Observational Method for Ground Treatment of Tunnel Cross Passages in Complex Ground Conditions

This paper focuses on the design and review of the ground treatment and rock fissure grouting required to excavate tunnel Cross-Passages in the Liantang / Heung Yuen Wai Boundary Control Point Site Formation and Infrastructure Works – Contract 2 in Hong Kong SAR. The Cross-Passages were expected to go through Tuff in various degrees of weathering (Grade V to Grade III/II). The Site Investigation, SI, showed that SPTs numbers generally ranged from 30 to 50 for the Completely to Highly Decomposed Tuff, CDT / HDT, with localised values as low as 6. Ground Treatment consisting of permeation and rock fissure grouting as well as 120° pipe roof / canopy tubes, was required to ensure not only the stability during excavation but also limit the groundwater inflow. The SI determined in-situ permeabilities ranging from 1x10-5 to 1x10-6 m/s for the CDT and a 21m long probe hole recorded a water inflow in excess of 60 l/minute. A discussion relative to the methods employed for drilling, e.g. pressure balance drilling system, drilling alignment tools used, and grouting techniques, e.g. microfine cement, chemical grout is presented in this paper. The use of drilling survey tools integrated with 3D representation models of the cross-passage and the ground treatment is discussed. A review of the overall performance of the Cross-Passage, e.g. groundwater inflow, stability, is undertaken.