The hole cut is about 3 inches in diameter and the same size as that cut by a standard NX bit. The core, 1⅞ inches in diameter, is a little smaller than that from a standard NX bit but, nevertheless, the core recovery obtainable in difficult ground is much superior to that obtainable from the use of NX bits and stationary double-tube core barrels. NM core barrels are now adopted as standard equipment for exploratory core drilling, They have given good results in closely jointed sedimentary and metamorphic rocks, granite, crushed granite and in weathered granite and gneiss where previously high core losses would have occurred.

Every effort is made to impress the drillers with the idea that maximum information from the drilling, particularly the highest possible recovery of core, is much more important than the footage of hole drilled. The drillers are required to keep a daily log sheet on which details of each lift are recorded. Cores are logged frequently during drilling by the engineering geologists and plotted on standard sheets to a scale of 1 inch : 10 feet and also 1 inch : 50 feet.

Testing of Diamond Drill Holes:

Water Pressure Tests.—Diamond drill holes are systematically tested in sections, usually 20 feet long, by water under pressure (Fig. 5). After the hole has been deepened 20 feet, a rubber packer is put down to 20 feet from the bottom and expanded mechanically against the wall of the hole, and water pumped into the 20-ft. section between the bottom of the hole and this seal, at a pre-determined pressure. The quantity of water passing out of the hole is measured. The test is repeated at pressures increasing by 25 lb. per sq. in. up to a maximum pressure limited to l lb. per sq. in. per ft. of depth of the rubber seal, to avoid lifting the rock.

In instances where artesian flows of water are encountered, the seal is placed above the point where the water is entering the drill hole and the pressure which it develops is measured on the pressure gauge.

The test as described above can only be applied to uncased sections of diamond drill holes. It cannot be used in unconsolidated sand or gravel or in completely or highly weathered rock, in which the walls of the drill hole need to be supported by casing. The test is carried out in stages during drilling so as to test the walls of the hole before the cracks and fissures become blocked with drill cuttings and grease.

The purpose of the water pressure test in the case of dam sites is to obtain a quantitative measure of the permeability of the dam foundation and abutments, to serve as a basis for estimating the amount of leakage which could occur from the reservoir, and for judging the need for foundation treatment, depth of grout holes and amounts of grout to be allowed for in estimates. In the case of shaft, tunnel and underground power station sites, it provides a basis for judging the amount of water inflow to be expected during excavation. Furthermore, the results of water pressure tests considered in conjunction with an examination of the core assist materially in judging whether the joints and fractures seen in the cores represent open fissures in the ground, or whether they are closed. This is important in judging the probable stability of the rock mass intersected by these joints and fractures, when chambers are excavated into it. Since the tests are carried out according to this standard procedure, different sites can be compared in respect to permeability.