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South Africa

Fj = Total Shaft Friction q _c = Cone Resistance

Fig. 5. Results of Dutch static cone pe-netration test in estuarine sands and clayey sands.

The side friction sleeve for direct mea-surement of lateral friction over a short length above the cone tip is not in generał use in South Africa, the only one, to the author's knowledge, being at Richard¹s Bay where it has provided data for design of foundations in normally Consolidated clay8 some 40 m deep.

In carrying out tests with the Dutch static cone penetrometer it not infreguently happens that the point refuses further penetration on a dense sandy stratum that is known, from the results of borings, to be underlain by very much weaker soils.

In such situations the dense sands are sometimes penetrated by wash boring and the static penetrometer then operated through Steel casing from ground level. Satisfactory results have been obtained in this way but, because of the possibility of buckling of the laterally unsupported outer tubę of the penetrometer, the load capacity of the apparatus may be reduced.

Estuarine and alluvial sandy sediments in places in the Natal Coastal area comprise abrasive angular grains and frequent checks are necessary to ensure that neither the geometry of the cone nor its surface texture are significantly changed.

The procedurę in performing the Dutch static cone penetrometer test is in sub-stantial accord with A.S.T.M. Tentative Standard Method for Deep Quasi Static-

Cone Penetration Test. (Section D.18, 02,07 - August, 1971).

INTERPRETATION AND EVALUATION OF TEST RESULTS.

Standard Penetration Test.

Because of the many factors that can af-fect the validity of the SPT N value (Fletcher, 1965, de Mello, 1971) the test is limited largely to establishing generał qualitative Information relating to the type and distribution of subsoil strata. In the deep sands and clays of the Natal coast and in the sands of the Cape Flats it is almost always eraployed in conjunction with the Dutch static cone penetrometer test. The results of the standard penetration test are rarely used as a direct measure of the shear strength of clays or sands for design purposes, but for their qualitative evaluation clas-sifications of sands and clays in terms of SPT N value, such as those given by Ter-zaghi and Peck (1948) are employed. The graphical method of Figurę 6 is sometimes used for assessroent of allowable ground bearing pressure for shallow foundations where the depth of the water table is at least a distance B below founding level.

In very fine or silty sands below ground-water level SPT N values in excess of 15 are reduced to compensate for the effects of negative porę pressures during testing in accordance with the expression

N = 15 + ^ł/4 (N' - 15)    (1)

where N* is the SPT blow count recorded in the test. Following the work of Gibbs and Holtz (1957) a further correction is madę to allow for the overburden pressure. This correction is approximated by the expression originally developed by Teng (1964),

p + 70

where N' is the penetration resistance actually recorded and p is the effective overburden pressure in kN/m². No allow-ance is madę for the length of the rods. The corrected N values are entered in Figurę 6, and allowance for the water table level raade in accordance with the recomraendations of Peck and Bazaraa (1969).

Also employed in estimating settlement, which is usually the controlling factor rather than bearing capacity in the design of earthworks and foundations in sands, are the methods of Schultze and Menzen-bach (1961) using the expression

l/ra_y * 71 + 4,9N kg/cm² (3) and Webb (1969) using the expression

E* = 537(N + 15) kN/m²

in saturated fine to medium sand and

(4)