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BANKA DRILL OPERATING MANUAL
Sand - Clay - Gravel
For working fast in wet sand, loam and gravel, the sand bailer with flap valve No.4-47, provided with bevelled shoe No.4-54, is used; the passage along the flap valve being as large as possible. The small tongue drill No.4-24 is used in looser ground, such as sandy clay, sandy loam and dry compact sand.
The spiral auger No.4-21 gives better results in firm grounds like stiff clay, loam, and decomposed bedrock. When drilling in stiff compact clay, care should be taken that the auger does not become stuck. If so, a rod turner is firmly clamped to the drill rod so that it rests upon the top of the platform socket. While turning the string of tools again, the auger cannot descend, cohesion between the clay and the auger is broken and it can then be pulled out easily.
The flat bit No.4-19 or the cross bit No.4-20 are used for breaking harder layers. In heavy ground it may require driving with a rammer, which is usually a locally cut log, provided with an iron ring and handles No.4/6-45, weight about 60~100 kg. Before ramming, the drive cap No.4/6-57 is placed on the platform socket. Do not drive deeper than 20 to 30 cm each time.
Quicksand (running sand)
Quicksand may sometimes rise in the casing faster than the bailers can remove it. The water level inside the casing should then be kept as high as possible and the casing driven down into firmer ground as quickly as possible. If this cannot be achieved by means of ramming, it can be done by drilling under the casing shoe, when each stroke of the bailer will result in lowering that shoe 5 to 10 cm. The difficult part of this operation is to drill through the column of quicksand in the casing so as to reach the casing shoe. This can be done by continuing drilling even when the bailer is already filled; it then works as a pump, pumping sand from under the shoe, keeping it in suspension for a short time as long as the bailer is moved up and down rapidly. (When the bailer rests on the bottom for a short time, or when it is operated too slowly, the sand will settle again and tools and casings must be removed together. This implies that the borehole is lost).
Boulders and buried timbers
If a boulder is too large and hard to be broken into pieces, it is usually better and more economical to abandon the hole and to start a new one nearby. However, before abandoning a deep hole, it may be desirable to first determine the depth of the bedrock. In that event the obstacle may be removed by exploding half a stick of dynamite in the hole after the casing has been pulled up about one metre to prevent damage.
Rafal Swiecki, geological engineer:
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