Ground Treatment by Combination of CSM and Jet-Grouting in Hong Kong

The Tuen Mun-Chek Lap Kok Link Project involves the design and construction of a dual two lane subsea tunnel of approximately 5-km long (3.1-mile) between Tuen Mun Area and the future Hong Kong Boundary Crossing Facilities (HKBCF) on Lantau Island. Two tunnel boring machines (TBM) of 14-m (46-ft) and 17.6-m (57.8-ft) diameter started operation in March 2015 from northern side of the reclaimed land at Tuen Mun. In order to improve the ground conditions for the world’s largest 17.6-m diameter TBM to break in, mass treatment with a combination of CSM panels and jet-grouting columns was employed to carry out the ground treatment works at the location ahead of the launching shaft. Break-in plug properties, as designed by the main contractor, were an unconfined compressive strength of 3 MPa (435 psi) minimum in reclaimed sand, 1.5 MPa (220 psi) in marine deposit and alluvium layer with a minimum 1x10^-6 m/s global permeability of the treated ground. A mass treatment by overlapping cutter soil mixing (CSM) panels was proposed, associated to a layer of deeper jet-grouting columns made to anchor the break-in treatment into decomposed granite underneath. A row of overlapping jet-grouting columns were also used to guarantee a good continuity between the launching shaft D-Wall and ground treatment. This paper presents the design considerations and the application of the CSM technique at the project site Tuen Mun–Chek Lap Kok Link Northern Connection Subsea Tunnel Section. Additional technical points and results based on experience will also be discussed.