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YOUNATHAN YOUASH Department of Geology, University of Baghdad, Adhamiya, Iraq

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YOUNATHAN YOUASH Department of Geology, University of Baghdad, Adhamiya, Iraq YOUNATHAN YOUASH Department of Geology, University of Baghdad, Adhamiya, Iraq Tension Tests on Layered Rocks Abstract: Tension tests were performed on four layered rocks: a shale, a gneiss, and two sand- stones. Cores 2.125 by 4.25 inches were prepared, with the layers dipping at 0°, 15°, 30°, 45°, 60°, 75°, and 90° to the short cylinder axis. All tests were carried to failure, and strain was measured by gauges mounted on the cores. Rupture strength in tension is highly affected by the orientation of layering. In general, as the dip of the layers increases, the rupture strength increases correspondingly. For 0° to 60° cores, failure occurs along layering; for 75° and 90° cores, failure occurs across layering. Rupture strength in tension is lower if failure occurs along layering than if it occurs across it. Therefore, it is postulated that during rock deformation, if rocks are subjected to tension, it is more likely that failure will occur along pre-existing weakness planes, such as joints, faults, bedding, lamination, and foliation, than for failure to occur across layering and weakness planes. The different theories of failure which assume that material is homogeneous and isotropic, whether in tension or in compression, are not applicable on rocks that are characterized by definite planes of weakness and anisotropy. Introduction sults have already been reported (Youash, 1964). There has been extensive work done on the mechanical properties of rocks during the last Acknowledgments few years. Rocks were tested under various The writer is very thankful to Prof. W. R. confining pressures, temperatures, and loading Muehlberger of the University of Texas, who rates (Balmer, 1953; Robertson, 1959; Handin supervised this work and offered many helpful and Hager, 1957, 1958; Griggs and Handin, suggestions. The writer would like to also 1960; Heard, 1963; and Handin and others, thank Dr. John Handin and Dr. Fred A. 1963). In most of these studies, the rocks tested Donath who read the article and assisted in its were homogeneous and statistically isotropic, improvements. The writer is also very grateful and most tests were performed under com- to the U.S. Geological Survey in Denver, pression on small cores (usually .5 by 1.0 Colorado, for their assistance and for the use of inches). their laboratory equipment in the Engineering Because most rocks are layered in their Geology Branch. natural condition, that is, bedded, laminated, jointed, faulted, and foliated, experiments on Tension Test Equipment and Sample Assembly layered rocks will have more geologic and Tension tests were made on sandstones of the engineering significance than those on homo- Lyons and Blackhawk Formations, a shale geneous and isotropic rocks. Some work has (laminated marlstone?) of the Green River already been done on the effect of layering Formation, and a gneiss of the Idaho Springs (Bott, 1959; Jaeger, 1959; and Donath, 1961). Formation. Cores of 2.125 inches in diameter These studies were again of small samples run and 4.25 inches long were prepared with the under compression. layers dipping at 0°, 15°, 30°, 45°, 60°, 75°, In the present work, the effect of layering and 90° to the short cylinder axis. The equip- was studied systematically by preparing 2.125- ment consists of a tension-testing machine and by 4.25-inch cores oriented at 15° intervals to sample assembly. While the machine is operat- the layering and then testing them under ten- ing, the tensile force is recorded continuously sion. Four layered rocks were tested: shale, on an automatic plotter. The force at failure gneiss, and two sandstones. All tests were run is read from the plot directly. at atmospheric pressure. Some of the early re- The sample assembly consists of platens, Geological Society of America Bulletin, v. 80, p. 303-306, 2 figs., 1 pi., February 1969 303 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/80/2/303/3428091/i0016-7606-80-2-303.pdf by guest on 28 September 2021 304 YOUASH—TENSION TESTS ON LAYERED ROCKS steel rings, and steel plates. The platens are 2.125 inches in diameter, the same as that of the cores. The sample is glued to the platens by epoxy cement. After gluing, the sample and platens are put in an oven and kept at 165°F for two hours. This cures the cement, and upon cooling, the core adheres to the platens. These are attached to the steel plates with several screws. The plates have hooks which are pulled by tell rods. In some tests, strain was measured by resistance gauges mounted on the cores. Stress Analysis Stress analysis for a tensile specimen is shown N= Pcos 0 in Figure 1. If the circular cross section of the Q= Psin 0 sample is assumed to_be A, then the normal P si n 2 0 stress on cross section ab due to a force P is COS 0 2A P Figure 1. Determination of normal and shear stresses on a bar tested in tension. A (1) The stresses on an oblique cross section cd, cutting the sample_ at an angle (j> with the nor- diminishes, until when 0 = 90°, ft = 0. Thus, mal cross section ab, are shown in Figure la. An no normal stress exists at right angles to the isolated part of the sample to the left of the sec- axis of the cylindrical sample. The shear stress tion cd is shown in Figure lb as a free body. along the axis of the cylindrical sample will also From the equilibrium conditions, the internal equal zero. On the other hand, with an increase force S must be equal, opposite, and colinear in the angle <$> to 45°, the normal and shear with the external force P. Resolving the force stresses will be into components N and Q, normal and tangential, respectively, to the plane cd, gives 0 = 2A (9) N = P cos <p, (2) P Q = P sin (/>. (3) (10) 2A Since the area of the oblique section cd is A', In other words, the shear stress increases to A its maximum value when the angle of inclina- A' = • (4) COS (j) ' tion reaches 45° (Table 1). The corresponding normal and shear stresses Experimental Results are So far the discussion has applied to homoge- neous materials, without planar anisotropy. In 2 = A / = A cos 6* ', (5) rocks with definite planes of weakness, it is possible that failure occurs neither on planes of maximum shear stress, at 45°, nor across the -sin 2(j). A' 2A (6) plane normal to maximum normal stress, at 0°. Failure may be due to a combination of Their magnitudes are a function of angle <t>. both normal and shear stress. Even for aniso- When 0 = 0°, the section cd coincides with the tropic rocks, the nature of the plane of weak- normal section, and equations 5 and 6 become ness is important, and its influence on the mechanism of failure differs widely. 08) max = (7) The first rock tested was the Lyons Sand- A stone, which is very well bedded and laminated. T = 0. (8) Inspection of thin sections reveals the variation in the grain size between adjacent laminae, However, as <f> increases, the normal stress whereas grain size within a single lamina is uni- Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/80/2/303/3428091/i0016-7606-80-2-303.pdf by guest on 28 September 2021 NOTES AND DISCUSSIONS 305 TABLE 1. NORMAL AND SHEAR STRESSES AT VARIOUS The fourth rock tested was Idaho Springs INCLINATIONS OF LAYERING TO THE SHORT CYLINDER Gneiss. Foliation is produced by parallel ar- Axis. rangement of elongated biotite, quartz, and feldspar. Cores were prepared at 15° intervals to this pronounced foliation which is not truly planar. In some cores failure occurs along planes p 0 — Max. 0 of anisotropy. This is true especially in 0°, 15°, A and 30° cores (PI. 2, fig. 2). In the steeper cores P foliation affected rupture strength to a lesser 45 Max 2 A TK ' degree. The rupture strength of the gneiss in- 90 0 0 creased with the increase of inclination of folia- tion to the short cylinder axis (Fig. 2). form. Failure in tension is parallel to bedding Summary and Conclusions for 0°, 15°, 30°, 45°, and some 60° cores (PI. 1, The effect of layering on tensile strength of fig. 1). On the other hand, failure is across bed- rock was studied systematically. Four rocks ding for some 60°, and all 75° and 90° cores. were tested: a "shale," a gneiss, and two sand- For orientations of 0° to 45°, the bedding is not stones. Cores of 2.125 by 4.25 inches were pre- connected to either the top or bottom of the pared at inclination of layering of 0°, 15°, 30°, steel platens, and failure always occurs along 45°, 60°, 75°, and 90° to the axis of the bedding. For some 60° and all 75° and 90° cylindrical sample. All tests were performed cores, no free beds exist; therefore, failure dry, at room temperature and pressure, and across bedding is preferred. Figure 2 is a plot of were carried to failure. tensile strength versus inclination of layering. Tensile strengths versus inclinations of layer- For the Lyons Sandstone, note the gradual in- ing for all four rocks tested were plotted in one crease of the maximum principal stress with an diagram for comparison. Cores of 0°, 15°, 30°, increase in inclination, particularly for the 0° 45°, and some 60° orientations failed along to 45° cores. For 60° to 90° cores the maximum layering. As inclination increased, rupture principal stress increases greatly, because failure strength increased correspondingly.
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