MRT2 on track despite terrain challenges

The MRT Chan Sow Lin station which goes down five levels to 40m is the second deepest underground station in the MRT2 Line after MRT Ampang Park (43m). The two cylindrical contraptions on the bottom surface are the tunnel boring machines (TBMs).

THE Klang Valley Mass Rapid Transit (KVMRT) has undoubtedly transformed, and will continue to change, the way a large number of people in the city travel.

The latest addition to the infrastructure to further improve connectivity – the MRT Sungai Buloh-Serdang-Putrajaya (SSP) Line – is well underway for completion in 2022, despite a number of technical difficulties due to its underground geological challenges.

Also known as MRT2, the SSP alignment spans 52.2km and 35 stations, with 13.5km (11 stations) underground.

This isn’t to say that it has been an easy journey thus far.

Hidden beneath the bustling metropolis of Kuala Lumpur is a highly variable terrain comprising granite, karstic limestone, and the Kenny Hill sedimentary formations.

Karst is a geological ground condition characterised by caves, caverns and rock-head pinnacles, which render any underground construction highly challenging as these can cause sinkholes and ground subsidence during tunnelling and excavation works, among other issues.

“As much as 60% of the entire MRT2 underground geological structure is of karstic limestone, ” said geotechnical consultant Datuk Ir Dr Gue See Sew.

‘As much as 60% of the entire MRT2 underground geological structure is of karstic limestone, ’ said Gue.‘As much as 60% of the entire MRT2 underground geological structure is of karstic limestone, ’ said Gue.

“Hence some early mitigation and safety measures were designed to mitigate occurrences of sinkholes and subsidence.”Deep underground

Such is the case with the MRT Chan Sow Lin station, an interchange station with LRT Chan Sow Lin.

The station spans five levels: platform level, plant room, concourse and two retail levels earmarked for a future shopping mall development. Atop the station is a proposed transit-oriented development which is still in the proposal stage.

Prior to starting construction works, MMC Gamuda conducted a microgravity survey – a geophysical method to detect sub-surface cavities – and further refined the results with boreholes to provide more information on the ground composition of the site in early 2016.

What it found was extreme karstic conditions, including a deep valley area with multiple cavities and deep rock heads resembling pinnacles 35m to 36m below ground level.

It also undertook ground treatment in the form of compaction grouting for cavities, as well as a combination of deep soil mixing to strengthen the weak soil for areas with shallow rock heads, and secant bored piles to tackle the deep valley locations.

Station manager Abdul Wakil said this demonstrated value engineering, where a problem presented is then turned into an opportunity for optimisation.

Due to the erratic nature of limestone, there was a rock slip incident near the tunnel entrance in June last year due to unpredictable joint formations behind the exposed rock face. It was originally planned for the breakthrough of the tunnel boring machine (TBM) going from Bandar Malaysia to Chan Sow Lin.

Crisis response measures to stabilise the collapsed area started immediately, while investigation studies and review activities were conducted at the same time.

“What we needed to do, firstly, was rectification works on the slip area with rock bolts and mass concrete. We managed to clean back the slope to have a safe and stable rock surface.

“Then, we had to resequence the excavation and the TBM retrieval to move it 50m to the north from the original breakthrough area and further from the slip area, ” he said, adding that they had to adjust the station design.High-risk factors

Apart from tough ground conditions, there are also other high-risk factors, including heavy traffic and close proximity to LRT Chan Sow Lin and its live railway track.

What appeared to be a simple bottom-up construction of an underground station turned out to be a multi-tiered conundrum.

In the same month, a sinkhole was discovered on the LRT Chan Sow Lin track and it had to be backfilled with grout and topped up with ballast materials with cooperation from the LRT team.

Last October, there was another sinkhole, which was dealt with in the same way to stabilise the area.

During both incidences, temporary stop-work orders causing a 10-week delay were issued by the Land Public Transport Commission (now the Land Public Transport Agency) amid concerns that the construction activity would further aggravate the situation.

Eventually, the authorities were satisfied with the rectification efforts and lifted the stop-work order. Nonetheless, there was no disruption to LRT train operations during the suspension period as works were done outside of the LRT operation hours.

Despite the progress it has made, there are still challenges looming on the horizon, one of which Abdul Wakil anticipates will be when it links to the LRT station. MMC Gamuda obtained permission from Prasarana Malaysia Bhd to build a foundation between the two LRT tracks, as the link bridge between the stations will be an elevated one.

“We have started the additional and alteration work for the elevated link bridge, but we have a very limited time frame between 1.30am and 4.30am daily, so we need to work fast.

“We also need to work in small teams and with small tools, instead of heavy machinery, due to the confined space, ” he added.

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