Standard Test Method for Determination of In Situ Stress in Rock Mass by Overcoring Method—Three Component Borehole Deformation Gauge
5.1 Either the in situ stresses or the stresses as influenced by an excavation may be determined. This test method is written assuming testing will be done from an underground opening; however, the same principles may be applied to testing in a rock outcrop at the surface.
5.2 This test method is generally performed at depths within 50 ft (15 m) of the working face because of drilling difficulties at greater depths. Some deeper testing with this gauge has been done, but should be considered developmental. This test method has a long and proven record and considered very accurate relative to many other techniques, both new and old, out there. Other overcoring methods that use instruments that are different, but follow much of the same basic concepts are now available and can go deeper; however, the pros and cons of each method need to be carefully compared to this test method.
5.3 It is also useful for obtaining stress characteristics of existing concrete and rock structures, such as mass concrete dams, for safety (such as alkali aggregate issues), vetting of computer models, and modification investigations.
5.4 This test method is difficult in rock with fracture spacings of less than 5 in. (130 mm). A large number of tests may be required in order to obtain data.
5.5 The rock tested is assumed to be homogeneous and linearly elastic. The moduli of deformation and Poisson's ratio of the rock overcore are required for data reduction. The preferred method for determining modulus of deformation values involves biaxially testing the recovered overcores, as described in Section 8. If this is not possible, values may be determined from uniaxial testing of smaller cores in accordance with Test Method D7012. However, this generally decreases the accuracy of the stress determination in all but the most homogeneous and isotropic rock. Modulus of deformation results may be used from other in situ tests, such as Test Methods D4394 and Test Method D4395, D4971 or other test methods that can determine the modulus of deformation in specific directions.
5.6 The physical conditions present in three separate drill holes are assumed to prevail at one point in space to allow the three-dimensional stress field to be estimated. This assumption is difficult to verify, as rock material properties and the local stress field can vary significantly over short distances. Confidence in this assumption increases with careful selection of the test site.
5.7 Local geologic features with mechanical properties different from those of the surrounding rock can influence significantly the local stress field. In general, these features, if known to be present, should be avoided when selecting a test site location. It is often important, however, to measure the stress level on each side of a large fault. All boreholes at a single test station should be in the same formation or rock mass.
5.8 Since most overcoring is performed to measure in situ stress levels, the boreholes should be drilled from a portion of the test opening and the testing performed at least three excavation diameters from any free surface. The smallest opening that will accommodate the drilling equipment is recommended; openings from 8 to 12 ft (2.4 to 3.6 m) in diameter have been found satisfactory.
5.9 A minimum of three nonparallel boreholes is required to determine the complete stress tensor. The optimum angle each hole makes with the other two (trihedral arrangement) is 90°. However, angles of 45° provide satisfactory results for determining all three principal stresses. Boreholes inclined upward are generally easier to work in than holes inclined downward, particularly in fractured rock.
Note 2: The quality of the result produced by this standard is dependent on the competence of the personnel performing it, and the suitability of the equipment and facilities used. Agencies that meet the criteria of Practice D3740 are generally considered capable of competent and objective testing/sampling/inspection/ and the like. Users of this standard are cautioned that compliance with Practice D3740 does not in itself assure reliable results. Reliable results depend on many factors; Practice D3740 provides a means of evaluating some of those factors.
1.1 This test method covers the determination of the ambient local stresses (principal and secondary) in a rock mass and the equipment required to perform in situ stress tests using a three-component borehole deformation gauge (BDG) that was developed by the U.S. Bureau of Mines (USBM); see Note 1.
1.2 The test procedure and method of data reduction are described, including the theoretical basis and assumptions involved in the calculations.
1.3 A section is included on troubleshooting equipment malfunctions.
Note 1: The gauge used in this test method is commonly referred to by users as a USBM gauge (U.S. Bureau of Mines three-component borehole deformation gauge).2
1.4 The values stated in inch-pound units are to be regarded as standard, except as noted below. The values given in parentheses are mathematical conversions to SI units, which are provided for information only and are not considered standard. Reporting of test results in units other than SI shall not be regarded as nonconformance with this test method.
1.5 This standard does not purport to address all of the safety problems, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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