The relationship between the shape of a disced core and three-dimensional in-situ stresses estimated by a tensile principal stress analysis
Koji MATSUKI
Professor, Department of Geoscience and Technology, Graduate School of Engineering, Tohoku University
Ko HONGO
Graduate student, Tohoku University
Kiyotoshi SAKAGUCHI
Research associate, Department of Geoscience and Technology, Graduate School of Engineering, Tohoku University
J. fo MMIJ, Vol.113, No.5, pp.317-324 (1997)
Based upon the assumption that core discing results from tensile stresses within and below a core during boring, the direction of the principal tensile stress was analyzed in detail for the stress conditions where core discing is likely to occur to investigate the relationship between the shape of a disced core and in-situ stresses for the case of a long disced core. Main results obtained in this study are summarized as follows:
1)In the central parts of the end surfaces, a relatively flat plane is formed. The azimuth of the normal direction of the plane coincides with that of the minimum principal stress, s3. The inclination, fm of the normal direction from the core axis is approximately one thirds of that, f3 of s3. By using the two equations, (1) and (3), a method for estimating more accurate f3 (±23%) was proposed.
2)By combining the additional equation, (1) on the magnitudes of s3, the mean stress, sm and the stress in the direction of the core axis, sZ with the previously proposed equation, (2), which is the condition of core discing, two of the above stresses can be determined if fm is measured and if one of them is determined independently. For the vertical borehole, by assuming sZ to be an overburden pressure, sm and s3 can be determined from the two equations.
3)When the difference between the maximum principal stress, s1 and the intermediate principal stress, s2 is large enough, a saddle shaped disc is formed and the shape becomes more distinct with the difference. For theupper end surface of the disced core, the azimuth of the concave upwards isapproximately that of s1 and that of convex upwards is approximately that of s2. However, this does not hold strictly except the special stress conditions.
4) When the difference between s1 and s2 is small, the disced core has almost flat end surfaces.
5)The symmetry in the shape of a disced core coincides with that of in-situ stresses with respect to the axes on the core.
6)A method was proposed to estimate all of the directions of the principal stresses from the symmetry of the disc shape, the normal direction of the central plane and the azimuth of the concave axis or the convex axis when one of the principal stresses is in one of the directions of the core axis and the axes perpendicular to the core axis.
KEY WORDS: core discing, three-dimensional stresses, tensile principal stress, saddle shape