This paper outlines the feasibility assessment conducted to explore the possibility of constructing two submarine water supply pipelines beneath the Ma Wan Channel in Hong Kong using either horizontal directional drilling or tunnelling. The Ma Wan Channel is a major shipping channel in the Northwest New Territories, which separates Ma Wan Island from the mainland. The challenges faced by the project's designers included space limitations on either side of the channel, marine department restrictions on construction activity within the channel and selection of a suitable crossing alignment and profile given the channel's depth, irregular sea floor topography and complex geology. The existing geological information indicated the proposed alignment would cross granitic and volcanic formations, which would likely be faulted._x000D_
The primary concerns with horizontal directional drilling were: the length, diameter and depth of the crossing were at the limit of precedent experience in rock; the lack of available space at both the entry and exit sites; and whether the technique would be flexible enough to cope with the anticipated range of ground conditions._x000D_
The primary concerns for tunnelling were: the possibility of encountering large inflows of high pressure groundwater; and the possibility of encountering disturbed ground within the fault zones which could flow if not properly treated._x000D_
As a means to evaluate and mitigate these technical risks for both construction options, a directionally drilled pilot hole was used to investigate the ground conditions along the proposed pipeline alignment. The pilot hole results indicated that either directional drilling or a closed face TBM drive were technically feasible crossing options at a crossing depth of 80 m. The results also indicated that either an open face TBM or a drill blast drive would need to be constructed at much greater depth because of the presence of highly permeable, disturbed ground in mid-channel at this crossing elevation.