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  Modification designs for TBM Assembly of Mae Tang-Mae Ngat-Mae Kuang Water diversion tunnel project, Chiang Mai Province, Thailand

  Mae Tang-Mae Ngat tunnel project in Chiang Mai Province, Northern Thailand is a part of Mae Kuang Reservoir Inflow Augmentation project. The main objective of the whole project is to diverse annually the surplus water about 147 million cu.m from Mae Tang river to Mae Ngat reservoir through a tunnel with a length of 25.4 km and a diameter of 4.0 m, and then transfer the water from Mae Ngat reservoir to Mae Kuang reservoir with a tunnel length of 22.726 km and a diameter of 4.20 m. The transferred water will cope with an increase in water demand for agriculture, domestic uses and industry in the lower Mae Kuang basin. The construction contract was divided into 4 individual contracts. Right Tunnelling Co. Ltd. won the first section of the Mae Tang-Mae Ngat tunnel which is starting from Mae Tang diversion weir to km 13+600. The construction plan called for Tunnel Boring Machine to be excavated from Adit 2 at km 13+526 to Adit1 at km 3+641, a total length of 9,885meters. The alignment of Adit2 is originally perpendicular to the main tunnel axis with the radius curvature of only 90 meters. As it is impossible to assemble the TBM outside the tunnel. In order to assemble the TBM and its backup system in the tunnel, the contractor was forced to modify the original design: the tunnel size, grading and curvature to accommodate the TBM assembly chamber and launching tube to be within the original main tunnel with minimum deviation from the original tunnel alignment. The scope of this paper is to present various aspects of the designs and analyses of the modify components using the same design methodology, the Austrian guideline for the geotechnical design of underground structures; OeGG 2010, used originally by the owner designer.

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