Segmental construction: it's something Bilfinger & Berger really, really believes in. As contractor for a 21m high elevated freeway snaking 55km south-east from Bangkok, the German firm is putting that belief to the test on an extraordinary scale. The project is a symphony in segments.
Units are up to a monstrous 27.6m, or six lanes wide - bigger than any previously cast. Approach-road segments are two or sometimes three lanes wide. There are also square box-section portal segments that make up beams supporting the freeway's 16 approach/exit ramps and two toll plaza areas. These are segments to hold up segments.
Asia's largest precast yard, which was set up to make the units, covers 650,000m2. It employs 2,000 workers to bend steel, assemble reinforcement frames, pour concrete and move, store and deliver the match-cast segments.
The segments are erected dry, without epoxy glue, and are held together purely by pre-stress. To erect them the contractor has assembled 14 segment erection trusses.
Big under-slung trusses support the main highway units as they are erected. Overhead trusses, from which segments are hung via cables until they are stressed, are used to lift the approach lanes. Bilfinger & Berger has also brought in an exotic under-slung/overhead truss. It slides along underneath the road but pivots 90degrees to erect the portal frames.
A lifting gantry has been mobilised for hauling the big steel frame trusses up and into position. 'Heavy construction cranes are ruled out,' explains Bilfinger & Berger construction manager Eckehard Stosch, 'because for virtually the whole length of the project we have to work within the narrow central reservation of the highway'.
The baht 30bn (£0.51bn) expressway is being built in the existing transport corridor out of Bangkok towards Bang Na and Bang Pakong. Client Express Transit Authority used its close relationship with the Department of Highways, the owner of the alignment, to gain easy right of way in a country where European-style compulsory purchase does not exist. Access can be fraught.
However, the department stipulated that neither temporary nor permanent works should disrupt traffic on the existing, and significant, route. Nor should it oppressively overshadow the lower road.
A consortium of Bilfinger & Berger, German company Dyckerhoff & Widmann and one of Thailand's two big contracting firms, Charn Karnchang, won the turnkey contract in June 1995. Design was subcontracted to US firm Jean Muller International. Dyckerhoff & Widmann had wanted to build a structure using precast U-beams. However, the group selected the segmental solution, with deck elements elevated to between 19m and 21m and supported by central columns 16-18m high.
Segments allow most of the work to take place above ground, says Stosch. Gantries can jump from pier to pier by self propulsion and form a complete span 'which you can be walking on two days later', he says. While traffic continues to flow at ground level, segments for further spans can be delivered along completed deck sections. Asphalting can be carried out and a slipformed parapet installed.
Deck is carried by a pair of arms that branch from the top of each pier. An under-slung truss was used to support and stress together the giant segments without over-loading the arms. The truss sits in the V between the arms, loading the column directly. For lightness and elegance piers also have an inverted V at the bottom. Special steel formwork was developed for casting them.
'The formwork weighs about 100t and it takes 110m3 of concrete to fill,' says Stosch. There are 16 sets manufactured to B&B design in Thailand. Schwing pumps fill the forms in a three stage pour.
Piers rise from longitudinal pile caps supported on between 14 and 16 precast spun concrete tube piles, up to 40m long and 800mm diameter. Ground is marine clay and sand. Work has been tightly confined within the central reservation.
'The problem is not only traffic disruption but severe differential settlement in this flood plain,' says Stosch. Any piled structure starts to stand proud after a period and this would cause road bumps.
A factory has been set up by the consortium for delivery of the tubes which come in 12m lengths. Two or three must be welded for each pile.
'This job is about factory production and quantities,' says Stosch. 'One baht on the price can make a big difference.'
The project has gone well so far. The first 25km is open and collecting tolls. In line with its turn-key contract B&B must hand over in eight phases. 'We are doing 18-20 spans of 42-43m per week,' says Stosch.
Sadly the collapse of the Thai baht in 1997 has hammered finances. Despite being able to buy in Thailand the project still requires large-scale imports. 'We will lose,' says Stosch calmly, 'but you must ask headquarters how much'.