Getting Balambano dam on form again

The tropical island of Sulawesi is approximately 1000km north of Australia and is one of the larger islands in the Indonesian Archipelago, the largest system of island groups to be found anywhere in the world. Located northwest of the existing Batubesi dam at Lake Towuti, the Balambano dam is being constructed on the Larona river.

A gravity RCC hydroelectric structure, the new dam is being built to supply additional energy for PT INCO’s (PT International Nickel Indonesia’s) expanded pyrometallurgical nickel processing plant at Soroako, where output is currently set to double to 68,000t a year. The power provided by the dam — 137MW — will reduce the sensitivity of the Soroako mining facility to the fluctuations in the world price for nickel. The dam will provide additional low cost power for mining operations, reducing operating costs and allowing the mine to operate profitably even when world prices for nickel are low.

The dam, 90m high with a crest length of 350m, comprises 546,000m3 of lean concrete with impermeability guaranteed by a PVC membrane installed on the upstream face. A 38m wide radial gated spillway is positioned mid way across the dam and 45,000m3 of conventional concrete was used in the construction of the spillway and power intake structure. Water is conveyed to the powerhouse by two 5m diameter metal penstocks and the powerhouse has two Francis turbines with a combined capacity of 137MW.

Construction is being carried out by Astaldi Thiess Joint Operation. PT Fluor Daniel Indonesia is project managing the work, while the design and resident engineering is being provided by Pacific Rim Power.

Getting on track

When the joint venture contractors approached formwork specialist RMD about working on the project — by this time the original contractor, Astaldi, an Italian company with extensive worldwide dam building experience, had been joined by Thiess — all parties were keen to maintain schedules. Time was a precious commodity and formwork solutions had to be devised, supplied, implemented and supervised. This task was made all the more difficult by the distances involved and the sheer remoteness of the site.

Detailed designs — more than 300 to date — and calculations for the spillway had first to be prepared and submitted for approval to Pacific Rim Power. On average, it took up to ten days to transport equipment from the company’s base in Jakarta to the site. This involved loading equipment onto barges in Jakarta for the sea crossing to Ujung Pandang, the capital of the island and its main sea port some 300km south of the Balambano dam. Equipment was then unloaded before making what can only be described as the arduous trek by road to the dam construction site.

So far RMD has supplied 57 containers, housing 530t, of equipment — all of which will need to make the return journey to Jakarta, as the Balambano formwork is supplied on a hire contract.

Formwork and falsework is delivered to the site in component form. It is assembled on site, usually in the position where it is required. Some large or complicated sections are pre-assembled on site and then craned into position, such as the Balambano wall forms and the curved and shaped sections of formwork that were used to construct the pier and abutment nosing.

RMD’s work on site started in November 1998 and is now fast nearing completion, in line with the Astraldi Thiess programme which specified that the dam must be fully operational by November 1999. Throughout almost the entire ten-month programme RMD has, as part of its contract, had two experienced formwork engineers supervising the safe and efficient use of the equipment.

To complete the project against such a tight deadline, RMD utilised a substantial quantity of formwork and falsework systems. Extensive use was made of the Super Slim Soldiers formwork, along with Alform aluminium beams and the Rapid Stage access and falsework system. The project also called for the use of a wide selection of formwork accessories and the design and manufacture of special steel formwork for constructional elements. This included the off-site manufacture of 20t of special curved and shaped sections of formwork that were used to construct the pier and abutment nosings on the upstream face.

Super Slim Soldiers formwork was chosen because of its ready availability, its high strength-to-weight ratio, and the system’s ability to accommodate complex geometric shapes. Available in nine standard lengths from 10mm to 3600mm, it incorporates webs that feature a series of holes along the entire length to allow the easy attachment of a host of accessories. Individual Soldiers can be bolted together to form longer members and adjustable node assemblies allow the Soldiers to be connected at any point along their length. Bracing is most frequently provided using adjustable Super Slim prop jacks or turnbuckles.

At the Balambano dam, the Super Slim Soldier system was teamed up with Alform beams, which were used as secondary members spanning between the Soldiers to form plywood-decked platforms. The beams incorporate timber infills to allow easy fixing of the formwork facing. The facing was also supplied by RMD as part of its total supply solution. It was either locally sourced plywood or, for certain applications, high strength plywood specially imported for the task from Australia. Steel facing was, however, used for the special pier and abutment nosings.

Super Slim Soldier and Alform Beam wall forms were craned into position and were restrained using a range of cast-in anchors and tiebars welded to the concrete reinforcement bars. Where necessary, special shear brackets were used to space the formwork off completed lower sections of the concrete structure. Super Slim Soldiers were also utilised as support brackets and raking shores.

For working access to the chute walls and pier abutments the modular Rapid Stage was used. The system was also put to work as temporary falsework on various parts of the project. It was particularly suited to the project — bearing in mind the pressure to keep the contract to schedule — as it is highly adaptable and reduces construction times.

The Balambano dam is just the latest in a number of dam construction projects that RMD has been involved with around the world in recent years. Others include the Ord hydro power station and the North Dandalup dam in Australia, the Pak Phang dam in Thailand, Yong dam in Korea, Houay Ho hydro project in Laos and the Pergau hydroelectric project in Malaysia.