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Reverse-circulation drilling is designed specifically for the construction of production water wells, but is readily adaptable to soil exploration uses, foundation projects, and underwater drilling. The method is an adaptation of the dredge to vertical holes, which permits construction of moderate to large diameter holes of any depth to several hundred feet. The water-filled holes are clean and of uniform diameter, and can be maintained uncased so long as the water level in the hole is higher than the piezometric head in the formation drilled. Drill cuttings are clean, virtually uncontaminated, and often in chunks representative of the materials being drilled. Travel time of cuttings from bit to discharge is only a few seconds, thus permitting close depth control. The method is also well suited to core sampling, which can often be done through the drill pipe without removing the drill tools from the hole. Considering this ease of sampling and the usually excellent sample quality, the cost for exploratory work may be less than with many other drilling methods. Most existing reverse-circulation drilling equipment is not readily adaptable to holes much less than 14 in. in diameter, which may be a disadvantage. However, the ability to construct and maintain clean, uncased holes up to at least 60 in. in diameter in unconsolidated materials and to great depth, could be a distinct advantage.
Burnham, W. L.
Assistant district geologist, U. S. Geological Survey, Sacramento, Calif.