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Penetration testing in South Africa    esopt

D.L. Webb    State-of-the-art report

The most widely used forms of penetration test in South Africa are the Dutch static cone penetration test and the standard penetration test. Several other forms of penetration test are also employed but to a much lesser extent.

In the deep estuarine and aeolian sands of the Coastal areas greater reliance is placed on the results of the static cone penetration test and here the methods of analysis have generally followed those de-veloped in Holland and Belgium. During recent years other, semi-empirical, methods have also been employed in evalua-tion of the results of static cone penetration tests.

Details of the various penetrometers employed and the procedures followed in sub-soil penetration testing are briefly des-cribed, and the current methods of eva-luating the results in both transported and residual soils are reviewed.

GEOLOGICAL CONDITIONS IN SOUTHERN AFRICA.

As shown in Figurę 1 by far the greater part of Southern Africa is underlain at shallow depth by bedrock strata carrying residual soils. It is only where deeper soils of transported origin occur, as in the alluvium of the major river valleys of the interior of the sub-continent, or in the sand dune areas and drowned estuaries of rivers around the coast, that penetrometers for subsoil testing have been employed to any great extent. Thus far there appears to have been little or no demand for penetration testing in the deep transported fluviatile and aeolian sedi-ments of the Kalahari basin.

The residual and transported soils of the sub-continent both frequently contain appreciable quantities of gravel, boulders and talus materiał which prevent the use of penetrometers. Pedogenic materials such as calcrete, ferricrete and silcrete also occur extensively in the form of strata and lenses in the shallow residual soils overlying rock, and in these materials subsoil exploration is generally limited to inspection shafts, andaugeror rotary drill holes.

Other than the raain rivers of the interior, which flow across broad peneplains, the rivers of the sub-continent are cha-racteristically steeply graded except in the estuarine reaches of their lower courses and in the Coastal plains of the eastem part of Southern Africa. The Coastal belt is characterised by dunes of various ages and degrees of weathering which in many places may rise to heights of up to 150 m above the underlying bedrock.

The estuarine parts of the rivers are in-filled with variable sediments ranging

from coarse gravels through fine sands and silts to clays. These sediments are related to Pleistocene and Recent glacio-eustatic sea level changes and in places may be as much as 70 m deep. Due to desiccation and to migration of dunes during their deposition, the estuarine sediments in the peripheral rivers of the Coastal area frequently have a complex stress history. Thus, for example, clays of these estuarine sediments may vary from stiff overconsolidated clays to very soft sensitive clays, or slurries, still in the process of consolidating un-der their own mass.

Some of the major urban and industrial areas of Southern Africa are located in such infilled areas, for example Cape Town, Port Elizabeth, Durban and Lourenco Marques, while the futurę industrial city and harbour of Richards Bay are also located in these materials. It is for this reason that penetration testing has found its greatest application in estuarine situations in Southern Africa up to the present time. Elsewhere in the country subsoil exploration usually in-volves rotary core drilling or excavation of trial holes and shafts. Because of the very shallow depth of the ground-water, the use of trial holes and shafts in the Coastal estuarine areas is generally limited to the dune materials which, however, often occur extensively to distances of up to several kilometres from the coastline. In places, such as the Cape Flats, the Port Elizabeth area and the eastem, or Zululand Coastal plain, dune areas can be many tens of kilometres in width.

Of extensive occurrence under the Coastal Durban area and under the Zululand Coastal plain are deep deposits of partially ce-mented silty and clayey fine sands of Cretaceous age. This materiał is highly overconsolidated, morę than 300 m of superincumbent similar sediments having been removed by erosion in the geologie past. Penetration testing in this materiał, except in its shallow upper wea-thered zonę, is ordinarily not applicable, because of the high point resistance en-countered.

USE OF PENETROMETERS.

Dynamie penetration tests, in which the penetrometer i6 driven into the soil by percussion, and static penetrometer tests, in which the penetrometer is pushed stea-dily into the soil under hydraulic pres-sure, are both in widespread use in South Africa.

Dynamie penetration tests are carried out in the following soils.

1. Alluvial and estuarine sediments com-prising loose to dense sands and gra-vels, soft normally Consolidated clays to stiff overconsolidated clays and