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South Africa

The CBR penetrometer is intended for use where access is difficult for conventional truck-mounted insitu CBR testing eąuipment. It also provides results ąuickly. Nume-rical values of the CBR are related to the penetration of the cone in mm per blow at any depth by the calibration given in Table 1.

Calibration of CBR Penetrometer.

(After van Vuuren 1969)

TABLE 1

mm/blow	CBR	mm/blow	CBR	mm/blow	CBR
<4	50+	13	17	28	7
5	50	14	16	33	6
6	40	15	14	38	5
7	33	16	13	45	4
8	28	18	12	60-70	3
9	25	19	11	80-90	2
10	22	20	10	>100	1
11	20	23	9
12	18	25	8

The recommended procedurę in fine grained soils is to perform three penetrometer tests at each position and average the results. In gravelly soils van Vuuren re-commends increasing the number of tests and disregarding any values that are ob-viously out of linę, the penetrometer ob-yiously having come in contact with a stone. In two comparisons of CBR's by conventional tests and those by the CBR penetrometer, the penetrometer values significantly exceed the conventional va-lues at depths below about 200 mm. Calibration of the penetrometer for parti-cular site conditions and particular depths would thus appear to be necessary if control testing is to be based on quan-titative and not qualitative results. Sanglerat (1972) draws attention to the importance of using CBR penetrometers in conjunction with conventional tests and not to replace them.

Dynamie cone penetrometers are employed without casing, and thus, as pointed out by Schultze (1957) the penetration resis-tance is affected by shaft friction below ground-water level, particularly in clays.

Dutch Static Cone Penetrometer.

The only static penetrometer used in South Africa to any extent at the present time is the Dutch static penetrometer. Kantey (1951), however, describes a static penetrometer constructed by the National Building Research Institute which uses either an open cone or a mantle cone. The sectional area of the cone is 9,08 cm² to coincide with the cross-sectional area of the E rods used for the outer tubę.

The basie components of the Dutch static cone penetrometer, as employed in South Africa, have been described by Kantey (1951) and Sanglerat (1972) . These pene

trometers are of the movable shielded cone type, the cone having an apex angle of 60 degrees and a sectional area of lO cm² with a diameter of 3,57 mm. The penetration rods are actuated by a moto-rized hydraulic piston, with a stroke in excess of a metre, connected to a Iow rangę and a high rangę Bourdon type pres-sure gauge. The Iow pressure gauge is graduated in intervals of 0,2 bars to 10 bars and the high pressure gauge in inter-vals of 1,0 bars to 40 bars, both gauges being filled with glycerine to provide greater accuracy. Outside and inside diameters of the outer Steel tubing, which is flush jointed, are 33,3 and 20,3 mm respectively. The inner penetration rods are of solid stainless Steel with a diameter of 19,0 ram in lengths of 914 mm, with accurately sguared ends, which are simply placed on top of one another inside the outer tubę. Overall length of the cone and sleeve assembly is 161,1 mm and the inside and outside diameters of the upper end of the shield are 27,1 and 29,4 mm respectively.    Some cone assem-

blies have been redesigned recently to inerease the length of travel from 117,8 ram to 228,6 mm. In most cases.the capa-city of the penetrometer equipment, in terms of total resistance, is 10 000 kg.

It is held in position on the ground by helical earth anchors. Sometimes penetrometers are loaded by concrete blocks to provide reaction to the thrust of the penetration rods, but this usually limits the capacity of the penetrometer to we11 below 10 000 kg. Dead loading of this sort is used where the shallow subsoil is too soft for helical anchors. In the estuarine loose sands and soft clays depths of up to 40 m are often attained. The ratę of penetration is 2 cm per sec-ond, and recording of the hydraulic pressure on the gauges is madę at intervals of 20 to 25 cm. Total resistance and point resistance are determined from the hydraulic pressure, and the results plot-ted, as shown in Figurę 5, to show varia-tion in total resistance, point resistance and shaft frictional resistance with depth.

It is common practice to put down a bor-ing with standard penetration tests and a deepsounding with the Dutch static cone penetrometer together, at a few locations on a site, to obtain correlations to en-able the static penetration test results to be used to assess soil types. Simi-larly, borings and static penetration tests are often altemated on a grid vary-ing from 15 to 50 m or morę depending on the extent of the site and the subsoil conditions. In estuarine clays the cone resistance, q_c, does not usually inerease appreciably over the stratura thickness. When the clays are normally Consolidated q_c is relatively Iow. In contrast the value of q_c in sands generally inereases markedly with depth.