A Tale of Instability

Not so long ago we did a job down in Amanzimtoti. Routine we thought, and due to there being a crunch on and none of the youngsters were available, I decided that a day on site would be great and that we would get through the work quickly and be out of there by mid afternoon. Well, I dug the first test pit without anything untoward, and similarly with the second, but thereafter the situation began to change rapidly. To understand the implications of what was appearing in the test pits one needs an understanding of the local geology.

Stratigraphically Berea Red sand overlies the Basal Boulder Bed which overlies shales of the Pietermaritzburg Formation. Well, as the day went on I began to find boulders mixed in with weathered shales and the Berea sands, which was not tying up with the geology of the area. Then I began to find evidence of disturbance in the shales themselves and just after lunch came across a gun-metal grey clay horizon exposed in all four sidewalls of the pit. Some digging with the geological hammer and there lay exposed a highly polished, or what is known in the trade as a slickensided, surface. Hard evidence indeed for a land slip on the site at some stage in its geological past. To develop the site would have been foolhardy but unfortunately some of the stands had already been sold and the earthworks were in full swing, so of course my news wasn’t what the developer wanted to hear.

Well, we went back to that site twice more with a large excavator to prove to the client and the engineer what we already knew. There were of course some politics attached to the geotechnical report, and we were at one stage pressured to play down the significance of the findings, but could not for obvious reasons. There were several lessons to be learned from this experience. Firstly my blood ran cold at the thought that one of our young geologists might have done the fieldwork and may not have picked up on the significance of the disturbed geology, and that it was imperative that they be made aware of these kinds of situations and would be able to recognise them in the field should they arise.

Secondly, one needs an understanding of geology and the local stratigraphy to be able to subtle but vital changes in the local conditions. Thirdly, from a developers and civil engineering point of view it is imperative that a geotechnical investigation be carried out right at the outset to determine the underlying ground conditions. In our case the geotech had been done belatedly which caused a great deal of trouble for the developer but actually saved his bacon in the longer term, as a catastrophic slide of houses into the valley below would have had him tied up in litigation for years. 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. I think that I have made my point.

Geotechnical investigations are always carried out for large scale structures – tunnels, bridges, dams or large buildings. But too often it is neglected in the smaller projects, an approach, as we have seen, can be fraught with unforeseen hazards. Geotechnics has been part of my life for over twenty years, in a career that has included site investigation work in England, Germany, Southern Africa and Middle East. Some of these international jobs have been the large projects – both large dams on the Lesotho Highlands Project, a dam site in Oman, tunnels in the Italian Dolomites, the upgrade of Chapmans Peak drive, the massive bridge crossings for the proposed new N2 in the Transkei and finally the Gautrain Mass Rapid Transport project. Then of course there is a myriad of smaller jobs ranging from groundwater contamination assessments to 100 km long pipeline investigations for the local water utility. We are well positioned then to assist with any engineering project and it would be fantastic if we could be involved to prevent any unforeseen surprises from rearing their heads during the design life of a structure.

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