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Malaysia waiting for monsoon rains to see how a new combined road and flood relief tunnel performs

In Malaysia, engineers are completing the hydrostatic water pressure tests on a vast stormwater storage and diversion tunnel, ready for the upcoming monsoon rains.
The bored tunnel is the direct result of damaging heavy floods in April 2001, and the project is designed to hold the flood flows from the Ampang and Klang rivers that merge to the northwest of the capital Kuala Lumpur.

The 9.7km long, 11.8m diameter flood diversion tunnel will draw off floodwater, carrying it south, under the city centre, to join the Kerayong River before it then merges with the Klang downstream of Kuala Lumpur.

As well as acute flooding, Kuala Lumpur suffers from chronic traffic congestion. The government had been looking for a way to increase road capacity and was keen to reduce its outlay on what promised to be an expensive piece of new flood relief infrastructure. So it engaged in an optioneering exercise in which the construction industry was invited by the government to suggest how the flood relief tunnel should be designed and built.

Contractors MMC and Gamuda found an innovative idea of using the central 3km of the tunnel to carry traffic during dry weather, relieving congestion on the roads above. Motorists could be charged to use the new subterranean expressway; a private sector company could take responsibility for building that section, recouping its costs through a 40-year toll road agreement. The dual-purpose structure was dubbed the stormwater management and road tunnel (Smart). Work on Smart started in summer 2001. Local firm Sepakat Setia Perunding was appointed as prime designer, joined by Mott MacDonald supply specialist tunnel and hydraulic design services.

The rivers that converge on Kuala Lumpur have always been flood prone. The area of Kuala Lumpur at flood risk is 5700ha. But the frequency of life-threatening floods has increased dramatically, because of development. There were five severe floods between 2000 and 2004, compared with four in the 1990s, three in the 1980s, and one a decade earlier.

Smart is designed to cope with three flood modes – floods of up to 70m3/s, up to 150m3/s, and of more than 150m3/s.

As well as diverting floodwater under central Kuala Lumpur, the tunnel will provide 3M.m3 of storage capacity. At the head of the tunnel is a holding basin, and at its toe is a 1.4M.m3 pond. This will enable the peak of the flood to be held back until water levels have subsided slightly downstream of Kuala Lumpur.

Putting 3km of road into a flood relief tunnel posed some interesting strength, durability and operational challenges. It is anticipated that the tunnel will be flooded up to 12 times a year, with frequency increasing over time. “The concession company wants to recoup its outlay as fast as possible so any time the road is closed because of flooding is time that it is not generating revenue from tolls,” says Mott MacDonald head of tunnelling Arthur Darby.

Traffic runs through the tunnel on two decks – each one carrying traffic in a different direction, with two running lanes and an emergency lane. Clearance height is only 2.55m, restricting traffic to cars and light vans.

Water is able to flow through the tunnel, under the lower deck, without interrupting traffic flow in most flood conditions. Double sets of guillotine-style steel gates at either end of the road section seal it off from the rest of the tunnel, enabling it to remain dry and operational even when the sections either side have been filled. Outer gates measuring 7m high and weighing 40t provide a first line of defence. Each deck is individually protected by inner gates weighing 26t (see diagram).

It is possible to fill the rest of the tunnel and maintain traffic over the central section. But this posed a structural challenge. When full, there can be as much as 20m head of water above the level of the lower deck, subjecting it to an uplift pressure of 2bar.

This has been resisted by heavily reinforcing the joint between the decks and the tunnel walls. Cast in situ walls have been created within the tunnel’s segmental lining. At the top and bottom of the walls are shoulders supporting the decks, which have been constructed by pouring a reinforced concrete topping over precast concrete planks. The decks and walls form a box structure that is both fixed and wedged immovably within the tunnel.


Operating the tunnel involves constant meteorological surveillance. It is thought that the operator will have 45 minutes warning of an impending flood. If expecting a category three flood, requiring road decks to be filled, the gates will normally be raised, allowing water levels to rise slowly and steadily.

“But we had to anticipate vehicles breaking down or crashing,” Darby says. The upstream holding pond and the capacity of the dedicated flood-only section of the tunnel will offer an additional hour to clear the road decks.

“Then the gates can be opened.” But, this scenario raised a new concern. When the tunnel is full of water the road section is rather like a submarine, with water pressing at the downstream and upstream gates. As soon as they are raised it will surge along the road decks from both ends.

“We were concerned that pressure generated by the surge could exceed ground pressure,” Darby says. Although segments making up the tunnel lining are bolted together, the lining is held firm by the constant pressure of the surrounding ground, like a masonry bridge is by gravity.

“There was a scenario where you had water racing down the tunnel from the top and up from the bottom, in which the lining could have been blown apart,” he says.
“We couldn’t even contemplate that, and incorporated surge shafts to ensure pressure is dissipated,” Darby says.

The surge shafts serve dual roles. Two are provided at the ends of the road section by the on and off ramp structures, and another two, mid-tunnel, double as ventilation shafts.

Floods typically last only a few hours, but peak water velocities are expected to be as high as 4.7m/s.

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