Correlation between point load index and compressive strength for quartzite rocks

Summary The relationship of the point load test with uniaxial compressive strength was examined using quartzite rocks to substantiate the existing correlations.     

Validity of uniaxial compression tests for indirect determination of long term strength of rocks

Summary The validity of using laboratory uniaxial compression tests to determine the long-term strength of rock is considered. Results of an experimental study on three types of rocks are analysed, and the usefulness and limitations of the approach are discussed. Different methods of estimating strength are used and the results compared.     

Random field characterization of uniaxial compressive strength and elastic modulus for intact rocks

Rock properties exhibit spatial variabilities due to complex geological processes such as sedimentation,metamorphism, weathering, and tectogenesis. Although recognized as an important factor controlling the safety of geotechnical structures in rock engineering, the spatial variability of rock properties is rarely quantified. Hence, this study characterizes the autocorrelation structures and scales of fluctuation of two important parameters of intact rocks, i.e. uniaxial compressive strength(UCS) and elastic modulus(EM).UCS and EM data for sedimentary and igneous rocks are collected. The autocorrelation structures are selected using a Bayesian model class selection approach and the scales of fluctuation for these two parameters are estimated using a Bayesian updating method. The results show that the autocorrelation structures for UCS and EM could be best described by a single exponential autocorrelation function. The scales of fluctuation for UCS and EM respectively range from 0.3 m to 8.0 m and from 0.3 m to 8.4 m.These results serve as guidelines for selecting proper autocorrelation functions and autocorrelation distances for rock properties in reliability analyses and could also be used as prior information for quantifying the spatial variability of rock properties in a Bayesian framework.     

Influence of width/height ratio on post-failure behaviour of coal

Conclusions The post-failure slope immediately after strength failure decreases on the negative side with the increase inw/h ratio. Atw/h equal to around 10 or 11 this slope becomes almost zero and then again the slope increases but on the positive side with further increase inw/h ratio.At loww/h ratios, the post-failure characteristic touches the abscissa ultimately. Forw/h greater than 4 or 6 after an initial fall, the broken mass starts reconsolidation on further loading due to high lateral constraints provided at the centre and the curve starts rising again. Forw/h ratios greater than 10 or 11, there is no loss in the post-failure strength.     

Weathering effects on the strength and deformational behaviour of crystalline rocks under uniaxial compression state

Uniaxial compression tests were performed on different categories of weathering of three lithological units: Malanjkhand granite; Nagpur basalt; and Delhi quartzite, occurring in central and northern parts of India. The deformational behaviour is studied in terms of variation in tangent modulus (E_t50) and initial modulus (E_i) due to weathering. The power relationship between uniaxial compressive strength (σ_c) and E_t50 shows strong correspondence for weathering sequence of common rock types. This relationship has been established by regression analysis and significant correlation parameter (coefficient of determination, r²=0.87) for crystalline rocks. It is shown that there is a systematic decrease in stiffness ratio, that is, ratio of tangent modulus and uniaxial compressive strength with increased weathering state. Comparison of E_t50 and E_i values has shown that E_t50 decreases more gradually than E_i, and reduction is more drastic for E_i values with an increased degree of weathering in all the three rock types. The mode of failure has been found to be influenced by weathering extent in rocks. A brief account is given of the intrinsic characteristics of fresh and weathered rocks and mineralogical changes produced by weathering investigated quantitatively. Correlation drawn between the petrographical and mechanical indices has shown that mechanical properties are apparently dependent on the intrinsic characteristics of weathered rocks.     

Correlation between petrographic characteristics and failure duration in clastic rocks

Being the inherent attributes of rock, the petrographic characteristics such as mineral composition and microstructure are the constructive parameters which influence its bursting potential. In the laboratory, rock specimens with different petrographic characteristics are tested to study their rock strength and failure duration, and correspondingly, composition and microstructure of each rock specimen are analyzed in thin sections to determine the mineral composition, content, grain size, cement and contact modes between grains. The results show that the petrographic characteristics of clastic rock have an intrinsic influence on its strength and failure duration. With the increase of the rock detritus content (or quartz), the uniaxial compressive strength increases according to a logarithmic expression and the failure duration decreases according to a negative logarithmic expression. With increasing average size of detritus grains in rock, the stiffness and brittleness of rock strengthens. The uniaxial compressive strength of rock has a logarithmic variation with its average size of detritus grains and the failure duration decreases with its increasing average size of detritus grains according to a negative logarithmic expression.     

岩石点荷载强度与单轴抗压强度线性关系试验

针对目前岩石点荷载试验大部分采用规则试件进行,试件加工制作及试验的过程繁琐、成本高的问题,通过自制点荷载试验仪以及RMT-150B岩石力学伺服试验机,对煤矿顶底板常见岩性进行不规则岩石试件点荷载试验和规则岩石试件单轴抗压试验。试验结果表明：不规则岩石点荷载强度基本上满足正态分布规律,同时,点荷载破坏载荷与破坏面积之间呈线性关系,不同载荷破坏面积对点荷载试验中的尺寸、形状效应基本上没有太大的影响。通过分析点荷载强度与单轴抗压强度两者间的关系,得出通过点荷载强度确定单轴抗压强度的经验公式。     

点荷载强度指数估算与岩浆岩的单轴压缩强度的关系

单轴压缩强度是岩石工程建设中广泛使用的力学参数,直接确定单轴压缩强度相对耗时且较为麻烦。点荷载试验可以间接估算岩石的单轴压缩强度（UCS）,试验方式简洁。通过收集到的岩浆岩点荷载试验成果,分析结果表明,（1）大部分转换公式在点荷载强度指数较大时得到偏高的单轴压缩强度值,特别是对部分幂函数;（2）ISRM（American Society for Testing and Materials）建议的取值范围仍然高估了岩石的单轴压缩强度值。为更准确地估算岩石的单轴压缩强度,建议采用点荷载强度指数 的二次函数（见式（1））估算岩石的单轴压缩强度,适用范围为0.0 MPa < <15.0 MPa。     

液氮冻结对岩石抗拉及抗压强度影响试验研究

液氮温度极低（?195.8℃）,当与储层岩石接触时,能够改变岩石物性并对岩石结构产生损伤致裂,因此,可用于储层压裂改造。为了研究液氮压裂时低温对岩石力学性能的影响,分别对不同含水状态（干燥与饱和）的不同类型（大理岩、砂岩和花岗岩）岩石进行液氮冻结处理,并对冻结前、后岩样进行抗拉及单轴抗压强度对比测试。结果表明,经液氮冻结后,岩石的单轴抗压强度、抗拉强度和弹性模量都降低;岩石在干燥状态下,液氮冻结对大理石强度的影响大于对红砂岩的影响;岩石饱和水状态下,液氮冻结对红砂岩强度的影响大于对大理岩的影响;饱和水状态岩石经液氮冻结后,其应力-轴应变曲线在弹性变形阶段出现一个拐点;对于同种类型岩石,饱和水状态能加剧液氮冻结并对岩石损伤,岩石强度影响显著;对3种岩样微观结构进行了电镜扫描（以大理岩为例进行分析）,发现经液氮冻结后在矿物颗粒之间生成了微裂隙。研究结果可为进一步研究液氮压裂机制提供试验依据。     

Depth Variability of Compressive Strength Test Results of Toki Granite from Shobasama and Mizunami Construction Sites,Japan

This paper studies the depth variability of uniaxial compressive laboratory test results on intact Toki granite (i.e., sound rock without macroscopic fractures) from the Shobasama and Mizunami Construction Sites, Japan. Some of the depth variability observed in the laboratory results can be indirectly attributed to the high fracture frequency of the “upper highly fractured rock domain” from which some of the samples were taken. For samples taken from the “lower sparsely fractured rock domain,” however, the uniaxial compressive strength of the granite seems to be very strongly correlated to the level of in situ rock stress (i.e., maximum shear stress) determined by measurement results obtained from hydrofracturing tests. The correlation between the laboratory results and the level of in situ stress is explained by the damage due to the complex stress path that the cores undergo during drilling, besides the stress concentrations at the drill-bit/rock contact, which can also affect the microcracking of the samples. An attempt to adjust laboratory test results to estimate the in situ intact rock strength of Toki granite based on its correlation with in situ stresses was carried out.     

Determination of the elastic modulus set of foliated rocks from ultrasonic velocity measurements

Ultrasonic measurements of compressional and shear wave velocities under hydrostatic pressure up to 70 MPa were carried out on cylindrical specimens cored across and along the foliation planes. Our measurements revealed that the foliation of the metamorphic rocks induces a clear velocity anisotropy between two orthogonal directions; faster along the foliation plane and slower across the plane in most rock types. All velocity components monotonically increase with the confining pressure, probably due to the closure of microcracks distributed in rock specimens. We determined the complete set of dynamic moduli of foliated metamorphic rocks with two assumptions; transverse isotropy due to the foliation and ellipsoidal seismic energy propagation from a point source. The calculated elastic moduli referring to different directions could be valuable for the design of various engineering structures in planar textured rock mass.     

Determination of unconfined compressive strength and Young's modulus of porous materials by indentation tests

The formation of a compacted zone under the indenter seems to be the major factor controlling the indentation process in porous rocks. In the case of very porous materials, where the pore structure fails and deformation (by structural collapse) proceeds with almost no increase in the applied load and with very limited damage to the surrounding material, no chipping is observed. The extent of the compacted zone is controlled by the porosity of the material and by the strength of its porous structure. This paper presents an interpretation model developed by the authors to obtain the uniaxial compressive strength of porous materials from the results of indentation tests. It is based on the model proposed by Wilson et al. (Int. J. Mech. Sci., 17, 1975, 457) for the interpretation of indentation tests on compressible foams and on an estimation by the authors of the extent of the compacted zone under the indenter. The results of indentation tests can also be used to obtain the Young's modulus of the material with a model proposed by Gill et al. (Proceedings of the 13th Canadian Symposium on Rock Mechanics, 1980, 1103). Uniaxial compression and indentation tests have been performed on artificial porous materials showing porosities varying between 44 and 68%. The uniaxial compressive strength values obtained from both types of test show a very good agreement. For the Young's modulus, the values obtained from the two types of test are different but the variation of the moduli with porosity is the same. Finally, a parameter called permanent penetration modulus is proposed as a means of characterizing the uniaxial compressive strength of porous materials.     

Factors affecting the durability of selected weak and clay-bearing rocks from Turkey, with particular emphasis on the influence of the number of drying and wetting cycles

Weathering can induce a rapid change of rock material from initial rock-like properties to soil-like properties. The resistance of a rock to short-term weathering is described through a durability parameter called the slake durability index. As durability is an important engineering parameter, particularly for weak and clay-bearing rocks, it was assessed by a number of tests. The main purpose of this study is to assess the influence of the number of drying and wetting cycles and controls of mineralogical composition and strength on durability. For this purpose, 141 samples of different types of weak and clay-bearing rocks were selected from different parts of Turkey, and relationships between the above-mentioned rock characteristics were statistically investigated. The samples were subjected to multiple-cycle slake durability testing, X-ray diffraction (XRD) analysis and uniaxial compression testing. In addition, to assess the influence of mineralogical composition on durability, the mineral contents of the original material and the material passing from the drum of the slake durability apparatus after each cycle were also determined by XRD. The results indicate that the type and amount of clay minerals are the main factors influencing the variations of the slake durability index in all samples. The durability of the clay-bearing rocks studied correlates best with the amount of expandable clay minerals. A strong relationship between the uniaxial compressive strength and the fourth-cycle slake durability index is found only for the marls among the rock types studied. Assessment of gradation results of the spoil pile materials consisting of clay-bearing rocks also reveals that the increase in percentage of fines in old piles is indicative of material degradation, as is evident by multiple-cycle slaking. It is emphasized that two-cycle conventional slake durability testing did not appear to offer an acceptable indication of the durability of weak and clay-bearing rocks when compared with multiple-cyclic wetting and drying. Comments on the performance of the test are made that aim to make the testing process and interpretation of the results more reliable.     

Prediction of uniaxial compressive strength of sandstones using petrography-based models

The uniaxial compressive strength of intact rock is the main parameter used in almost all engineering projects. The uniaxial compressive strength test requires high quality core samples of regular geometry. The standard cores cannot always be extracted from weak, highly fractured, thinly bedded, foliated and/or block-in-matrix rocks. For this reason, the simple prediction models become attractive for engineering geologists. Although, the sandstone is one of the most abundant rock type, a general prediction model for the uniaxial compressive strength of sandstones does not exist in the literature. The main purposes of the study are to investigate the relationships between strength and petrographical properties of sandstones, to construct a database as large as possible, to perform a logical parameter selection routine, to discuss the key petrographical parameters governing the uniaxial compressive strength of sandstones and to develop a general prediction model for the uniaxial compressive strength of sandstones. During the analyses, a total of 138 cases including uniaxial compressive strength and petrographic properties were employed. Independent variables for the multiple prediction model were selected as quartz content, packing density and concavo–convex type grain contact. Using these independent variables, two different prediction models such as multiple regression and ANN were developed. Also, a routine for the selection of the best prediction model was proposed in the study. The constructed models were checked by using various prediction performance indices. Consequently, it is possible to say that the constructed models can be used for practical purposes.     

预测岩石单轴抗压强度的神经网络方法

本文提出一种根据岩石点荷载强度、密度、岩石类型、孔隙率和粒度预测单轴抗压强度(Rc)的神经网络方法.以反向传播算法和包含96个样本的训练集训练网络模型,以包含71个样本的试验集检验网络.最后,以本模型预测黑沟铁矿钙质千枚岩的30个试件的抗压强度.结果表明,对于预测的单轴抗压强度,神经网络模型能给出比回归模型高得多的精度,比试验方法节省成本和时间.     

Geomechanical characterisation of rock masses in Alpine regions: the Basque Arc (Basque-Cantabrian basin, Northern Spain)

The geomechanical characterisation of rock masses in Alpine regions is one of the main challenges currently facing rock mechanics. These rock masses, made up of very different materials and frequently tectonically disturbed, are affected by engineering works, giving rise to potentially important problems. Thus, in the Basque Country, these materials are at the cause of important fatalities occurring in recent years. In this context, we aim at characterising these materials with the ultimate purpose of predicting their behaviour. The characterisation is initially approached based on the GSI, m_i and σ_ci parameters, following the Hoek-Brown failure criterion. This information is supplemented with field and laboratory measurements, and by obtaining the RQD and RMR indices. Using the information obtained from 99 metering stations, we develop a graphic classification procedure, which allows us to distinguish materials based on their characteristics, behaviour, type and degree of associated problems, and type and effectiveness of solutions provided. Eight classes of material are established.     

岩石疲劳强度试验和弹性理论分析初步研究

通过单轴压缩试验研究了岩石强度,并用空间弹性力学理论进行分析比较,采用高频振动疲劳试验研究了岩石的疲劳强度,得出了试验中所用材料603号花岗岩的纵向应力-应变曲线以及疲劳强度区间,并对其疲劳现象进行了分析。