Understanding Uncertainty: Assessment and Management of Geotechnical Risk in Tunnel Construction

Abstract

Risk management is fast becoming routine in civil engineering as owners, engineers, and contractors recognize the benefit of capturing, assessing, and planning for risks on their projects. Recent experience has shown that risk management techniques employed on tunnel projects can help reduce unexpected cost impacts and delays. However, capturing these risks and accurately predicting their impact is not a simple task as the nature of the ground is difficult to predict and this uncertainty can often have the greatest influence on the success of the project. This paper discusses some of these techniques in detail and addresses how they can be used to help mitigate the most common cause of cost overruns and delays on tunnel projects, the ground and its behavior during construction. Details from case histories are presented to illustrate the influence that geotechnical risks can have on tunnel projects and the techniques implemented to mitigate these risks are outlined. BACKGROUND Unlike conventional surface construction, tunnel construction takes place in an unknown environment where ground conditions are difficult to predict prior to construction. Tunnel engineers are consistently faced with the difficult task of investigating geotechnical conditions along the project alignment, understanding and predicting ground behavior during construction, and planning for changing, and sometimes unforeseen, ground conditions. For many tunnel projects, this is a task that requires careful examination by experienced professionals and substantial investment (much more than traditional civil engineering projects). It is not uncommon for the construction means and methods for a tunnel project to account for over one-half of the project cost. This is largely due to the significant investment required for large and highly specialized equipment, i.e., tunnel boring machines (TBMs), and the need for experienced labor to operate this equipment. Decisions related to the contractor’s means and methods are of great importance to the financial success of the project, but also intimately dependent on the nature of the ground and the related geotechnical risks contained therein.