The successful execution of a large number of specialty geotechnical construction techniques necessitates the efficient and safe drilling of holes through any and all ground conditions. Inappropriate means and methods may, in fact, worsen the ground properties or structural conditions the construction technique is intended to remedy. There are a potentially bewildering variety of drilling methods and associated technical concerns. This review is intended as a fundamental guide to various aspects of the technology, including drilling methodologies, flushing, drill hole deviation, monitoring and specifications.
The specialty geotechnical construction processes of grouting, anchoring, micropiling, soil nailing and ground freezing all require the drilling of holes through overburden and/or rock. Such holes typically are 3 inches to 12 inches in diameter and are rarely more than 200 feet deep. Holes may range in inclination from vertically upwards to vertically downwards, with most holes for grouting, micropiling and freezing being within 30 degrees of vertical, and most holes for anchoring and nailing being within 30 degrees of horizontal. Although rock masses naturally are variable in terms of strength and structure, overburden - from the drilling viewpoint - usually poses far greater difficulties to the drilling contractor. For the purposes of this review, overburden is regarded as any non-lithified material, either deposited or formed by nature, or placed or created by man. Such material may range from soft and loose to hard and dense, and from dry to saturated. Overburden may contain artificial and/or atypical inclusions or horizons, which will be problematical to penetrate - for example, boulders or deep foundations in soils, and utility trenches or shallow foundations in fills. Such conditions will challenge the drilling contractor who, for financial reasons, will always want to drill the holes as quickly as possible, with the minimum practical “footage” cost. Equally, however, specific project needs may impose significant restrictions or performance requirements.