Long-term stability of a backfilled room-and-pillar test section at the Buick Mine, Missouri, USA

Rock mechanics instruments have been providing data in a backfilled room-and-pillar test section of the Buick Mine near Boss, Missouri, USA, for nearly 16 years. Host rock instruments include borehole extensometers installed in the mine roof and pillars, and biaxial stressmeters used in pillars and abutments. Embedment strain gauges, extensometers, and earth pressure cells were installed in the cemented backfill. The instruments monitored stability of the test section for two years while the pillars were extracted, and 14 years after pillar extraction to monitor long-term stability. Of the transducers that were not mined out when the pillars were extracted, 68% still function. Data from these instruments demonstrate that backfill improves long-term underground safety by supporting the mine roof and maintaining the strength of support pillars. For example, backfill significantly limited the dilation of a remaining support pillar by providing confinement on one side of the pillar. Post-mining stress and strain in the backfill account for 35% and 28% of the total stress and strain that was measured, respectively. Earth pressure cell stress measurements confirmed visual observations that the backfill remained stable. Post-mining stress measurements from the earth pressure cells fit natural log equations as a function of time with r-squared values ranging from 0.76 to 0.98. Natural log equations also described post-mining strain behavior of the backfill with r-squared values ranging from 0.30 to 0.99. Stresses calculated for the backfill by a three-dimensional numerical model of the test area were consistent with those that were measured by earth pressure cells.     

Numerical simulation of coal pillar strength

The present paper describes an approach for estimation of strain-softening constitutive behaviour of coal-mass through calibration of a numerical model with field cases. A three-dimensional finite element model was used, in which the coal-mass was considered to be a Hoek–Brown strain-softening material. Dilation behaviour was considered to be a function of confinement and plastic shear strain. The numerical model was calibrated by satisfying the (1) various failed/stable cases of coal pillar of Indian coal mines and (2) post-failure behaviour of large-scale coal pillars determined by various researchers through conducting in-situ tests. The calibration of the model was done by taking various permutation and combination of strain-softening parameters. It has been concluded that the yielding of the pillars start almost at 2/3 of the peak strength. The statistical expressions for estimation of pillar strength for Indian coal mines and post-failure modulus have been developed by analyzing the results of the simulations. For Indian coal pillars having width-to-height ratio (w/h) less than five, the pillar strength was almost linearly dependent on w/h, and non-linearly dependent on uniaxial compressive strength of the coal specimen. The post-failure modulus of coal pillars was non-linearly dependent on w/h, and not significantly dependent on the uniaxial compressive strength.     

建筑垃圾细料生产流动化回填材料的性能

以灰砂比0.03、0.05和0.08,粉砂比0、0.05、0.1、0.15和0.2为设计参数,对建筑垃圾回填材料进行设计。通过试验对回填材料的流动性(流动度、泌水率)、无侧限抗压强度以及应力应变曲线、本构关系模型和弹性模量等进行研究。研究结果表明:回填材料的流动度受水固比影响较大,两者接近线性关系;流动度在200~250mm范围,泌水率在4%~8%之间;回填材料抗压强度与灰砂比和水固比之间存在很好的幂指数关系;回填材料应力应变曲线形状与普通混凝土的相似,在此基础上提出回填材料的本构关系模型;回填材料无侧限抗压强度与弹性模量之间存在很好的指数关系。     

等效岩体技术在岩体工程中的应用

 以某露天矿边坡为工程背景,结合岩石力学参数室内和现场原位试验,基于颗粒流理论和PFC3D程序,运用等效岩体技术,建立充分反映节理分布特征并考虑细观破裂效应的等效岩体模型,采用Fish语言编制加卸载命令流,研究岩体的强度和力学效应。研究结果表明：(1) 等效岩体在单轴压缩时,轴向应力–应变曲线分为弹性变形阶段、非稳定破裂塑性阶段和破裂后阶段,抗压强度和弹性模量与标准岩块试样相比,有较大幅度降低;(2) 随围压增大,等效岩体抗压强度和残余强度明显提高,延性特征增强,逐渐向理想塑性过渡;(3) 等效岩体技术能有效地描述岩体受节理分布影响而表现的各向异性特征;(4) 等效岩体内部微裂纹主要沿节理走向分布并扩展,破坏形式受主导节理面产状及性质控制。     

Variability of intact rock mechanical properties for some metamorphic rock types and its implications on the number of test specimens

Intact rock strength and stiffness properties are commonly used in rock mass mechanical characterization, and their evaluation is usually based on laboratory tests. Due to the variability that affects strength and stiffness parameters, the determination of the number of laboratory-tested specimens required to obtain a reliable reference value is very useful. However, many studies reported in apposite literature focused only on the variability of strength parameters. This study investigates the variability of some of the most important strength and stiffness properties (unconfined compressive strength, indirect tensile strength, tangent and secant Young’s moduli, Poisson’s ratio) by applying statistical methods (statistical decision theory and statistical inference theory). A data set of 451 laboratory tests was used, performed on three rock types. The statistical analyses were applied with the aim of assessing how closely intact rock laboratory data follow a normal distribution and determining the minimum number of specimens required to obtain a reliable average value of the parameters in relation to a targeted precision index for a confidence level of 95 %. The results indicate that the minimum number of samples needed varies depending on rock and test types. Among the stiffness properties, tangent Young’s modulus has a lower variability than both the secant modulus and the Poisson’s ratio, whereas in terms of strength parameters, unconfined compressive strength is subject to greater variability than indirect tensile strength.     

A geo-engineering classification for rocks and rock masses

In this article, an attempt is made to assess the reliability of predicting the uniaxial compressive strength and the corresponding modulus of a rock mass by current approaches. These two basic engineering properties, when estimated from rock mass rating (RMR), Q and geological strength index (GSI), indicate hardly any change in the modulus ratio with the change in the quality of the rock mass from very good to very poor. However, the modulus ratio obtained from the relations involving the joint factor, J_f, indicate a definite decrease in the modulus ratio with a decrease in the quality of the rock mass. The strength and modulus in the unconfined and confined states, the modulus ratio and failure strain in the unconfined case were linked to J_f in earlier publications based on a large experimental database. Some of these relations were adopted to verify the response of jointed test specimens, the response of the rock mass during excavations for mining and civil underground chambers, in establishing ground reaction curves including the extent of the broken zone, and the bearing capacity of shallow foundations.The joint factor is now linked to RMR, Q and GSI. The prediction of compressive strength and modulus of the rock mass appears to be more suitable. For classifying the rock, based on these properties, the Deere and Miller engineering classification, applicable to intact rocks, has been suitably modified and adopted. The results of different modes of failure of jointed specimens establish definite trends of changes in the modulus ratio originating from the intact rock value on the modified Deere and Miller plot. A geo-engineering classification is evolved by considering strength, modulus, quantifiable weathering index and lithological aspects of the rock.     

岩体变形破坏过程的能量机制

 叙述岩体单元变形破坏过程中能量耗散与强度、能量释放与整体破坏等概念。在循环压缩载荷下,实测岩石的能量耗散及损伤,数据拟合表明,基于能量耗散分析建立的岩石损伤演化方程可以较好地描述岩石的损伤演化过程。在循环压缩载荷下同时实测不同加载速度及不同载荷水平下岩体内可释放应变能、耗散能、卸荷弹性模量及卸荷泊松比的变化规律,给出复杂应力条件下卸荷弹性模量的变化公式。基于可释放应变能建立岩体单元的整体破坏准则,该准则与大理岩的双压试验结果符合得比较好。对工程中常见的层状岩体,提出基于畸变能与广义体积膨胀势能而建立的层状岩体破坏准则,该准则与层状岩的双压试验也符合得比较好。     

Determination and applications of rock quality designation(RQD)

Characterization of rock masses and evaluation of their mechanical properties are important and challenging tasks in rock mechanics and rock engineering.Since in many cases rock quality designation(RQD)is the only rock mass classification index available,this paper outlines the key aspects on determination of RQD and evaluates the empirical methods based on RQD for determining the deformation modulus and unconfined compressive strength of rock masses.First,various methods for determining RQD are presented and the effects of different factors on determination of RQD are highlighted.Then,the empirical methods based on RQD for determining the deformation modulus and unconfined compressive strength of rock masses are briefly reviewed.Finally,the empirical methods based on RQD are used to determine the deformation modulus and unconfined compressive strength of rock masses at five different sites including 13 cases,and the results are compared with those obtained by other empirical methods based on rock mass classification indices such as rock mass rating(RMR),Q-system(Q) and geological strength index(GSI).It is shown that the empirical methods based on RQD tend to give deformation modulus values close to the lower bound(conservative) and unconfined compressive strength values in the middle of the corresponding values from different empirical methods based on RMR,Q and GSI.The empirical methods based on RQD provide a convenient way for estimating the mechanical properties of rock masses but,whenever possible,they should be used together with other empirical methods based on RMR,Q and GSI.     

Strength of massive to moderately jointed hard rock masses

The Hoek-Brown(HB) failure criterion and the geological strength index(GSI) were developed for the estimation of rock mass strength in jointed and blocky ground where rock mass failure is dominated by sliding along open joints and rotation of rock blocks. In massive, veined and moderately jointed rock in which rock blocks cannot form without failure of intact rock, the approach to obtain HB parameters must be modified. Typical situations when these modifications are required include the design of pillars,excavation and cavern stability, strainburst potential assessment, and tunnel support in deep underground conditions(around s1/s ci 0.15, where s1 is the major principal compressive stress and s ciis the unconfined compressive strength of the homogeneous rock) in hard brittle rocks with GSI ! 65. In this article, the strength of massive to moderately jointed hard rock masses is investigated, and an approach is presented to estimate the rock mass strength envelope using laboratory data from uniaxial and triaxial compressive strength tests without reliance on the HB-GSI equations. The data from tests on specimens obtained from massive to moderately jointed heterogeneous(veined) rock masses are used to obtain the rock and rock mass strengths at confining stress ranges that are relevant for deep tunnelling and mining;and a methodology is presented for this purpose from laboratory data alone. By directly obtaining the equivalent HB rock mass strength envelope for massive to moderately jointed rock from laboratory tests,the HB-GSI rock mass strength estimation approach is complemented for conditions where the GSIequations are not applicable. Guidance is also provided on how to apply the proposed approach when laboratory test data are not or not yet available.     

Measurements of induced stress and strength in the near-field around a tunnel and associated estimation of the Mohr–Coulomb parameters for rock mass strength

The state of induced stress measured by the compact conical-ended borehole overcoring technique in the immediate roof of an approach tunnel excavated under high rock stress is described. During the measurements, core disking was observed. An X-ray Computed Tomography (CT) scanner was used to select strain data uninfluenced by the core disking; then the induced rock stress was estimated from selected strain data. From these results, it is shown that the non-destructive investigation using X-ray CT is effective for visualization of the fracturing within cores and the selection of strains measured during overcoring. Furthermore, the Mohr–Coulomb failure criterion parameters for the rock mass were estimated by comparing the measured stresses with the shear strength of in situ rock and the uniaxial compressive strength determined in laboratory tests.     

高强钢棒螺旋箍筋约束混凝土圆形截面柱受力性能试验研究

为研究高强钢棒螺旋箍筋约束混凝土柱的受力性能,设计了32个约束混凝土柱,对其进行轴心受压试验。试件主要设计参数包括混凝土强度等级为C50、C60、C70、C80,高强钢棒抗拉强度标准值为800、970MPa,体积配箍率为0.9%、1.2%、1.6%、2.0%。试验结果表明：约束混凝土柱在轴向荷载作用下呈腰鼓形破坏或单折剪切破坏,对于确定的非约束混凝土抗压强度和箍筋抗拉强度,约束箍筋体积配箍率较小时发生单折剪切破坏,体积配箍率较大时发生腰鼓形破坏;约束箍筋拉应变随混凝土弹性模量与非约束混凝土抗压强度比值增大而增大,随箍筋体积配箍率增大而减小;约束混凝土柱的体积配箍率大于某一量值时,会出现约束混凝土达到抗压强度时箍筋拉应变小于其屈服应变的情况。基于试验结果,建立了用于判别腰鼓形破坏和单折剪切形破坏的界限,并给出了相应的计算式;建立了约束混凝土柱发生轴压破坏时约束箍筋拉应变计算公式及箍筋受拉屈服对应的最大体积配箍率计算公式,为合理设计高强钢棒螺旋箍筋约束混凝土圆形截面柱提供参考。     

The Äspö Pillar Stability Experiment: Part II—Rock mass response to coupled excavation-induced and thermal-induced stresses

A 1-m-thick pillar was subject to coupled excavation- and thermal-induced stresses to induce brittle rock mass yielding. The yielding strength of the heterogeneous and fractured rock mass consisting of Äspö diorite was evaluated at eighteen discrete locations using data from the displacement, acoustic emission, and thermal monitoring systems. The average rock mass yielding strength was determined to be 0.59 of the uniaxial compressive strength.The onset of dilation in uniaxial laboratory tests, determined from strain gauge data, was found to occur at approximately 0.45 of the uniaxial compressive strength. It was shown that that the onset of acoustic emission events in situ also occurred when the tangential stress exceeded 0.43 of the uniaxial compressive strength. For sites with absence of in situ data it is recommended that this lower-bound value determined from laboratory data may be used for assessing the in situ rock mass yielding strength.Visual observation and displacement monitoring showed that extent of rock mass yielding is sensitive to small changes in the tangential stress magnitudes. It was determined using three-dimensional modelling that changes in the tangential stress magnitude of approximately 1 MPa was sufficient to cause yielding of the pillar to propagate in what appeared to be intact rock. Observations suggest that without this small stress change yielding of the rock mass would not occur. In other words, there appeared to be a well defined boundary, and if the stresses reached this boundary yielding was observed. However, if stresses were only slightly below this boundary yielding or time-dependant processes were not observed over the monitoring period used in the experiment.     

Ground reaction curves for tunnels excavated in different quality rock masses showing several types of post-failure behaviour

Rock mass behaviour model selection and, in particular, rock mass post-failure behaviour are key issues in analysing tunnel stability, in particular in terms of the correct application of design techniques such as the convergence–confinement method and also numerical modelling. Three different quality rock masses (good, average and poor) were defined in which simulated standard tunnels were excavated. Different behaviours – including elastic perfectly plastic, elastic brittle and three strain softening behaviours – were modelled for each type of rock mass and increasingly realistic parameters were calculated, along with the corresponding ground reaction curves. The results obtained demonstrate the importance of adequate post-failure behaviour model selection for tunnel analysis. Also assessed are the effects of the standard support and reinforcement.     

混合侵蚀与冻融交替作用下混凝土性能劣化试验

以质量分数为3％的氯化钠和5％的硫酸钠混合溶液为侵蚀介质,采用慢冻法,对立方体抗压强度为54.7 MPa的混凝土试件进行0,100,200,300次冻融循环试验,并测定各组试件的质量损失、抗压强度、相对动弹性模量和应力、应变等试验数据.结果表明:随着冻融循环次数的增多,混凝土试件表面剥蚀加剧,表观质量和相对动弹性模量无...     

水平旋喷固结体力学性能试验及分析

对在通常旋喷参数条件下形成的水平旋喷固结体进行了多种试验，包括无侧限抗压试验、劈裂抗拉试验、点荷载试验等，得到无侧限抗压强度、抗拉强度、抗压强度软化系数，变形模量，粘聚力和内摩擦角等水平旋喷固结体的物理力学指标，并讨论了变形模量，抗拉强度和无侧限抗压强度之间的关系。     

花岗岩高温疲劳效应研究

我国花岗岩分布十分广泛,花岗岩岩体稳定性直接关系到其上部或内部构筑物的安全。目前,关于花岗岩在高温循环作用后物理力学性质变化规律研究较少,为研究高温循环作用对花岗岩岩体物理力学性质的影响,在对花岗岩进行高温—淬火循环(100~700 ℃,循环1~7次)处理后,对花岗岩岩样进行单轴压缩试验,分析其抗压强度与弹性模量变化规律;对花岗岩岩样进行巴西劈裂试验,分析其抗拉强度变化规律。结果表明:(1)在100~700 ℃范围内,随温度升高抗拉强度、抗压强度、弹性模量变化均呈现下降趋势,其中抗拉强度降幅不断加大、弹性模量和抗压强度降幅呈现由大到小再增大的趋势;(2)在循环1~7次范围内,随循环次数增加,抗拉强度、抗压强度、弹性模量均呈现下降趋势,变化幅度在10％以内;(3)抗压强度与弹性模量变幅波动均为V字形,且V字开口方向相同,其中,随温度升高V字开口向下、随循环次数增加V字开口向上。     

A mine shaft case study on the accurate prediction of yield and displacements in stressed ground using lab-derived material properties

Continuum models provide a useful tool for the prediction of stress re-distribution due to excavation and induced yielding, and are used as a key analysis tool in the design of many underground excavations. Recent developments in the study of rock strength and post-yield behaviour have played a key role in improving our understanding of how plastic constitutive models can also be used to practically replicate observed phenomena in brittle rocks. In particular, new models for rock dilatancy can help to improve the applicability of plastic constitutive models as a predictive tool for excavation design. In this study, laboratory data for a heterogeneous, brittle, conglomerate unit from a mine shaft has been analysed. Using parameters from this analysis, brittle strength and dilatancy models have been implemented in a finite-difference code to predict not only stress re-distribution and yield around the shaft, but also to obtain realistic displacement values. Comparison of the modelling results to displacements measured using borehole extensometers show that the constitutive model and lab-derived parameters used were effective in predicting the rockmass behaviour. Parameters were further optimized through back analysis. One interesting finding of this analysis is that the in-situ rockmass dilation decay rate (as a function of plastic strain) appears to be faster than estimated based on laboratory data, which may be indicative of the influence of rockmass-scale natural fractures and other geological structures on the dilation decay process. It also appears possible to model the in-situ dilation decay rate using a single parameter, instead of separate parameters for unconfined and confined conditions. To conclude the study, more numerical results obtained using alternative dilatancy models are presented to illustrate the problem of non-uniqueness in plasticity back analyses.     

Effect of Rock Wool Waste on Compressive Behavior of Pumice Lightweight Aggregate Concrete After Elevated Temperature Exposure

In the present study, the mechanical properties and the residual stress–strain behavior of lightweight concrete (LWC) containing pumice coarse aggregate and rock wool waste (consisting of mineral fibers) were explored prior to and following thermal loading. Key variables included the volume percentage of rock wool waste (0%, 2.5%, 5%, 7.5%, and 10%) and exposure temperature (20°C, 200°C, 400°C, and 600°C). Here, parameters playing a role in the compressive performance of LWC containing rock wool waste were examined. These parameters included the elastic modulus, compressive strength, strain at peak stress, ultimate strain, toughness index, stress–strain relationship, and failure mode. Then, several empirical relationships were proposed to predict different mechanical characteristics in terms of temperature and volume percentage of rock wool. Furthermore, the compressive strength, elastic modulus, and strain at peak stress values were compared to the prediction results of the ACI 216, EN 1994-1-2, ASCE, and CEB-FIP 1990 codes. The results demonstrated that the mechanical properties of the LWC specimens were degraded with temperature. The highest degradation in the temperature range under study occurred at 600°C, with around 50% and 80% drop in the compressive strength and elastic modulus, respectively. Furthermore, after exposure to 600°C, an increase of 2 to 2.8 folds occurred in the strain at peak stress and an increase of 2.6 to 3.4 folds occurred in the ultimate strain of the specimens relative to those at the ambient temperature. In the end, two stress–strain models were presented to capture the compressive performance of LWC including rock wool waste after elevated temperature exposure based on the empirical equations obtained for the mechanical characteristics. These models showed a relatively good correlation with the experimental data.

     

基于分层计算的层间剪切带原位变形试验研究

由于含层间剪切带复合岩体的非均匀性,传统的承压板变形试验只能得到复合岩体的综合模量,而无法准确得到层间剪切带的变形参数。介绍了黄河中游红色碎屑岩地区某大型水利枢纽工程坝址区层间剪切带的发育特征,选择典型勘探平硐进行刚性承压板中心孔法原位变形试验,在预留上部保护岩层条件下,通过在中心孔内设置不同深度的液压锚头,直接得到复合岩体、层间剪切带上下缘岩体的压力-变形关系曲线。利用推导的深部岩体压缩变形公式,分别计算其变形参数。根据分层计算原理,利用层间剪切带上缘岩体和下缘岩体变形差值计算得到层间剪切带的等价变形参数。研究结果表明,层间剪切带的变形参数低于上下缘岩体及复合岩体,其等价变形模量为0.72GPa,等价弹性模量为1.41GPa。     

循环载荷下岩体能量特征及变形参数分析

 以RMT–150C岩石力学测试系统为平台,进行不同应力水平和加载速率条件下不同种类岩石材料的循环载荷试验。在对加、卸载应力–应变曲线进行分区的基础上,定义卸载过程中的能量消耗率,探讨岩石损伤破坏过程中能量的转化,同时定量分析弹性模量,泊松比和残余应变等变形参数的变化规律。研究结果表明：应力水平、加载频率越高,岩性越强,则滞回圈的面积,加载过程中做的功,卸载过程中释放的弹性能越大,岩石越软,能量消耗率越高;岩体未到达峰值抗压强度前,应力水平越高,弹性模量就越大,这说明在岩体抗压强度范围内,越高的载荷下,空隙和微缺陷被压密后,岩体相同载荷增量下的变形能力明显减弱。同时分析认为：频率升高到一定程度会导致疲劳强度的降低。整体而言,岩石轴向及横向相对残余应变均随着循环次数的增加而逐渐减小。