Brunei Darussalam is hoping its first ever mega-project – a 27km bridge – will boost the country’s capacity for eco-tourism and development.
The oil-rich state of Brunei – the world’s fourth richest country per capita – lies on the northeast coast of the Borneo, South East Asia, surrounded geographically by Malaysia and Indonesia.
The Temburong District and the capital, Bandar Seri Begawan, are currently separated by Brunei Bay, which can take up to an hour to cross by boat. Alternatively, motorists can take the drive through Limbang town in the Malaysian state of Sarawak, but this takes up to four hours during peak times.
A new dual two lane motorway is expected to cut this connection to 30 minutes.
Brunei 3 cropped
Temburong District could be considered the “green jewel” of Brunei: densely forested and populated by about 10,000 people across 1,300km2 – an area about the size of Greater London.
Economically, the area does not contribute a large share of wealth. The country’s riches are almost entirely based on energy reserves, which are expected to last for another 50 years. To reduce its dependency on oil and gas revenue the government is promoting eco-tourism in Temburong as one option, while a range of other development opportunities are also expected to come from the new bridge link.
The link will require 14.6km of marine viaducts, two cable stayed bridges over shipping channels, and 12km of viaducts across peat swamp forest in Temburong District.
It is a truly international project, and is the first major project in East Asia designed to Eurocodes. It draws on expertise from all over the world. Arup was commissioned to undertake a feasibility study for the project in 2010, then subsequently appointed to carry out the detailed design, while HR Wallingford led the environmental impact assessment work in 2013. The marine viaducts and bridges contracts have been awarded to Daelim-Swee, a joint venture between Korean construction giant Daelim Industrial and Brunei construction company Swee. Finally, the elevated structure through Temburong will be built by a Brunei-China joint venture between China State Construction Engineering Corporation (CSCEC) and Ocean Quarry & Construction – this marks the first mega infrastructure project won by CSCEC through international competitive bidding.
While the total cost of the project has not been officially divulged, various sources list the marine viaducts and bridges contract alone as between £110M and £350M.
After selecting the optimal route, a fast-tracked procurement strategy was set in motion by the Brunei Government. Contract packaging and schedule phasing gave priority to the contracts with the longest construction activities. Construction of the marine viaducts, longest in length and relatively repeatable and straightforward to design and construct, started first, in 2014; work on the cable stayed bridges began in the same year – these were expected to take longer to design but require less time to construct. Work on the Temburong District elevated structures began last, as environmental impact assessments and ground investigations pushed back the start date.
The marine viaducts are formed of twin post-tensioned single cell concrete, box girders.
With construction starting 2014, and the client looking for a rapid programme, the viaducts are designed to be built fast. Span-by-span erection using precast concrete segments was employed. Continuity between the separate spans is achieved by a 200mm wide cast in-situ stitch.
An innovative gantry supplied by Dorman Long is in use to erect the bridge decks. The gantry, believed to be the first in the world of its type, simultaneously erects the two parallel 50m long 870t precast concrete bridge deck girders. The gantry has three legs and uses an innovative launch sequence to avoid placing heavy loads on the deck beams when moving forward onto the next span.
To transport the precast sections out to the gantries by barge, extensive dredging was required so that barges did not run aground. The entire bay is shallow, scarcely exceeding 2m in depth, except at the two navigation channels where it is up to 10m. Below the water is typically 20m to 30m of soft clay.
HR Wallingford principal scientist John Baugh led the marine modelling study supporting the project and says the construction phase was the main focus of this work.
“The bridge itself, once constructed, with quite slender piles, will have a marginal effect. A lot of our work ended up being centred on the construction phase,” says Baugh.
“They had to work a lot from the marine side, and didn’t have the water depth to use a typical large crane barge. So the solution proposed was to dredge a trench along the route of the bridge for the plant.”
typical cross section
The trench was about 120m wide – in all about 1.6M.m³ of material was removed.
With the fast-track impetus coming from the top, Baugh says his team were asked to contribute beyond the traditional contract relationship, into plans for carrying out the required work. “One example of this is on the disposal of sediments. They wanted to do it as sensitively as possible. And while the country does have offshore disposal sites available to them, which have been used for a long time, they wanted to think of ways not to take their spoil so far. So we looked for an in-bay site, being a benefit, with plans for reclamation.”
The marine viaduct is mostly supported by 1m diameter concrete spun piles – cylindrical precast hollow piles of high density concrete. Steel tubular piles of 1m to 1.6m diameter, 30m to 70m deep were adopted for locations where the ground is harder or where piers could be subject to higher ship impact forces near the navigation channels.
One of the key drawbacks of concrete spun piles is their brittleness and limited bending resistance, an important consideration in this part of the world, where earthquakes are common. Ground investigation information was unavailable until the detailed design had reached a late stage, further complicating matters. To reduce seismic load on the piles, the final design incorporates high damping rubber bearings acting as isolators.
Vessel movements are scarce in the bay but two channels – Brunei Channel and Eastern Channel – are navigable by larger ships. These will be spanned by two cable stayed bridges. The bridges are 110m tall and feature Islamic star and crescent symbols. The four articulated ribs on the towers’ arches symbolise each of the four districts of Brunei, unified into one after the bridge opens.
digital temburong 2
Brunei Channel Bridge will have a main span of 145m to take it across the 130m wide Brunei Channel. Soffit level is about 23m above sea level. The Eastern Channel Bridge has a main span of 260m, to take it across the 235m wide Eastern channel. Soffit level for this structure is about 33m above sea level. Despite differences in span and width, the Eastern Channel Bridge (ECB) is conceptually the same as Brunei Channel Bridge (BCB) but with two towers instead of one.
It is no accident that the bridges show a similar architecture and geometry – the bridges will be built one after the other, so Arup has suggested the same equipment could be re-used or adapted. This uniform approach also assisted in design stages, with a parametrical analysis, common model and verification tools shared across the two structures.
Both bridges have a concrete ladder beam deck which comprises two edge girders and cross beams between them, with concrete slab on top spanning between the cross beams. The stay cables are anchored into the edge girders, which together with the cross beams are post-tensioned. Foundations for the piers and approach spans are 2.2m diameter concrete bored piles.
Each crossbeam is typically post-tensioned with two tendons per beam, anchored at the deck edges. The spacing of the crossbeams along the axis of the bridges was chosen so that the same cross section and a similar tendon arrangement (tendon number profile and types) could be adopted for both bridges.
To make edge-of-deck detail more compact the cable stay anchorage blocks are incorporated within the width of the edge girders.
cable stay deck anchorage arrangement
At the towers, the vertically curved concrete side legs are hollow with constant wall thickness from foundation level to the deck diaphragm, and then again up to the first stay saddle. Then the legs become solid higher up as the stresses from the cables increase.
The upper tower infill is made up of two separate walls with a cavity between them, making the structure lighter, minimizing material use, as well as providing an inspection route.
The tower construction sequence is conventional, but with additional complexities due to the shape. The tower legs can be cast in constant height lifts of about 4m using a jump formwork system, with minor adjustments for the inner “ribbed” design.
bridge deck casting sequence
To construct the bridge decks, a cast in-situ cycle has been developed. The first portions of the deck are cast outwards from the tower, with the sections between the and the first stays cast on falsework. For further segment casting, a balanced cantilever erection cycle is proposed. For this traveller forms would be launched forward into position to support casting of the segments. Then all segment reinforcement, cable stay anchors, stressing anchors and ducts can be placed, followed by concrete casting and stay cable installation.
One of the most challenging parts of the project is to construct the elevated viaduct across the Temburong peat swamp forest using a top-down construction method to reduce environmental impact. The elevated structures are constructed from 12m long spans supported by concrete spun piles, pile caps, piers and beams, all of which are precast. Piling and transportation of material are all carried out at deck level. The precast components will be stitched together by in situ pour on site to form a 10-span monolithic structure module.
Arup associate Sammy Yip says intelligent sequencing of the construction has been employed to maximise the efficiency of the precast construction of the bridge girder segments. At maximum construction rate, it is estimated that a 2.5-day cycle can be achieved for each span.
“There’s two parallel express ways, with temporary cranes in front for piling, then cranes doing the deck behind you, so then four cranes working in parallel on each work front,” says Yip.
Construction of this section has yet to begin. The project is expected to open in late 2019.
Total bridge length over water = 14.4km
Number of bridge spans over water = 296
Total bridge length for Temburong viaduct (through forest) = 11.8km
Number of spans on Temburong viaduct = 968
Total concrete volume = 860,596 m³
Total reinforcement weight = 140,233t