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Absolute stress measurements at the rangely anticline, Northwestern Colorado
Inf. ], Rot'& 3fevh..~hn. S,J. & Geomeeh. Ahttr. Vol. 10, DO. 37~)-374. Pt=gamton Prt=s 197J. Printed in Great Brttam
DISCUSSION Discussion of R. V. DE LA CRUZ and C. B. RALEtGrt'S* Paper Absolute Stress Mea.~urements at the Rangely Anticline, Northwestern Colorado by G. T. BULLOCKt [ r ts TIlE author's opinion that an increasing number of people are coming to distrust the overcoring, strain-relief methods of determining ground stresses. This distrust arises from a re-examination of the assumptions made in calculating stress from strain or deformation measurements. Although the authors may well have satisfied themselves that the assumptions are valid t\~r the Mesa Verde sandstone in the Rangely anticline, they do not produce any results that allow others to satisfy themselves. For the benefit of others considering the use of overcoring techniques this writer wishes to point out how the assumptions can affect the results. A good survey of stress measuring techniques can be found in the work of FAIRIIURST [I ], and HALL and HOSKINS [2]. ..lssumption I. l h e rock is isotropic Most rocks are not isotropic. Even the assumption of transverse isotropy not only complicates the solution of the stress-strain relationship, but it is also only valid for some bedded rocks. Any anisotropy causes a rotation of the plane of principal stress with respect to the plane of principal strain [3]. This rotation is dependent on the elastic constants. Even the measure|nent of the five elastic constants required to solve the stress-strain relationships in a transversely isotropic body is a complicated, tedious procedure outside the scope of many laboratories. in the case of the Mesa Verde sandstone isotropy cannot be assumed as shown by KING [41. Therein King compares the properties of Boise sandstone and Berea sandstone. ..I.~sumption 2. The rock mlt.¥t be linearly elastic Very few rocks, in fact very few materials are linearly elastic, i.e. stress is proportional to strain. At low stress levels the closure of pores and flaws gives rise to large deformations and a non-linear relationship between stress and strain at the outset of (compressionai) loading. Towards the yield point another non-linearity can occur. Depending on the material the linear range may vary from non-existent to quite extensive. In the case of the Mesa Verde sandstone, in the absence of other information, it is safe to assume that, with stresses of the order of less than 200 psi maximum principal stress, that the rock would behave non-linearly. Since E (Youngs modulus) is the gradient of the stress-strain curve what value of E does one use ? Assumption 3. The rock is homogeneous In the case of the Mesa Verde sandstone it is admitted that the rock is inhomogeneous. The effects of a discontinuity are difficult to quantify, but any discontinuity makes the elastic solution invalid as the discontinuity allows inelastic deformations to occur. * Int. J. Rock Mech. Min. Sci. 9, 62.5-634 (1972). ~"Cominco Ltd., Kimberley, British Columbia, Canada. 373
374
DISCUSSION
In banded and conglomerate rocks inhomogeneity is caused by the various constituents. each with its own elastic properties. The effects of inclusions in the rock matrix, and the effects of discontinuities are dramatically illustrated by the use of an overcoring technique with a photoelastic strain gauge. One technique is described by HAWKES [5]. It is the author's opinion, based on personally supervising several hundred overcorings with the photoelastic strain gauge described by Hawkes, that the photoelastic gauge is the most underrated overcoring technique available. By a simple visual examination it is possible to tell if meaningful results have been obtained. If an unsymmetical pattern is observed, the results are meaningless. The unsymmetrical pattern is due to glue failure (because of moisture), inclusions in the borehole face, discontinuities, etc. It is the author's experience that a 5-10 per cent success rate can be anticipated. Since the technique is no different from ot herlovercoring techniques then this success rate should apply to all techniques. Since most techniques give no redundant readings a statistical test of the reliability of the results is not available. The photoelastic disk gives three redundant readings since only one quadrant of the gauge pattern is necessary for an analysis. The photoelastic strain-gauge technique is not the most sensitive, but it has so many advantages that it cannot be ignored as a rock mechanics tool for determining whether an area is more highly strained than another. The advantages include: I. Cheapness. 2. Reliability. 3. Re-usability. REFERENCF2~ 6. FAIRIIUS'rC. Ah'thodv o/ l)ett'rmininx, in situ Rock Stre.vses at Great Depths. Mi~,iouri River Division.
Corps of Enginccrs. Omaha, Nebraska, Technical Re,port No. 1-68. Rcprodm.-ed by National Technical Information .~¢rvice, February (I~K~8). 2. |I^LL C. J. and HOSKINS J. R. A Comparative Stud), off St.lected Ro~'k Str¢.v.s' aml Property Mea~lring ht~truments. Technical Relxwt No. UI-BMR-2. Spon.~rt.xl by U.S. Government Advanced Research P r o ~ t s Agtmey, June (1972). 3. H~.^R.~,~ R. F. S. An intrmhu'tion to Applied Aui.volr.pic Eluaticity, p. 6, O.U.P. (1961). 4. KI,~; NI. S. Static and Dynamic Moduli of Rocks und©r Pl'casur¢, Proceedi~x o f the Eleventh Sympo.~ium on Rock Mechanic.~. Rock Mechmlics Tiu.ory and Prm.ti<'e. p. 330, A.I.M.E. (1970). 5. HAWK~S I. "rhctwy of the photoclastic biaxial gauge and its applications in rt~ck str~s nmasuremcnts. Strain 3, (3) 36-43 July {1967).
DISCUSSION Discussion of R. V. DE LA CRUZ and C. B. RALEtGrt'S* Paper Absolute Stress Mea.~urements at the Rangely Anticline, Northwestern Colorado by G. T. BULLOCKt [ r ts TIlE author's opinion that an increasing number of people are coming to distrust the overcoring, strain-relief methods of determining ground stresses. This distrust arises from a re-examination of the assumptions made in calculating stress from strain or deformation measurements. Although the authors may well have satisfied themselves that the assumptions are valid t\~r the Mesa Verde sandstone in the Rangely anticline, they do not produce any results that allow others to satisfy themselves. For the benefit of others considering the use of overcoring techniques this writer wishes to point out how the assumptions can affect the results. A good survey of stress measuring techniques can be found in the work of FAIRIIURST [I ], and HALL and HOSKINS [2]. ..lssumption I. l h e rock is isotropic Most rocks are not isotropic. Even the assumption of transverse isotropy not only complicates the solution of the stress-strain relationship, but it is also only valid for some bedded rocks. Any anisotropy causes a rotation of the plane of principal stress with respect to the plane of principal strain [3]. This rotation is dependent on the elastic constants. Even the measure|nent of the five elastic constants required to solve the stress-strain relationships in a transversely isotropic body is a complicated, tedious procedure outside the scope of many laboratories. in the case of the Mesa Verde sandstone isotropy cannot be assumed as shown by KING [41. Therein King compares the properties of Boise sandstone and Berea sandstone. ..I.~sumption 2. The rock mlt.¥t be linearly elastic Very few rocks, in fact very few materials are linearly elastic, i.e. stress is proportional to strain. At low stress levels the closure of pores and flaws gives rise to large deformations and a non-linear relationship between stress and strain at the outset of (compressionai) loading. Towards the yield point another non-linearity can occur. Depending on the material the linear range may vary from non-existent to quite extensive. In the case of the Mesa Verde sandstone, in the absence of other information, it is safe to assume that, with stresses of the order of less than 200 psi maximum principal stress, that the rock would behave non-linearly. Since E (Youngs modulus) is the gradient of the stress-strain curve what value of E does one use ? Assumption 3. The rock is homogeneous In the case of the Mesa Verde sandstone it is admitted that the rock is inhomogeneous. The effects of a discontinuity are difficult to quantify, but any discontinuity makes the elastic solution invalid as the discontinuity allows inelastic deformations to occur. * Int. J. Rock Mech. Min. Sci. 9, 62.5-634 (1972). ~"Cominco Ltd., Kimberley, British Columbia, Canada. 373
374
DISCUSSION
In banded and conglomerate rocks inhomogeneity is caused by the various constituents. each with its own elastic properties. The effects of inclusions in the rock matrix, and the effects of discontinuities are dramatically illustrated by the use of an overcoring technique with a photoelastic strain gauge. One technique is described by HAWKES [5]. It is the author's opinion, based on personally supervising several hundred overcorings with the photoelastic strain gauge described by Hawkes, that the photoelastic gauge is the most underrated overcoring technique available. By a simple visual examination it is possible to tell if meaningful results have been obtained. If an unsymmetical pattern is observed, the results are meaningless. The unsymmetrical pattern is due to glue failure (because of moisture), inclusions in the borehole face, discontinuities, etc. It is the author's experience that a 5-10 per cent success rate can be anticipated. Since the technique is no different from ot herlovercoring techniques then this success rate should apply to all techniques. Since most techniques give no redundant readings a statistical test of the reliability of the results is not available. The photoelastic disk gives three redundant readings since only one quadrant of the gauge pattern is necessary for an analysis. The photoelastic strain-gauge technique is not the most sensitive, but it has so many advantages that it cannot be ignored as a rock mechanics tool for determining whether an area is more highly strained than another. The advantages include: I. Cheapness. 2. Reliability. 3. Re-usability. REFERENCF2~ 6. FAIRIIUS'rC. Ah'thodv o/ l)ett'rmininx, in situ Rock Stre.vses at Great Depths. Mi~,iouri River Division.
Corps of Enginccrs. Omaha, Nebraska, Technical Re,port No. 1-68. Rcprodm.-ed by National Technical Information .~¢rvice, February (I~K~8). 2. |I^LL C. J. and HOSKINS J. R. A Comparative Stud), off St.lected Ro~'k Str¢.v.s' aml Property Mea~lring ht~truments. Technical Relxwt No. UI-BMR-2. Spon.~rt.xl by U.S. Government Advanced Research P r o ~ t s Agtmey, June (1972). 3. H~.^R.~,~ R. F. S. An intrmhu'tion to Applied Aui.volr.pic Eluaticity, p. 6, O.U.P. (1961). 4. KI,~; NI. S. Static and Dynamic Moduli of Rocks und©r Pl'casur¢, Proceedi~x o f the Eleventh Sympo.~ium on Rock Mechanic.~. Rock Mechmlics Tiu.ory and Prm.ti<'e. p. 330, A.I.M.E. (1970). 5. HAWK~S I. "rhctwy of the photoclastic biaxial gauge and its applications in rt~ck str~s nmasuremcnts. Strain 3, (3) 36-43 July {1967).