Well, the nights are drawing in now and the mornings have a bit of a nip in the air so autumn is upon us, although the days don’t seem to be cooling down that much. I made a promise that I would write a monthly newsletter and my time has almost run out for March, so best I stick to my commitments and get those fingers flying over the keyboard. What has been happening at the GeoZone? Well, apart from the smaller geotechnical projects we also installed a rain gauge and evaporation pan loggers at Matla Power Station near Kriel which went off successfully. Of course it tipped with rain the day we were meant to work which was great for the mielie farmers but pretty dismal when it comes to installing high tech loggers in that sticky Highveld mud. We chickened out though by loading the instruments into the truck and configuring the logger back in the BnB – which was a far more civilised arrangement.
Brink Volume 2
Keeping with the Brink theme I was blown away by the response I got to my first email. Indeed the breadth and reach of his work was amazing, but we also have to acknowledge the work of the pure geologists who mapped south Africa and on whose foundation Brink built his work. From an engineer’s point of view I think Brink is the only geologist who registers on the radar, but all the geological formations that Brink describes and the interpretations thereof were done by geologists out there in the hot sun and freezing cold, mapping, sketching, photographing and interpreting. And in the early days some of that work was done even before the surveyors had mapped it, so they had to make their own topographical maps. No air photos, Google maps, GPS units, air conditioned four wheel drive vehicles are fancy caravans to assist them in their endeavours. So here’s to geologists all and the dedication to their craft.
Having said that, let us return to Brink and the next layer in the sedimentary pile. Last time we dealt with the wind blown deposits of the Bluff and Berea Formations and to a lesser extent the estuarine deposits of places such as Richard’s Bay and Durban. Moving onto more terrestrial matters Brink addresses the concept of colluvium, which is a broad and inclusive term for soils which have been transported by gravity and surface wash. Colluvium can comprise fine grained hillwash and coarse grained talus and anything in between. Generally speaking slopes are either concave or convex with concave slopes predominating in South Africa, often influenced by the hard capping layers found atop koppies and plateaux. Which forces me to dive into some of that oh-so- boring matric geography – the elements of a slope – namely the waxing slope (crest), the free face, the talus or debris slope and the pediment or foot slope. So the free face, which in common English most of us know as a cliff, retreats due to erosion. Material accumulates on the talus slope whose profile is governed by the angle of repose (the angle of friction) of the material. A natural winnowing process takes place where the finer material is carried downslope and away from the cliff to create the pediment slopes with angles of less than 5 degrees.
Colluvium on these concave slopes can attain thicknesses of many metres. The coarser material closer to the cliff has a factor of safety close to 1 and caution needs to be exercised when engineering these materials. Further downslope cohesion may increase due to a reduction in particle size. Gravity is the propelling mechanism and in association with water can have disastrous consequences at worst. So those of us who ply our trade on the eastern side of the country need to be doubly aware of the issues of water, drought conditions aside. The colluvium of KZN and the Eastern Cape can be extremely thick and highly expansive and once again caution needs to be exercised when engineering these materials. And then of course there is the problem of settlement.
And there I must leave it once again as I have used up my quota of words. Thank you Mr Brink for providing us with your knowledge, experience and insight, and for illuminating the path ahead so that we are less inclined to stumble on those rocky outcrops and talus boulders that lie in our path.
Marmaduke the Tyrannosaur
On another note Marmaduke the T Rex has moved to the office and is drawing attention from the kids. Six metres of ornery beast are now occupying loads of linear space next to the window – where else – so looking forward to having some fun with him. He had his second car ride today to the hoots and waves of passers-by. Fame is such a wonderful but ephemeral thing. It must have been those Mesozoic hips and long legs that were attracting all the attention.
Do have a very splendid day.