Article - Issue 30, March 2007
A Dual-Purpose Tunnel
A dual-purpose tunnel
Recurring floods and ongoing traffic congestion have an adverse economic impact on Kuala Lumpur’s central business district in Malaysia. But now, a single solution to both problems is taking shape in the form of the world’s first dual-purpose tunnel capable of carrying vehicles and channelling storm water. The Stormwater Management and Road Tunnel (SMART) opening this spring, is the first of its kind in the world. Arthur Darby, Head of Tunnels at Mott MacDonald, tells the story behind this unique building project.
Major floods affecting Kuala Lumpur’s city centre occurred in 1926 and 1971 but, after rapid urbanisation and expansion of the city since 1985, flooding has become an almost annual occurrence. Until 1985, the average annual flood in the Klang river was constant at about 148 m3/s. After 1985 it increased rapidly so that by 1995 the average annual flood was 440 m3/s. Kuala Lumpur’s urbanisation included the encroachment of infrastructure into the river channels, pushing flood river levels even higher. Since the major flood of 1971, the Malaysian government has continuously monitored this situation. The Klang Basin Flood Mitigation Project has been developed and implemented incrementally involving a range of measures, including creating holding ponds and increasing river channel capacity. However, such measures alone would not be sufficient to prevent the flooding of the commercial centre of Kuala Lumpur. Detailed studies led to the concept of a tunnel for storm water storage but, at the design stage, the novel idea arose of also using it to tackle traffic congestion.
In 2001 the Government sought proposals from major development companies for a solution that would allow a typical flood of three to six hours’ duration to occur without inundating the city centre. A tunnel enabling floods to bypass the centre was one way of achieving this, providing it was coupled with temporary storage facilities to keep flows downstream of Kuala Lumpur within the capacity of the river channel.
A group led by major Malaysian company Gamuda engaged SSP, a large Malaysian consultant engineering firm, and Mott MacDonald UK to develop proposals for a tunnel with holding ponds at upstream and downstream ends of the tunnel. For legal reasons, the tunnel had to run for most of its length under Government owned land – in practice under roads. Under competitive pressure to come up with the most cost-effective proposal, Gamuda considered whether the normally empty tunnel could also be used to carry some of the traffic that was otherwise in nose-to-tail jams on the roads above. The tunnel could be tolled, normal for expressways in Malaysia, and the income would reduce the requirement for Government funding.
Plans were drawn up for a tunnel fitted with two road decks, one for traffic going north, and the other for traffic travelling south. Each deck would be wide enough for two traffic lanes (with a hard shoulder for breakdowns) and the headroom would be sufficient for cars and light vans only (see Figure 1). A two-deck, low-headroom tunnel was a unique idea and the concept of an alternate use of the same space by floods and traffic was also new.
Requirements for dual-purpose use
For this concept to work, three special requirements had to be met. Safety – whilst in use as a highway it had to be certain that floodwater could not enter the highway space. Preparing the tunnel for floods – it had to be possible to predict floods in sufficient time to clear the tunnel and convert it to flood use before the flood peak arrived. Returning the tunnel to road use – after the flood had passed, the tunnel had to be able to be put back into highway service quickly in order to minimise both the loss of toll revenue and build up of congestion on alternative surface roads.
The highway tunnel occupies the central, 3km long section of the 9.7km long flood tunnel. To ensure that flood water could not enter the highway space, service gates at both ends of the highway tunnel (see Figure 2) close the upper and lower road decks. A second barrier, the emergency gate, closes both road decks. In highway operation all these vertical lifting gates are closed and remain so under self-weight. Normally the holding pond will be empty so there will be no water pressure on the gates.
Preparing the tunnel for floods
As part of its Klang River Basin Flood Mitigation Project, the Malaysia Department of Irrigation, is developing an extensive flood warning system. The system will have rain gauges spread throughout the flood catchment area. These will feed information in real time to the Stormwater Control Centre, located at the point where water is diverted from the Klang River into a holding pond and into the tunnel. The computers in the Control Centre will use the rainfall and river level information, to predict probable flood magnitude which will lie in one of three ranges (see Figure 3).
Mode 1 – Flood. If the peak flow past the intake is predicted to be less than 70 m3/s, no water will be diverted into the holding pond and tunnel.
Mode 2 – Flood. If the peak flow is forecast to be between 70 and 150m3/s, which occurs about ten times a year, radial gates will open enough to divert water into the holding pond and reduce river flow to 50 m3/s. Once the water in the holding pond reaches the crest of a bellmouth weir, it spills over into the tunnel. Flows of this magnitude are passed through the void under the lower road deck and do not require the tunnel to be taken out of traffic use.
Mode 3a – Flood. If the peak flow is forecast to exceed 150 m3/s (which occurs about once a year) then it will advise the Motorway Control Centre that it will be necessary to use one or both road decks for flood flow. Between 70 and 100 minutes after the start of the storm, depending on its intensity, the radial gates will be lowered to divert water into the holding pond. Traffic will be stopped from entering the tunnel and the passage of vehicles already in the tunnel will be monitored on CCTV. The Motorway Control Centre will send a vehicle through the road tunnel to confirm that it is empty, and any broken down vehicles will be towed out. 45 minutes is allowed for these activities. The gates at the ends of the road deck will be lifted while the tunnel is still dry. Between 50 and 60 minutes after water has first been diverted into the holding pond, it will spill over the bell mouth and into the tunnel.
Mode 3b – Flood with delayed opening of tunnel. If it takes longer than 45 minutes to clear the tunnel, water will continue to fill the holding pond and run through the tunnel and under the lower road deck. Water pressure will build up on the emergency gates at both ends of the highway decks. Once the road decks are eventually cleared the gates can be opened.
An abrupt transition from open channel water flow, with air between the water surface and the tunnel crown, to pressurised ‘pipe full’ flow could generate high transient pressures capable of damaging the structure. Advanced numerical modelling was thus undertaken to determine the safe opening sequence for the gates. It was found necessary to provide surge pressure relief at the four ventilation shafts along the tunnel, as well as providing a relief shaft downstream of the road decks.
Returning the tunnel to road use
Once the flood has passed, the diversion gates are raised so that no more water enters the holding pond. It will take about three hours for most of the water to drain out of the tunnel under gravity. Pumps at the downstream end of the water tunnel can fully empty the road section in a further five hours. The ventilation system will be employed to assist drying.
Minimal work is anticipated to return the tunnel to service. A trash barrier will have prevented floating debris entering the tunnel. All fittings within the tunnel have been designed to withstand a water velocity of 5 m/s, and the electrical fittings have been specified to IP68 to withstand continuous immersion. The emergency telephones are located in escape staircases, connecting the decks, which will be closed off with watertight doors. The holding pond at the upstream end will act as a settling pond to remove sand and grit, and the velocity within the tunnel is sufficient to ensure remaining fine particles stay in suspension and are carried through to the attenuation pond. The road decks will be washed down to remove a possible film of fine sludge and then sprayed with an anti-bacteriological solution to eliminate odour.
Overall, a period of 48 hours has been allowed to bring the tunnel back into service after a flood. A flood requiring use of the tunnel will only occur on average about once a year, so interference with the road operation will be tolerable.
A ventilation system that will survive flooding
While it was possible to specify that the tunnel lighting and CCTV equipment should be waterproof, this was not feasible for the ventilation fans. Consequently, they were located in four shafts at 1km intervals. In extreme situations, the water level in the shafts could rise to the level of the fans, so they will be protected behind watertight doors that are closed before the gates are raised.
The ventilation fans can inject 105 m3/s of fresh air into the road decks at the shafts through Saccardo nozzles at up to 20 m/s. In an emergency situation, after a multiple vehicle fire for example, passengers would be instructed by the public address system to leave their cars and walk to the nearest escape. Because the lower road deck has only three metres of headroom, there was concern that the high air velocities from the nozzles might discourage people from reaching the ventilation shafts, which contain emergency escape stairs. A three dimensional model that could simulate turbulence was built, using computational fluid dynamics and showed that very high velocities were restricted to the top of the tunnel above head level.
Geology and tunnel alignment
The geology, along the tunnel route, consists of alluvium overlying dolomitic limestone in which dissolution by water has created many cavities filled with soft material or water. As such, the surface of this karstic limestone is extremely irregular.
Ideally, from a construction point of view, the tunnel would have been aligned deep enough to remain wholly within the limestone. However, the elevation of the tunnel was fixed by hydraulic considerations at both ends, and while about 70% of the tunnel was excavated in rock there were many places, particularly towards the northern end, where the upper part of the tunnel was located in soft alluvium or ground disturbed by tin mining. Such variation between hard rock and very soft waterlogged soil made for difficult tunnelling, particularly below a busy city where ground settlement had to be controlled to prevent damage to existing infrastructure.
The tunnelling principle: (1) behind the cutting wheel with muck bucket lips and cutter tools is a steel cylinder, the shield (2). It is within the protection of this shield that the tunnel is excavated. The space in front of the pressure buckhead (3) is filled with a bentonite suspension which seals the existing soil. The pressure necessary to support the tunnel face is produced by means of a compressed air cushion (4) in the excavation chamber, which is divided by a submerged wall (5). The excavated soil is pumped into the slurry line (6) together with the suspension. Large rocks are broken down by a stone crusher (7). The suspension is supplied via the feed line (8). Protected by the shield, the reinforced concrete segments (9) are installed by an erector (10). To continue the advance, the machine presses against each previously installed segmental ring with hydraulic thrust cylinders (11). The annular gap between the segmental ring and the ground is continuously grouted with mortar as the machine advances. All operations are controlled from the control panel.
Tunne; boring machines
Simple tunnel boring machines, of the kind used in clay under London, follow the 19th century concept of a rotating cutter head on which are mounted picks and a cylindrical shield to support the ground where necessary. This concept would not have worked in Kuala Lumpur, because water and loose soil would have poured uncontrollably past the cutter head into the tunnel. Therefore, it was necessary to use a more modern and more complex design of machine with a bulkhead behind the rotating cutter head closing off the tunnel face. This enables the water and soil to be kept at a pressure sufficient to prevent movement of ground towards the tunnel face.
In the type of machine selected, spoil is removed from the tunnel face in a bentonite slurry which is pumped to the surface, passed through a separation plant utilising cyclones to separate the spoil, and then the cleaned slurry is returned to the tunnel face. A feature of the Herrenknecht mixshield machines that were used is that a compressed air bubble (see 4, Figure 4) is maintained in a chamber behind a bulkhead behind the cutterhead. If the cutterhead suddenly encounters a soil-filled cavity, as happens in karstic limestone, the magnitude of any pressure drop in the bentonite is reduced by the bubble expanding.
Very soft soil, below the invert level of the machine,was grouted from the surface to ensure that the machine would not sink under its own weight. Movement of the ground surface was monitored by precise levelling, and ground water levels were monitored, as falls in groundwater level could lead to settlement in the soft soils.
In practice, settlements caused by machine tunnelling were generally very small (a matter of millimetres), but very occasionally combinations of ground and machine operation led to local over-excavation.The diameter of the bore is 13.2m, the height of a five storey building. This was, at the time of ordering the machine, one of the largest in the world.
Programme to completion
The lower road deck has been pressure tested by building a bulkhead below the lower deck and filling the invert with water. Static testing of the whole tunnel by this method is not feasible, and the performance of the structure and systems will be carefully monitored during the first floods that pass through. It is expected that operating rules will require optimisation in the light of experience.
Opening of the highway tunnel is scheduled at the end of March 2007. At that time, the northernmost section of the flood tunnel will still be under construction and the opening of the flood tunnel, will follow at the end of June 2007. Kuala Lumpur will then be able to enjoy the full benefits of this innovative engineering.
BIOGRAPHY – Arthur Darby
Arthur Darby is a graduate in engineering from Cambridge University and has a Master’s degree in soil mechanics from Imperial College. He has wide experience, in the UK and internationally, of management roles on both water and tunnel projects including: the Channel Tunnel and Heathrow Terminal 5. He is a Divisional Director of Mott MacDonald’s Metro and Civil Division, and Head of Tunnels.