Monday, 5 May 2014

Making tunnelling safe

Early investigation: Bore hole sampling at the Pasar Rakyat MRT station site two years ago. While bore holes provide valuable information for geotechnical engineers, they also have to use geophysical surveys to glean as much data as possible about the underlying strata of areas between two bore holes. -MGKT

Early investigation: Bore hole sampling at the Pasar Rakyat MRT station site two years ago. While bore holes provide valuable information for geotechnical engineers, they also have to use geophysical surveys to glean as much data as possible about the underlying strata of areas between two bore holes. -MGKT

Building the Klang Valley MRT has reached a vital stage under busy Bukit Bintang.

THERE is only one more year to go before the tunnel-boring work of the Klang Valley MRT’s Sungai Buloh-Kajang line (SBK Line) concludes. By the end of this year, only two tunnel-boring machines (TBMs) will still be mining, and the last unit is set to be retrieved from the ground in May 2015.

At present, five TBMs are working on forming the 9.5km-long pair of tunnels.

Supervised by project owner Mass Rapid Transit Corporation Sdn Bhd, the contractor for the underground portion, MMC Gamuda KVMRT (T) Sdn Bhd (MGKT), is currently working around the clock to ensure that the TBMs are running at optimal efficiency.

“Tunnelling work is easier when the TBM encounters relatively uniform ground conditions. These machines are geological condition-specific, meaning they are always made to order to suit the unique ground conditions of each project. But things get a bit interesting when those ground conditions vary,” says Dr Ooi Lean Hock, head of geotechnics for MGKT.

The ground conditions for this particular project are particularly challenging because the SBK Line tunnels have to pass through two major soil formations: the Kenny Hill sedimentary rock and sand formation and the Kuala Lumpur limestone formation.

While the former is a typical and more predictable geological formation found widely within the vicinity of Kuala Lumpur and in the Klang Valley, the Kuala Lumpur Limestone formation is highly weathered karst. This form of extreme karst is quite beautiful to look at – think of the limestone hills of Ipoh that are filled with nooks, crannies, and passageways, some interconnected with others through a maze that not even experienced cavers dare to explore. When viewed from a 3D perspective, though, karstic formations look like a piece of Swiss cheese, with the voids often filled with groundwater; this, together with the surrounding ground, forms a system that is in equilibrium.

“The great challenge comes when a TBM punctures through into these voids – equilibrium is rapidly disturbed. The groundwater and the slurry required by the machine to maintain working pressure will dissipate quickly through the network of channels,” says Ubull Din Om, MGKT’s general manager for underground works.

“This is why building a large tunnel through karst is such a demanding job; lots of painful lessons were learned during the construction of the 9.7km-long Stormwater Management and Road Tunnel (SMART),” he adds.

LEAD PIX: Keeping tabs: Surveying is carried on a daily basis along Jalan Bukit Bintang to detect any form of settlement or ground movement during and after a tunnel boring machines passes through. Pavilion Kuala Lumpur is in the background, highlighting the sensitivity of the area that the TBMs are passing through.
The Bukit Bintang area is surveyed daily for ground settlement or movement and will continue to be monitored during and after tunnelling. Pavilion Kuala Lumpur is in the background, highlighting the sensitivity of the area that the tunnel-boring machines are passing through. -MENG YEW CHOONG/The Star

Two TBMs took four years, from 2003 to 2007, to complete a single tunnel for SMART, which opened officially in 2010. In the course of that project, which was built almost exclusively through karst, there were many instances of ground subsidence, including some sinkholes – thankfully, though, the SMART tunnel travels under relatively sparsely populated areas in the valley.

“While mining through karst is already very challenging, it becomes doubly difficult when a TBM has to pass through karst that is overlayered with other soil profiles. Tunnel-boring machines are at their most efficient when tackling uniform ground conditions – be it all rock, all soft soil, or all sand. But men and machine will be pushed to their limits when presented with mixed ground conditions,” Ubull says.

This month, a pair of TBMs will enter an area where undulating karst is layered with the material of the Kenny Hill formation – and this interface comes in rather unpredictable patterns, which tunnellers hate.

The difficult area lies close to the shopping mall Fahrenheit88 and Menara Keck Seng, both on Jalan Bukit Bintang. At this location, the tunnels will be stacked one on top of the other, with the crown of the upper tunnel just 16m below the ground’s surface.

“We have to be extremely watchful when the TBMs tunnel through this area. The TBM operator has to choose the most suitable ‘driving mode’, and be extra vigilant to minimise chances of causing any disturbance to the surface of the surrounding vicinity as he guides the machine through this challenging stretch of rock-soil mix,” says Ubull.

Minimising risks

Every element of the MRT project, whether it involves tunnels or not, starts with comprehensive ground investigations carried out by collecting samples from bore holes and other geophysical techniques.

“Conventional studies may not always yield enough information to assess factors such as the variability within a block of rocks, the exact location of fault zones, or the stand-up times (a measure of how long the tunnel wall will hold its shape right after excavation) of softer ground. All of these factors are important when it comes to boring the large diameter tunnels required for Malaysia’s MRT system – the external diameter of a typical tunnel is 6.67m, way taller than a double-decker bus!” shares Dr Ooi.

MGKT carried out initial geological studies, beginning in 2006, by conducting preliminary soil sampling along the underground alignment of the tunnels; the stretch under Jalan Bukit Bintang proved to be particularly difficult to sample (without some form of traffic diversion) as it was nearly all built over.

In addition to this, there is also a near-absence of good record keeping by the relevant authorities as far as buried utilities are concerned, with contractors performing excavations routinely surprised to find electricity cables and water pipes when none were expected – even after completion of ground scanning using state-of-the-art equipment to detect buried objects.

Dr Ooi Lean Hock preaches constant vigilance when it comes to tunnelling work, warning that one should always have a healthy respect for the ground.

On average, the tunnels run about 30m below ground, with the deepest portion at the Pasar Rakyat MRT station with depths of 45m.

According to Ubull, preliminary soil investigations during the initial stages were not easy. “But we are grateful that there were corporations and companies who were understanding, and would grant us access when tests had to be carried out at specific locations.

“Work on building protection faced similar constraints, but we are glad that most property owners have been cooperative with our requests and this helped us perform the works required,” says Ubull.

Dr Ooi says preliminary site investigation studies included 69 bore holes and geophysical surveys of sections near the pair of tunnels as well as stations located in karst.

“Additional ground investigation was carried out during the detailed design and early construction stage, with more than 400 bore holes and geophysical surveys covering in excess of 9km. We had to collect disturbed and undisturbed soil samples for visual inspection and laboratory testing,” Dr Ooi explains.

To investigate soil conditions at station sites located within limestone, bore holes were drilled to depths of 10m below the final excavation level or through 10m of continuous, cavity-free, coring into limestone, whichever was deeper.

Critical bore holes were bored up to 1.6 times the excavation depth, which, in the case of the Cochrane MRT station site, went down another 20m below the final excavation depth of 32m.

“At the limestone bedrock, our geotechnics team collected core samples to assess rock quality by looking at weathering conditions and fracture states, with quality defined by bedrock water permeability, and hydraulic conductivity resulting from fractures.

“However, information obtained from bore holes have to be treated with caution as it had not been possible for us to put in as many bore holes as ideally desired along the entire tunnel alignment. To fill in the gap in information for stretches between bore holes, we had to conduct geophysical surveys to provide bridging information,” says Dr Ooi.

Such geophysical surveys included microgravity, seismic refraction, multichannel analysis of surface wave, resistivity, and seismic cross hole tomography.

“All these measures will reduce – though never completely eliminate – the risk of encountering unforeseen ground conditions,” cautions Dr Ooi, adding that there is no room for complacency, and everyone must maintain a healthy respect for the ground at all times.

On the bright side, since station excavation and tunnelling works began in 2012, MGKT has not encountered any major geological problems, strongly hinting that initial geological surveys and calculations are correct.

Advanced ground treatment

MGKT has to also take into consideration erratic ground conditions along the TBMs’ path.

“As part of best practices, where major or significant anomalies are detected in the ground, ground treatment work will be undertaken to stabilise the ground so that the TBM can pass through the area as uneventfully as possible. Such works include grouting, which involves controlled injection of grout into existing fissures and cavities within the rock formation to stop water infiltration,” says Dr Ooi.

During tunnelling, the real-time tunnel face pressure monitoring is done both within the TBM control room, as well as in monitoring centres above.

“Buildings located within the ‘zone of influence’ of tunnelling will be installed with settlement markers and other automated instruments that will help surveyors to determine how much settlement or movement has taken place, if any. With this information, it is possible to determine the level of settlement (if any) during and after the TBM works its way through the area. This paves the way for the next course of remedial action, if required,” shares Dr Ooi.

Despite the use of state-of-the-art equipment, construction techniques and comprehensive management, Bukit Bintang remains a highly formidable stretch to tackle.

Naturally, the mark of “success” will be when no one notices anything as the two TBMs tunnel past Fahrenheit88 over the next two months.

Tags / Keywords: Lifestyle , MRT works , tunnelling , Gamuda , tunnel boring machines


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