ABSTRACT

This paper draws from experience gained from a series of risk assessments and on site managements of such risks from recently completed, major rock tunnel projects in Hong Kong. The following projects are selected to identify a variety of adverse geological hazards encountered by a range of rock tunnelling methods:

  • Kowloon Southern Link – TBM excavation using slurry for tunnelling in extreme mixed face conditions below Kowloon Peninsular;

  • Eagles Nest Highway Tunnel - Drill and blast excavation of 18 – 20m span rock tunnels with low rock cover and mixed face conditions at portals requiring pre-support and heavy support;

  • Tai Po potable water aqueduct - 12kms of TBM rock tunnel with steel liner, excavated at 700m depth in hard volcanic and granitic rocks with high water inflows.

A state of the art Risk Management Plan and Matrix was employed for the first two of these projects. Key geological risks successfully managed during construction of all three tunnelling works are presented together with the appropriate mitigation measures adopted in order to maintain effective control of the works. Back analysis of costs of tunnelling in different IMS rock classes are presented to demonstrate the importance of geological conditions on cost and programme.

1 INTRODUCTION

Hong Kong's tunnelling industry is relatively mature, highly mechanized and sophisticated. To date most modern tunnelling methods have been or are being employed. At depth the terrain is largely granitic and volcanic in nature giving stable tunnelling in extremely hard rock types up to 400Mpa in strength with occasionally up to 300m overburden cover. In the case of the Tai Po to Butterfly Valley aqueduct cover was up to 700m. No substantial insitu stress or deformation of tunnels have been recorded. However much tunnelling is undertaken at shallow depth in extreme mixed face conditions often below intensively developed urban areas. Water inflow and associated settlement is recognized as being of major concern to Government, but is leading to overly tight specification of allowable inflows for rock tunnelling in mountainous areas. This is leading to unnecessary high costs to the public as well as carrying the risk of delayed project implementation and litigation.

From back analysis of geological hazards and risks encountered on three major projects completed, each at least partly in extreme ground conditions it has been possible to identify many of the key geological risks being encountered locally as well as the specific and appropriate mitigation measures undertaken.

2 RISKASSESSMENT AND MANAGEMENT

A formal and well developed risk management plan and matrix has been employed in this analysis as described by McFeat-Smith and Harman, (2004) and McFeat-Smith, (2006 and 2008) and conforms with the code for Risk Assessment and Management by Anon / ITIG (2006). This was employed in two out of the three cases cited from which experience of tunnelling in extreme geological conditions is drawn.

Throughout the development of tunnel projects the parties involved were required to hold monthly meetings to review risks associated with the various planning, design and construction methodologies and strategies.

This content is only available via PDF.
You can access this article if you purchase or spend a download.