continued construction, so the tunnel was abandoned during the winter of 2004 to gather required data and make decisions. This article describes the rock mass litho-mechanical characterization along nearly 10.4 km of the QLT project based on extensive engineering geology and rock mechanics studies including surface geology, joint-fabric study, borehole drilling and logging, and field and laboratory geotechnical testing.

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