Presented paper

IMWA2019 Students work

Integrating Hydrogeology With Geotechnical Engineering

Waterhouse, John Douglass
Golder Associates Pty Ltd, Australia

Integrating physical hydrogeology with geotechnical engineering is a valuable part of slope design in open pit mines and can be a critical safety aspect, particularly in underground mining.

In the past and perhaps now, this integration is not always being carried out properly. Designs are still being made with assumptions about drained conditions or simplistic assumptions are being made about pressure distributions within saturated rock masses or soils below a “phreatic line”.

For many years, some of us hydrogeologists have been working to provide stronger integration of hydrogeology and geotechnical engineering in the mining area.

What are the key issues and how should we think about hydrogeology in association with slope stability? The first aspect to consider is whether the highest water pressure likely to occur would affect stability. Designs for some mines with strong and poorly-fractured rock masses will be essentially independent of groundwater pressures. Others, for example in weak, layered sedimentary rocks may be extremely sensitive to groundwater pressures This consideration is the first point of integration of hydrogeology with geotechnical engineering.

Safety is a key issue. Water pressures that exceed those included in design calculations can cause failures which, if sudden, can kill people. Sudden failures and catastrophic water inrushes should be avoided in underground and surface mines.

Measurements of groundwater pressures, without undue reliance on numerical models, with regular critical review of monitoring data, are keys to minimising the risks of failure.

Apart from safety, slope design and underground design and support can be optimised with a proper understanding of the magnitude and distribution of groundwater pressures around mines. Such optimisation may be a key economic aspect of a mine, making the difference between a feasible project and one that fails economically.

Understanding the distribution of groundwater pressures requires adequate hydrogeological investigation and conceptualisation. In some situations, numerical modelling is essential to predict groundwater behaviour as a mine is developed and after closure.

Integration of the disciplines requires good communication. In the author’s experience, this must often be driven from the hydrogeological side to the engineering team.