Getting the Geotech Right – a Tale of Instability

I had an ex boss whose mantra was ‘you will get a geotechnical investigation whether you want one or not’ and he was right. Settlement, subsidence, slope instability, collapse, influx of groundwater, or the failure of road pavements are just some examples of the type of damage that might occur to infrastructure founded on unforgiving soils or rock. Trouble may come knocking suddenly during a period of heavy rain, or may take place over a number of years as a house founded on an expansive clay is systematically demolished as the soils take up moisture or dry out over the seasonal wet and dry cycles. Only by carrying out a thorough assessment of the geology can one be sure that the long-term integrity of the structure is ensured.

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Life on the Slippery Slope

I have just had the pleasure of assessing the stability of the sidewalls of an open cast coal mine in Mpumalanga. The excavations are in excess of 20 metres deep in a number of instances and the mine wanted to whether they could reduce the number of benches cut in the sidewalls without comprising safety. Removing tens of thousands of unproductive overburden, stockpiling it and then returning it at the end of the mining operation is both expensive and time consuming and to be able to reduce this would assist greatly in reducing operating costs.

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Collapsible Soils - that sudden sinking feeling

Great care must be taken in identifying problem soils on site, and once identified, preventing collapse. Various methods are available for determining if soils are potentially collapsible, beginning with identifying the geology, in situ observation of voids in the soil, cone penetrometer tests, density tests, and more comprehensively but perhaps not more conclusively, collapse potential and one-dimensional triaxial tests. If problem soils are identified on a site, then the necessary precautions need to be taken to prevent damage occurring, which include drainage precautions, removing and recompacting the offending soil horizons below the structure, attending to drainage or perhaps piling through the collapsible soils if the building is particularly sensitive.

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My Geotechnical Fantasy

Wouldn’t it be absolutely fantastic to have a well written geotechnical report based on a decent number of test pits, dynamic probing and laboratory tests? If a multi-storey structure is to be built, well, I would like to drill boreholes and carry out Standard Penetration Tests. If I am looking at the stability of a slope I need to know the shear strength parameters of the soils which make up that slope and so undrained and drained triaxial tests are necessary. It might of course be a cutting in rock – again I need to characterise the shear strength of the discontinuities.

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