A key driver to the success of a tunnelling project (the project) is the
management of hydrogeological risk. The hydrogeological risk, if not managed,
may cause significant impacts on the overall performance of the project. Impacts
may include groundwater quality impairment due to contaminant transport and acid
drainage from disturbed geology to groundwater, and groundwater lowering. The
groundwater lowering and quality impairment may cause tunnel failure, with
consequences such as injury to humans, as well as reduced property and
environmental values, potentially costing millions of dollars to sponsors of the
Depending on the nature of the contaminants and level of hydraulic connection
of bedrock fracture along the length of tunnel, the contaminant transport may
reduce structural strength of the tunnel due to corrosive action of contaminants
such as acid drainage. The groundwater lowering can lead to varying degrees of
ground settlement, posing threat of tunnel collapse.
The management of hydrogeological risk includes:
- detection of potential pathways for contaminants into and around the
- identification of factors responsible for the lowering of
- development of strategic measures to
control groundwater lowering and contaminant transport.
The paper presents an in-house hydrogeological risk management model named
Vedanta-THRM (tunnel hydrogeological risk management). The model uses Risk
Management Standard AS/NZS 4360:2004 (Standards Australia, 2004), Pressure State
Response framework developed by Organisation of Economic Cooperation and
Development, and Interaction Matrices.
The Vedanta-THRM identifies important characteristics, processes and events
of potential hydrogeological risk and develops control measures for managing
hydrogeological risk throughout the life cycle of the project.
The Vedanta-THRM was applied for a project in the early stages of planning
and found to be useful in assessing, evaluating and reducing the hydrogeological
risk in a managed approach that potentially increased the performance of the
project. The application results also provided management options for use by
project sponsors and insurance industry, which included tunnel risk financing
options and options for equity investors to choose among various returns under
different tunnel credit ratings.