Groundwater processes are recognised as critical to design and construction of major tunnels. These issues have achieved prominence in many tunnelling projects due to mistakes in the past. The full range of groundwater investigation and analysis tools are now required to define impacts and to ensure safe and acceptable tunnelling projects. This talk will focus on groundwater fundamentals in tunnel investigations, analysis and modelling tools, design approaches and acceptable groundwater impacts, and control and mitigation strategies.
Determining the effective hydraulic conductivity along the tunnel continues to be a challenge as investigation methods (principally slug tests and packer tests) produce widely varying results. Interpretation approaches of these test results (including from pumping tests) often produce very different answers. A suggested analysis approach is proposed. The determination of design groundwater levels requires the evaluation of extreme and long term hydrogeological processes. These levels drive the design in many cases and even relatively small changes in levels have significant financial implications.
Analytical and numerical analysis methods are often hampered by a lack of data for the development of a good conceptual hydrogeological model. Nonetheless these methods are very useful for weighing up various options. The currency of numerical models during construction is often poor and regular updates of models are needed, although rarely undertaken. Calibration of numerical models against pumping tests continues to be a necessary evil, as frequently there are no alternatives to undertake a transient calibration. Grouting is both a fundamental design tool and a remedial measure. Grouting behaviour can also provide a fundamental investigation tool and predictor of groundwater impacts.
These approaches all lead to hydrogeological judgements which are based on science but heavily influenced by construction practices and operational functionality and the perception of what is an acceptable impact. A case study of Melbourne’s City Link will illustrate the often difficult decisions to be faced in the art of hydrogeology as applied to tunnelling.