Cutter Soil Mixing as Ground Improvement Technology in Under-Consolidated Clays-Paper

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The paper presents the use of Cutter Soil Mixing (CSM) to stabilize under-consolidated marine clay at a reclaimed site on Lantau Island, Hong Kong, before constructing diaphragm walls for two tunnel ventilation shafts and a TBM parking plug. The site contained about 16 m of sand fill over roughly 18 m of very soft marine clay, with shear strength ranging from nearly zero to 30 kPa. Because environmental restrictions prevented dredging and the construction schedule did not allow time for consolidation, the clay posed a serious risk of trench collapse and soil intrusion during diaphragm-wall excavation.<br /><br />The project installed 540 CSM panels, generally 32–36 m deep, in two rows along the diaphragm-wall alignments. The panels served as deep guide walls and trench support. A two-phase construction process used water or bentonite slurry during penetration and cement grout during withdrawal. Slag cement (CEM III/A) was selected after laboratory trials because it produced higher strength than ordinary Portland cement and suited the site conditions.<br /><br />Quality control included slurry and grout testing, equipment monitoring, and core sampling. Core tests showed generally homogeneous treatment, an average recovery rate of 97.7%, and panel-average 28-day unconfined compressive strength above the required 1.0 MPa. Strength continued to increase over time. Wet grab samples proved unreliable in the cohesive clay, so coring was adopted for acceptance testing. The diaphragm-wall works were completed without trench-stability problems, demonstrating CSM’s suitability for improving soft, under-consolidated ground under demanding schedule and environmental constraints.

Keywords

Cutter Soil Mixing

CSM

marine clay stabilization

diaphragm walls

Lantau Island

Hong Kong

under-consolidated clay

slag cement

trench stability

quality control