Core Damage Caused by Locked-in Residual Stresses in a Shallow High-strength Rock Formation

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Department of Civil, Environmental, and Geospatial Engineering


Residual stresses refer to those stresses from sources other than contemporary tectonic forces that are locked within the rock formation. These stresses could develop and be stored within the rock during the cooling process of rock formation, metamorphism periods, previous tectonic activities, and other deformation phenomena. If relieved, by any excavation activity, for example, they can cause immediate or time-dependent (or both) deformations due to strain relaxation. If such stresses existed in any high-strength unfractured rock formations (such as a metamorphosed iron formation) in the path of an exploration core drilling, these strain relaxations could be large enough to break up the core into small pieces, reducing the rock quality designation (RQD) values even down to zero. If these stresses exist and are ignored, the RQDs from the coring could be very misguiding which results in over-designing and unnecessarily increased time and expenses of projects. In this paper, a numerical modeling approach was used to investigate a case study where an RQD of zero was reported for a shallow high-strength unfractured rock formation in Michigan. As the geologic data indicate this formation is not fractured at all, the zero value of RQD could be the result of core damage during drilling. The results of the numerical modeling conducted in this study confirmed that the release of locked-in residual stresses during the drilling process could explain the observed core damage and observed zero value for the RQD.

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Geotechnical and Geological Engineering