Influence of intermediate principal stress on failure mechanism of hard rock with a pre-existing circular opening

Based on particle flow theory, the influences of the magnitude and direction of the intermediate principal stress on failure mechanism of hard rock with a pre-existing circular opening were studied by carrying out true triaxial tests on siltstone specimen. It is shown that peak strength of siltstone specimen increases firstly and subsequently decreases with the increase of the intermediate principal stress. And its turning point is related to the minimum principal stress and the direction of the intermediate principal stress. Failure characteristic(brittleness or ductility) of siltstone is determined by the minimum principal stress and the difference between the intermediate and minimum principal stress. The intermediate principal stress has a significant effect on the types and distributions of microcracks. The failure modes of the specimen are determined by the magnitude and direction of the intermediate principal stress, and related to weakening effect of the opening and inhibition effect of confining pressure in essence: when weakening effect of the opening is greater than inhibition effect of confining pressure, the failure surface is parallel to the x axis(such as σ2=σ3=0 MPa); conversely, the failure surface is parallel to the z axis(such as σ2=20 MPa, σ3=0 MPa).

New artificial neural networks for true triaxial stress state analysis and demonstration of intermediate principal stress effects on intact rock strength

Simulations are conducted using five new artificial neural networks developed herein to demonstrate and investigate the behavior of rock material under polyaxial loading. The effects of the intermediate principal stress on the intact rock strength are investigated and compared with laboratory results from the literature. To normalize differences in laboratory testing conditions, the stress state is used as the objective parameter in the artificial neural network model predictions. The variations of major principal stress of rock material with intermediate principal stress, minor principal stress and stress state are investigated. The artificial neural network simulations show that for the rock types examined, none were independent of intermediate principal stress effects. In addition, the results of the artificial neural network models, in general agreement with observations made by others, show （a） a general trend of strength increasing and reaching a peak at some intermediate stress state factor, followed by a decline in strength for most rock types; （b） a post-peak strength behavior dependent on the minor principal stress, with respect to rock type; （c） sensitivity to the stress state, and to the interaction between the stress state and uniaxial compressive strength of the test data by the artificial neural networks models （two-way analysis of variance; 95% confidence interval）. Artificial neural network modeling, a self-learning approach to polyaxial stress simulation, can thus complement the commonly observed difficult task of conducting true triaxial laboratory tests, and/or other methods that attempt to improve two-dimensional （2D） failure criteria by incorporating intermediate principal stress effects.

Formation mechanism of high-quality reservoirs in intermediate volcanic rocks:A case study of Longfengshan area in Songliao Basin

The basin margin slope area of Changling fault depression in Songliao Basin is dominated by intermediate-basic volcanic rocks of Huoshiling Formation,and there is still a lack of in-depth understanding of the development pattern and formation mechanism of high-quality reservoirs in intermediate-basic volcanic rocks in this area.Taking the intermediate volcanic rocks in Longfengshan area of Changling fault depression as an example,a comprehensive study was undertaken via core observations,thin section porosity analysis,rock physical property tests,scanning electron microscopy and X-ray diffraction analysis,combined with well log and seismic d ata.As a result,formation mechanism of a high quality reservoir was made clear based on dissolution modification effects of the volcanic rocks,and the development model of the high quality reservoir under t he dual control of lithology and fluid was established.The research shows that the volcanic rocks in this area consist of six lithologies:andesitic hydrothermal breccia,andesite,andesitic volcanic breccia,andesitic tuff,dacitic tuff and sedimentary tuff,which are controlled by structure,eruptive source,eruptive action and palaeotopography.The proximal facies of the volcanic edifice were located on the basin margin,distributed along faults mainly consisting of overflow facies intermixed with eruptive facies,and gradually transformed into thick eruptive facies(middle-source facies)and volcanic sedimentary facies(distal facies).The reservoir space of volcanic rocks was dominated by dissolution pores,which were formed by weathering and dissolution by organic acidic fluids.The thermal debris flow subphase in the middle-source facies of the volcanic edifice had a large amount of easily dissolved volcanic ash,which was close to the source rock in the centre of the basin and was a favourable area for the development of dissolution pores.This study is providing a guide for oil and gas exploration in the intermediate-basic volcanic rocks in the slope of Changling fault depression in Songliao Basin.

Effect of intermediate principal stress on strength of soft rock under complex stress states

A series of numerical simulations of conventional and true triaxial tests for soft rock materials using the three-dimensional finite difference code FLAC3D were presented. A hexahedral element and a strain hardening/softening constitutive model based on the unified strength theory(UST) were used to simulate both the consolidated-undrained(CU) triaxial and the consolidated-drained(CD) true triaxial tests. Based on the results of the true triaxial tests simulation, the effect of the intermediate principal stress on the strength of soft rock was investigated. Finally, an example of an axial compression test for a hard rock pillar with a soft rock interlayer was analyzed using the two-dimensional finite difference code FLAC. The CD true triaxial test simulations for diatomaceous soft rock suggest the peak and residual strengths increase by 30% when the effect of the intermediate principal stress is taken into account. The axial compression for a rock pillar indicated the peak and residual strengths increase six-fold when the soft rock interlayer approached the vertical and the effect of the intermediate principal stress is taken into account.

Geochemical Characterization of Intermediate to Silicic Rocks in the Global Ophiolite Record

Ophiolites consist predominantly of mafic-ultramafic rocks but also contain in subordinate amounts intermediate to silicic intrusive–extrusive rocks in them.These rocks,although not in large volumes in comparison to mafic–ultramafic units,reveal significant clues about melt evolution,magmatic accretion processes,and partial melting of pre-existing oceanic crust during ophiolite evolution.They also yield ample accessory minerals,such as zircon and monazite,which are widely used to date the timing of oceanic crust generation.We present a global synthesis of the occurrence of such leucocratic,intrusive and volcanic rocks from 150 Phanerozoic to Archean ophiolites,and evaluate models for their genesis during the development of oceanic crust in different tectonic environments.In the ophiolite complexes we have investigated,intrusive and extrusive rocks show a wide range in composition.Intermediate rocks（with Si O2 between 52and 63 wt.%）include andesite and diorite,whereas the silicic rocks（with Si O2 more than 63 wt.%）include dacite,trondhjemite,tonalite and granite.For subduction-relatedandsubduction-unrelated MORB-type ophiolites in the Phanerozoic orogenic belts,the most commonly reported intermediate to silicic intrusive rocks,independent of their original tectonic setting,are trondhjemite,plagiogranite and tonalite,and less commonly quartz-diorite and diorite.These rock types have been documented from 80 percent of the 104ophiolites included in this study.Intermediate to silicic lavas and/or pyroclastic rocks（basaltic andesite,andesite,dacite and rhyolite）are less abundant,and have been reported from only 35 percent of the 104 ophiolites.Trachyte occurs in some of the Rift/Continental Margintype,subduction-unrelated ophiolitesEvolvedrocksinsubduction-unrelated,Rift/Continental Margin ophiolites are predominantly basaltic andesite and andesite,whereas MOR type（mid-ocean ridge）ophiolites exhibit nearly equal proportions of basaltic andesite/andesite and rhyodacite and Plume/MOR type ophiolites are characterized by rhyolites.Intermediate to silicic volcanic uints in the Backarc sub-group of subduction-related ophiolites are characterized by similar amounts of basaltic andesite/andesite and rhyodacite,whereas in the Backarc to Forearc,Forearc,and Volcanic Arc sub-groups they are mainly basaltic andesite/andesite.Intermediate to silicic rocks in Rift/Continental Margin and Plume/MOR type ophiolites are generally LREE-enriched,whereas those in the MOR type vary from LREE-depleted to LREE-enriched.The Backarc and Backarc to Forearc types are similar to the MOR type;silicic rocks of the Forearc and Volcanic Arc types are generally LREE-enriched.The main process in the formation of the majority of the intermediate to silicic rocks in both subduction-unrelated and subduction-related ophiolites is partial melting of basaltic and/or gabbroic rocks beneath the spreading centers,whereas a minor volume in subduction-related ophiolites are adakites that were produced from partial melting of a subducting slab.Silcic to intermediate rocks in Plume/MOR type ophiolites are generated by fractional crystallization of basaltic melt.The incompatible,non-conservative elements,such as Ba and Th,are weakly to strongly enriched in subduction-related ophiolites as a result of shallow to deep enrichment associated with subduction zone processes.The field occurrence and the geochemical character of leucocratic rocks in ophiolites show considerable variations,providing additional constraints on the petrogenesis of ophiolites in different tectonic settings.Fractional crystallization appears to have been an important process in the formation of the intermediate to silicic rocks in Rift/CM and P/MOR ophiolites in the subduction-unrelated class,as well as VA ophiolites in the subduction-related class.In these types of ophiolites magma chambers might have operated as closed-systems for a sufficient amount of time for fractionation and compositional zoning of the magma to take place（Fig.1C1）.Fractional crystallization was of subordinate importance during the igneous construction of the MOR,BA,FA,and BA-FA ophiolites.Instead,partial melting of hydrothermally altered crust was the most common process resulting in the majority of the intermediate to felsic rocks of the Rift/CM（estimated to 60%）,MOR（estimated to 95%）,and the BA,FA and BA-FA（estimated to 75%）ophiolites,as well as a high proportion of the VA ophiolites（;0%）(Fig.1C2).Slab-generated melts that result in the production of intermediate to silicic rocks of adakitic character represent a relatively small fraction（;5%in Phanerozoic ophiolites）of subduction-related ophiolites（Fig.1C3）.Some adakites are very low in incompatible elements（e.g.,La）,suggesting that these melts were generated from highly depleted MORB.Precambrian ophiolites contain a higher percentage of adakites（ca.35%）,indicating that physical conditions were more favorable for slab melting during the Precambrian than in the Phanerozoic.This phenomenon may have resulted from higher geothermal gradient along relatively shallow-dippingsubductionzonesduringthe Precambrian.

U and Th Contents and Th/U Ratios of Zircon in Felsic and Mafic Magmatic Rocks:Improved Zircon-Melt Distribution Coefficients

High-precision data on U and Th contents and Th/U ratios of zircon obtained using secondary ion mass spectrometry analysis have been collected from the literature. Zircon in the granitic rocks has median values of 350 ppm U, 140 ppm Th, and Th/U=0.52; the recommended zircon-melt partition coefficients are 81 for Du and 8.2 for DTh. In zircon from mafic and intermediate rocks, the median values are 270 ppm U, 170 ppm Th, and Th/U=0.81, and the recommended zirconmelt partition coefficients are 169 for Du and 59 for DTh. The U and Th contents and Th/U ratios of magmatic zircon are low when zircon crystallizes in equilibrium with the melt. Increasing magma temperature should promote higher Th contents relative to U contents, resulting in higher Th/U ratios for zircon in mafic to intermediate rocks than in granitic rocks. However, when zircon crystallizes in disequilibrium with the melt, U and Th are more easily able to enter the zircon lattice, and their contents and Th/U ratios depend mainly on the degree of disequilibrium. The behavior of U and Th in magmatic zircon can be used as a geochemical indicator to determine the origins and crystallization environments of magmatic zircon.