Estimating the Strength of Jointed Rock Masses

Determination of the strength of jointed rock masses is an important and challenging task in rock mechanics and rock engineering. In this article, the existing empirical methods for estimating the unconfined compressive strength of jointed rock masses are reviewed and evaluated, including the jointing index methods, the joint factor methods, and the methods based on rock mass classification. The review shows that different empirical methods may produce very different estimates. Since in many cases, rock quality designation (RQD) is the only information available for describing rock discontinuities, a new empirical relation is developed for estimating rock mass strength based on RQD. The newly developed empirical relation is applied to estimate the unconfined compressive strength of rock masses at six sites and the results are compared with those from the empirical methods based on rock mass classification. The estimated unconfined compressive strength values from the new empirical relation are essentially in the middle of the estimated values from the different empirical methods based on rock mass classification. Similar to the existing empirical methods, the newly developed relation is only approximate and should be used, with care, only for a first estimate of the unconfined compressive strength of rock masses. Recommendations are provided on how to apply the newly developed relation in combination with the existing empirical methods for estimating rock mass strength in practice.     

Design of Raft Foundations for High-Rise Buildings on Jointed Rock

This paper presents calculations of displacements and bending moments in a 2-m-thick reinforced-concrete foundation slab using three-dimensional finite-element software. A preliminary paper was presented by Justo et al. (Rock Mech Rock Eng 43:287–304, 2010). The slab is the base of a tower of 137 m height above foundation, supported on jointed and partly weathered basalt and scoria. Installation of rod extensometers at different depths below foundation allowed comparison between measured displacements and displacements calculated using moduli obtained from rock classification systems and three material models: elastic, Mohr–Coulomb and hardening (H). Although all three material models can provide acceptable results, the H model is preferable when there are unloading processes. Acceptable values of settlement may be achieved with medium meshing and an approximate distribution of loads. The absolute values of negative bending moments (tensions below) increase as the rock mass modulus decreases or when the mesh is refined. The paper stresses the importance of adequately representing the details of the distribution of loads and the necessity for fine meshing to obtain acceptable values of bending moments.     

The Use of Rock Mass Classification Systems to Estimate the Modulus and Strength of Jointed Rock

Three-dimensional, elastic and elasto-plastic finite element (FE) programs have permitted calculation of the displacements and the factor of safety (FOS) for the excavation for a tower, 132.70 m high (above foundation) on the island of Tenerife. The tower is supported by a 2 m thick reinforced concrete slab on jointed, vesicular and weathered basalt and scoria. The installation of rod extensometers at different depths below the slab has permitted comparison between measured and calculated displacements and the estimation of in situ deformation modulus. The moduli deduced from the simple empirical equations proposed by Hoek et al. (In: NARMS-TAC, 2002) and Gokceoglu et al. (Int J Rock Mech Min Sci 40:701–710, 2003) as a function of GSI, and Nicholson and Bieniawski (Int J Min Geol Eng 8:181–202, 1990) as a function of RMR, provide an acceptable fit with the measured settlements in this type of rock. Good correlation is also obtained with the empirical equation presented by Verman et al. (Rock Mech Rock Eng 30(3):121–127, 1997) that incorporates the influence of confining stress in the deformation modulus. The FOS obtained from different correlations with geomechanical classifications is within a relatively narrow range. These results increase our confidence in the use of classification schemes to estimate the deformation and stability in jointed rock.     

Prediction of Elastic Modulus of Jointed Rock Mass Using Artificial Neural Networks

Two artificial neural network models for the prediction of elastic modulus of jointed rock mass from the elastic modulus of corresponding intact rock and joint parameters have been demonstrated in this paper. The data collected from uniaxial and triaxial compression tests on different rocks with different joint configurations and different confining pressure conditions, reported in the literature are used as input for training the networks. Important joint properties like joint frequency, joint inclination and roughness of joints are considered separately for making the network more versatile. Two different techniques of artificial neural networks namely feed forward back propagation (FFBP) and radial basis function (RBF) are used to predict the elastic modulus ratio.     

冻融循环作用下路基土回弹模量试验研究

为研究季节冻土地区冻融循环作用对路基土回弹模量的影响,以2种路基土作为研究对象,通过风干和加湿的方法,使高50mm、直径为50mm无侧限圆柱体试件达到5种预期含水率;采用承载板法进行回弹模量试验,分析了路基土在0～12次冻融循环作用下,回弹模量随冻融循环次数变化的规律.结果表明:随着冻融循环次数的增加,土体的回弹模量减小,到第6次冻融循环后土体回弹模量衰减基本稳定.在进行季节冻土地区路面设计时,应考虑冻融循环作用对路基土的影响,建议选取第6次冻融循环后的土体回弹模量作为路基强度设计值.     

Study on Smooth Blasting Results in Jointed and Fractured Rock

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Multivariate Adaptive Regression Spline (Mars) for Prediction of Elastic Modulus of Jointed Rock Mass

This article presents multivariate adaptive regression spline (MARS) for determination of elastic modulus (E_j) of jointed rock mass. MARS is a technique to estimate general functions of high-dimensional arguments given sparse data. It is a nonlinear and non-parametric regression methodology. The input variables of model are joint frequency (J_n), joint inclination parameter (n), joint roughness parameter (r), confining pressure (σ₃) and elastic modulus (E_i) of intact rock. The developed MARS gives an equation for determination of E_j of jointed rock mass. The results from the developed MARS model have been compared with those of artificial neural networks (ANNs) using average absolute error. The developed MARS gives a robust model for determination of E_j of jointed rock mass.     

岩体弹性模量的尺寸效应初步研究

通过建立单轴压缩情况下含裂纹岩石试件的功能平衡方程,运用概率统计的方法,推导了岩体弹性模量尺度效应的表达式,揭示了岩体弹性模量尺度效应的内部结构本质。     

节理岩体中锚索加固作用分析

简要介绍了锚索加固技术的应用,并对全长粘结式锚索在节理岩体中的锚固作用机理进行初步分析,同时对节理岩体中锚索加固的抗拉粘结强度及抗剪切强度的影响因素进行了分析。     

成型软岩填料路堤强夯补强现场试验研究

软岩的主要特点是粘土矿物成分含量高、强度低、性能不稳定、易风化、易软化、且易崩解,路用性能较差,在铁路、公路中的应用受限制。国内对软岩填料的工程性质认识不足,其填筑的铁路或公路路基,经多年运行后,在交通循环荷载作用或自然条件侵蚀下产生严重的路基病害。试验研究运用强夯法对成型软岩路堤进行补强,加强路堤填筑密实度以改善其路用性,检测证明其强夯效果显著。     

Mechanical Tests on Pervasively Jointed Rock Material: Insight into Rock Mass Behaviour

Summary In the exploration phase for the design of an underground cavern in a limestone formation a large number of triaxial compression tests were carried out on laboratory specimens which were characterized by a variable degree of fracturing. The data were analyzed to investigate the influence of fracture intensity and of confining stress on the mechanical parameters. In particular, investigations focused on the relationships between the parameters of the strength criteria, respectively in residual and peak conditions, on the decay of the Young modulus with stress level in the prepeak phase of the test, and on the brittleness of the rock in the postpeak phase. The tested rock can be considered as a small-scale model of a jointed rock mass, and the laboratory data therefore provide useful insight into the mechanical behaviour of rock masses, especially the relationships between residual and peak strength parameters, which are required in many analytical models and in numerical codes for the analysis of underground excavations.     

节理岩体三维偶应力弹性理论

节理的存在使得岩体的力学特性十分复杂。偶应力理论考虑了介质内颗粒尺度的影响，它以具有一定尺度的微元体为研究对象，引进了偶应力及弯曲曲率的概念，从而考虑了弯曲效应对介质变形特性的影响，可以用来解决岩土工程中的地下洞室底鼓、顶板开裂及反倾向层状岩体的弯曲倾倒等弯曲变形及弯曲破坏问题。本文考虑了节理间距的影响，对节理岩体的空间间题提出了弹性偶应力理论并建立了有限元计算模式，最后通过算例说明用偶应力理论解决上述问题的有效性。     

节理岩体爆破数值模型及模拟研究

运用多刚体系统动力学及接触冲击理论建立了节理岩体弹塑性动态本构模型。基于DYNA－2D程序原理及其框架,利用所建模型,模拟了节理岩体的爆破过程,量化了节理岩体在爆炸载荷作用下不同时刻的应力场。     

Strength and Deformational Behaviour of a Jointed Rock Mass

Summary An assessment of the strength and deformational response of jointed rock masses is an essential requirement in the site selection, design and successful execution of Civil and Mining Engineering projects. A quick estimate of these properties for preliminary evaluation of alternate sites, will reduce considerable expenditures for field tests. An attempt has been made in the present study to develop a link between strength and deformability of jointed block masses with the properties of intact specimens, obtained from simple laboratory tests, taking into account the influence of the properties of the joints. Extensive experimentation has been carried out on large specimens of jointed block masses under uniaxial compression. The model material represents a low strength rock. Various joint configurations were introduced to achieve the most common modes of failure occurring in nature. A coefficient called Joint Factor has been used to account for the weakness brought into the intact rock by jointing. Methods of computing the Joint Factor for various modes of failure of a jointed mass in an unconfined state have been established. The effect of Joint Factor on strength and tangent modulus of the mass has been studied and the values have been correlated with those of intact rock. Guidelines for assessing probable modes of failure of a jointed mass will enable one to estimate the relevant strength and tangent modulus of the mass.