Applicability of the theory of hollow inclusions for overcoring stress measurements in rock

Summary Whenever solid or hollow inclusions are used as instrumented probes in overcoring techniques, residual stresses remain in the overcored rock sample and in the probes. When using such devices for computing the in-situ stress field components from measured strains or displacements, it is common practice to assume that the overcoring diameter is infinite and that there is a perfect bonding between the rock and the probes. The validity of these assumptions depends on the magnitude of the residual stresses at the rock-probe contact as compared to the tensile and shear strengths of the rock-probe bond material. It also depends on the distribution of residual stresses in the overcored sample.In comparison to previous work, new expressions are proposed in this paper for the residual stresses associated with solid or hollow inclusion type stress probes in anisotropic ground. These expressions are presented in dimensionless form and are used to show that the distribution and magnitude of residual stresses depend on the isotropic-anisotropic rock character, the degree and type of rock anisotropy, the orientation of the rock anisotropy with respect to the hole in which the probes are located and the relative deformability of the rock with respect to the deformability of the material comprising the probes. The conditions that are required for neglecting the overcored sample diameter are also discussed. This is shown for rocks that can be described as isotropic, transversely isotropic and orthotropic materials.     

Generalization of two-dimensional discontinuous deformation analysis for forward modelling

This paper provides the theoretical basis for a forward numerical model of rock block systems. This method was developed to compute stress, strain, sliding and opening of rock blocks; rigid body movement and deformation occur simultaneously. Input data consist of block geometry, loading forces, the deformability constants E and v, and the restraint or boundary conditions of the block system. Output data give the movements, deformations, stresses and strains of each block, and the sliding and detachment or rejoining of blocks. The forces acting on each block, from external loading or contact with other blocks, satisfy the equilibrium equations. Equilibrium is also achieved between external forces and the block stresses. Furthermore, this analysis fulfills constraints of no tension between blocks and no penetration of one block into another. To acheive equilibrium and satisfy these constraints, simultaneous equations are solved repeatdly with partial changes of the coefficients each time the contact constraints are chosen.     

用解析延拓求取剩余磁异常来圈定浅部隐伏磁铁矿脉

剩余磁异常求取是一种对磁场强度进行化极,解析延拓后提取浅部磁性介质异常的高通滤波方法。对于处于强磁性围岩的磁铁矿脉来说,浅部磁铁矿脉的异常往往被大规模的磁性围岩所引起的异常所掩盖。磁场强度与场源到测点距离的平方(二度体)或者立方(三度体)成反比,进行向上延拓的时候,由浅部场源引起的高频异常衰减快,深部场源引起的低频异常衰减的慢,因此地面实测磁场去掉向上解析延拓后的磁场值,可突出反映近地表磁性体引起的高频异常。这里通过解析延拓求取剩余异常,实现浅部磁铁矿脉异常信息的提取,为寻找隐伏的磁铁矿脉提供依据。并且利用此方法在秘鲁共和国Morritos铁矿圈定隐伏磁铁矿脉,验证了剩余异常在大规模的高磁性围岩中寻找隐伏磁铁矿脉的应用效果。     

Approaches to stress monitoring in deep boreholes for future CCS projects

There is no monitoring technology available to observe possible changes of stress in the rock mass of a CO₂ reservoir or its cap rock formations. Any development of a stress-monitoring technique must be related to the natural regional stress conditions and must be adjusted to the possibly changing in situ stress conditions due to CCS activity. As a step towards an in situ stress-monitoring probe, a lab scale device was developed and used for investigations on the practicability of a hard-inclusion tool for stress monitoring. In situ stress conditions, as deduced from the Altmark Gas Field, were applied to evaluate the efficiency and the limits of this stress-monitoring technique. At lab-scale the applied stresses resp. stress differences with moderate amounts of 9?C15?% of the vertical stress component Sv coincide sufficiently with the resulting strain answers of the hard inclusion tool (i.e., a steel tube corresponding to the liner in the borehole). Therefore, it was possible to re-calculate the stresses and to compare them with the applied ones. The resulting coincidence, however, can be disturbed at high pressure levels due to rock failure around the borehole with extended deformations. In addition, the results are influenced by the mechanical behaviour of the surrounding rock mass type. Nevertheless, a further development of a hard inclusion probe for monitoring of stress changes in deep boreholes can be successful and may be the only possible way to detect stress changes without fracturing damages in deep boreholes.     

测定岩石经历的最高古应力状态实验研究

当前国内外尚无测定三维古构造应力场的理想方法.提出了一种新方法: 从待测地点取大岩心或新鲜石块, 从6个以上方向取岩石试样, 每方向至少10个试样, 以声发射的广义抹录不净现象, 测定岩石经历的各主要构造运动期的最大主应力值.而后, 统计出岩石记忆的最高古应力状态的最大主应力值σ_mt.由于岩石对最高古应力状态在各取样方向的正应力σ_mn尚有记忆, 只不过隐藏于各期较低古应力状态的最大主应力值之间, 不易区分, 故以σ_mt为目标, 通过“AE反演搜索法”, 则可将隐藏的各σ_mn搜索出来, 从而计算出岩石经历的最高古应力状态.以准平面应力状态模拟实验检验了该新方法的可行性.      

Non-linear analysis for calculating the support of a rock block with dilatant joint faces

In jointed rock masses the instability of individuals, discrete blocks is a concern. In underground excavations it is necessary to know how much support is required for “key blocks” in the roof. As these blocks fall, dilatancy of the block's bounding joints tends to keep the block in contact with the surrounding rock mass. Shear stresses devloped along the joints as a result of this contact and help to support the block. By knowing the initial stress field and the shear properties of the joint, as a function of stress and displacement, a non-linear analysis can determine the normal and shear stresses on the bounding joints as a function of the displacement path. The amont of support required to keep the block in the roof can then be determined as a function of displacement.     

Numerical modelling of the three-dimensional stress field in southwestern Japan

It has been inferred from the focal mechanism of earthquakes and their hypocenter distribution (Shiono, 1977) that the stress field in southwestern Japan indicates complicated features; a NW-SE compression at shallow depths along the Nankai trough, an E-W or ESE-WNW compression in the inland crust, an extension parallel to the leading edge of the Philippine Sea plate at subcrustal depths in the region from the southern Chubu to northwestern Shikoku, and a down-dip tension beneath the Kyushu island.In order to investigate possible sources of these complex features of the stress state, a three-dimensional finite element method is employed to model the configuration of the subducting Philippine Sea plate, taking into consideration the following three possible types of forces:

1. (1) A negative buoyancy due to the density contrast between the subducting plate and the surrounding mantle.

2. (2) A northwestward compressive force generated by the movement of the Philippine Sea plate.

3. (3) A westward compressive force due to the movement of the Pacific plate.

For various combinations of different magnitudes of these forces, and of different elastic moduli, the stresses at a number of selected sites are calculated, and their directions are compared with those inferred from the focal mechanism and other geophysical information.It is found that the observed extensional stresses parallel to the leading edge of the subducting Philippine Sea plate may be caused mainly by the negative buoyancy. The northwestward compressive force may not play an important role in generating the complex stress field in southwestern Japan. The observed E-W compressional stress field prevailing in the inland region appears to result mainly from the westward-moving Pacific plate. The present results suggest that if a thin low-velocity transitional layer exists just above the subducting Philippine Sea plate, it could give appreciable effects on the mechanism of low-angled thrust faulting off the Kii peninsula and the Shikoku island.The magnitude of the shear stress in the continental crust and in the subducting plate is estimated to be of the order of several hundred bars, although the calculated shear stresses are considerably affected by the configuration of the subducting plate and also by the applied forces.It is interesting that the stress distribution appears to have some relations to seismicity of subcrustal earthquakes, and to the rupture process of large thrust earthquakes along the plate boundary.     

Anisotropic growth in the olivine-spinel transformation of Mg2GeO4 under nonhydrostatic stress

The olivine-spinel phase transformation in Mg₂GeO₄ does not occur by a martensitic mechanism. The evidence, from samples transformed in a Griggs-type solid medium deformation apparatus, are:

1. (1) lack of microstructural features in the olivine phase which can be specifically associated with a martensitic mechanism

2. (2) the orientation relationship between the two phases that is predicted by the martensitic mechanism does not occur nor is there any apparent consistency of relative orientations

3. (3) application of a differential stress to the transforming sample resulted in an anisotropic growth rate for the spinel phase indicating that growth was externally controlled rather than crystallographically controlled.

Anisotropic growth of the spinel phase results in elongation of the residual olivine phase grains in the plane normal to the direction of maximum principal compressive stress. A velocity ratio of 1.7_−0.7^+5.4 has been determined for the growth rate of the spinel from measurements on residual olivine grains. The interphase grain boundary in samples transformed under stress has cusp-shaped fingers of spinel with a blunt end separated by thin spikes of olivine. Samples transformed isostatically do not exhibit this feature providing further confirmation of anisotropic growth of the spinel. The preferred growth of the spinel is consistent with a theory of phase transformation under nonhydrostatic stress. The predicted spinel finger shape based on this theory is generally consistent with observed shapes except for the blunt end. The discrepancy may be due to surface energy which has not been considered here, or to local deviations of the applied macroscopic stress.     

罗布莎铬铁矿区成矿构造条件的研究

矿区位于藏南超基性岩带罗布莎超基性岩体,主要矿床成因类型有:岩浆晚期熔离矿床和正岩浆期矿床。后者未构成具工业价值铬铁矿体,前者为矿区主要矿床成因类型。本文仅探讨岩浆晚期熔离矿床的成矿构造条件。     

Coupling of BEM with a large displacement and rotation algorithm

In stability analysis of rock blocks, the deformability of the blocks can conveniently be simulated using the boundary element method (BEM). However, all boundary conditions are given as stresses. Thus, the displacement solution is not unique. In this paper, an algorithm is proposed to remove rigid body motions in the solution of the boundary form of Somigliana identity discretized by the direct BEM formulation. The algorithm is applied to the calculation of the normal stiffness of rock blocks and coupled with BS3D, large displacement and rotation algorithm for the general stability of rock blocks. Copyright © 2010 John Wiley & Sons, Ltd.     

基于多尺度直剪试验的岩石模型结构面抗剪强度特征研究

开展岩石模型结构面抗剪强度特征的多尺度（尤其大尺寸）直剪试验研究对于理解岩石结构面力学特性具有重要的理论价值和实践意义。首先,基于多尺度直剪试验仪(MSJ-DST),对20 cm×20 cm、40 cm×40 cm、60 cm×60 cm、80 cm×80 cm和100 cm×100 cm的岩石模型结构面试样采用法向应力分别为200～1 000 kPa进行直接剪切试验;然后,研究不同尺寸岩石模型结构面抗剪强度的特征。结果表明：不同法向荷载作用下模型的受力变形特点相近,峰值剪切位移总体上随着某一数值附近上下浮动;在同一法向应力作用下,不同尺寸结构面试样的峰值抗剪强度表现出在某一数值附近上下浮动的特征,残余抗剪强度则表现出随尺寸的增加有小幅度增加;5级法向应力作用下,不同尺寸的峰值抗剪强度和残余抗剪强度随着法向应力的变化规律均近似相同,抗剪强度残余值与峰值的比值随着法向应力的增大逐渐增大并趋于稳定。     

对称荷载正多边形孔洞弹性应力解析逼近方法

提出一种简单有效的计算方法,求解对称荷载作用下含正多边形孔洞无限平面体的弹性应力解。将孔洞内边界向两侧延伸至应力可忽略的远处,其外侧构成半无限平面体。相邻两个半无限平面体有一个公共域,其中一个半无限平面体顶边延伸至相邻两个半限平面体体内。孔洞内边界上面力是已知的,延伸至两相邻半无限平面体体内的顶边上面力可先假设,由弹性理论求出半无限平面体体内应力,再计算相邻半无限平面体顶边上面力,根据对称性得到本半无限平面体顶边上面力改进值,迭代求解直至收敛。该方法具有计算过程简单、精度高等优点。算例分析表明,该方法求解的工程尺度下孔洞周边应力场与复变函数方法、有限元方法计算结果吻合,拟合计算的孔边角点处应力奇异性次数与理论解基本一致。     

Petrochemical Investigation of the Antique Ophiolite (Philippines): Implications on Volcanogenic Massive Sulfide and Podiform Chromitite Deposits

Abstract: The Antique ophiolite, located in Panay island (west‐central Philippines), corresponds to several tectonic slices within the suture zone between the Philippine Mobile Belt (PMB) and the North Palawan Block (NPB). It includes dismembered fragments of a basaltic sequence, dominantly pillow‐lavas with minor sheet flows, rare exposures of sheeted dikes, isotropic gabbros, subordinate layered mafic and ultramafic rock sequences and serpentinites. Most of the ophiolite units commonly occur as clasts and blocks within the serpentinites, which intrude the whole ophiolitic body, as well as, the basal conglomerate of the overlying Middle Miocene sedimentary formation. The volcanic rock sequence is characterized by chemical compositions ranging from transitional (T)‐MORB, normal (N)‐MORB and to chemistry intermediate between those of MORB and island arc basalt (IAB). The residual upper mantle sequence is harzburgitic and generally more depleted than the upper mantle underlying modern mid‐oceanic ridges. Calculations using whole‐rock and mineral compositions show that they can represent the residue of a fertile mantle source, which have undergone degrees of partial melting ranging from 9‐22.5 %. Some of the mantle samples display chondrite‐nor‐malized REE and extended multi‐element patterns suggesting enrichments in LREE, Rb, Sr and Zr, which are comparable to those found in fore‐arc peridotites from the Izu‐Bonin‐Mariana (IBM) arc system. The Antique ultramafic rocks also record relatively oxidizing mantle conditions (Δlog f_O2 (FMQ)=0.9‐3.5). As a whole, the ophiolite probably represents an agglomeration of oceanic ridge and fore‐arc crust fragments, which were juxtaposed during the Miocene collision of the PMB and the NPB. The intrusion of the serpentinites might be either coeval or subsequent to the accretion of the oceanic crust onto the fore‐arc. Volcanogenic massive sulfide (VMS) deposits occur either in or near the contact between the pillow basalts and the overlying sediments or interbedded with the sediments. The morphology of the deposits, type of metals, ore texture and the nature of the host rocks suggest that the formation of the VMS bodies was similar to the accumulation of metals around and in the subsurface of hydrothermal vents observed in modern mid‐oceanic ridge and back‐arc basin rift settings. The podiform chromitites occur as pods and subordinate layers within totally serpentinized dunite in the residual upper mantle sequence. No large coherent chromitite deposit was found since the host dunitic rocks often occur as blocks within the serpentinites. It is difficult to evaluate the original geodynamic setting of the mineralized bodies since the chemistry of the host rocks were considerably modified by alteration during their tectonic emplacement. A preliminary conclusion for Antique is that the VMS is apparently associated with a primitive tholeiitic intermediate MORB‐IAB volcanic suite, the chemistry of which is close to the calculated composition of the liquid that coexisted with the podiform chromitites.     

结构面对程潮铁矿西区地表变形的影响分析

以程潮铁矿西区为例,通过对现场结构面和裂缝分布特征的调查,以及结合地表变形监测数据的分析,揭示了矿区结构面对地表变形的影响。研究结果表明,在矿区较大的水平应力条件下,岩体结构面改变了地表拉伸变形分布和破坏形态,加剧了岩体变形,在ⅰ区(剖面Ⅲ以东区域),目前最外侧裂缝以内的岩体沿着NNW结构面发生倾倒滑移破坏,地表变形以快速变形为主,而最外侧裂缝以外的岩体沿着NNW结构面发生弯曲变形,地表变形以线性稳定增长为主,进入了倾倒破坏阶段,该区域的岩体主要发生水平位移;在ⅱ区(剖面Ⅲ以西区域),岩体在开采沉陷引起的南北向应力作用下,分离成平行的块体,地表变形以快速变形为主,在局部区域,产生的一部分平行块体在近东西向的应力作用下,沿着NNW结构面产生倾倒破坏。所得成果对类似金属矿山的地表征地及安全高效生产具有实际指导意义。     

A ground reaction curve based upon block theory

Summary Discontinuities in a rock mass can intersect an excavation surface to form discrete blocks (keyblocks) which can be unstable. Once a potentially unstable block is identified, initial normal and shear stresses on each block face are calculated using elastic theory, and are then modified by discontinuity deformations as the keyblock displaces. The modified stresses are summed into resultant forces to evaluate block stability. Since the resultant forces change with displacement, successive increments of block movement are examined to see whether the block ultimately becomes stable or fails. Calculated keyblock stability increases with larger in situ stress magnitudes, larger lateral stress ratios, and larger shear strengths. Discontinuity stiffness controls block displacement more strongly than it does stability itself. Large keyblocks are less stable than small ones, and stability increases as blocks become more slender.     

Fracture analysis near the mid-ocean plate boundary, Reykjavik-Hvalfjördur area, Iceland

The geometry and thermal history of fractures have been determined at 59 stations from Reykjavik to Hvalfjördur in southwestern Iceland. The data provide information on crustal stress regimes in the vicinity of mid-ocean ridges.Two major, generalized fracture orientations are present

1. (1) a northeast system, trend 010°–030°, except on Akranes where the orientation is 040°–060°

2. (2) a broad east—west system containing one or more sets with strike between 070°–130°.

Thermal history of the host rock and fractures was determined from secondary minerals in vugs and fractures. The thermal history indicates that the northeast fracture set opened while the area was within the relatively hot axial zone of active volcanism and rifting. Some of the east—west trending fractures also opened at this time but many formed later, after the area had begun to cool and drift from the active zone.The northeast fracture set is essentially parallel to the trend of dikes and normal faults in southwestern Iceland. They have been interpreted as extension fractures (resulting in about 0.4% maximum extension) forming generally from the same stress field associated with normal faulting and dike injection in the active zone. Fracturing in an east-west direction (estimated 0.1% maximum extension), mainly near the edge and outside the active zone, indicates a reorientation of this stress field. The dominant mechanism related to the origin of the east—west fractures may be thermoelastic stresses arising from axial and basal accretion and cooling of lithospheric plates.Both fracture systems are inferred to have formed, in the Griffiths idealization, under nearly biaxial effective compressive loading on the order of 200 bar. The discrepancy between this value and the kilobar-order strengths of short-time laboratory tests reflects such factors as high temperature stress corrosion and fatigue. Fracture propagation is assumed to have been stable, but governed primarily by lateral load-diminishing mechanisms rather than by progressive loading. These relaxation mechanisms may have been episodic (northeast-system fissure swarm activity) or steady-state (thermoelastic contraction) in time.     

采用环剪仪对超固结黏土抗剪强度特性的研究

利用大型高速环剪仪,对不同超固结比、法向应力和剪切速率下的超固结饱和黏土的峰值强度和残余强度特性进行了研究,并对循环荷载作用前后残余强度的变化进行了初步探讨。试验结果表明,（1）超固结比对超固结黏土的峰值强度和残余强度有着明显的影响;（2）在剪切速率相同的条件下,土体达到残余强度时的位移取决于现存的应力状态,而与应力历史无关;（3）剪切速率越大,峰值强度随之增大,达到稳定残余强度时的剪切位移也随之增加,但剪切速率的变化对残余强度值几乎没有影响;（4）在循环荷载作用下残余强度不同程度的降低,最大降低幅度达12.2 %;当土体剪切面为不规则剪切带时,施加循环荷载后出现残余强度上升的现象。     

湖南东坡半隐伏岩体的侵入方向及其空间形态特征

东坡(千里山—王仙岭)半隐伏岩体是湖南郴州地区与锡钨、铅锌成矿关系密切的重要岩体。为了弄清它们的侵入方向,根据不同尺度的剩余重力异常中心的位置与出露岩体中心的位置,判断出岩浆的侵入方向;通过重力异常的二维、三维反演,揭示了千里山岩体、王仙岭岩体的连接关系、二维空间形态特征以及与围岩的接触关系;推断认为:形成王仙岭岩体的岩浆是由北东—东南—北西方向侵入的;形成千里山岩体的岩浆是由北东往南(西)方向侵入的。而且两个岩体在深部相连,并且呈大岩基产出。     

Raman spectroscopic estimation of depth of diamond origin: technical developments and the application

Residual pressure around mineral inclusions in diamond can provide useful information on the depth of diamond origin. Differential stress between an inclusion and host diamond arises from differences in thermal expansion and compressibility between host diamond and minerals. We determined residual pressure around mineral inclusions in a diamond from the Internationalnaya pipe, Yakutia, Russia, using the three-dimensional Raman mapping system developed recently by our group. The maximum residual pressures around the olivine and chromite inclusions were determined to be 0.69 GPa and 0.75 GPa, respectively. We proposed an advanced method for determining simultaneously pressure and temperature conditions where the mineral inclusions were trapped in the host diamond. The obtained values were 3.0 GPa and 447 °C, but these values are lower than typical P-T conditions in the mantle. Several technical possibilities for the discrepancy are discussed.     

圆形巷道围岩层裂或板裂化的等效连续介质模型及侧压系数的影响

为了避免均质和非均质模型不能较好地模拟围岩的层裂或板裂化现象,将岩石视为等效连续介质,即颗粒体材料。颗粒被视为弹性材料,而颗粒之间的界面破坏后被视为摩尔-库仑材料。颗粒和界面均被离散为正方形单元。采用FLAC研究了不同侧压系数时圆形巷道围岩中的剪切应变增量、最小、最大主应力等的分布规律。研究发现,颗粒体材料模型在压应力作用下诱发出的拉应力值接近于在模型边界上所施加的最大压应力,而最大压应力是所施加的最大压应力的数倍。这些结果意味着均质和非均质模型的计算结果是偏于不安全的。另外,最小主应力和剪切应变增量的等值线图均显示,这些量的高值区的距离大致相等,这与V形岩爆坑内的板裂化现象类似。围岩层裂现象的原因是环向的高压应力和径向的高拉应力的共同作用的结果。