Horizontal Soil Mixing for A Platform Link Under NSL Orchard Station, Singapore-Paper

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The T219 Thomson-East Coast Line project required a new platform link beneath Singapore’s operating NSL Orchard Station. Constructing the link involved sequential mining and underpinning approximately 30 m below ground, directly beneath the existing station’s piled-raft base slab. Because trains continued to run above, settlement limits were stringent: no more than 15 mm total settlement and a maximum differential settlement of 1:1000.<br /><br />Excavation removed soil support beneath the slab and altered how its loads were carried, creating a particular risk in areas where the raft supported most of the load. Ground improvement was therefore installed ahead of the excavation to strengthen and stiffen the soil. Jet grouting was avoided because its high installation pressures could cause settlement. Instead, crews installed horizontal, 780 mm-diameter soil-mixing columns from the underground top-heading excavation, using modified, low-headroom drilling rigs and Soletanche Bachy’s Springsol tool.<br /><br />Obstructions and access constraints prevented construction of a uniform treated block. The team created “soil mixing walls” from overlapping, high-strength columns, with the arrangement modelled to provide equivalent support. Trial and production testing showed 28-day unconfined compressive strengths of 2–2.4 MPa and Young’s moduli of 480–550 MPa.<br /><br />Real-time monitoring tracked slab and track movements, while strain gauges measured loads in underpinning columns. Maximum track-level settlement was 3.7 mm at the end of construction—well within the 15 mm limit. The horizontal soil-mixing walls provided critical support to the existing foundation and helped enable the mining and underpinning works without disrupting railway operations.

Keywords

Thomson-East Coast Line

Orchard Station

underpinning

sequential mining

soil mixing

ground improvement

settlement control

horizontal columns

real-time monitoring

railway excavation