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1.
In-situ Rock Spalling Strength near Excavation Boundaries   总被引：2，自引：0，他引：2  
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.  相似文献   

2.
The influence of mineral grain and grain boundary strength is investigated using a calibrated intact (non-jointed) brittle rock specimen subjected to direct shear with a particle-based distinct element method and its embedded grain-based method. The adopted numerical approach allows one to independently control the grain boundary and mineral grain strength. The investigation reveals that, in direct shear, the normal stress (σ n) applied to a rock specimen relative to its uniaxial compressive strength (UCS) determines the resulting rupture mechanism, the ultimate rupture zone geometry, and thus its shear stress versus horizontal displacement response. This allows one to develop a rupture matrix based on this controlling parameter (i.e., σ n/UCS). Mineral grain strength reductions result in the lowering of the apparent cohesion intercept of the peak linear Coulomb strength envelope, while grain boundary strength reductions change the peak linear Coulomb strength envelope to a bi-linear or curved shape. The impact of grain boundary strength is only relevant at σ n/UCS ratios <0.17 where tensile and dilatant rupture mechanisms dominate. Once shear rupture begins to be the dominant rupture mechanism in a brittle rock (i.e., at σ n/UCS ratios >0.17), the influence of weakened grain boundaries is minimized and strength is controlled by that of the mineral grains.  相似文献   

3.
To deeply understand the cracking mechanical behavior of brittle rock materials, numerical simulations of a rock specimen containing a single preexisting crack were carried out by the expanded distinct element method (EDEM). Based on the analysis of crack tips and a comparison between stress- and strain-based methods, the strain strength criterion was adopted in the numerical models to simulate the crack initiation and propagation processes under uniaxial and biaxial compression. The simulation results indicated that the crack inclination angle and confining pressure had a great influence on the tensile and shear properties, peak strength, and failure behaviors, which also showed a good agreement with the experimental results. If the specimen was under uniaxial compression, it was found that the initiation stress and peak strength first decreased and then increased with an increasing inclination angle α. Regardless of the size of α, tensile cracks initiated prior to shear cracks. If α was small (such as α ≤ 30°), the tensile cracks dominated the specimen failure, the wing cracks propagated towards the direction of uniaxial compression, and the propagation of shear cracks was inhibited by the high concentration of tensile stress. In contrast, if α was large (such as α ≥ 45°), mixed cracks dominated the specimen failure, and the external loading favored the further propagation of shear cracks. Analyzing the numerical results of the specimen with a 45° inclination angle under biaxial compression, it was revealed that lateral confinement had a significant influence on the initiation sequence and the mechanical properties of new cracks.  相似文献   

4.
工程开挖面附近卸荷扰动区的岩体,受结构面和拉应力共同影响作用,其变形和破坏具有拉剪复合特征。为研究节理岩体的拉剪力学特性,基于颗粒离散元法针对共面断续节理岩体开展了系列数值模拟研究。通过假设粒间接触的力学参数服从Weibull分布表征岩体的非均质性,探讨了非均质性、均质度、法向拉应力和节理连通率对节理岩体拉剪强度和破坏模式的影响。研究表明:拉剪应力条件下非均质性节理岩体主要沿阶梯型破裂面破坏,剪应力-水平位移曲线可以分为线性变形阶段、非线性变形阶段、峰值及峰后阶段;随均质度提高,节理岩体的剪切强度逐渐增加且提升幅度逐渐减弱,趋于均质岩体,岩体中微裂纹由弥散型分布向破裂面集中;节理岩体峰值剪切强度和法向拉应力的大小呈非线性负相关关系;岩体剪切强度随节理连通率增加而显著降低。  相似文献   

5.
The uniaxial compressive strength (UCS) of intact rock, which can be estimated using relatively straightforward and cost-effective techniques, is one of the most practical rock properties used in rock engineering. Thus, constitutive laws to represent the strength and behavior of (intact) rock frequently use it, along with additional intrinsic rock properties. Although triaxial tests can be employed to obtain best-fit failure criterion parameters that provide best strength predictions, they are more expensive and require time-consuming procedures; as a consequence, they are often not readily available at early stages of a project. Based on the analysis of an extensive triaxial test database for intact rocks, we propose a simplified empirical failure criterion in which rock strength at failure is expressed in terms of confining stress and UCS, with a new parameter which can be directly estimated from the UCS for a specified rock type in the absence of triaxial test data. Performance of the proposed failure criterion is then tested for validation against experimental data for eight rock types. The results show that strengths of intact rock estimated by the proposed failure criterion are in good agreement with experimental test data, with small discrepancies between estimated and measurements strengths. Therefore, the proposed criterion can be useful for preliminary (triaxial) strength estimation of intact rocks when triaxial tests data are not available.  相似文献   

6.
The brittle failure behavior of an over-consolidated clay shale (Opalinus Clay) in undrained rapid triaxial compression was studied. The confining stress levels were chosen to simulate the range of confining stresses relevant for underground excavations at the Mont Terri Underground Research Laboratory, and to investigate the transition from axial splitting failure to macroscopic shear failure. Micro-crack initiation was observed throughout the confining stress range utilized in this study at a differential stress of 2.1 MPa on average, which indicates that friction was not mobilized at this stage of brittle failure. The rupture stress was dependent on confinement indicating friction mobilization during the brittle failure process. With increasing confinement net volumetric strain decreased suggesting that dilation was suppressed, which is possibly related to a change in the failure mode. At confining stress levels ≤0.5 MPa specimen rupture was associated with axial splitting. With increasing confinement, transition to a macroscopic shearing mode was observed. Multi-stage triaxial tests consistently showed lower strengths than single-stage tests, demonstrating cumulative damage in the specimens. Both the Mohr–Coulomb and Hoek–Brown failure criteria could not satisfactorily fit the data over the entire confining stress range. A bi-linear or S-shaped failure criterion was found to satisfactorily fit the test data over the entire confinement range studied.  相似文献   

7.
This study investigates the causes and failure mechanism of the Aksu landslide that occurred during the construction of the Giresun–Espiye road between KM: 1 + 030–1 + 170 in northern Turkey and recommends proper stabilization techniques. For the purpose of investigating the causes and mechanism of this slope failure, engineering geological mapping, geotechnical investigation and rock mass characterization were performed. From top to bottom, weathered tuffite, tuffite, flysch, and dacitic tuffite were the major units in the study area. The disturbance of the slope by the excavations performed at the toe of the slope (i.e., due to the foundation excavation for the Tünel restaurant building and for the road cut) led to a “translational slide”. The “translational slide” occurred in completely weathered tuffite due to the disturbance of the stability of the slope by the excavations performed at the toe of the slope, particularly for the foundation excavation of the Tünel restaurant building and for the road cut along the Giresun–Espiye road. The rise in the groundwater level was also another important factor that has contributed to the occurrence of the landslide. After establishing the geometry of the landslide in detail, the shear strength parameters of the failure surface were determined by the back analysis method. Sensitivity analyses were performed and landslide failure mechanisms were modeled to quantify the contributing factors that have caused the formation of the Aksu landslide. The influence of an earthquake was investigated through pseudostatic slope stability analysis. Toe buttressing, ground water drainage, and surface water drainage alternatives were considered for stabilizing the slope.  相似文献   

8.
An Empirical Failure Criterion for Intact Rocks   总被引：1，自引：1，他引：0  
The parameter m i is an important rock property parameter required for use of the Hoek–Brown failure criterion. The conventional method for determining m i is to fit a series of triaxial compression test data. In the absence of laboratory test data, guideline charts have been provided by Hoek to estimate the m i value. In the conventional Hoek–Brown failure criterion, the m i value is a constant for a given rock. It is observed that using a constant m i may not fit the triaxial compression test data well for some rocks. In this paper, a negative exponent empirical model is proposed to express m i as a function of confinement, and this exercise leads us to a new empirical failure criterion for intact rocks. Triaxial compression test data of various rocks are used to fit parameters of this model. It is seen that the new empirical failure criterion fits the test data better than the conventional Hoek–Brown failure criterion for intact rocks. The conventional Hoek–Brown criterion fits the test data well in the high-confinement region but fails to match data well in the low-confinement and tension regions. In particular, it overestimates the uniaxial compressive strength (UCS) and the uniaxial tensile strength of rocks. On the other hand, curves fitted by the proposed empirical failure criterion match test data very well, and the estimated UCS and tensile strength agree well with test data.  相似文献   

9.
岩石非连续变形分析方法DDARF成功实现了模拟岩体裂纹萌生、扩展、贯通、破碎的全过程,但算法只考虑了岩体的线弹性本构模型,没有分析岩体的非线性应力-应变关系。为更符合真实岩体工程,同时扩展岩石非线性本构模型的应用范围,分别采用摩尔-库仑强度准则和最大拉应力强度准则对岩体进行剪切和拉伸破坏判断;对理想弹脆性本构模型进行算法分析,并在VC++平台下程序实现;对“自定义”的岩体非连续变形分析方法弹脆性本构模型与室内单轴压缩试验进行比较分析;将岩体非连续变形分析方法弹脆性本构模型应用于大型地下洞室开挖,并与线弹性本构模型进行对比。结果表明,非连续变形分析方法中自定义岩体弹脆性本构模型是可行的,它能够反映岩体变形的非线性特征,与室内试验吻合度较好;非连续变形分析方法弹脆性本构模型应用于大型地下工程,能更安全且真实地分析洞室围岩的稳定性,进而更好地指导地下洞室的防护措施。  相似文献   

10.
梁宁  伍法权  王云峰  包含 《岩土力学》2016,37(Z2):329-336
甘肃省关山隧道是一条受高地应力影响的大埋深硬脆性闪长岩铁路隧道,位于青藏高原东北缘,构造活跃,运动速率较大,且方向变化显著的六盘山挤压隆升构造区。在隧道开挖过程中围岩变形破坏现象明显,围岩等级低于前期岩体质量分级,表现出强烈的岩体质量劣化和各向异性。针对该问题,除了采用矿物成分和微结构分析寻找原因,还通过现场结构面统计分析对围岩质量劣化和各向异性进行描述,同时运用自行研发的钻孔电视进一步分析开挖前后一定时间间隔内围岩的渐进式变形和破坏。钻孔电视试验结果表明,尽管闪长岩作为一种硬脆性岩体,单轴抗压强度（UCS）高于现场地应力值,但其变形和破坏却普遍发生,开挖过程中新生裂隙迅速发育,原先在高地应力下闭合的裂隙也会重新张开和发展,围岩劣化,稳定性降低。为了进一步分析围岩的变形破坏过程,设计了变压力大小和方向的单轴抗压试验,试验中闪长岩的单轴压力值低于单轴抗压强度,试验结果与钻孔电视试验观测结果吻合,证明了在开挖引起的地应力剧烈变化条件下硬脆性闪长岩结构劣化,存在变形破坏的可能性。在大埋深高地应力条件下,除了岩体的各向异性,地应力的变化也是硬脆性围岩稳定性的重要考量因素。  相似文献   

11.
One of the aims of rock mechanics analysis is to predict fallouts in underground excavations. The objective of this paper was to study the relative importance of different strength parameters and their significance on the simulation of brittle failure and fallouts. This work was conducted as a parametric study, using numerical modelling and a number of approaches. The results were compared with observed fallouts. More obvious and distinct shear bands could be observed with decreased element sizes close to, and at, the boundary. The maximum shear strain was the most reliable indicator for fallout prediction. The results of the (instantaneous) cohesion softening friction softening models were sensitive to changes of the peak strength parameters and less sensitive to variations in residual parameters. The result from the cohesion-softening friction-hardening (CSFH) model, when using a peak cohesion equal to the intact rock strength, best captured the observed rock behaviour.  相似文献   

12.

A stability analysis of a laterally confined slope model, lying on an inclined bedding plane, was presented to evaluate the lateral shear resistance by considering the loading paths and failure envelopes. Two slope models were prepared on a bedding plane by compaction, one with and one without lateral confinement. The compacted models are related to the geological conditions at shallow depths where brittle deformation can occur and an excavation can induce horizontal field stress that significantly influences the stability of the slope. Three distinct loading paths, controlled by either tilting the angles or increasing the surcharge loads, were applied to achieve the failure of the slope models. Rankine’s passive earth pressure due to compaction was reduced by the shear strength reduction ratio. The shear strength reduction ratio was estimated through the least-squares fitting method based on the results of model tests at failure when the loading paths intersected the failure envelope. Provided that the effect of lateral confinement in a rock mass can be described by the shear strength reduction ratio, the proposed equations will be beneficial for slope stability analyses of laterally confined slopes on bedding planes. A case study of an undercut pit wall in an open-pit mine was demonstrated by showing that the unknown shear strength reduction ratio can be back-analyzed from the rainfall-induced landslide case. Therefore, the design of other undercut slopes with different geometries and groundwater conditions in the rock mass, which have undergone the same geological process as the back-analyzed case, is possible.

  相似文献   

13.
Surface-parallel slabbing is a failure mode often observed in highly stressed hard rocks in underground excavations. This paper presents the results of experimental studies on slabbing failure of hard rock with different sample height-to-width ratios. The main purpose of this study was to find out the condition to create slabbing failure under uniaxial compression and to determine the slabbing strength of hard rock in the laboratory. Uniaxial compression tests were carried out using five groups of granite specimens. The mechanical parameters of the sample rock, Iddefjord granite from Norway, were measured on the cylindrical and Brazilian disc specimens. The transition of the failure mode was studied using rectangular prism specimens. The initiation and the propagation of slabbing fractures in specimens were identified by examining the relationship among the applied stress, strain and the acoustic emission. The stress thresholds identified were compared to those reported by other authors for crack initiation and brittle failure. It is observed that the macro failure mode will be transformed from shear to slabbing when the height/width ratio is reduced to 0.5 in the prism specimens under uniaxial compression. Micro σ 1-parallel fractures initiate when the lateral strain departs from its linearity. Slabbing fractures are approximately parallel to the loading direction. Labotatory tests show that the slabbing strength (σ sl) of hard rock is about 60% of its uniaxial compression strength. It means that if the maximum tangential stress surrounding an underground excavation reaches about the slabbing threshold, slabbing fractures may take place on the boundary of the excavation. Therefore, the best way to stop or eliminate slabbing failure is to control the excavation boundary to avoid the big stress concentration, so that the maximum tangential stress could be under the slabbing threshold.  相似文献   

14.
ABSTRACT

In this study, over 1000 data from the literature was used to characterize and compare the density, strengths, modulus, fracture toughness, porosity and the ultimate shear strengths of the gypsum, limestone and sandstone rocks. The compressive modulus and Mode-1 fracture toughness of the gypsum rock, limestone rock and sandstone rocks varied from 0.7 GPa to 70 GPa, and from 0.03 MPa.m0.5 to 2.6 MPa.m0.5  respectively. Vipulanandan correlation model was effective in relating the modulus of elasticity, fracture toughness with the relevant strengths of the rocks. A new nonlinear Vipulanandan failure criterion was developed to quantify the tensile strength, pure shear (cohesion) strength and to predict the maximum shear strength limit with applied normal stress on the gypsum, limestone and sandstone rocks. The Vipulanandan failure model predicts the maximum shear strength limit was, as the Mohr-Coulomb failure model does not have a limit on the maximum shear strength. With the Vipulanandan failure model based on the available data, the maximum shear strengths predicted for the gypsum, limestone and sandstone rocks were 64 MPa, 114 MPa and 410 MPa respectively.  相似文献   

15.
The effect of joint overlap on the full failure behavior of a rock bridge in the shear-box test was numerically investigated by means of the particle flow code in two dimensions (PFC2D). Initially, the PFC2D was calibrated by use of data obtained from experimental laboratory tests to ensure the conformity of the simulated numerical model’s response. Furthermore, validation of the simulated models was cross-checked with the results from direct shear tests performed on non-persistent jointed physical models. By use of numerical direct shear tests, the failure process was visually observed and the failure patterns were seen to be in reasonable accordance with experimental results. Discrete element simulations demonstrated that macro shear fractures in rock bridges are because of microscopic tensile breakage of a large number of bonded discs. The failure pattern is mostly affected by joint overlap whereas the shear strength is closely related to the failure pattern. The results show that non-overlapping joints lost their loading capacity when nearly 50 % of total cracks developed within the rock bridge whereas the overlapping joints lost their loading capacity as soon as cracks initiated from the joint walls. Furthermore, progressive failure or stable crack growth was seen to develop for non-overlapped joints whereas brittle failure or unstable crack growth was seen to develop in overlapped joints.  相似文献   

16.
Rock burst is a complex dynamic process can lead to casualties, to failure and deformation of the supporting structures, and to damage of the equipment on site; hence, its prediction is of great importance in underground construction. We present a novel empirical method to predict rock burst based on the theory of logistic regression classifiers. An extensive database collected from the literature, which includes observations about rock burst occurrence (or not) in underground excavations in projects from all over the world, is used to train and validate the model. The proposed approach allows us to compute new class separation lines (or planes) to estimate the probability of rock burst, using different combinations of five possible input parameters—tunnel depth, H; maximum tangential stress, MTS; elastic energy index, W et; uniaxial compressive strength of rock, UCS; uniaxial tensile strength of rock, UTS—among which it was found that the preferable model could be developed in H–W et–UCS space. The proposed model is validated with goodness-of-fit tests and nine-fold cross-validation; results show that its predictive capability compares well with previously proposed empirical methods and confirm that, as expected, the probability of rock burst increases with excavation depth, and that both W et and UCS have a similarly significant influence on rock burst occurrence. Finally, expressions are proposed for identification of conditions associated with several reference values of rock burst probability, which can be employed in preliminary risk analyses.  相似文献   

17.
Summary The primary objective of this work was an examination of the complimentary roles of tensile damage and confinement reduction (or stress relaxation) on excavation response of hard rockmasses. Tensile damage and relaxation are examined with respect to structurally controlled or gravity driven failure modes as well as to strength controlled or stress driven rockmass damage and yield. In conventional analysis of both structurally controlled and stress driven failure, the effects of tensile damage and tensile resistance as well as the elevated sensitivity to low confinement are typically neglected, leading to erroneous predictions of groundfall potential or rock yield. The important role of these two elements in underground excavation stability in hard rock environments is examined in detail through a review of testing data, case study examination and a number of analytical and numerical analogues including discrete element simulation, statistical theory and fracture mechanics. This rigorous theoretical treatment updates, validates and constrains the current use of semi-empirical design guidelines based on these mechanisms.  相似文献   

18.
The drill and blast method (D&B) is perhaps the most common excavation method for rock mass, and intense blasting vibrations would induce an excavation damage zone (EDZ) around the excavated space. The tensile failure of rock mass in EDZ at diverse rupture velocities results in various geological disasters in engineering practices. The objective of this paper is to investigate the effect of blasting on the tensile strength of sandstone rock and the influence of loading rate on the disk specimens affected by blasting. We firstly performed a D&B exercise on a sandstone block with a size of 600 mm × 600 mm × 120 mm. Then, a total number of 49 standard disk specimens were prepared from large fragments of this blasting sandstone block and an undamaged block. A series of Brazilian split tests was carried out using these specimens to determine their indirect tensile strength, and to assess the effects of the distance from the blasting source and loading rate (varying from 1.67 × 10?5 to 8.33 × 10?2 mm s?1). The results show that the tensile strength of specimens exhibits an upward trend with increasing distance from the blasting source, to approach that of undamaged rock, following a power function with a positive exponent (0~1). The loading rate affects the tensile mechanical behaviors of disks, in terms of the convergence of microscopic defects, the main load-bearing area, and the absorbed energy at the fracture moment of specimens. Both the tensile strength and absorbed energy have positive linear correlations with the natural logarithm of the loading rate. In addition, the fragmentation degree of disk specimens also increases due to an increasing brittleness of sandstone with the loading rate.  相似文献   

19.
基于一种脆性指标确定岩石残余强度   总被引：1，自引：0，他引：1  
彭俊  荣冠  蔡明  彭坤 《岩土力学》2015,36(2):403-408
岩石的残余强度是岩石力学的重要指标,准确地评价岩石残余强度对于评价地下工程的稳定性以及优化岩体支护设计具有重要意义。基于岩石的三轴力学特性提出一种表征岩石峰后强度衰减行为的力学指标--岩石强度衰减系数,该指标可反映岩石的脆性程度,并提出岩石强度衰减系数与围压关系的幂函数模型。对22组不同成因的岩石常规三轴压缩试验数据进行幂函数模型参数拟合,发现不同岩石拟合所得参数离散性较大,分析其原因主要与岩石矿物组成和岩石结构特征等因素相关。在此基础上提出基于强度衰减方法确定岩石残余强度的方法,分析表明,该方法能够很好地拟合岩石残余强度试验数据,并能反映岩石结构性质对残余强度的影响。  相似文献   

20.
高地应力区地下岩体工程开挖常形成围岩拉-压应力状态,发生岩体张性破坏灾害。本文针对传统PFC平行黏结模型不能模拟脆性岩石高单轴压缩与拉伸强度比的问题,建立双抗拉强度参数的平行黏结强度准则,开展岩石拉-压数值模拟试验,得到了与物理试验接近的拉-压强度,实现了岩石高压拉强度比的模拟,并深入分析了破坏机制。研究结果表明随着围压的增加,破裂面倾角逐渐增大,由拉伸破裂转化为拉-剪破裂,发现了拉-压应力状态下破裂面处的雁行裂纹。根据细观颗粒位移场揭示了破裂面力学性质,随着围压的增加(破裂面倾角逐渐增大),破裂面张性逐渐减弱而剪性增强。可将拉-压应力状态下岩石损伤演化过程大致分为弹性变形阶段、稳定破裂发展阶段、不稳定破裂发展阶段和整体破裂阶段(峰后应力跌落及残余阶段)。围压较大时弹性变形和稳定破裂发展阶段相对较短,不稳定破裂发展阶段相对较长较剧烈,峰后残余阶段破裂面摩擦更强、应力波动较大。  相似文献   

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