Rock Mass Properties and Their Potential Impacts on Trenchless Projects

By David J. Scarpato, P.E., Haley & Aldrich Inc. | August 2012, Vol. 67 No. 8
HDD pilot bore road crossing exit pit in moderately weathered tuffaceous siltstone in Utah.
  • Near-field drill hole deviation – Also sometimes referred to as drill hole “wander” or “deflection.” Drilling involves cutting or comminution of the rock, followed closely-behind by placement or installation of a linear product like drill casing or plastic pipe, for example. As the length of the drill string or product pipe increases in down-hole length, there is a corresponding reduction in the flexural rigidity of the linear member, which in-turn imparts a bearing force directly on the geologic material that forms the outside diameter of the drill hole. If the material is rock, then the rock needs to be able to impart an equal and opposite force back on the pipe in order to resist bending. In cases where a weak rock mass is present or where discontinuities in the rock are persistent, open and frequent, “unanticipated” drill hole deviation may result. For smaller diameter bores as may be the case with many HDD projects, open joints and seams in the bedrock could result in a preferred path of least resistance for the drill string, potentially resulting in excessive drill steel deflection along the drill path.
  • Hydraulic continuity problems – If persistent and open discontinuities intersect the proposed bore path, then the possibility of a “frac-out” along a discrete joint plane cannot be ruled-out. A frac-out has implications for both the drilling operation and the surrounding environment. There can be potential permit violations and water quality impacts, in addition to the loss of a critical lubrication and cutting transport agent. If anticipated, drilling fluids can be conditioned or in more extreme cases, ground pre-treatment via pressure grouting could also be undertaken.
  • Geostatic stress state – Understanding in-situ stress state at depth can be helpful in determining the required drilling fluid pressure and drill mud properties that are needed to ensure a stable hole, and to minimize chances of frac-out. In cases of overburden soils, geostatic stress at depth may be estimated by simply multiplying soil unit weight by the depth of soil between ground surface and proposed bore depth. This is sometimes also used as a crude initial estimate for geostatic stress estimation in bedrock; however, in the event that the rock mass either contains relic horizontal stresses or where rock mass conditions are highly variable, rock mass classification correlations and/or down-hole pressure meter testing may be beneficial in evaluating in-situ stress state, particularly for projects with advanced level of risk.

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