Tensile Strength of Rock Under Elevated Temperatures

Rock strength affects the behaviour of rocks differently under different conditions such as temperature, time, pressure, presence of fluids, rock mass characteristics and stress history of rock in a natural environment. It is not always easy to replicate such conditions of undisturbed rock in a laboratory scale. Hence, it is imperative to study the rock behaviour with respect to every such condition which the rock experiences since its time of formation. Temperature is one of the key parameters which influence the rock throughout its history, ranging from the conditions of formation, experience of depth (loading/unloading) or deformational and metamorphic history. Also, increase in rock temperature; say due to the thermal stress changes like disposal of spent nuclear fuels affects the strength of the surrounding rock. In this work, the effects of temperature on the tensile strength of rock have been studied. The results obtained were interesting as the strength of rock is found to increase considerably up to a particular temperature after which it starts falling by as much as 70% around 250°C.     

Effects of Testing Conditions on Intact Rock Strength and Variability

The strength of intact rock is a fundamental and integral part of many aspects of engineering and geologic practice. It is necessary to understand how testing conditions influence this strength and its variability, especially for use in reliability-based design. In this study, the effect of sample diameter and moisture content on the strength and its variability was examined in detail. Correlations are given for assessing the effect of sample diameter on the laboratory uniaxial compressive strength (q_u), Brazilian indirect tensile strength (q_t-Brazilian), and point load strength (I_s), and the associated direct correlations among these tests. Correlations also are given for assessing the effect of sample moisture content on q_u, q_t-Brazilian, and I_s. The statistics of data scatter suggest these correlations can introduce large uncertainties. The effect of sample diameter and moisture content on the coefficients of variation was examined subsequently, and it was found to be relatively minor.     

圆环试样内径对抗拉强度的影响

运用RFPA2D软件对岩石的单轴拉伸强度试验、不同内径的圆环试样的劈裂试验进行了模拟。比较模拟结果 ,重点研究了圆环试样内径对试件抗拉强度的影响 ,确定了最接近单轴拉伸试验结果内径的范围 ,利用回归分析确定了内径与试样抗拉强度之间的关系。     

A Discrete Element Model for Predicting Shear Strength and Degradation of Rock Joint by Using Compressive and Tensile Test Data

A discrete element model is proposed to examine rock strength and failure. The model is implemented by UDEC, which is developed for this purpose. The material is represented as a collection of irregular-sized deformable particles interacting at their cohesive boundaries. The interface between two adjacent particles is viewed as a flexible contact whose constitutive law controls the material fracture and fragmentation properties. To reproduce rock anisotropy, an orthotropic cohesive law is developed for the contacts, which allows their shear and tensile behaviors to be different from each other. Using a combination of original closed-form expressions and statistical calibrations, a unique set of the contact microparameters are found based on the uniaxial/triaxial compression and Brazilian tension test data of a plaster. Applying the obtained microparameters, joint specimens, made of the same plaster, are simulated, where the comparison of the obtained results to laboratory data shows a reasonable agreement.     

Creep Testing of Thin Interbedded Clayey Seams in Rock and Determination of Their Longterm Strength

First in this paper, two types of apparatus in laboratories, the simple shear plastometer and the relaxation plastometer, and the methods of simple shear testing and relaxation testing are introduced. Then according to the test results of thin interbedded clayey seams in rock, the upper yield value f3-ritical stress suggested by Dr. Tan Tjongkie is the limit long-term stress to rocks or soils. For engineering purposes it is important to know their long-term creep strength.\     

岩体中泥化夹层的流变试验及其长期强度的确定

文中首先介绍两种室内试验设备--单剪流变仪和剪应力松弛仪（松弛流变仪）及其试验方法,然后根根岩体中泥化夹层的试验成果,证实陈宗基教授提出的最高屈服值f3--强度的临界值即长期强度值,工程设计要重视它。     

冻结黄土抗拉强度的试验研究

冻结饱水黄土的拉伸试验表明，拉伸破坏均属脆性破坏类型，拉断面上矿物颗粒剪移错粒，裂隙发育。拉伸的应力－应变过程，视荷载作用的快慢大致可分为粘弹－塑性、粘－弹性有脆性破坏三类，它们可用统一的方程形式加以描述，此外，对饱水冻结黄土的峰值应力、破坏应变和破坏时间等特性进行了分析，对抗拉、抗压强度作分析比较后指出，它们的比值为０．４４－０．６３，与负温和加载速率的关系不大。     

Dynamic Tensile Failure of the Rock Interface Between Tuff and Basalt

The dynamic tensile strength properties of the rock interface and its host rocks sampled from the Baihetan Hydropower Station from Western China were measured using a split Hopkinson pressure bar (SHPB). The results were compared with those for its two host rocks. The dynamic tensile strengths of the two host rocks, tuff and basalt have typical loading rate dependence. However, the dynamic response of the rock interface is much more complicated and at a given loading rate, varies between those of tuff and basalt. To explain the observation, numerical simulation using the discrete element method (DEM) was conducted to determine the detailed tensile failure process of the rock interface. The numerical simulation verifies that the variation of the dynamic tensile strength of the rock interface is a result of the variation of the interface geometry.     

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.     

Tensile Strength of Cement-Treated Champlain Sea Clay

Geotechnical and Geological Engineering - Champlain Sea clay, a sensitive marine clay, is commonly found along the Saint Lawrence River basin in Eastern Canada. According to recent studies, cement...     

纤维加筋MICP固化钙质砂的抗拉强度特性研究

针对微生物诱导碳酸钙沉积 (MICP)固化钙质砂脆性强、抗拉强度低等问题,通过制备“8”字形MICP固化钙质砂试样并开展直接拉伸试验,对纤维加筋的改善作用、纤维-MICP联合加固机理及纤维掺量、纤维长度等影响因素进行了研究。结果表明：纤维加筋能够显著提高抗拉强度、峰值位移和残余强度,减轻峰值强度点的脆性破坏现象,但受纤掺量和长度的影响,总的来说,抗拉强度随纤维掺量的增加和长度的加长呈先增后减的趋势。相比无纤维试样,添加最优纤维掺量（0.6%）时,试样的抗拉强度增长了172.4%,峰值变形提升了158.1%。机理可解释为纤维增加了微生物的吸附量,促进碳酸钙在纤维与钙质砂之间以及纤维表面的沉积,增大纤维与钙质砂之间的界面作用力,整体提升钙质砂的抗拉强度特性。纤维的添加能够显著改变试样的变形特征,无纤维添加试样曲线仅有初始误差阶段和弹性阶段两个阶段,添加纤维后曲线表现为四个阶段包括初始误差阶段、弹性阶段、损伤破坏阶段和残余阶段。纤维掺量影响的内因是纤维与钙质砂的界面作用力和纤维空间分布状态随纤维掺量的变化而变化,纤维长度的影响主要和破坏面附近纤维数量和单位长度所能承担的拉应力相关。研究成果对以钙质砂为地基的岛礁工程的稳定性、安全性具有一定的指导意义。     

On the Measurement of the Tensile Strength of Soft Rocks

Summary. This paper reports on a comparative study of various types of experimental tests for measuring the tensile strength of rocks and rock-like materials. A critical assessment is presented of some widely used laboratory techniques on the basis of experimental data from the literature and from the laboratory investigation performed in this study.Tests were carried out using a triaxial apparatus recently set up at Milan University of Technology. This was designed to reduce random misalignments between specimen and loading frame, which are typical of conventional triaxial cells with external tie bars. The apparatus was then modified to perform various types of tests for determining the tensile strength of materials. An artificial building stone and a natural calcarenite of the Gravina di Puglia geological formation, sampled at Montescaglioso (Matera-Italy) were tested in this research. The experimental investigation included various types of tests, namely the uni-axial, the Brazilian, the ring, the three and four points bending and the Luong test. Specimens of both materials were also compressed to failure in unconfined conditions and loaded cyclically in unconfined tension and compression, Young’s moduli being measured by means of local instrumentation.     

Importance of Tensile Strength on the Shear Behavior of Discontinuities

In this study, the shear behavior of discontinuities possessing two different rock wall types with distinct separate compressive strengths was investigated. The designed profiles consisted of regular artificial joints molded by five types of plaster mortars, each representing a distinct uniaxial compressive strength. The compressive strengths of plaster specimens ranged from 5.9 to 19.5 MPa. These specimens were molded considering a regular triangular asperity profile and were designed so as to achieve joint walls with different strength material combinations. The results showed that the shear behavior of discontinuities possessing different joint wall compressive strengths (DDJCS) tested under constant normal load (CNL) conditions is the same as those possessing identical joint wall strengths, but the shear strength of DDJCS is governed by minor joint wall compressive strength. In addition, it was measured that the predicted values obtained by Barton’s empirical criterion are greater than the experimental results. The finding indicates that there is a correlation between the joint roughness coefficient (JRC), normal stress, and mechanical strength. It was observed that the mode of failure of asperities is either pure tensile, pure shear, or a combination of both. Therefore, Barton’s strength criterion, which considers the compressive strength of joint walls, was modified by substituting the compressive strength with the tensile strength. The validity of the modified criterion was examined by the comparison of the predicted shear values with the laboratory shear test results reported by Grasselli (Ph.D. thesis n.2404, Civil Engineering Department, EPFL, Lausanne, Switzerland, 2001). These comparisons infer that the modified criterion can predict the shear strength of joints more precisely.     

Estimation of the Strength of Stratified Rock Mass

Stratified rock formations are often encountered in engineering structures with large dimensions, such as tunnels, caverns, etc. In the present work, the question of how and when such materials fail is addressed. It is argued that average values cannot provide appropriate measures for the strength of layered rock, as local stresses and strains differ greatly in the various constituents and one may fail, while others remain intact. The present work attempts, therefore, to formulate a solution covering both elasticity and failure in three dimensions, though simple enough to be easy to use in engineering applications. To this end, the local stress and strain state in each constituent were evaluated, assuming the constituents to be orthotropically elastic up to the point of failure. Subsequently, several failure criteria are considered and an example is presented to illustrate the model’s response. A discussion on the effect of geometry and on the values of local versus global quantities follows.