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1.
Understanding how the strength of basaltic rock varies with the extrinsic conditions of stress state, pressure and temperature, and the intrinsic rock physical properties is fundamental to understanding the dynamics of volcanic systems. In particular it is essential to understand how rock strength at high temperatures is limited by fracture. We have collated and analysed laboratory data for basaltic rocks from over 500 rock deformation experiments and plotted these on principal stress failure maps. We have fitted an empirical flow law (Norton’s law) and a theoretical fracture criterion to these data. The principal stress failure map is a graphical representation of ductile and brittle experimental data together with flow and fracture envelopes under varying strain rate, temperature and pressure. We have used these maps to re-interpret the ductile–brittle transition in basaltic rocks at high temperatures and show, conceptually, how these failure maps can be applied to volcanic systems, using lava flows as an example.  相似文献   

2.
The Cretaceous Normal Superchron is a period of great interest to investigate global scale variations of the geomagnetic field. Long periods of single polarity are still a matter of debate: up to now there are two contradicting theories, which try to relate geomagnetic field intensity and reversal rate. We aim to shed light on the geomagnetic field strength during the Cretaceous Normal Superchron because data are still scarce and of dissimilar quality. To obtain reliable, absolute paleointensity determinations we investigate volcanic rocks from the Western Cordillera of Colombia. Several age determinations allow relating the samples to an age of about 92.5 Ma. To characterize the samples, we investigate rock magnetic properties and determine the characteristic remanent magnetization behavior. To determine paleointensities, we use a multimethod approach: first, we apply the classic Thellier-Coe protocol, and then, the relatively new multispecimen method. Rock magnetic measurements indicate magnetite as the main ferrimagnetic mineral, a stable magnetization revealed by reversible and nearly reversible thermomagnetic curves, and grain sizes that are either in the pseudosingle domain range or a mixture of single and multidomain grains. Alternating field and thermal demagnetization are rather complex, although we observe a few vector diagrams with a single, essentially uni-vectorial component with a small viscous overprint. Paleointensity determination with the Thellier-Coe protocol was unsuccessful, while with the multispecimen protocol we obtained four successful determinations out of 20. The failure of the Thellier-Coe protocol can be attributed to multidomain grains, which were observed during demagnetization and in rock magnetic experiments, and to the inhomogeneity of the volcanic rocks. Our multispecimen paleointensity determinations support low field strength at around 90 Ma during the Cretaceous Normal Superchron.  相似文献   

3.
Weathering reduces the strength of rocks and so is a key control on the stability of rock slopes. Recent research suggests that the geotechnical response of rocks to weathering varies with ambient stress conditions resulting from overburden loading and/or stress concentrations driven by near-surface topography. In addition, the stress history experienced by the rock can influence the degree to which current weathering processes cause rock breakdown. To address the combined effect of these potential controls, we conducted a set of weathering experiments on two sedimentary lithologies in laboratory and field conditions. We firstly defined the baseline geotechnical behaviour of each lithology, characterising surface hardness and stress–strain behaviour in unconfined compression. Weathering significantly reduced intact rock strength, but this was not evident in measurements of surface hardness. The ambient compressive stress applied to samples throughout the experiments did not cause any observable differences in the geotechnical behaviour of the samples. We created a stress history effect in sub-sets of samples by generating a population of microcracks that could be exploited by weathering processes. We also geometrically modified groups of samples to cause near-surface stress concentrations that may allow greater weathering efficacy. However, even these pronounced sample modifications resulted in insignificant changes in geotechnical behaviour when compared to unmodified samples. The observed reduction in rock strength changed the nature of failure of the samples, which developed post-peak strength and underwent multiple stages of brittle failure. Although weakened, these samples could sustain greater stress and strain following exceedance of peak strength. On this basis, the multi-stage failure style exhibited by weaker weathered rock may permit smaller-magnitude, higher-frequency events to trigger fracture through intact rock bridges as well as influencing the characteristics of pre-failure deformation. These findings are consistent with patterns of behaviour observed in field monitoring results. © 2019 John Wiley & Sons, Ltd.  相似文献   

4.
The remote sensing observational study for infrared radiation of rocks was proceeded during the loading on rocks until failure. The major instruments used in experiments were transient spectrum apparatus, intelligent spectrum apparatus, infrared radiation thermometer, infrared spectrum radiometer, and infrared thermal imaging system. The experiments for 26 kinds of rocks were made. The studies show that infrared radiation temperature of rocks increases along with increasing of stress. The amplitude of infrared radiation spectrum of rocks also increases along with increasing of stress. The observational results of infrared thermal imaging of rocks are consistent with infrared radiation temperature. Before formation of major faults for some rocks, the belt-shape thermal imaging of temperature anomaly displaies in position of future major faults. This study has led the new technology of remote sensing into rock mechanics and tend to establish a new field in rock mechanics — remote sensing rock mechanics (or remote sensing rock physics). The application of remote sensing rock mechanics in prediction of earthquake and rock burst, and in measurement of stress field in rock mass is expected. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,14, Supp., 645–652, 1992. Jin-Shen HAO, Ji-Han LI, Xiao-Hong LIU, Yi-Qiao ZHI, Jin-Kai ZHANG, Yong-Hong Lü, Yi LIU, Yun-Shen YU, He ZHANG, Quan-Quan JI, Xiao-Fan ZHU and Ning CHEN took part in this work. This subject is supported by the Chinese Joint Seismological Science Foundation (91006). Work of Institute of Geophysics, SSB (93A0009).  相似文献   

5.
Thermal loading of rocks at high temperatures induces changes in their mechanical properties. In this study, a hard gabbro was tested in the laboratory. Specimens were slowly heated to a maximum temperature of 1,000°C. Subsequent to the thermal loading, specimens were subjected to uniaxial compression. A drastic decrease of both unconfined compressive strength and elastic moduli was observed. The thermal damage of the rock was also highlighted by measuring elastic wave velocities and by monitoring acoustic emissions during testing. The micromechanisms of rock degradation were investigated by analysis of thin sections after each stage of thermal loading. It was found that there is a critical temperature above which drastic changes in mechanical properties occur. Indeed, below a temperature of 600°C, microcracks start developing due to a difference in the thermal expansion coefficients of the crystals. At higher temperatures (above 600°C), oxidation of Fe2+ and Mg2+, as well as bursting of fluid inclusions, are the principal causes of damage. Such mechanical degradation may have dramatic consequences for many geoengineering structures.  相似文献   

6.
To better understand the mechanisms leading to different radon background levels in volcanic settings, we have performed two long-term deformation experiments of 16 days using a real-time setup that enables us to monitor any variation of radon activity concentration during rock compression. Our measurements demonstrate that, in the case of highly porous volcanic rocks, the emanating power of the substrate changes as a function of the volcanic stress conditions. Constant magmatic pressures, such as those observed during dike intrusions and hydrothermal fluid injections, can result in pervasive pore collapse that is mirrored by a significant radon decrease until a constant emanation is achieved. Conversely, repeated cycles of stress due to, for example, volcano inflation/deflation cycles, cause a progressive radon increase a few days (but even weeks and months) before rupture. After rock failure, however, the formation of new emanation surfaces leads to a substantial increase of the radon signal. Our results suggest that surface deformation in tectonic and volcanic settings, such as inflation/deflation or constant magmatic pressures, have important repercussions on the emanating power of volcanic substrates.  相似文献   

7.
It is evident from the laboratory experiments that shear moduli of different porous isotropic rocks may show softening behaviour upon saturation. The shear softening means that the shear modulus of dry samples is higher than of saturated samples. Shear softening was observed both at low (seismic) frequencies and high (ultrasonic) frequencies. Shear softening is stronger at seismic frequencies than at ultrasonic frequencies, where the softening is compensated by hardening due to unrelaxed squirt flow. It contradicts to Gassmann's theory suggesting that the relaxed shear modulus of isotropic rock should not depend upon fluid saturation, provided that no chemical reaction between the solid frame and the pore fluid. Several researchers demonstrated that the shear softening effect is reversible during re-saturation of rock samples, suggesting no permanent chemical reaction between the solid frame and the pore fluid. Therefore, it is extremely difficult to explain this fluid–rock interaction mechanism theoretically, because it does not contradict to the assumptions of Gassmann's theory, but contradicts to its conclusions. We argue that the observed shear softening of partially saturated rocks by different pore fluids is related to pore-scale interfacial phenomena effects, typically neglected by the rock physics models. These interface phenomena effects are dependent on surface tension between immiscible fluids, rock wettability, aperture distribution of microcracks, compressibility of microcracks, porosity of microcracks, elastic properties of rock mineral, fluid saturation, effective stress and wave amplitude. Derived equations allow to estimate effects of pore fluids and saturation on the shear modulus and mechanical strength of rocks.  相似文献   

8.
Crustal structure in a number of Kamchatka volcanic regions is deduced from geophysical data. Anomalous structure and physical properties of the crust are found beneath some volcanic groups. Beneath the Klyuchevskaya and Avachinskaya volcanic groups crustal layers have high elastic properties. There is a thick transition layer from the crust to the upper mantle which has lower clastic properties and electrical resistance. These data, supported by experimental investigations of elastic properties of xenoliths in volcanic rocks at high pressures and temperatures, enable the probable substance composition of the crustal layers to be defined. The feeding zones and magma chambers of individual volcanoes are deduced from anomalies in gravity, electrical conductivity and seismic wave propagation.  相似文献   

9.
Rock shear strength plays a fundamental role in volcano flank collapse, yet pertinent data from modern collapse surfaces are rare. Using samples collected from the inferred failure surface of the massive 1980 collapse of Mount St. Helens (MSH), we determined rock shear strength via laboratory tests designed to mimic conditions in the pre-collapse edifice. We observed that the 1980 failure shear surfaces formed primarily in pervasively shattered older dome rocks; failure was not localized in sloping volcanic strata or in weak, hydrothermally altered rocks. Our test results show that rock shear strength under large confining stresses is reduced ∼20% as a result of large quasi-static shear strain, as preceded the 1980 collapse of MSH. Using quasi-3D slope-stability modeling, we demonstrate that this mechanical weakening could have provoked edifice collapse, even in the absence of transiently elevated pore-fluid pressures or earthquake ground shaking. Progressive strength reduction could promote collapses at other volcanic edifices.  相似文献   

10.
变形过程中岩石P波速度场的空间变化   总被引：3，自引：0，他引：3       下载免费PDF全文
陈yong  杨咸武 《地震学报》1990,12(1):54-59
在一次地震之前,在不同观测点测量岩石中弹性波速度,经常得到不同的结果.这一现象提示我们:岩石破坏前,其物理性质的变化在空间上的分布是不均匀的,破坏前兆的获得应该建立在对岩石性质的空间分布,即对物理场分析的基础上. 本实验观测了岩石样品在单轴压缩下,P波走时空间分布变化的全过程.实验表明:在岩石的破坏过程中,岩性的空间分布具有同步性变化的中长期前兆和差异性变化的短期前兆的特点.在岩石接近破坏时,岩性的变化表现出严重的不均匀性.岩石的整个破坏过程可以看成岩石的物性分布是由无序向有序的演化,即是一个熵减小的过程.   相似文献   

11.
This paper reports an experimental study of the evolution of permeability during rock brittle failure and a theoretical analysis of rock critical stress level. It is assumed that the rock is a strain-softening medium whose strength can be described by Weibull’s distribution. Based on the two-dimensional renormalization group theory, it is found that the stress level λc (the ratio of the stress at the critical point to the peak stress) depends mainly on the homogeneity index or shape parameter m in the Weibull’s distribution for the rock. Experimental results show that the evolution of permeability is closely related to rock deformation stages: the permeability has a rapid increase with the growth of cracks and their surface areas (i.e., onset of fracture coalescence point), and reaches the maximum at rock failure. Both the experimental and analytical results show that this point of rapid increase in permeability on the permeability-pressure curve corresponds to the critical point on the stress-strain curve; for rock compression, the stress at this point is approximately 80% of the peak strength. Thus, monitoring the evolution of permeability may provide a new means of identifying the critical point of rock brittle fracture.  相似文献   

12.
Loading Rate Dependence of Tensile Strength Anisotropy of Barre Granite   总被引：8，自引：0，他引：8  
Granitic rocks usually exhibit strongly anisotropy due to pre-existing microcracks induced by long-term geological loadings. The understanding of the rock anisotropy in mechanical properties is critical to a variety of rock engineering applications. In this paper, Brazilian tests are conducted statically with a material testing machine and dynamically with a split Hopkinson pressure bar system to measure both static and dynamic tensile strength of Barre granite. To understand the anisotropy in tensile strength, samples are cored and labelled using the three principle directions of Barre granite to form six sample groups. For dynamic tests, a pulse shaping technique is used to achieve dynamic equilibrium in the samples during the dynamic test. The finite element method is then implemented to formulate equations that relate the failure load to the material tensile strength by employing an orthotropic elastic material model. For samples in the same orientation group, the tensile strength shows clear loading rate dependence. The tensile strengths also exhibit clear anisotropy under static loading while the anisotropy diminishes as the loading rate increases, which may be due to the interaction of pre-existing microcracks.  相似文献   

13.
本文利用两个频率通道同时接收花岗岩标本及石灰岩标本在不同单轴加压方式下的声发射信号,研究了从加压到破坏的整个形变过程中,不同频道接收到的声发射率的变化特征.发现在各种加压方式下,花岗岩和石灰岩在破坏前,由不同频率通道接收到的声发射率均急剧增长;而在破坏的瞬间,高频通道接收的声发射率增长快于较低频率通道的声发射率.这两点可用来预测岩石的破坏.声发射率随轴向应力的变化与加压方式有关.匀速加压,加压过程中保持一段时间恒压或降压均对应一定的声发射率状态.有可能通过追踪声发射情况,推测应力变化趋势.   相似文献   

14.
In mining and civil engineering projects, physico-mechanical properties of the rock affect both the project design and the construction operation. Determination of various physico-mechanical properties of rocks is expensive and time consuming, and sometimes it is very difficult to get cores to perform direct tests to evaluate the rock mass. The purpose of this work is to investigate the relationships between the different physico-mechanical properties of the various rock types with the P-wave velocity. Measurement of P-wave velocity is relatively cheap, non-destructive and easy to carry out. In this study, representative rock mass samples of igneous, sedimentary, and metamorphic rocks were collected from the different locations of India to obtain an empirical relation between P-wave velocity and uniaxial compressive strength, tensile strength, punch shear, density, slake durability index, Young’s modulus, Poisson’s ratio, impact strength index and Schmidt hammer rebound number. A very strong correlation was found between the P-wave velocity and different physico-mechanical properties of various rock types with very high coefficients of determination. To check the sensitivity of the empirical equations, Students t test was also performed, which confirmed the validity of the proposed correlations.  相似文献   

15.
刘贵  周永胜 《地震地质》2012,34(2):365-383
在总结岩石变形机制与岩石流变学实验进展的基础上,讨论了岩石流变学数据的重复性。虽然高温高压流变学实验积累了大量的数据,但中、上地壳长英质岩石和早期获得的石英集合体的流变实验数据重复性比较差,而近年来发表的石英、长石的流变学实验数据重复性相对较好。虽然利用经验理论模型,根据端元组分可以拟合两相矿物集合体的流变律,但并不能满足定量确定复杂组分和特殊流变性的长英质岩石流变参数的需要。因此,利用长英质岩石流变参数估计大陆地壳流变强度剖面时,即使在相同地温和应变速率条件下,给出的流变曲线、脆-塑性转化带深度也有一定差别,还需要通过大量实验给出更精细的长英质岩石流变学实验数据。根据近年来流变学实验研究的新进展,讨论了在实验室条件下影响长英质岩石流变的各种因素,重点分析了流体、岩石成分、样品粒度和组构对流变的影响。微量结构水对岩石流变有显著的弱化作用,而熔体对流变的影响与熔体含量和分布相关,只有熔体呈薄膜状湿润颗粒边界时,熔体的弱化作用才显著。成分对岩石流变的影响不仅体现在样品的应力指数等流变参数的变化方面,还体现在样品从半脆性变形向塑性变形的转化温度方面。粒度主要影响岩石的变形机制,其中,细粒样品在扩散蠕变域具有应力与粒度线性负相关特性,是理想的应力计,可以用来定量确定韧性剪切带的流变强度;而在位错蠕变域,应力与粒度没有依存关系,这为将实验室条件得出的流变数据外推估计地壳流变提供了重要依据。组构和各向异性是地壳中岩石存在的普遍现象,但关于层状组构对多相矿物组成的岩石流变影响的研究非常少,需要通过新的实验来深入研究。  相似文献   

16.
Rock brittleness directly affects reservoir fracturing and its evaluation is essential for establishing fracturing conditions prior to reservoir reforming. Dynamic and static brittleness data were collected from siltstones of the Qingshankou Formation in Songliao Basin. The brittle–plastic transition was investigated based on the stress–strain relation. The results suggest that the brittleness indices calculated by static elastic parameters are negatively correlated with the stress drop coefficient and the brittleness index B2, defined as the average of the normalized Young’s modulus and Poisson’s ratio, is strongly correlated with the stress drop. The brittleness index B2, Young’s modulus, and Poisson’s ratio correlate with the brittle minerals content; that is, quartz, carbonates, and pyrite. We also investigated the correlation between pore fluid and porosity and dynamic brittle characteristic based on index B2. Pore fluid increases the plasticity of rock and reduces brittleness; moreover, with increasing porosity, rock brittleness decreases. The gas-saturated siltstone brittleness index is higher than that in oil- or water-saturated siltstone; the difference in the brittleness indices of oil- and water-saturated siltstone is very small. By comparing the rock mechanics and ultrasonic experiments, we find that the brittleness index obtained from the rock mechanics experiments is smaller than that obtained from the ultrasonic experiments; nevertheless, both decrease with increasing porosity as well as their differences. Ultrasonic waves propagate through the rock specimens without affecting them, whereas rock mechanics experiments are destructive and induce microcracking and porosity increases; consequently, the brittleness of low-porosity rocks is affected by the formation of internal microcrack systems.  相似文献   

17.
The influence of the elastic and hydrodynamic properties of a rock on the time attenuation coefficient of an acoustic wave, which is reflected from the borehole wall, is considered using the methods of the mechanics of saturated porous media. The calculations were performed for a system consisting of an acoustic logging tool of a finite with a finite diameter, a fluid-filled borehole, and a porous permeable rock. The performed simulation showed that in rocks with a low hydrodynamic permeability, the acoustic-reverberation time is determined by the acoustic impedance of the borehole wall (product of the rock density and the longitudinal-wave velocity in it). In the case of rocks with a permeability of about several hundred millidarcy, the time signal’s attenuation coefficient substantially depends on the rock’s permeability.  相似文献   

18.
明亮 《地震研究》1989,12(2):155-160
我们对含水岩石进行了双剪摩擦滑动实验。所用样品为济南辉长岩、周口店石灰岩,小浪底砂岩和点苍山大理岩。本实验得到的主要结果表明,水的存在增大了岩石的摩擦强度,减小了粘滑时的应力降和错距,促进了位移强化效应。这说明水对岩石摩擦的影响是一个复杂的问题。还应深入地研究。  相似文献   

19.
Burial stress on a sediment or sedimentary rock is relevant for predicting compaction or failure caused by changes in, e.g., pore pressure in the subsurface. For this purpose, the stress is conventionally expressed in terms of its effect: “the effective stress” defined as the consequent elastic strain multiplied by the rock frame modulus. We cannot measure the strain directly in the subsurface, but from the data on bulk density and P‐wave velocity, we can estimate the rock frame modulus and Biot's coefficient and then calculate the “effective vertical stress” as the total vertical stress minus the product of pore pressure and Biot's coefficient. We can now calculate the elastic strain by dividing “effective stress” with the rock frame modulus. By this procedure, the degree of elastic deformation at a given time and depth can be directly expressed. This facilitates the discussion of the deformation mechanisms. The principle is illustrated by comparing carbonate sediments and sedimentary rocks from the North Sea Basin and three oceanic settings: a relatively shallow water setting dominated by coarse carbonate packstones and grainstones and two deep water settings dominated by fine‐grained carbonate mudstones and wackestones.  相似文献   

20.
A wide range of rock magnetic properties have been determined from two collections of mid-Cretaceous basalts; one from Israel, the other from the Rajmahal traps in northeast India. Deuteric oxidation is rare in both collections, with titanium-rich titanomagnetite being the principal remanence carrier in most cases. There are a number of differences in rock magnetic properties between the two groups. Some of these seem to be primary, whereas others appear to be caused by hydrothermal alteration and weathering, which are more prevalent in the Indian rocks. These rocks are being used in palaeointensity experiments, from which it is hoped to determine the strength of the Earth's magnetic field during the long period of normal polarity in the mid-Cretaceous. Thellier palaeointensity experiments have been performed on two samples from each site. The degree of agreement between the two results is highly variable. The low blocking temperatures and the presence of secondary viscous components in many samples make Thellier palaeointensity experiments very difficult. A further problem is that of thermal alteration, two main types of which are observed. The first manifests itself as a large and sudden increase in partial thermoremanent magnetization (pTRM) capacity, and the second as a steady decrease in the size of pTRM with increasing temperature.  相似文献   

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