Under the sky of Kuala Lumpur City Centre (KLCC), you can see many luxury hotels, fashion malls and the futuristic Petronas Twin Towers. Under the KLCC what you can’t see at the moment is the SSP Line under construction in Malaysia.
This line will connect the towns of Sungai Buloh, Serdang and Putrajaya and it will be the second part of the Klang Valley Mass Rapid Transit (KVMRT) project. The first line Sungai Buloh-Kajang (SBK) was completed in 2017 and it is now operational.
The alignment of the SSP Line will have a length of 52.2km, consisting of 38.7km of elevated tracks and 13.5km running through underground tunnels. There will be 10 interchange stations to transfer from the SSP Line to existing and future rail lines, including the future Kuala Lumpur- Singapore High Speed Rail. The cost of the underground stretch is RM13.11bn (USD 3bn).
The main involved parties include government-owned company MRT Corporation as client while the contractor for the underground works is MMC Gamuda KVMRT. The lead designer is Aecom.
The works of the SSP Line commenced in June 2016 and the completion of the underground works is scheduled by 2022. The excavation of stations has been substantially finished; tunnelling works are at more than 70% complete.
Some mechanical and electrical works have already started but others will begin this year followed by the track works and other systems works, such as operational and communication systems and design, manufacturing, testing and commissioning of rolling stock.
Hau Wei Ng, deputy project director at MMC-Gamuda explains, “The SBK and SSP lines first run from the North to the South of Kuala Lumpur and they go through the city centre of Kuala Lumpur and that’s one of the reasons they need to go underground.”
The tunnel is being constructed in a twin bored tunnel configuration and it has an internal diameter of 5.8m.
The depth of the alignment is around 30m but the deepest section of the tunnel is at around 60m close to the Tun Razak Exchange development. At this point the tunnel of the SSP Line goes under the tunnel of the SBK line.
The deepest station is around 40-45m below the ground level at Ampang Park station. There are 16 tunnelling drives in total as they are twin-bored tunnels and six tunnels have been completed.
Some stations are constructed adopting diaphragm walls as primary retaining walls for excavation with top down method while other stations with secant bore piles as temporary retaining structures, using the bottom up method.
NG explains, “We decided to choose between these two methods according to the geology and the work sequence required to interface requirement at each station.
“If stations are in soft ground, we normally use diaphragm-walls with top down method, where the structure is built from the ground level to the bottom. If the excavation is in good rock conditions, we opt for a bottom up method, where first we construct bore piles as temporary retaining structures and then excavate the rock by controlled blasting.
NG adds that top down and bottom up methods are related to the location of stations, for example if they are placed under the road or within a greenfield land.
“If a station has to be constructed directly under the road, we prefer to use the top down method because we have less disruptions to traffic,” says NG. “We can construct the top slabs before, and then we can carry out the excavation under the structure while allowing the traffic to run on top. Top down stations with traffic running above are Sentul Barat station and Kampung Baru station.”
Regarding the stations to be excavated in rock, NG adds, the procedure is similar to tunnels excavated with drill and blast. A vertical excavation has been done to drill into the rock and to do some controlled blasting.
“All stations have been excavated except for Titiwangsa station, which is still having some controlled blasting,” he says. “With controlled blasting we mean that the procedure is designed to stay within a certain limit of vibrations and noise. Thus, blasting mats, tyres and other insulating materials will be applied over the area to contain the blast, dust and noise generated.”
NG says that top down and bottom up methods are also considered depending at which station the TBM has to be launched. “For the launch of the TBM the bottom up method is applied because it allows us to reach the bottom faster and to prepare the TBM launch in advance,” he says. “We can also continue the structure at the same time.”
The geology consists of a variety of different formations. In the northern section there is a portion of granite while in the middle there is some Kenny Hill soil, including clay, silts and traces of quartz. Kenny Hill formation is also present in the southern section of the alignment while the rest of the route is in limestone.
NG explains that Earth Pressure Balance machines (EPB TBMs) are put at work only at sections within competent clay and Kenny Hill formations. “We apply variable density machines (VD TBMs) at some sections with granite and karst limestone,,” says NG. “The karst limestone formation in Kuala Lumpur is generally grade 5, which represents the worst karst condition with a lot of cavities and channels in the ground. That’s why slurry machines have to handle these formations.
NG adds that there are fault zones with mixed limestone and Kenny Hill formations. “We met some fault zones but we didn’t have major problems with them so far,” says NG. “We do have problems during the transition from limestone to Kenny Hill where the ground is very blocky and it is made more even more difficult if we couldn’t implement ground treatments due to the presence of utilities ranging from telecommunication lines to drainages.
“In our VD TBMs we have several modes of working conditions that we can apply, for example we can vary from low-density slurry support to higher density slurry support.
“We can also have a transition in and out of EPB dry mucking or EPB with a slurry muck transport system.
“When the ground is blocky, we can choose to switch to EPB mode to better deal with the situation. In very fractured conditions, we can apply very high density slurry to improve face support, hence providing better safety for interventions.”
Geological investigations have been undertaken by MMC Gamuda contractors. Some boreholes are required along the alignment, averaging at around 50m intervals.
For challenging locations some geophysical methods have been applied including seismic waves, seismic refraction and microgravity methods.
Tunnels are totally excavated by 12 Herrenknecht TBMs, including 2 EPBs and 10 VD machines. All the TBMs have an outer diameter of 6.6m, and a length of 130m. There are 5 TBMs launching locations, which have been chosen with the intent to break the alignment in equal sections, but it would be subjected to land availability or suitability on the surface for launching and operating the TBMs. NG says that in most cases, they do not purposely build shafts for launching the TBMs as they are mostly launched from the station box, which will be a permanent structure later on.
In terms of lining, segments have 275mm of thickness with a 7+1 configuration as universal ring.
“We use only steel fibres in our tunnel lining segments,” says NG. “At certain locations due to specific loading requirements, we do need some rebar reinforcement. These locations are normally situated within poor ground conditions or where the overburden is minimum. Some locations would require additional steel cage reinforcements due to future development loadings specified by our Client.”
Lining segments are manufactured by four precast factories Eastern Pretech, MDC, SPC and KomT segments based around Kuala Lumpur.
NG says that they require a storage up to 2000 rings during the peak time of the tunnelling works.
Most ground improvement will be carried out in limestone due to a lot of fractures, fissures and channels, which pose a great risk during excavation. It is also required at stations to mainly prevent groundwater loss during construction of d-wall or also during the rock excavation to avoid the ingress of water and muck.
Challenging Locations
Most of the underground sections are placed under the city centre, and a section of the drive is next to a theatre and near a hospital. Thus, engagements with stakeholders were necessary to put in place special requirements to protect these buildings. “Near the hospital we have very strict noise control requirements and this presents a big challenge in terms of progress,” says NG.
“We usually blast during the day and the last blast is before 6pm. During the night, blasting is not permitted.
“In terms of vibrations, the limits are usually higher than those of that residential area or stakeholders would consider for annoyance.”
All the buildings are monitored with real time instrumentation, including optical settlement markers, extensometers for stations. There are also 24-hours surveying automated stations where they scan all the markers almost every hour and then the data is transmitted automatically to the computers where the client and the contractors check them immediately.
“To avoid any congestion on site, we generally have limited space for stockpile,” says NG. “For tunnelling we can only stockpile on site for a few days. We have a 24-hour transport regime via trucks for spoils.” The final destination for spoils will be at approved dumpsites around the Klang Valley. As TBM breakdowns are a key risk to the project, NG clarifies that they have kept the TBMs utilisation high so far. “We have had some interruptions traversing sections through granite,” he says. “The abrasivity of the ground wore the TBM more than anticipated and at one point who had to stop the TBM for a month for repairs.”
To ensure the water tightness of tunnels, EPDM gaskets with hydro-filling elements have been used. These gaskets have been manufactured by Normet and BASF Malaysia.
Water proofing requirements are at a high level and any visibly dripping or flowing waters will need to be plugged. As the water table in Kuala Lumpur is only a metre below the ground surface, the pressure to be resisted by the waterproofing installed in the project may need to withstand up to 6bar at the deepest point.
The tunnels go underneath rivers at least at three locations. Nevertheless, the cover between the tunnel crown and the riverbed were high and have been assessed to required minimal treatment. Monitoring of leakages into the river during crossing were stringent, hence support pressures when tunnelling under river were carefully designed and observed during the crossings.
Hyperbaric Interventions
Hyperbaric interventions are required to maintain the cutting tools during the tunnelling. Along the limestone alignment, pre-treatments are required to enable an intervention due to existence of fissures whose pressure could leak. “Sometimes, when conditions after pre-treatment, still fail to enable a successful intervention, decisions have to be made to either move on or re-treat.”
Compressed air interventions are usually required every 150-200m within the limestone. There are manlocks in the TBM and a resident medical officer in place on the surface to facilitate all the diving operations in compressed air.
“Hyperbaric interventions remain the most dangerous activity in the mechanised tunnelling ,” says NG. Routes to reach the nearest hospital certified for hyperbaric illnesses were planned out in advance in case of emergency and project dedicated ambulances and medics are available on site. Standard ventilation is in place where air is pumped by ducting into the TBM under pressure.
