In-situ Rock Spalling Strength near Excavation Boundaries

It is widely accepted that the in-situ strength of massive rocks is approximately 0.4 ± 0.1 UCS, where UCS is the uniaxial compressive strength obtained from unconfined tests using diamond drilling core samples with a diameter around 50 mm. In addition, it has been suggested that the in-situ rock spalling strength, i.e., the strength of the wall of an excavation when spalling initiates, can be set to the crack initiation stress determined from laboratory tests or field microseismic monitoring. These findings were supported by back-analysis of case histories where failure had been carefully documented, using either Kirsch’s solution (with approximated circular tunnel geometry and hence σ _max = 3σ ₁ ?σ ₃) or simplified numerical stress modeling (with a smooth tunnel wall boundary) to approximate the maximum tangential stress σ _max at the excavation boundary. The ratio of σ _max /UCS is related to the observed depth of failure and failure initiation occurs when σ _max is roughly equal to 0.4 ± 0.1 UCS. In this article, it is suggested that these approaches ignore one of the most important factors, the irregularity of the excavation boundary, when interpreting the in-situ rock strength. It is demonstrated that the “actual” in-situ spalling strength of massive rocks is not equal to 0.4 ± 0.1 UCS, but can be as high as 0.8 ± 0.05 UCS when surface irregularities are considered. It is demonstrated using the Mine-by tunnel notch breakout example that when the realistic “as-built” excavation boundary condition is honored, the “actual” in-situ rock strength, given by 0.8 UCS, can be applied to simulate progressive brittle rock failure process satisfactorily. The interpreted, reduced in-situ rock strength of 0.4 ± 0.1 UCS without considering geometry irregularity is therefore only an “apparent” rock strength.     

Threshold of sediment motion under unidirectional currents

Carefully selected data for the threshold of sediment movement under unidirectional flow conditions have been utilized to re-examine the various empirical curves that are commonly employed to predict this threshold. After a review of the existing data, we employed only that data obtained from open channel flumes with parallel sidewalls where flows were uniform and steady over flattened beds of unigranular, rounded sediments. Without these restrictions, an unmanageable amount of scatter is introduced. This selected data is used to develop a modified Shields-type threshold diagram that extends the limits of the original diagram by three orders of magnitude in the grain-Reynolds number. The equally general but more easily employed Yalin diagram for sediment threshold is also examined. Although the Shields and Yalin diagrams are general in that they apply to a wide range of different liquids, in both cases somewhat different curves are obtained for threshold under air than for the liquids. The often used empirical curves of the friction velocity u_*, the velocity 100 cm above the bed u₁₀₀, the bottom stress θ_t, and Shields’ relative stress θ_t, all versus the grain diameter D, are limited in their ranges of application to certain combinations of grain density, fluid density, fluid viscosity and gravity. These conditions must be selected before the curves are generated from either the more general Shields or Yalin curves. For example, on the basis of the data selected for use in this paper, empirical threshold relationships for quartz density material in water are where the velocity u₁₀₀ measured 100 cm above the sediment bed is given in cm/sec and the grain diameter D is in cm. The limitations on any of the threshold relationships are severe. These limitations should be properly understood so that the empirical curves and relationships are not improperly employed.     

埋置简谐扭转荷载作用下广义Gibson饱和地基动力响应

考虑地基为饱和半空间,研究了广义Gibson饱和地基内作用简谐扭转动荷载时地基的动力响应问题。从Biot饱和地基固结理论出发,结合扭转振动的特点,建立了剪切模量随深度线性变化的饱和地基扭转振动的动力微分方程,通过Hankel变换求解此微分方程,给出了Hankel变换域内的切向位移和剪应力关于待定系数的表达式。根据饱和地基表面为自由表面,荷载作用面位移连续、剪应力差等于动荷载大小,波的辐射条件等边界条件求解出待定系数,借助Hankel逆变换给出地基内的位移和应力的表达式。通过数值算例研究发现:在同一水平面内,地基内的切向位移和剪应力曲线的实部和虚部都呈现出非常明显的波动变化规律;在竖向平面内,动荷载作用面上部区域内随深度逐渐增大时,地基内切向位移和剪应力曲线的实部逐渐增大,而在动荷载作用面下部区域则正好相反;扭转动荷载的影响范围主要是荷载作用面上下2倍半径区域。     

Buckling stability of a viscous single-layer system,oblique to the principal compression

A new theory is developed for single-layer buckling, where the layer is not parallel to the principal stresses. The model chosen consists of a single layer with Newtonian viscosity η embedded in an infinite matrix of viscosity η₁. The layer lies at an angle θ to the bulk principal compressive stress in the embedding medium. It is deformed in equal-area plane strain, with the direction of no strain and the third principal bulk stress, parallel to the layer; hence the obliqueness to the principal stresses is only in two dimensions. It is shown that stress refraction is a necessary condition for this system, and an expression is derived for its value in terms of η, η₁ and θ. Buckling stability equations are completely developed which satisfy the Navier-Stokes equilibrium equations for the buckling layer, and the condition of stress continuity at the layer-embedding medium interface. The dominant wavelength of the buckles is shown to be independent of θ, but the stress required increases with θ.The results of this work have an important bearing on natural folds, since there is no evidence that rock layers are initially parallel to the stresses which fold them, an assumption made in former buckling theories. It is suggested that refraction of stresses and the resulting incremental strains gives rise to the finite structure of cleavage refraction so common in deformed rocks, and that the progressive development of folds in layers oblique to the principal bulk stresses gives rise to asymmetry.     

断层面摩擦强度对工作面开采影响数值模拟分析

断层面摩擦强度是评价煤炭开采中应力扰动诱发断层滑动危险性的依据。依托晋城矿区成庄井田,采用理论分析和数值模拟计算方法,分析了断层面摩擦强度对深部地应力的约束机制,研究了成庄井田F13断层及其在不同摩擦强度条件下对回采工作面顶板稳定性、超前支承压力分布和断层滑动的影响规律。研究结果表明:地壳深部最大与最小主应力比值受断层面摩擦强度的限制,当其达到临界方向断层的摩擦强度极限时,断层就会发生滑动;断层破碎带的存在导致初始应力场扰动,形成断层带低应力区及高应力集中区,在回采过程中将直接影响煤层顶板移动变形和采动应力分布;断层面摩擦强度较小时,工作面开采至断层附近顶板下沉量及断层上下盘错动位移较大,支承压力峰值由大变小明显,断层面上剪应力与正应力的比值易达到断层面的摩擦系数,断层滑动的危险性较大。      

Energy relations of density-current flows: an experimental investigation*

Three series of density-current experiments were performed in a 5.76 m flume. In the first series, the flume was horizontal, and in the second and third, it was inclined with a positive slope and negative slope, respectively. Energy relations during successive stages of density-current movement were computed from observed data, which showed an appreciable frictional energy dissipation. The computed friction factors of our experimental density-flows were compared to the friction factors for pipe flows (Moody diagram), and while the calculated friction factor increases with increasing Reynold's number within the range of our experiments (Re 2 × 10³?2 × 10⁴), it is concluded that with increasing Reynold's number above about 5 × 10⁴ the friction factor decreases. For natural turbidity currents, the Moody diagram gives a reasonable estimate of the friction factor between the current and sediment bed. The value of the friction factor for the interface between the current and overlying water was found to be about 0.2 times the friction factor for the current and flume. However, due to errors inherent in measuring the depth of the current, a value of 0.4 would be more reasonable for density-currents in our range of Reynold's number. Friction tends to decrease the value of the dimensionless coefficient in Keulegan's law of saline front and to decrease the thickness of the flow. In contrast, the presence of a slope in the direction of flow tends to compensate the effect of friction. The angle θ^c that provides the potential energy to exactly offset the energy losses incurred during movement by the density-currents in our experiments has a calculated value of 31′. An empirical formula φ= 0.935θ—0·57 relating friction, in terms of the hydraulic gradient φ, to the slope angle θ was obtained. Since the thickness of the current can be computed from the relationship between φ and θ, we estimated the thickness of naturally occurring density-currents in Swiss lakes. The results suggest the applicability of our experimental results to small turbidity currents in nature. Our analysis further indicates that large turbidity currents have a small φ and can be expected to flow very long distances on a flat abyssal plain.     

Bed forms in bimodal sand–gravel sediments: laboratory and field analysis

Bed forms were studied in Goodwin Creek and a laboratory flume channel. The bed sediment of the field site and flume had median diameters of 8·3 (modes of 0·4, 22·6 mm) and 1·82 mm (modes of 0·5, 5·6 mm), respectively. The laboratory and field channels had similar values of bimodal parameters, ratios of flow depth to median bed material diameter, and ratios of shear stress to critical shear stress and were judged to be comparable in the transport of bed load sediment and the resulting bed forms. Three groupings of bed forms from the laboratory flume experiments (ripple-like bed forms, bed load sheets, low-relief bed waves) were identified using the height and period of the bed forms. For the range of flow depths and discharges investigated in the flume, bed forms became higher and longer with increasing bed shear stress. Bed forms from Goodwin Creek were similar to those from the flume with comparable ratios between bed form length, height, and flow depth. The bed forms in the flume provide a positive link between rate and size fluctuations measured in the field and the bed forms. The smaller bed forms identified were sediment starved and are not considered to be dunes, while the largest bed forms in which all of the bed material sizes were mobilized in the field and laboratory were judged to be dunes.     

A study of the behaviour of brittle rocks subjected to confined stress based on the Mohr–Coulomb failure criterion

In this study the relationship between brittle rocks’ behaviour and uniaxial compression stress is obtained based on the Mohr-Coulomb failure criterion and the behavioural characteristics of brittle rocks under uniaxial compression stress are investigated. According to the laboratory uniaxial compression tests on granite rocks, and also available results in the literature, required parameters in the study are obtained. It is indicated that the behaviour of brittle rocks which is affected by uniaxial compression is function of cohesion. In the other words, cohesion changes as the uniaxial compression increases. Also, in yield point where its stress is equivalent to the uniaxial compressive strength, the cohesion maximises. By suggesting a new viewpoint to the Mohr’s circle, normal stresses on failure plane, shear stresses tangential to the failure plane and hydrostatic stresses are investigated. Results show that, normal stress on the failure plane in yield point of the behavioural curve equals zero and shear stress tangential to the failure plane is maximum which is equal to the maximum rocks’ natural strength. Also, in this point the strength is equi-pressure, therefore the stress is of hydrostatic type.     

地下洞室围岩脆性破坏时的应力特征研究

在高应力作用下,岩爆、钻孔崩落、片帮都是地下空间硬脆围岩中常见的破坏现象,这三类现象本质上均可归于完整岩体的脆性破坏,它们分别反映了高应力作用下完整岩体不同的破坏程度。通过对前人关于岩爆判据、钻孔崩落判据和片帮应力强度比判据研究成果的类比分析可知,这些脆性破坏现象在破坏时具备相同的应力背景条件。脆性破坏的应力条件可以用地下空间周边切向最大应力与岩石单轴抗压强度之比（ / ）或者工程区最大主应力与岩石单轴抗压强度之比（ / ）来描述,两种指标本质上反映了相同的应力背景条件。对于 / , / = 0.4 ± 0.1是发生脆性破坏的应力临界条件;对于 / , / = 0.15 ± 0.05是发生脆性破坏的应力临界条件。大量的工程实例和基于Hoek-Brown强度准则的力学分析也证明了这一背景条件的正确性。这里两种指标都取了一个范围,主要是由于不同的岩体分级、岩性和工程地质条件会对指标的界定产生较为显著的影响。     

Water assisted dynamic recrystallization and weakening in polycrystalline bischofite

Artificially prepared specimens of bischofite (MgCl₂-6H₂O) have been experimentally deformed at temperatures between 20 and 100°C, strain rates between 10⁻⁴ and 10⁻⁸⁸ s⁻¹, and confining pressures between 0.1 and 28 MPa. Development of microstructure with strain was studied by in-situ deformation experiments, and results of these were correlated with observations made on thin sections of deformed samples.In a first series of experiments the effect of grain size, impurity content and water content on the flow behaviour was investigated. Addition of about 0.1 wt.% water to dry samples was found to decrease the flow stress by a factor of 5. This effect was found to be associated with the formation of a thin fluid film on grain boundaries, strongly enhancing dynamic recrystallization due to the movement of high-angle grain boundaries, and possibly also to enhanced intracrystalline plasticity due to excess water present in the lattice. In a second series of experiments the strain-rate sensitivity of the flow stress of selected samples was investigated. Two regimes could be distinguished: one with a stress exponent n = 4.5 in the power law creep equation for values of the differential stress above 2.0 MPa, and one with n = 1.5 for stresses below this value.The main deformation mechanisms were intracrystalline slip, twinning, and grain-boundary sliding. Recrystallization occurred by subgrain rotation and high-angle grain-boundary migration. The rates of grain-boundary migration fell into two different regimes, one regime being distinguished by extremely fast migration rates. The applicability of the experimentally found flow law to the behaviour of bischofite rocks in nature is discussed.     

Frequency distribution of river bed materials

The size distribution of river bed material plays an important role in the study of fluvial hydraulic processes. It is not always easy to sample the river bed material for the determination of its size distribution, but knowledge of the frequency distribution and of bed material sizes can be used to predict its size distribution. Therefore, data on bed material size distributions of 63 streams from 10 different countries have been analysed to determine the frequency distribution which can be best fitted to them. The log normal distribution cannot be fitted to the bed material mixtures when the entire range of sizes is considered. The method of power transformation is found to normalize these mixtures. Relationships have been proposed between the parameters of the normalization and the stream slope. These relations can be used to predict the size distribution of the river bed material.     

A laboratory study of anisotropic geomaterials incorporating recent micromechanical understanding

This paper presents an experimental investigation revisiting the anisotropic stress–strain–strength behaviour of geomaterials in drained monotonic shear using hollow cylinder apparatus. The test programme has been designed to cover the effect of material anisotropy, preshearing, material density and intermediate principal stress on the behaviour of Leighton Buzzard sand. Experiments have also been performed on glass beads to understand the effect of particle shape. This paper explains phenomenological observations based on recently acquired understanding in micromechanics, with attention focused on strength anisotropy and deformation non-coaxiality, i.e. non-coincidence between the principal stress direction and the principal strain rate direction. The test results demonstrate that the effects of initial anisotropy produced during sample preparation are significant. The stress–strain–strength behaviour of the specimen shows strong dependence on the principal stress direction. Preloading history, material density and particle shape are also found to be influential. In particular, it was found that non-coaxiality is more significant in presheared specimens. The observations on the strength anisotropy and deformation non-coaxiality were explained based on the stress–force–fabric relationship. It was observed that intermediate principal stress parameter b(b = (σ ₂ ? σ ₃)/(σ ₁ ? σ ₃)) has a significant effect on the non-coaxiality of sand. The lower the b-value, the higher the degree of non-coaxiality is induced. Visual inspection of shear band formed at the end of HCA testing has also been presented. The inclinations of the shear bands at different loading directions can be predicted well by taking account of the relative direction of the mobilized planes to the bedding plane.     

波,流联合作用下的近岸海底沙波稳定性研究进展

近岸海底大型沙波是一种研究较为薄弱的地貌单元，但它的迁移对港口、航道和海底电缆及钻井平台、输油管道等海洋工程设施建设造成的危害极大。在综述目前国内外有关研究文献的基础上，概要地介绍了作者提出的一种较为简便、省力和有效的沙波迁移速率和沙波稳定性定量评价和预测方法，旨在提高航道通航能力和海洋工程建设的灾害防治和预测能力     

Source parameters of earthquakes in the reservoir-triggered seismic (RTS) zone of Koyna–Warna, Western India

New empirical relations are derived for source parameters of the Koyna–Warna reservoir-triggered seismic zone in Western India using spectral analysis of 38 local earthquakes in the magnitude range M _L 3.5–5.2. The data come from a seismic network operated by the CSIR-National Geophysical Research Institute, India, during March 2005 to April 2012 in this region. The source parameters viz. seismic moment, source radius, corner frequency and stress drop for the various events lie in the range of 10¹³–10¹⁶ Nm, 0.1–0.4 km, 2.9–9.4 Hz and 3–26 MPa, respectively. Linear relationships are obtained among the seismic moment (M ₀), local magnitude (M _L), moment magnitude (M _w), corner frequency (fc) and stress drop (?σ). The stress drops in the Koyna–Warna region are found to increase with magnitude as well as focal depths of earthquakes. Interestingly, accurate depths derived from moment tensor inversion of earthquake waveforms show a strong correlation with the stress drops, seemingly characteristic of the Koyna–Warna region.     

Spalling Experiments on Large Hard Rock Specimens

Specimens of coarse-grained Äspö diorite were axially compressed to observe stress-induced spalling. The specimens had a novel design characterized by two manufactured large radius notches on opposite sides. The tangential stress occurring in the notches aimed to represent the tangential loading around a circular opening. Fracture stages were monitored by acoustic emission measurements. Rock chips were formed similar to those found in situ, which indicates a similar fracture process. Slabs were cut out from the specimens and impregnated using a fluorescent material to visualize the cracks. The cracks were subsequently examined by the naked eye and by means of microscopy images, from which fracture paths could be identified and related to different minerals and their crystallographic orientations. The microscopy analyses showed how the stress field and the microstructure interact. Parallel cracks were formed 2–4 mm below the surface, sub-parallel to the direction of the maximum principal stress. The crack initiation, the roles of minerals such as feldspar, biotite and quartz and their grain boundaries and crystallographic directions are thoroughly studied and discussed in this paper. Scale effects, which relate to the stress gradient and microstructure, are discussed.     

地震作用下岩盐层套变规律研究

5.12汶川特大地震发生后,某气田在岩盐层段发现了多口井套变,为了能有效地治理,急需了解其套变的原因和规律。为此,利用真三维接触有限元分析方法研究了震前套管应力和变形分布规律,并在原有边界条件基础上,在模型的顶面进一步施加不同震级地震载荷,重新分析套管应力和变形的分布规律。对比二者分析的结果发现：无论是震前还是震后都显示岩盐层段套管的应力和变形水平明显高于套管其他部位,且基本规律是震级越高套管在岩盐层段的应力水平和变形越大。对比有限元计算震前和震后套管应力和变形的结果还发现,仅5.12特大地震载荷的竖向分量所产生的应力水平已经超过了套管的屈服强度极限,因此,可以判定此次地震是导致该气田盐岩层段出现套变的真正原因。     

Microboudin structures of piemontite along the Sambagawa metamorphic belt, Japan: implications for lateral variation of differential stress during regional metamorphism

Evidence is presented of a lateral variation in differential stress during metamorphism along a regional metamorphic belt on the basis of the proportion of microboudinaged piemontite grains (p) in a quartz matrix in metacherts. It is proposed that p is a practical indicator of relative differential stress. Analysis of 123 metacherts from the 800 km long Sambagawa metamorphic belt, Japan, reveals that p‐values range from < 0.01 to 0.7 in this region. Most samples from Wakayama in the mid‐belt area have p‐values of 0.4–0.6, whereas those from western Shikoku have p‐values of < 0.1. This difference cannot be explained by variations in metamorphic temperature, and is instead attributed to a regional, lateral variation in differential stress during metamorphism.     

新疆乌恰萨热克含铜盆地上侏罗统库孜贡苏组复合型冲积扇沉积相研究

新疆萨热克含铜盆地库孜贡苏组是复合型冲积扇沉积相,是重要的赋矿层。利用研究区24个钻孔和10个露头剖面资料,运用高分辨率层序地层学原理进行单孔沉积相、连孔沉积相、平面沉积相分析,对新疆萨热克含铜盆地库孜贡苏组(J3k)复合型冲积扇内部垂向和3个中期旋回期间沉积相亚相的平面分布特征进行了研究。研究表明,该区库孜贡苏组垂向分布大体可分为3个中期基准面旋回和若干短期基准面旋回,据此建立了高分辨率层序地层对比格架。3个中期旋回期间,沉积亚相平面发育模式差异大。铜矿(化)体垂向、平面分布特征明显。     

Lithomorphic stresses and cleavage of loess

Lateral stresses existing in soil deposits are important clues to recent stress history because they tend to remain “locked in” following adjustments in the overburden load: Thus a high lateral stress is partly retained following a reduction in vertical stress, as by glacial melting or erosion. Conversely, during loess deposition a low lateral stress should be retained if the soil structure is preserved under increasing vertical stress. This has not previously been verified in the field because of the difficulty of measuring lateral stresses in situ in an hydrocollapsible material.

Tests were conducted at two sites in Missouri river valley loess using theK_o Stepped Blade, which uses data extrapolation to give stress on a zero thickness blade. In basal loess where the moisture content was at or above the liquid limit,K_o = 1.0 ± 0.02. In the upper to middle loess zoneK_o is as low as 0.2 to 0.4, lower than the values of 0.4 to 0.5 that would be expected if the soil were normally consolidated. Near the ground surface, lateral stresses are higher soK_o > 1, attributed to post-depositional weathering and the presence of smectite clay minerals. The vertical sequence of lateral stress from high to low to high again should contribute a tension-induced cleavage if lateral confinement is removed by excavation or erosion. This stress condition is most pronounced in loess close to its source, where the potential tension zone was found to extend to a depth of about 7 m compared to 4 m at the other site. Loess cleavage appears to result from stress relief and is not a unique directional property of this material.