GeoZone - Applied Geology & Geotechnics

GeoZone - A Tale of Instability - Getting the Geotech Right

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.

Polished Failure Surface in Ecca Shale

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.

GeoZone - A Tale of Instability - Getting the Geotech Right

GeoZone - Applied Geology & Geotechnics