A Model for the Mechanical Behaviour of Bentheim Sandstone in the Brittle Regime

— The mechanical behaviour of Bentheim sandstone, a homogeneous quartz-rich sandstone with porosity of 22.8%, was investigated by triaxial compression tests conducted on dry samples. At confining pressures up to 35 MPa, the failure mode was characterized by a typical brittle deformation regime, as the samples showed dilatancy and failed by strain softening and brittle faulting. Previous studies have shown that the mechanical behaviour and failure mode of brittle porous granular rocks are governed by the time-dependent growth of microcracks. We analyse this process using the “Pore Crack Model” based on fracture mechanics analysis. It is consistent with the microstructure of porous granular rocks since it considers the growth of axial cracks from cylindrical holes in two dimensions. These cracks grow when their stress intensity factors reach the subcritical crack growth limit. Interaction between neighbouring cracks is introduced by calculating the stress intensity factor as the sum of two terms: a component for an isolated crack and an interaction term computed using the method of successive approximations. It depends on crack length, pore radius, pore density, and applied stresses. The simulation of crack growth from cylindrical holes, associated with a failure criterion based on the coalescence of interacting cracks, is used to compare the theoretical stress at the onset of dilatancy and at macroscopic rupture to the experimental determined values. Our approach gives theoretical results in good agreement with experimental data when microstructural parameters consistent with observations are introduced.     

Microstructural Inhomogeneity and Mechanical Anisotropy Associated with Bedding in Rothbach Sandstone

This study present the result of conventional triaxial tests conducted on samples of Rothbach sandstone cored parallel, oblique (at 45 degrees) and perpendicular to the bedding at effective pressures ranging from 5 to 250 MPa. Mechanical and microstructural data were used to determine the role of the bedding on mechanical strength and failure mode. We find that samples cored at 45 degrees to the bedding yield at intermediate level of differential stress between the ones for parallel and perpendicular samples at all effective pressures. Strain localization at high confining pressure (i.e., in the compactive domain) is observed in samples perpendicular and oblique to the bedding but not in samples cored parallel to the bedding. However, porosity reduction is comparable whether compactive shear bands, compaction bands or homogeneous cataclastic flow develop. Microstructural data suggest that (1) mechanical anisotropy is controlled by a preferred intergranular contact alignment parallel to the bedding and that (2) localization of compaction is controlled by bedding laminations and grain scale heterogeneity, which both prevent the development of well localized compaction features.     

静水条件下砂岩渗透性的试验研究

在TRIAXIAL CELL V3岩石三轴伺服试验机上进行了砂岩的渗透试验,采用流量法测量不同围压和渗透压作用下的砂岩渗透率,揭示了围压和渗透压对砂岩渗透率的影响规律,并给出了围压与渗透率的拟合关系式。结果表明,在相同围压下,随着渗透压差的增加,砂岩渗透率呈不同程度的增加;围压越小,渗透压差对渗透率的影响越大;当渗透压差相同时,砂岩围压一渗透率曲线变化趋势基本一致,渗透率随围压的增加而减小,显示砂岩内部裂隙的闭合程度受围压的影响较明显。     

Failure Mode and Spatial Distribution of Damage in Rothbach Sandstone in the Brittle-ductile Transition

— To elucidate the spatial complexity of damage and evolution of localized failure in the transitional regime from brittle faulting to cataclastic ductile flow in a porous sandstone, we performed a series of triaxial compression experiments on Rothbach sandstone (20% porosity). Quantitative microstructural analysis and X-ray computed tomography (CT) imaging were conducted on deformed samples. Localized failure was observed in samples at effective pressures ranging from 5 MPa to 130 MPa. In the brittle faulting regime, dilating shear bands were observed. The CT images and stereological measurements reveal the geometric complexity and spatial heterogeneity of damage in the failed samples. In the transitional regime (at effective pressures between 45 MPa and 130 MPa), compacting shear bands at high angles and compaction bands perpendicular to the maximum compression direction were observed. The laboratory results suggest that these complex localized features can be pervasive in sandstone formations, not just limited to the very porous aeolian sandstone in which they were first documented. The microstructural observations are in qualitative agreement with theoretical predictions of bifurcation analyses, except for the occurrence of compaction bands in the sample deformed at effective pressure of 130 MPa. The bifurcation analysis with the constitutive model used in this paper is nonadequate to predict compaction band formation, may be due to the neglect of bedding anisotropy of the rock and multiple yield mechanisms in the constitutive model.     

Solar Flares in Extreme Ultraviolet and X-Ray Bands Based on SDO and TIMED Satellite Data

Geomagnetism and Aeronomy - Fluxes in flares are studied in soft X-ray (SXR) and extreme ultraviolet (EUV) bands based on data from GOES, the Solar Dynamics Observatory (SDO), and the Thermosphere,...     

Comparison of Measured and Modelled Hydraulic Conductivities of Fractured Sandstone Cores

— A new method for characterising the detailed fracture geometry in sandstone cores is presented. This method is based on the impregnation of samples with coloured resin, without significant disturbance of the fractures. The fractures are made clearly visible by the resin, thus allowing the fracture geometry to be examined digitally. In order to model the bulk hydraulic conductivity, the samples are sectioned serially perpendicular to the flow direction. The hydraulic conductivity of individual sections is estimated by summing the contribution of the matrix and each fracture from the digital data. Finally, the hydraulic conductivity of the bulk sample is estimated by a harmonic average in series along the flow path. Results of this geometrical method are compared with actual physical conductivity values measured from fluid experiments carried out prior to sectioning. The predicted conductivity from the fracture geometry parameters (e.g., fracture aperture, fracture width, fracture length and fracture relative roughness all measured using an optical method) is in good agreement with the independent physical measurements, thereby validating the approach.     

Compaction and Failure in High Porosity Carbonates: Mechanical Data and Microstructural Observations

We investigated systematically the micromechanics of compaction in two carbonates of porosity above 30%, Majella grainstone and Saint Maximin limestone. The composition, grain size and pore surface area of these rocks were determined. Hydrostatic compression experiments were performed under dry and wet conditions beyond the onset of grain crushing. A significant water weakening effect was observed in both rocks. A set of conventional triaxial experiments was also performed on both rocks under dry conditions at confining pressures ranging from 3 to 31 MPa. Microstructural observations were carried out on the deformed samples. The mechanical behavior of these high porosity carbonates is dominated by shear-enhanced compaction associated in most cases with strain hardening. Stress-induced cracking and grain crushing are the dominant micromechanisms of deformation in both rocks. In Majella grainstone, compactive shear bands appeared at low confinement, in qualitative agreement with the deformation bands observed in the field. At higher confining pressures, compaction localization was inhibited and homogeneous cataclastic flow developed. In Saint Maximin limestone, compaction localization was observed at all confining pressures. An increasing number of compactive shear bands at various orientations appeared with increasing strain. These new data suggest that compaction localization is important in the mechanical compaction of high porosity carbonates.     

Initiation of subduction by post-collision foreland thrusting and back-thrusting

While postulated causes of initial subduction and trench formation include underthrusting, controls on its location and age have not been determined. Consideration of the age of subduction zones bordering five collisional orogens suggests that subduction may have been initiated by foreland thrusts and back-thrusts. Foreland thrusts develop within a continental foreland on the subducting plate mostly within 50 my of collision with an arc system; where the foreland is narrow the thrusts may intersect the continent-ocean crust boundary. Back-thrusts develop in the fore-arc or back-arc area on the overriding plate within 10 to 20 my of collision, and can result in tectonic burial of the magmatic arc; where the arc system is oceanic the back-thrusts may intersect the arc-ocean crust boundary. Possible examples of subduction initiated by foreland thrusts are the start of subduction in the late Jurassic beneath the northern Sunda Arc, and at the end-Miocene in the Negros Trench. Examples of back-thrusts which have initiated or may initiate subduction are the late Cenozoic eastward translation of Taiwan over the Philippine Sea plate, the incipient southward subduction of the Banda Sea beneath Timor, and the W-dipping back-thrust comprising the Highland Boundary Fault zone and postulated early Ordovician thrusts to the SE in Scotland. The suggested relationship of subduction to collision helps to explain the persistence of Wilson cycles in the still-active late Mesozoic to Cenozoic orogenic belts and implies that orogeny will cease only with collision between major continents.     

Variation of Wave Velocity and Porosity of Sandstone after High Temperature Heating

This paper reports the variations of mass, porosity, and wave velocity of sandstone after high temperature heating. The range of temperature to which the sandstone specimens have been exposed is 25–850°C, in a heating furnace. It has been shown that below 300°C, porosity and wave velocity change very little. Above 300°C, there is a rapid increase in porosity, but the wave velocity decreases significantly. The results of thermo gravimetric analysis (TGA), differential scanning calorimetry (DSC) and mercury intrusion porosimetry (MIP) suggest that a series of changes occurred between 400 and 600°C in sandstone could be responsible for the different patterns of variation in porosity and wave velocity.     

Naturally Occurring Arsenic in Sandstone Aquifer Water Supply Wells of Northeastern Wisconsin

The U.S. Environmental Protection Agency (EPA) maximum contaminant level (MCL) of 50 μg/L for arsenic was exceeded in 86 of 2125 water supply wells sampled over a broad geographic range in pails of Drown. Outagamie and Winnebago Counties, Wisconsin. The hydrologic and geochemical properties of the area were examined and the source of arsenic was determined to be natural, Ground water collected from two geologic formations, the St. Peter Sandstone and the overlying Platteville/Galena Dolomite, was found to be the principal source of the elevated arsenic concentrations. These two Formations supply a large portion of eastern Wisconsin private wells with their drinking water.
Three wells were found within Outagamie County to have an unusually low pH. Results suggest that the cause of the low pH in these wells is of natural origin induced by the oxidation of iron sulfide minerals. In this reaction iron sulfide minerals are oxidized. forming sulfuric acid causing a low pH and a high concentration of various metals to leach from native rock formations into the water supply.
Based on the data gathered from this study, an arsenic advisory area was designated for both Outagamie and Winnebago Counties. Guidelines were developed for well drillers and owners constructing new wells within the advisory area to reduce the likelihood of arsenic presence in the water supply. Fifteen wells containing arsenic exceeding the MCL were successfully reconstructed or new wells were constructed based on the guidelines developed. These constructions substantially reduced arsenic levels in the well water supplies.     

Propagation and attenuation characteristics of various ground vibrations

In order to effectively control vibration related problems, the development of a reliable vibration monitoring system and the proper assessment of attenuation characteristics of various vibrations are essential. Various ground vibrations caused by train loading, blasting, friction pile driving and hydraulic hammer compaction were measured using 3D geophones inside of the borehole as well as on the ground surface, and the propagation and attenuation characteristics of various source generated vibrations were investigated by analyzing particle motions. For the geometric modeling of various vibrations, the types of various sources and their induced waves were characterized and the geometric damping coefficients were determined. The measured attenuation data matched well with the predicted data when using the suggested geometric damping coefficient, and the estimated soil damping ratios were quite reasonable taking soil type of the site and experiencing strain level into consideration.     

Propagation and dispersion of electrostatic waves in the ionospheric E region

Low-frequency electrostatic fluctuations in the ionospheric E region were detected by instruments on the ROSE rockets. The phase velocity and dispersion of plasma waves in the ionospheric E region are determined by band-pass filtering and cross-correlating data of the electric-field fluctuations detected by the probes on the ROSE F4 rocket. The results were confirmed by a different method of analysis of the same data. The results show that the waves propagate in the Hall-current direction with a velocity somewhat below the ion sound speed obtained for ionospheric conditions during the flight. It is also found that the waves are dispersive, with the longest wavelengths propagating with the lowest velocity.     

Simulation of Medium Wave Propagation in the Magnetosphere

Geomagnetism and Aeronomy - A model of the medium of the ionosphere and magnetosphere, including the distributions of the concentrations and temperatures, collision frequencies, and magnetic field...     

Tomographic Imaging of Lg and Sn Propagation in the Middle East

?—?Observations based on relatively limited data recorded by sparsely distributed stations have indicated that regional seismic phase propagation (Lg and Sn) is very complex in the Middle East. Accurate characterization of regional seismic wave propagation in this region necessitates the use of a large number of seismic stations. We have compiled a large data set of regional and local seismograms recorded in the Middle East. This data set comprises approximately four years of data from national short-period networks in Turkey and Syria, data from temporary broadband arrays in Saudi Arabia and the Caspian Sea region, and data from GSN, MEDNET, and GEOFON stations in the Middle East. We have used this data set to decipher the character and pattern of regional seismic wave propagation. We have mapped zones of blockage as well as inefficient and efficient propagation for Lg, Pg, and Sn throughout the Middle East. Two tomographic techniques have been developed in order to objectively determine regions of lithospheric attenuation in the Middle East.¶We observe evidence of major increase in Lg attenuation, relative to Pg, across the Bitlis suture and the Zagros fold and thrust belt, corresponding to the boundary between the Arabian and Eurasian plates. We also observe a zone of inefficient Sn propagation along the Dead Sea fault system which coincides with low Pn velocities along most of the Dead Sea fault system and with previous observations of poor Sn propagation in western Jordan. Our observations indicate that in the northern portion of the Arabian plate (south of the Bitlis suture) there is also a zone of inefficient Sn propagation that would not have been predicted from prior measurements of relatively low Pn velocities. Mapped high attenuation of Sn correlates well with regions of Cenozoic and Holocene basaltic volcanism. These regions of uppermost mantle shear-wave attenuation most probably have anomously hot and possibly thin lithosphere.     

岩石受压破裂的ULF和LF电磁前兆信号

实验观测到了4类岩性、18块样品在单轴压力下直至破裂发生全过程中的ULF和LF电磁前兆信号。这些不同频率的信号是零散出现的，其形态为一组组脉冲，宽度为4ms～16ms，幅度为0．1mV～1．4mV。实验发现具同一谐振频率的天线收到的信号频次与其相对于裂缝的位置和方位有关。结果还表明，电磁前兆信号的频度与岩样强度有关，抗压强度高的岩样电磁前兆信号多。微破裂可能产生了偶电层，在破裂过程中向外辐射，产生了LF频段的磁信号，ULF磁信号则主要由压磁效应产生。     

隧道纵向通风对火灾规模和火灾蔓延的影响

隧道纵向通风一方面会给火源提供大量氧气,扩大火灾规模,增加火灾蔓延的可能性;另一方面又可以带走大量热量,减小火灾蔓延的可能性。目前,这两方面影响的相对重要性还没有被很好地研究。本文对纵向通风对隧道火灾蔓延的影响进行了研究,首先分析了纵向通风对隧道火灾规模的影响,然后利用火灾动力学模拟程序FDS,对不同通风速率及相应火灾规模条件下隧道内车辆间的火灾蔓延进行了数值模拟,得出了不同通风速率条件下火灾蔓延的规律,并提出了控制隧道火灾蔓延的措施。结果表明,增加通风速率能较大地增加货车火灾的热释放速率,当通风速率小于2m／s时,火灾蔓延的距离随通风速率的增加而增大,当通风速率大于2m／s时,火灾蔓延的距离受通风速率的影响很小;对于客车火灾,通风对火灾的热释放速率影响较小,并且火灾蔓延的距离随通风速率的增大而减小。     

软土地基连续强夯置换碎石墩的数值分析

根据土体损伤力学,考虑在冲击荷载作用下土体失效机制,建立了基于ABAQUS软件的连续强夯碎石墩形成过程的大变形非线性轴对称有限元方法。根据室内模型实验和数值模拟对比研究,考虑了每一次冲击荷载下土体失效区域大小对下一击碎石进入土体的影响,得到了每次强夯作用下碎石墩性状和土体变形过程,分析了主要影响因素如不同夯击能、垫层厚度以及锤径条件下夯击次数与碎石墩高度之间的关系。本文结果对强夯工程实践具有指导作用。     

Wave Propagation Modeling and Amplitude-Variation-with-Offset Response in a Fractured Coalbed

Coalbed methane can be detected employing the amplitudevariation-with-offset technique. However, there are two issues in applying this technique to a coalbed: strong azimuthal anisotropy resulting from high-density fractures, and the seismic response being composed of many or several individual reflections within the coalbed. To overcome these difficulties, we present an exact solution for reflections in extensive dilatancy anisotropy media. First, we build a three-layer model and simulate the wave propagation in this model. Then we derive an exact P- and converted S-wave reflection coefficient equation based on boundary conditions. Finally, substituting given model parameters into the exact equation, we obtain the variation in the reflection coefficient with incidence angle. The results show that the fracture factors, wavelet frequency and thickness of the coalbed have different effects on the reflection coefficient. Furthermore, we create a synthetic seismogram by forward calculation, and the result fits well with results of the exact equation.