Crossing erratic ground
AS cities get crowded and busier, it is only natural for public transportation systems to be in favour of going underground. Malaysia has to some extent been onboard with the presence of the LRT and MRT systems in the Klang Valley; however, we are barely scratching the surface of metro travel.
The Sungai Buloh-Kajang line, completed in 2017, has a 9.5km-long tunnel with seven underground stations, while the Sungai Buloh-Serdang-Putrajaya line that is currently being built has a 13.5km underground portion with 11 stations. What have we learnt so far from these tunnelling experiences?
One thing is for sure, tunnelling in Kuala Lumpur’s geological landscape is no walk in the park.
Even with ample experience in building tunnels here such as the Stormwater Management and Road Tunnel (SMART), the Larut and Berapit tunnels in northern Perak for KTMB’s double-tracking project, as well as the tunnels for the Sungai Buloh-Kajang MRT Line, MRT tunnelling contractor MMC-Gamuda is not taking things lightly
“The paramount rule to observe is to treat each tunnelling project with caution. We always respect the ground,” said MMC-Gamuda’s geotechnical department head Dr Ooi Lean Hock.
“The fact is the SSP underground alignment is more difficult to design and build, not easier, compared to the SBK alignment. For the SBK, it was a rather well-defined zone of either Kenny Hill or KL limestone, with a difficult transition zone at the Bukit Bintang area.
“In the SBK Line, we were blessed with the relatively good kind of Kenny Hill (typically defined by the interbedding of sandstone and shale) for tunnelling. The presence of extreme karst (grades 4 and 5) were largely taken care of by the variable density TBMs and targeted ground treatment works at specific locations.
“All in all, we dealt with two fault zones as we crossed through 9.5km of underground terrain,” said Ooi.
The major challenge for the SSP tunnel and station builders come in the form of seven inferred fault lines (versus two in the SBK Line), and various anomalies in the ground that are too small to detect using the usual soil investigation techniques.
“’The geology is a lot more complex. The tunnels are typically deeper, there are more structures and objects underground, and stations have to be built close to existing high-value or important buildings,” said Ooi.
Fault lines are areas where rock movements occurred in the past. There is a tendency for these zones to undergo weathering and dissolution when mild acidic conditions are encountered, thus disrupting the continuity of conditions optimal for tunnelling.
When a tunnel boring machine (TBM) hits an interface consisting of a substantial mixed zone, which can just be no more than several metres wide, conditions can change rapidly to affect the face support pressure of a TBM.
Fault zones could also be filled with a lot of water as they are highly permeable zones, thus facilitating the movement of ground water.
All these factors mean it is very difficult to predict the TBM face support pressure and rock strength of the area that the TBM will have to pass through. For any tunnelling contractor, crossing a fault zone or a mixed zone is one of the riskiest manoeuvres during the project.
Intermittent changes of geological formations on the tunnel face and mixed-face conditions could also accelerate TBM cutterhead wear, ground settlement, tunnel face stability problems, muck conditioning problems, groundwater inflow and in some cases, even TBM steering problems.
“Other than fault zones, we also have to contend with some folded limestone. The synclines and anticlines inherently have many cracks and fractured zones, posing many unexpected challenges to tunnelling,” said Ooi.
“Based on interpreted geological profile, both TBMs from Bandar Malaysia North station to Chan Sow Lin station traversed mostly through limestone. Although preliminary geological studies indicate that the SSP karstic geology is similar to that of SBK Line, we experienced significantly more challenging ground conditions in terms of virtually undetectable geological troughs and mixed face conditions.
“True enough, the TBMs encountered localised drops in rockhead in the TBM horizon.
“Buried geological structures such as faults and folds are very difficult to detect and require intensive investigations to be defined. Two-dimensional geological profiles may not capture what is essentially three-dimensional geological anomalies where localised troughs, pinnacles or fractured rock could be missed out during discrete rock probing.
“Rock can also be folded in more than just two dimensions, thus complicating geological mapping,” added Ooi, who cited a 1993 KL geology map that showed a granite intrusion and some folding of limestone in the vicinity of the TBM’s path.
Tuesday’s breakthrough was a particularly proud moment for MMC-Gamuda as it proves that a refurbished machine could successfully complete a tunnelling job even after four years of hiatus where the refurbished machine underwent rigorous preservation works to maintain the vitality of its machine parts.
S776 (codename for TBM) was retrieved in 2014, after excavating the tunnels between Cochrane station to Tun Razak Exchange station of the Sungai Buloh-Kajang MRT Line.
‘’The fact that the machine performed flawlessly is testament to our maintenance regime, and engineering enhancements that made it even more effective in dealing with our karstic and other somewhat mixed ground conditions,” said Ooi, adding that by the middle of this year, a total of 12 TBMs, eight of them refurbished here, will be deployed to work on the SSP Line.
S776 successfully crossed 20m under the 14-lane Jalan Sungai Besi without impeding the flow of traffic at all, and the LRT operated by Rapid Rail kept on running even as the TBM crossed under the Ampang LRT line at a depth of 25m.
Secondly, S776 was piloted by an all Malaysian crew, testament to the depth and breadth of qualified manpower that Gamuda has built up over the years, beginning with the construction of the
SMART tunnel in 2003, the region’s first dual-purpose tunnel in Kuala Lumpur which is the longest stormwater drainage tunnel in South-East Asia and functions as a vehicles underpass when not in use as drainage.
A back of the envelope calculation showed that the TBM advanced at an average rate of almost 6m each day, showing the difficulty of the task for an experienced crew armed with the most sophisticated TBM available in the market now.
To this, Ooi said: “The geology is very complex, and very subtle. The tunnelling crew has to understand all these and make adjustments as they encounter them to manage the risks. This is not a task that anyone can just come in to take on.”