I have been remiss once again. Almost another month down and I am behind on my writing. It is a fresh and bright morning here in Hilton the world is on its way to work – blue buses chugging past, pedestrians striding by, and the beautiful golden light of a winters morning casting long bars of shadow across the tiled floor. But we aren’t here to discuss the beauty of a Monday morning I guess, so let’s get on with things.
The Aeolian Sands of the Interior
Up until now we have been looking at coastal and colluvial deposits, but Brink now turns his attention to the aeolian deposits of the interior. What an evocative name – Aeolus being god of the winds in Greek mythology. Can there be a better name for wind-blown deposits? I think geologists are all born romantics. Our American cousins have dropped the “A” which detracts from the origins of the name, but here in the colonies we are going to keep things as they should be.
I think we all know something about the aeolian sands of the interior – in common language we know them as the Kalahari sands. Anyone who has spent some time in Botswana knows how uniformly graded and at times treacherously soft these can be. Far more of the subcontinent is underlain by these sands than those of estuarine deposits, but the latter have received far more attention due to their distribution along strategically important zones – namely the harbours and attendant infrastructure that have been constructed over the years along our coastline.
Movies of Coke Bottles and San People
The distant Kalahari features large in movies on Coke bottles falling from the sky and the San people, and of course many of the wild life documentaries, but is not so famous from an engineering point of view. But these sands are significant in terms of road building and mine construction and as the Northern Cape and Botswana develop so a knowledge of their engineering behaviour becomes increasingly important. In these areas the basal deposits of the Kalahari Group comprise clayey gravels which are calcified and silicified in places. This lower unit can attain 100 m in thickness and is thought to be of Cretaceous or Palaeogene in age. Overlying this member is the Budin Clay Formation of up to 100 m of calcareous gravelly clays – active to the nth degree with plasticity indices in excess of 90 – which has created problems for shaft sinkers due to high lateral pressures.
Moving upwards we have the Eden Sandstone Formation – 80 metres of clayey, calcareous sandstones and finally, overlying much of the Kalahari, is a superficial cover of loose sands and fossil sand dunes, most of which have been colonised by thick bush which form the amazing wild ecosystems we see so much of on the Nat Geo channel. There are of course local variations within the Kalahari Group and if you are interested have a look at Brink, Volume 4, Chapter 6.
So how do these materials behave? Well, due to the development of iron oxide surfaces on the grains, and a small clay component due to in situ weathering, collapsible soil fabrics develop. In other areas the same problem arises due to the deposition of soluble salts between the grains such as gypsum and calcium carbonate. Leach out these components, apply a load, and hey, you may well have a foundation problem. So working in areas underlain by these sands the assumption should be that the soils do have a collapsible fabric, with the focus of the investigation being to quantify the amount of collapse to be expected.
The sands may have relatively high shear strength due to the intergranular cementing, have a good CBR when no clay is involved, but an awful one when there is, may settle under roads and airfields but can be improved with compaction and stabilisation. So once again we need to approach these sediments with a certain degree of caution when working in these areas. Forewarned is indeed forearmed.
Vibrating Wire Technology
However I must briefly touch on an altogether unrelated subject. One of the services GeoZone provides is geotechnical instrumentation and we have had some experience designing systems and installing instruments on the Gautrain, St Helena Airport, Metolong Dam, Katse and Mohale Dams, and for several smaller slimes dams and the like. One of the words bandied about is ‘vibrating wire’ but there seems little understanding of what this means exactly, so here is the quick explanation.
Each VW instrument is furnished with a high tension wire which is clamped at its ends and tensioned so that it is able to vibrate at its natural frequency. As with a guitar string, the frequency varies according to the tension in the wire. If the end clamps move due for instance to pressure being applied by a head of water, then the frequency of the vibration will change within the wire. A magnetic coil is used to pluck the wire near its midpoint and the frequency measured by a second coil which is then measured with a simple read out unit. Cheap, simple, long lived and ingenious to say the least. Vibrating wire technology is used in piezometers, pressure transducers, earth pressure cells, liquid level settlement gauges, load cells, and strain gauges.
If you would like an electronic copy of our instrument brochure please respond to my email and we will send one through forthwith.
Marmaduke the Dinosaur
And now for something completely different. As part of my fun projects Marmaduke the Dinosaur is essentially complete and now occupies a huge amount of office space in the HQ of GeoZone GeoServices. There is a smear of hand and nose marks on the glass at kid height as they sprint from their mother’s car to peer into my House of Wonders. Every so often they pluck up courage and come in to see all of the dinosaurs in the office, including myself I guess. If you want a copy of our DinoZone colouring book drop me an email and I will send one along directly.
If you found this interesting please forward on to colleagues and friends, and seeing there are no shortage of electronic copies of the dinsosaur colouring book, let all the kids know too.
Do have a very splendid morning.