Study on long-term uniaxial compression creep mechanical behavior of rocksalt-mudstone combined body

Rocksalt and mudstone are usually under common stress in salt storage caverns, resulting in different mechanical properties from pure rocksalt and mudstone. To accurately obtain the creep mechanical characteristics of rocksalt-mudstone combined body, we have made three different combinations. The long-term creep experiment of bedded rocks can more closely reflect the long-term mechanical behavior of surrounding rock of salt storage caverns. The experimental results indicated that the long-term creep curve of the combined body includes initial and steady creep stages, and even includes accelerated creep stage. The strain of mudstone layer in the combined body was lower than that of rocksalt because of the higher strength. With the increase of the height ratio of mudstone, the creep strain of the combined body and each rock layer decreased, but the creep rate increased. A new nonlinear creep-damage constitutive model was proposed, which can well describe the creep evolution characteristics of the experiment. Compared with the fitting curves of classical Burgers and Nishihara creep constitutive models, it is revealed that the proposed model is most consistent with the experimental data. The duration of the long-term creep experiment under lower stress has a highly significant effect on the accuracy of predicting rock creep results. This research will contribute to a deeper understanding of the long-term creep characteristics of bedded rocks in salt storage caverns.

Palaeomagnetic evidence bearing on the evolution of the Canadian Cordillera

Palaeomagnetic data from Permian, Triassic and Jurassic bedded rocks, to which attitudinal corrections can be applied, yield palaeolatitudes concordant with those of ancestral North America, but very large predominantly anticlockwise rotations about vertical axes. Data from Cretaceous rocks yield apparent palaeolatitudinal displacements that increase westward. Small or negligible displacements are obtained from the Omineca Belt. Intermediate displacements (1000-2000 km) from the Intermontane Belt, are based on data from Cretaceous bedded sequences. Further to the west in the Coast Belt, larger apparent displacements (greater than 2000 km) have been obtained from plutons for which no attitudinal control is yet available. Data from Eocene rocks are concordant. Possibilities to consider are as follows: (a) little or no displacement and tilting to the southwest at about 30°; (b) large (greater than 2000 km in the Coast Belt) northward displacement since mid-Cretaceous time preceded by southward displacement of comparable magnitude in Juro-Cretaceous time; (c) lesser (1000-2000 km) overall displacement coupled with variable and lesser tilts to the south and southeast of plutons of the Coast Belt. Under hypothesis (a) the western Cordillera was formed and has remained in approximately its present position relative to ancestral North America; data from bedded volcanics of the Intermontane Belt are not consistent with this hypothesis. From the evidence currently available we favour hypotheses (b) or (c), although more data from bedded sequences are required. It is noteworthy that hypotheses (a) and (c) predict tilt directions that differ by about 90° and hence ought to be distinguishable by geological studies.

Volcanogenic cherts in the late Precambrian Conception Group, Avalon Peninsula, Newfoundland

Bedded rocks of cherty appearance are locally abundant and conspicuous in the late Precambrian Conception Group of the Avalon Peninsula, southeast Newfoundland. Structural, petrographical, and chemical data from a well exposed locality near St. John's indicate that these beds are tuffaceous, predominantly of acid vitric composition, formed by pyroclastic material derived from contemporaneously active subaerial volcanic sources accumulating in sea water, and subjected to submarine transport. Appreciable quantities of acid material were thus erupted in 'Conception time.'

Mechanical Anisotropies of Laminated Sedimentary Rocks

The effects of bedding plane orientation on the elastic constants and the yield strengths of three laminated rocks (one sandstone and two shales) and one isotropic rock (a limestone) were studied. The directional dependence of the elastic properties of these rocks was experimentally evaluated using a triaxial compression cell and auxiliary stress - strain measuring equipment. Symmetry of Poisson's ratios within the bedding plane suggested that horizontal isotropy exists, but the bedding planes do give rise to an appreciable difference between properties in the horizontal and vertical directions. For the three bedded rocks studied, Young's modulus was lower normal to bedding than along bedding. Yield strengths were determined at confining pressures from 0 to 12,000 psi in a triaxial compression cell. The rocks studied showed strength reductions as high as 40 per cent when the test specimen was oriented at 20 degrees 30 degrees to the bedding planes. The mechanical behavior of these rocks suggested that the rock properties of shear strength and/or coefficient of internal friction can vary with direction, depending on the particular rock tested. Tensile strengths were also measured and found to be lowest when failure occurred along bedding. This work shows that bedded formations exhibit sizable directional variations in both their elastic constants and yield strengths. It is suggested that these variations may be accounted for by using the "elastic laminate" model and the "variable coefficients" failure model. Introduction The nature of rock deformation at elevated pressures has been studied by many workers; papers by Handin, and Robinson illustrate the present state of knowledge. Most investigators have either chosen rocks which were as isotropic as possible (in order to avoid complications of data interpretation and analysis) or they have oriented their samples so that the effects of anisotropies (such as bedding planes) have been avoided. Of the studies performed, few were concerned specifically with mechanical anisotropies. Griggs has presented limited data for specimens cut parallel, normal, and 45 to the bedding plane; he relates the strength anisotropy observed to the fabric (bedding) anisotropy. His tests were primarily concerned with large deformations (20 per cent strain); thus, no directional values of the elastic constants were reported. Handin has reported the results of similar experiments. Bott has discussed rock strength anisotropies due to faults, cleavage, or bedding. He was concerned primarily with determining the shear stress on such planes and did not mention the effect of friction. Jaeger later generalized Bott's work by taking friction into account and presented a limited theory for the failure of rocks having a "single plane of weakness", and also for rocks having a constant coefficient of friction, and a shear strength which varies with bedding plane orientation. Donath and Cohen, and Donath have evaluated rock strengths from shale and slate specimens cut normal and parallel to bedding. A dependence of cohesive strength (ro) on the specimen orientation was also shown. Adler has also studied this problem and lists similar results. He assumes that all bedded rocks behave according to Jaeger's single - plane - of - weakness theory. Kalinin and Belorussov list results for strength tests parallel and perpendicular to bedding and use this information as a basis for hole deviation analysis. From the literature it is apparent that sedimentary rocks have been tested under widely varying conditions of stress; however, the assumption of isotropy is generally, but not always, made. Since geologic sedimentation often deposits sediments in very definite layers, it seems that more systematic attention should be given to the possible effects of this natural bedding. Bedding, as used here, refers to visible regularities of grain size or orientation resulting from depositional processes. SPEJ P. 67ˆ