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South Africa

screwed to the end of a string of solid £lush jointed rods approximately 15 mra in diameter. A 9,1 kg (20 lb.) drop hainmer is raised manually into position and falls through a height of 450 mm to strike an anvil consisting of a collar, approximate-ly 25 mm in diameter and 50 mm high, screwed to the lower end of the upper rod which acts as a guide rod. The next rod, 1 m in length, is screwed firmly into the underside of the anvil collar. The num-ber of blows of the drop hammer reguired to drive the cone 300 mm into the soil is recorded.

BLOWS PER 300 mm 0    20    40    60    80

Fig. 4. Results of 25 mm diameter dynamie cone penetration test in sand.

In alluvial and estuarine sands, silts and clays and also in soft residual clayey materiał it is not unusual to carry out soundings with this apparatus to depths of up to 10 ra. It provides a useful generał qualitative indication of the relative density of sandy and silty materials and the consistency of clays. It is rapid and economical in operation and thus fre-quently used in conjunction with borings, standard penetration tests and static penetration tests to confirm the extent and depth of shallow founding or other strata, to control compaction, and to establish the existence of soft zones below founda-

tion excavations or in filis.

It is of interest to notę the effect of overburden pressure on the blow count as reflected in Figurę 4, and as often ob-served on construction sites, when the smali dynamie cone test is employed before and after excavation for foundation bases in sands.

Perth Penetrometer.

The Perth penetrometer, as described by Glick and Clegg (1965) and originally used by the Country Roads Board, Victoria, Australia, is used successfully in the Cape Flats area which forms the isthmus between the mountain ranges of the Cape Peninsula and the mainland. This area has been frequently uplifted and lowered relative to sea level, forming a generał soil profile consisting of a deposition-ary upper layer of sandy materiał over-lying the steeply dipping Malmesbury shales to a depth varying from 10 to 20 m. The upper sand layer contains freguent lenses and layers of clayey and silty sand deposited in ancient marshes and meandering rivers and streams.

The Perth penetrometer is used in two ways. Firstly it is used for testing and controlling the compaction of sand fili in conjunction with field density tests. Several soundings put down quick-ly across a site can give a good indication of the relative density of a fili, and can reveal poorly compacted layers or areas. Secondly it is used in site in-vestigations, often in conjunction with posthole augering. Again several soundings can give an indication of the sub-soil profiles, and can reveal soft clayey lenses and loose sands potentially danger-ous for footings. For both uses, a par-ticular advantage of the Perth penetrometer, and indeed also of the 25 mm cone penetrometer described above, is its ef-fectiveness below ground-water level when augering or field density testing cannot be performed.

CBR Penetrometer.

The CBR penetrometer, described by van Vuuren (1969) is a modified version of the Perth penetrometer and is intended for the rapid determination of the Cali-fornia Bearing Ratio (CBR) to depths of about lm, for road pavement design. When eguipped with a 3 m long rod it can also be used for generał subsoil investigation. It consists essentially of a 10 kg drop hammer which falls through a height of 460 ram to strike an anvil at the lower end of a 16 mm guide rod which is coupled to a 1 ra length of similar Steel rod at the lower end of which is a hardened Steel cone 20 mm in diameter. The cone is driven into the soil by the blows of the drop hammer and the penetration in mm per blow is reflected by graduations on the lower rod and recorded.