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the soli by a 63,5 kg (140 lb.) weight dropping through a height of 750 mm to a collar on the upper drill rod. B size rods of 15,8 mm internal diameter and 48,3 mm outside diameter in 3,5 m lengths are usually employed. The number of blows N reąuired for a penetration of 300 mm is recorded as the penetration resistance.

The sampler employed is the Raymond split spoon sampler 810 mm in overall length and containing a split inner cylindrical samp-ling tubę 600 mm long and 35,8 mm in internal diameter recessed about 88 mm above the cutting edge of the sampler, virtually as described by Hvorslev (1949).

In sands and soft to stiff clays boreholes are generally advanced by washing through flush jointed Steel casing having a saw-tooth cutting edge. BX size casing is commonly employed for holes to depths of about 30 m. For deeper holes NX size casing is employed initially reducing to BX beIow about 30 m depending on subsoil conditions. The NX casing employed has an outside diameter of 89,4 mm and an in-side diameter of 77,5 mm. Corresponding sizes for BX are 73,9 mm and 61,0 mm. Often loo mm diameter U4 or similar sam-pies of subsoil materiał are reąuired for laboratory testing and in such cases standard penetration tests are carried out in flush jointed Steel casing of 150 mm outside diameter.

The boring eąuipment is usually assembled in the form of a complete drilling rig mounted on a trailer. The "Dando" and ^MWay±arer" drilling rigs manufactured in Great Britain are in widespread use. In difficult terrain, including swamps, in-accessible to motor vehicles, individual items of eąuipment, such as tripods, win-ches and pumps are manhandled or skidded into position. For exploration of water covered areas the drilling eąuipment, including the standard penetration test eąuipment, is commonly supported on rafts constructed of pontoons or oil drums.

Wash borings on land or over water are ad-vanced in substantial accord with the pro-cedures described by Hvorslev (1949). The wash water is circulated through the hol-low drill rods at the lower end of which a light chopping bit may be fitted and the clays, silts and fine sands are removed by the water which is discharged on the surface of the ground and led away from the drilling rig in a smali channel. Sam-ples of subsoil materiał are taken in a baler, comprising a cylindrical tubę with a flap valve at its lower end, used for cleaning the bottom of the borehole.

Where water is in limited supply, a closed circulation system is employed and the soil-laden wash water discharged into a sump from which sediment can be recovered and examined. Soil samples retained for examination or laboratory testing are usually the split spoon samples obtained in the standard penetration test. When sands cannot be recovered in this way, however, baled samples are retained but clearly marked as such because of the strong possibility that they might be far from representative, due to loss of fine materiał in the baling water and segrega-tion and settlement of coarse particles at the bottom of the borehole.

When standard penetration tests are carried out in a wash boring particular care is taken to ensure that the bottom of the hole is carefully cleaned and that the underlying materiał is not disturbed. Excessive jetting or careless use of the bit can seriously weaken subsoil raate-rials to the depth reąuired for standard penetration testing particularly if these are fine sands or soft clays. In sands below ground-water level the water in the borehole is always maintained at or above ground-water level to prevent upward flow of sand into the borehole casing and loo-sening of underlying sand. Upward flow of sand can also occur as a result of suction caused by withdrawing the baler too rapidly. Such sand may becorae com-pacted in the casing during standard penetration testing to give falsely high N values. Apart from careful handling of the eąuipment, the level at which the standard penetration is coramenced in re-lation to the level of the bottom of the casing is also carefully checked.

Rotary Core Drilling.

When intact samples are reąuired from soils which are too friable, or too hard, for the Raymond spoon sampler or other open drive samplers, such as the Shelby tubę or U4 tubę, standard penetration testing is carried out in eonjunction with rotary core drilling. Such soils include highly weathered shales, phillites, hard clayey materials, lightly cemented sands, and various residual soils. Double tubę core barrels with core retainers are usually employed. To minimise damage and disturbance of the core due to torsional stresses a swivel head type is employed so that the inner tubę does not rotate during coring. When coring in soft or friable soils the double tubę core barrel is often provided with a thin-walled inner liner in which the sample can be sealed and transported.

Diamond coring bits of the bottom dis-charge type are normally employed, but bits with tungsten-carbide inserts are sometimes substituted in the less friable materials. Hydraulic feed drilling ma-chines are found to be necessary to permit fuli control of the bit pressure and the ratę of feed. The bit pressure due to the mass of the drill rods and core barrel may otherwise exceed that at which disturbance of the soil occurs below the core. Core size is most commonly NX, although, as in the case of wash boring, this is often reduced to BX at depths in excess of about 30 m below ground level.

To obviate erosion of the soil materiał at the bottom of the borehole and in the