Construction of underground power lines

Published:

Introduction

“The Metropolitan Electricity Authority of Bangkok (MEA) is a state enterprise under the Ministry of the Interior with responsibility for the distribution of electrical power. As part of a programme of improvement, reliability and expansion, the MEA instigated a project to install 230kV underground transmission lines to transfer power from Lardprao terminal station to Vibhavadi sub-station in Bangkok.”[1]

Brief

This project consists of the design and construction of 21 drive and reception shafts, approximately 8km of 2.6m internal diameter tunnels, installation of two circuits of 230kv oil filled cables within cable troughs with “cooling” pipes, low voltage cables, lighting, communication cables, ventilation system, a cooling plant building. There is also capacity for one circuit of 230kV cable and two circuits of 115kV cables to be installed in the future.

Shaft Design

For pipe jacking operations and installation of mechanical & electrical systems, each shaft was sized to provide, sufficient space to suitably accommodate the tunnelling machine and jacking equipment and to provide a fine balance between providing the structural strength to withstand jacking forces with minimal shaft movements, whilst minimising the weight of the structure, to prevent sinking under its own weight.

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Nineteen rectangular and two circular shafts have been constructed. These are of two types.

  • Ventilation shafts
  • Exhaust shafts

The inside plan dimensions of the rectangular shafts is 8.8m by 4.5m with wall thickness of 800mm. Inside diameter and wall thickness of the circulars shafts is 9m and 500mm respectively. The shafts are spaced at typically 500m centres along the route.

Techniques Used

Shafts were sunk to the required founding level using the Open Caisson System (open at the top and at the bottom). Single reinforced concrete elements (6 in total) were individually cast in-situ and sunk in sequence. This cycle of operation was repeated until the cutting edge had reached the required founding level. Continuity of reinforcement was provided at construction joints thereby making the elements an integral part of the shaft structure. During each sequence of the shaft sinking operation, excavation was kept to a limited depth leaving sufficient residual load at excavation base to provide basal stability. The rate of sinking was governed by a fixed cycle of operations. To reduce the magnitude of developed ground friction forces during sinking operations and to minimize sinking / excavation induced ground movements in the vicinity of the shafts, shaft walls were initially designed to set back for a distance of 75mm from the shoe. A total of 20 bentonite slurry injection ports were installed just above the cutting edge (or shoe level) to provide a membrane of slurry around the walls. In addition, similar injection ports were installed at 3rd and 5th elements.

Successive injections were made as caisson elements were sunk to their final level. The shaft wall set back value was later reduced to 30mm after the Contractor had gained a better understanding of the actual soil behaviour with respect to ground/structure interaction forces. Upon completion of shaft sinking operation the void behind the shaft wall was cement grouted.

The caisson sinking system used a hydraulic control system to lower each element using suspension links from piled staging. &rlquo;Bi-directional hydraulic cylinders were connected to high tensile stress bars which enabled the elements to be sunk either by being held and lowered when sinking in soft clay or jacked down when in stiff clay.”[2]

Verticality and eccentricity of each shaft were checked at sinking intervals of 500mm and were controlled by means of operating hydraulic cylinders either independently or in combination when necessary. Actual shafts inclinations and eccentricities were maintained within the specified values of 1:100 and ± 200mm respectively with the exception of one shaft where its long wall inclination reached 1:52.

After completion of sinking operations, a concrete plug was placed at shaft formation level to seal the shaft bottom. This was followed by placing a layer of hydraulically compacted sand and further followed by constructing the first stage concrete raft connected to the shaft wall by means of couplers.

During Construction

Major temporary and permanent utility diversion works were required to gain access to the shaft locations in the canal between the Vibhavadi Rangsit Road (East bank) and the footpath access to the adjacent buildings (West bank). It was therefore necessary to build a staging within the canal, which necessitated major water diversion works.

Safety Aspects

Ground moinitoring and movement

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Pipe jacking related shaft movements were assessed by carrying out numerical simulations using the “finite element program PLAXIS to determine anticipated shaft deformation at working loads and approximate deformations at failure.”[3] Staged construction features were used in order to simulate the step-by-step excavation during shaft construction as well as stress strain variations in the surrounding soil during construction and pipe jacking operations. Updated Langarian analysis was used to account for mesh deformations during the calculation process. The maximum predicted displacement at the active side of the shaft was predicted to be 110mm in the immediate vicinity of shaft wall when a maximum main pipe jacking load of 900 tons was used. Where possible, surface settlement points were installed to monitor ground movements. Actual ground settlements of up to 9mm in the vicinity of the shafts were experienced. Shaft movements at each drive shaft were regularly monitored when these shafts were subjected to maximum jacking forces. Maximum permanent movement of the drive shafts when subjected to the maximum jacking force of 900 tons was at 7mm. And for those shafts where the maximum jacking force was 500 tons, the maximum permanent movement was 5mm.