Tuesday, March 20, 2012

Tunnelling of Klang Valley Mass Rapid Transit

KVMRT brought the attention about the tunnelling of multi-billion My Rapid Transit (MRT).

Boring in the city (The Star)

Monday March 19, 2012

By Meng Yew Choong

Building the award-winning dual-purpose tunnel called SMART was difficult enough. But building the Klang Valley MRT tunnel will have engineers pushing the envelope even more.

By next month, engineers and contractors who are into tunnelling will be fixated on a 9.3km underground strip of land that runs through one of Kuala Lumpur’s most developed parts. The stretch, which forms a crucial part of the Klang Valley Mass Rapid Transit (KVMRT) infrastructure, starts near the intersection between Jalan Semantan and Jalan Duta, before snaking through KL Sentral, Pasar Seni, Bukit Bintang, Stadium Merdeka, and Pasar Rakyat, before reemerging near Taman Miharja.

Like all metro tunnels, the KVMRT’s underground infrastructure will be built in the form of twin tunnels mostly running side-by-side to enable trains to rapidly traverse both directions. Work will begin in earnest once the successful bidder for the multibillion-ringgit tunnelling portion of this massive project is announced by MRT Corporation.

The dual, or twin-bore, tunnels will enable trains to run up and down from Sungai Buloh to Kajang along a 51km-long track that is intended to transform the way Malaysians commute.

This Tunnel Boring Machine was used to create the SMART; the machine that will be used in the even more difficult KVMRT project will be similar, though probably more advanced.

The tunnels will connect seven underground stations at KL Sentral, Pasar Seni, Merdeka (after Stadium Merdeka), Bukit Bintang Sentral (near Lot 10), Pasar Rakyat, Cochrane, and Maluri (see kvmrt.com.my for exact location of stations).

Over the next five years, Klang Valley drivers can expect to see some road diversions to facilitate the construction of the track, both for the elevated as well as underground portions.

Work on the elevated portion would, of course, be easily visible to all, and hence, road users will have a sense of the progress of the construction; but tunnelling will remain largely hidden.

One may ask how difficult could tunnelling be, given the continuous technological advancement in the field, as well as extensive knowledge now available in the Klang Valley courtesy of tunnels built for the Putra LRT (now part of Rapid KL) and the award-winning 9.7km Stormwater Management and Road Tunnel (SMART).

However, those who are in the know think that the company that successfully bids for the KVMRT project will be facing an unprecedented challenge in completing the 9.3km tunnel running under some of Kuala Lumpur’s most heavily developed areas on time and on budget.

To get an idea of the difficulties posed by this project, we held a teleconference interview with a German tunnelling expert.

“Boring the SMART was slightly easier in the sense that it did not have to pass through developed areas characterised by deep foundations,” says Dr Bernhard Maidl, a tunnelling expert with Maidl Consulting Engineers located at Bochum, Germany.

Maidl certainly knows what he is talking about as he has assisted in building over 100 tunnels worldwide, and has extensive knowledge of the tough geological conditions found under Kuala Lumpur.

The difficulties in building the SMART were not really made known to the public while work was being done; it was only after its completion that we came to know that it was no walk in the park even though state-of-the-art (at that time) tunnel boring machines (TBMs) were used.

This sinkhole appeared in Jalan Chan Sow Lin, Kuala Lumpur, during tunnelling works for the karstic portion of the SMART in 2005. Knowledge and experience derived from that project as well as more sophisticated technology should minimise such incidents today.

Deep complications

The TBMs were needed to punch a smooth hole through the highly-weathered limestone on which Kuala Lumpur sits. These types of landforms are called karst by geologists, and are generally the result of mildly acidic water acting on weakly soluble bedrock such as limestone (which is mainly calcium carbonate).

Water, either rainwater or groundwater, will gradually dissolve the surface along fractures in the limestone bedrock. Over time, these fractures enlarge as the bedrock continues to dissolve. The openings in the rock will increase in size to form underground drainage systems that will, in turn, allow more water to pass through the area and accelerate the formation of underground karst features after thousands, if not millions, of years.

The erosive and corrosive power of acidic water works its magic to leave behind a labyrinth of passages, steep drops and huge caverns (just like those seen in Perak’s Gua Tempurung, or in other exposed limestone hills).

Engineering-wise, Kuala Lumpur’s karst is classified as “extreme karst” (5 on a scale of 1 to 5) – it has an intricate underground network of channels that can go in any direction, steep sided cliffs, soft in-filled valleys, and a highly irregular rock heads.

Cavers would know all these terms, as they love to explore such features – but they are a tunnel engineer’s absolute nightmare as these features are the same ones that can cause sinkholes and ground subsidence during tunnelling.

Karst sinkholes and ground subsidence happen when disturbances to the underground water table causes earth and other debris to shift; this can happen when the TBM pierces underground reservoirs or water-filled cavities, thus disrupting the fine equilibrium of the system. Another possible consequence are mudflows (known within the industry as blowouts), which can rise to the surface during the boring process.

With the exception of a few hundred metres at both ends, the SMART was basically bored through highly weathered karst, which provided the engineers with a good learning experience in constructing structures under Kuala Lumpur. Extreme karst gave the builders of the SMART plenty of headaches, as dozens of sinkholes and mudflows occurred unexpectedly during construction of the dual-purpose tunnel that lies at depths of between 10m and 16m from the surface.

During the excavation of a tunnel through soft, water-bearing ground, a temporary support is often required to maintain the stability of the working face. In some cases, this support is provided by slurry, a thick suspension of solids within a liquid (usually a pressurised mixture of bentonite and water). Occasionally, this muddy mixture can reach the surface through fissures in the ground, causing what is known as a blowout. Seen here is an instance of slurry blowout at Jalan Tun Razak, Kuala Lumpur, during the SMART’s construction.

However, experienced tunnellers know that they can, to a large extent, mitigate these incidents through careful and deliberate pre-excavation treatment, which largely entails filling up the cavities and voids with grout before the boring machine passes through that stretch. But in order to know where the cavities are, extensive soil sampling and geophysical tests (using sonar and electrical resistance, for example) are necessary to ferret out these anomalies; this is what the winning bidder for the multibillion-ringgit KVMRT project must do to minimise the possibility of ground subsidence in the heart of Kuala Lumpur.

Over 150 bore holes have already been dug for initial studies, and the data have been published in the tender document for the KVMRT project; and despite the enormity of the task, there are already five interested parties vying for the job, including MMC-Gamuda, the only local consortium shortlisted.

Whoever gets the job will face the prospect of tunnelling at an average depth of 30m while doing their utmost to avoid, for instance, a sinkhole appearing in Jalan Bukit Bintang – can you imagine the chaos caused by closing one of the country’s busiest – and, arguably, commercially most important – streets? Ground subsidence is also a no-no at places where there is existing critical infrastructure like railway lines, major highways and even existing tunnels – yes, the KVMRT tunnel will have to pass under the SMART.

Thankfully, preliminary soil sampling for the tunnelling portion has indicated that only half of the KVMRT tunnel will pass through karst formations. The other half of the tunnel will go through the much more predictable formation composed of sedimentary rocks called the Kenny Hill formation (see graphic below).

Most of Kuala Lumpur sits on either one of these formations, which hold very different implications for engineering, even for things as simple as putting in a single pile. For example, the Kuala Lumpur City Centre project that hosts the Petronas Twin Towers was shifted just so it could sit on the more predictable Kenny Hill formation, rather than being built on the riskier karst.

Sedimentary rocks are formed under relatively uniform conditions over relatively large areas; this permits relatively accurate interpretation between data points and projections. As such, tunnelling works through the Kenny Hill formation should be relatively easy to manage during the design and construction stages. This should be good news for fans of Jalan Sultan in the heritage enclave of Chinatown, as the stretch sits on the Kenny Hill formation, which means it is extremely unlikely that complications will arise when tunnelling through this portion.

Spiralling difficulty

To minimise ground subsidence in such challenging geological conditions, and particularly so in a densely urban environment such as Kuala Lumpur, whoever gets the contract is expected to use the most sophisticated tunnel boring machines available, coupled with extensive ground investigations and comprehensive pre-tunnelling soil treatment.

Dr Markus Thewes, a professor at the Institute for Tunnelling and Construction Management of Germany’s Ruhr-University (who joined Maidl at our teleconference), predicts that significant improvements in knowledge of karst terrain and tunnelling technology will come in handy for whoever builds the KVMRT tunnel.

“Many improvements in tunnelling technology have taken place over the past two years, and I would expect that these technological improvements should be in place for the project. Sinkholes arising from tunnelling, if they occur at all, will be very rare compared to previous tunnelling works in the same kind of formation.”

An outcome of the failure to allocate a rail transport corridor during the nascent years of Kuala Lumpur’s development means that engineers have little option even in tunnel alignment when they designed the KVMRT tunnel.

Other than the many deep foundations of tall buildings to avoid, there is also utility infrastructure like sewerage lines, telecommunications cables, electricity cabling, water pipes, and gas pipes to contend with. At Kampung Pandan, the KVMRT tunnel has to go down as deep as 50m to maintain a safe distance as it passes under the SMART.

At Bukit Bintang Central, the underground space is so narrow that the twin tunnels have to be stacked on top of each other, rather than being laid side-by-side as in all other areas. This means that one tunnel would have to be dug in such a way that it forms a mini spiral to one side, just so that it can be on top of the other tunnel, before curving back to the side-by-side formation after it passes the Bukit Bintang area!

According to Maidl, all these factors combine to create some of the most unique tunnelling challenges ever.

“Building the KVMRT tunnel will require an extremely high level of knowledge and experience, right from the selection of contractor and consultants, to the design as well as execution levels. Everyone on the project needs to be experienced, as the tunnel goes through very public areas with a lot of traffic.”