ABSTRACT:

An efficient design of foundations for transmission line towers has always been a challenge for the engineers due to the variety and cyclic nature of the loads. Foundations, especially for the four-legged towers, are subjected to all types of loads (compression, tension, torsion and shear) in different combinations. The cyclic nature of the loads further complicates the situation. Available design parameters proposed by different researchers are also mostly based on the monotonic loading conditions and are not directly applicable for tower foundations. This paper presents the analysis, design and construction practice of the barrettes used for the transmission line towers in Thailand. Adaptability of the barrettes under different site constraints like sensitive underground pipelines is also described.

INTRODUCTION

The 230kV-transmission line project presented in this paper involved construction of 40 transmission towers, 400m apart. The project site was located about 200km, east of Bangkok, Thailand. The transmission line was planned to convey electricity from the power plant to a sub-station. The alignment of 15.6km-long transmission line partially runs along the boundary of an Industrial Estate and thus the foundations of these towers are in close vicinity of existing underground gas pipelines and utilities above ground (Fig. 1). Foundation layouts, construction method and the work area were thus restricted by presence of these utilities. Initially, the structural designer proposed large diameter bored piles of 2.0m in diameter for tower foundations. Construction of such large bored piles would require heavy equipment and plant, and would induce unacceptable vibration level to the existing underground gas pipe lines located within and along the construction area. As an alternative construction method and foundation type to suit the site conditions, barrette construction that utilizes relatively less equipment and induces less vibration than conventional bored pile construction was adopted.

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