The power plant project is being built on a 'Power Island' just off the coast to allay environmental concerns. The recently reclaimed island is to be the site of the 3 x 700 MW plant. One of the most time critical structures of the project was the pump house and associated water pond. However, its design was complicated by the geology of the site. Although not far from the mainland, the reclaimed sand platform sits on soft marine clays overlying recent alluvial deposits, making it complex to excavate the 20 m deep sea water intake and pumping station in future. The initial design envisaged a sheet pile cofferdam. However, from a practical point of view the numerous strut levels would have made excavation and construction costly and very time consuming. Specialist geotechnical contractor Bachy Soletanche Malaysia developed an alternative scheme for the construction of the pump house which was adopted by the engineer in the ultimate design.


Figure of eight shaft -61 & 59m diameter with depths 30-40m

The solution consisted of using a diaphragm wall retaining wall to form the water pool and the pump house without any need for internal strutting. This was done by creating a figure of eight shape, with one section for the water pool and the other slightly larger section for the pump house proper. The figure of eight shaft consisted of two touching circles which are 61 m and 59 m in diameter with panel depths of 30 to 40 m deep. The use of the diaphragm wall to contribute load bearing capacity meant that only 31 barrettes foundations, 60 m deep, were required to found the heavy pump house structure and equipment. This also contributed to the savings in time as initially eighty large diameter bored piles had been anticipated. The diaphragm wall and foundation work was completed in about 12 weeks between June to August 2000. This allowed the rapid excavation of the water pool and pump house and the construction and installation of the critical mechanical equipment.

However, while the design is very efficient, the construction of any circular shaft needs to be carried out in accordance with strict tolerances to ensure the highest quality of work. Due to the nature of the circular retaining structures, their efficiency derives from the hoop action of the compressive forces. This means that the accuracy of the diaphragm wall panel joints must be controlled to ensure that the compressive forces are transmitted between the two structural elements. Any deviation between the panels reduces the surface area available and therefore the capacity of the hoop structure to resist the forces. To ensure compliance with the strict 1:200 tolerances, Bachy Soletanche Malaysia used the highly accurate KS3000 hydraulic grab to excavate the diaphragm wall panels and barrettes. Incorporated in the grab is an inclinometer that gives real time readout in the operator's cab showing the verticality in both planes. Any deviation outside can immediately be corrected. The KS3000 equipment is used regularly in Malaysia, Indonesia and Singapore but normally for its high productivity rate. This was one of the first instances it was used in Asia for such tolerance critical work. The compressive forces discussed above come together for both shafts at the two intersection points — another quality critical point.
Apart from the delicate excavation of these two Y shaped panels, the reinforcement cage detail was very important. For ease of construction and installation, the 'cage' consisted of four elements, a triangular central cage and three normal reinforcement cages for each leg'. All four elements had to be overlapping which made for complex manoeuvring of cranes on site." To enable the accurate placing of all this reinforcement, the triangular or 'spider' cage was placed in the intersection of the Y with its legs' extending to each side of the panel. Next the three normal reinforcement cages were lowered into each leg of the Y, interlocking with the triangular cage, which acted as a guide. Once all the cages were safely and accurately installed in the panel, universal beams were then placed in the pump house and water pool sides of the Y panel to further reinforce this 'pinch' point. Concreting was undertaken using three tremie pipes which were filled together to ensure the highest quality finish. In addition to these two special panels, there were a number of other special panels for the water intake tunnels and the pipe work leading to the power station from the pump. To ensure structural integrity around the future openings, specially designed reinforcement panels were installed into the wall. This innovative design and construction method is the first such application in Southern Asia, according to Bachy Soletanche. Similar but smaller projects of this nature have been carried out by Bachy Soletanche in Hong Kong and Korea.

The advantages to the programme and structure were significant. With the retaining structures being incorporated into the permanent works, further cost savings were obtained. Such structures are often used in Europe for storm water retention as the highly efficient design and the rapid construction time ensures minimum disruption to the surrounding properties. The equipment used incorporates sophisticated electronic and hydraulic systems, is reliable, and adapted to the works of this nature. It is a reliable alternative to the Hydrofraise diaphragm wall cutter for accuracy.

Extract from Southeast Asia Construction.