a dual - Ingenia

A DUAL - 

PURPOSE 

TUNNEL 

A DUAL - PURPOSE TUNNEL 

SUSTAINABILITY 

THE CREATION OF KUALA 

LUMPUR’S STORMWATER 

MANAGEMENT AND 

ROAD 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. 

© Mott Macdonald – Gamuda JV 

24 INGENIA ISSUE 30 MARCH 2007 

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 m 3 /s. After 1985 

it increased rapidly so that by 

1995 the average annual flood 

was 440 m 3 /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 

INGENIA ISSUE 30 MARCH 2007 25



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). 

Figure 1. Stormwater Management and Road Tunnel showing the flood and tunnel areas, with current congestion and a 

cross section of the tunnel showing the two floors for cars with a channel underneath collecting low level flooding 

© Sepakat Setia Perunding (Sdn) Bhd 

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. 

TUNNEL VISION 

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. 

SAFETY 

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 

Mode 1 – Flood. If the peak 

flow past the intake is predicted 

to be less than 70 m 3 /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 150 m 3 /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 m 3 /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 m 3 /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 

Figure 2. how SMART's service gates will operate © Mott Macdonald 

Figure 3. A summary of operational modes used to manage stormwater in the city © Sepakat Setia Perunding (Sdn) Bhd 

INGENIA ISSUE 30 MARCH 2007 27


FEATURE 


Two of the biggest tunnel boring machines ever seen in Asia are being used to 

create SMART. The tunneling machine bores a hole 13.2m diameter but the 

diameter inside the lining is 11.8m © Mott Macdonald 

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 m 3 /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. 

Figure 4. A numbered cross section of the Herrenknecht mixshield machine © Herrenknecht 

TUNNEL 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. 

INGENIA ISSUE 30 MARCH 2007 

29


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. 

Breakthrough on the north side of the tunnel. The overall length river to river is 

11.5km which includes both bored and cut and cover tunnel. The bored tunnel is 

9.7km long of which the central 3km also serves as a road tunnel 

© Mott Macdonald – Gamuda JV 

Entrance to road tunnel visible behind toll plaza – with Petronas twin towers in central Kuala Lumpur in background 

© Mott Macdonald 

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. 

30 INGENIA ISSUE 30 MARCH 2007

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