Abstract

This article highlights the geotechnical challenges faced during design and construction of Lot No. 6 of the Dez-Qomroud water transfer tunnel project in Iran. The main stratum along the tunnel path is a mud-supported weak conglomerate. Results from comprehensive rock mass characterization using different rock mass classification systems revealed that because of very weak strength properties of the conglomerate some tunnel sections will be stable for not more than ten hours. Some engineering geological considerations made to guarantee a safe construction of the tunnel such as reducing the tunnel diameter and relocating the tunnel route to avoid saturated ground. Implementing results from this study led to successful completion of the project.

1.
Introduction

Tunnels and underground structures can be considered as the most geologically dominated civil engineering projects undertaken so that tunnel geology can dictate whether construction of a tunnel is feasible or not technically. There are numerous examples in the literature from around the world where poor engineering geological investigations early in design stage of a tunnel or unexpected ground conditions during the construction stage led to severe construction problems, additional expenditures and delays. Having a clear picture of the ground behavior early in design and construction stages of tunnels and any other underground structures is central in conducting any engineering geological studies. In this article we demonstrate the geotechnical challenges faced during design and construction of Lot No. 6 of the Dez-Qomroud water transfer tunnel project in west central Iran known as the Qomroud Tunnel Project lot number 6 (QTP6). Dez-Qomroud water transfer tunnel project is planned to supply sufficient water for both drinking and irrigation purposes to vast areas in the central Iran. The QTP6 is situated in the Zagros Mountains and the main stratum along the tunnel path is a synorogenic mud-supported weak conglomerate. A comprehensive engineering geology and rock mechanics studies conducted in the study area in order to determine geological-geomechanical characteristics of the rock masses along the tunnel route. The physical and geomechanical properties of the conglomerate exposed in the study area determined in the laboratory based on ASTM standards and suggestions of ISRM. Different rock mass classification systems employed to classify and characterize the rock masses. Engineering geological studies revealed that the QTP6 will pass through a poor quality rock mass. This engineering geological fact early in design phase of the tunnel helped the designer to revise the previous design and avoid major instability problems in way of construction of the QTP6. Then a conventional tunneling method was adopted for construction of the QTP6. Most parts of the QTP6 excavated without any significant stability problem or incident. In some sections the tunnel reached a high water content conglomerate and lead to a collapse and it was closed for three months. A mitigation measure was planned and used in the tunnel site and lead to reopening the tunnel afterwards. In this paper engineering geology of the QTP6 is discussed. Also, the implemented tunneling techniques, instability problems and undertaken mitigation measures are discussed.

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