ABSTRACT:

The Qomroud Long Tunnel lot number 1 (QLT1), a 10.4 km tunnel, is under construction as part of the Dez to Qomroud water conveyance tunnel projects (total length 55 km) in west central Iran, Lorestan province. The project involves four tunnels, three small dams and a large dam. The longest tunnel in the Dez to Qomroud projects is known as Qomroud Long Tunnel (QLT) with length of nearly 36 km. QLT1, the subject of this study, was defined as the first 10.40 kilometers of the QLT. Construction of QLT1 stopped in June 2000 after severe stability problems because of the nature of geological strata along the tunnel. Additional engineering geological investigations were commissioned. In this article, we highlight engineering geological problems along the QLT1 tunnel route such as water inrush, water jet potential in meta-dolomite, tunneling in fault and crushed zones, potential for high water pressures, raveling, swelling and saturated material flow in alluvial sections, and ground squeezing in weak rocks. Also, some geotechnical design considerations are proposed as special measures to consider in design and implementation of a new construction method in this difficult ground condition.

1. INTRODUCTION

Tunnels are perhaps the most geologically dominated civil projects undertaken. There are numerous examples in the literature from around the world where poor engineering geological investigations or unexpected ground conditions led to severe construction problems. Clearly, a well-designed, comprehensive geotechnical study can examine directly only a negligible portion of the rock mass along a tunnel. The volume of samples taken from boreholes drilled along the tunnel axis is minuscule compared to the volume excvated. The rock volume from an aggressive drilling program compared to the excavated tunnel volume for even a small 3 m tunnel is perhaps one part in 50,000. Thus, there is always substantial uncertainty associated with the engineering geology data collected for a tunnel project, even with a comprehensive engineering geology investigation. Engineering geological problems associated with the nature of geomaterials in a tunnel project include high water inrush, water jet potential in karstic formations, tunneling through faults or heavily crushed zones, potential of high hydrostatic water pressure in saturated ground, raveling, flowing behavior (massive watertriggered erosion) in alluvial sections, swelling and ground squeezing in weak rocks. These problems often lead to human life loss or a project shut-down, causing huge unexpected costs for the client. Problem identification well in advance can minimize the risk of unexpectedly encountering severe engineering geological problems during construction. Central Iran is an arid area with limited water supply. Providing enough water to this area is a great challenge for the Iranian water supply industry. To help address the water needs, a project known as the Dez-Qomroudwill convey water from the adjacent Dez Highlands to the central Iran watershed; 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 length of 36 km. Describing engineering geological problems along the first lot of the QLT " QLT1 " is the subject of this article.

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