Risk of shear failure and extensional failure around over-stressed excavations in brittle rock

The authors investigate the failure modes surrounding over-stressed tunnels in rock.Three lines of investigation are employed:failure in over-stressed three-dimensional(3D) models of tunnels bored under 3D stress,failure modes in two-dimensional(2D) numerical simulations of 1000 m and 2000 m deep tunnels using FRACOD,both in intact rock and in rock masses with one or two joint sets,and finally,observations in TBM(tunnel boring machine) tunnels in hard and medium hard massive rocks.The reason for 'stress-induced' failure to initiate,when the assumed maximum tangential stress is approximately(0.4-0.5)σ_c(UCS,uniaxial compressive strength) in massive rock,is now known to be due to exceedance of a critical extensional strain which is generated by a Poisson's ratio effect.However,because similar 'stress/strength' failure limits are found in mining,nuclear waste research excavations,and deep road tunnels in Norway,one is easily misled into thinking of compressive stress induced failure.Because of this,the empirical SRF(stress reduction factor in the Q-system) is set to accelerate as the estimated ratio σ_(θmax)/σ_c 0.4.In mining,similar 'stress/strength' ratios are used to suggest depth of break-out.The reality behind the fracture initiation stress/strength ratio of '0.4' is actually because of combinations of familiar tensile and compressive strength ratios(such as 10) with Poisson's ratio(say0.25).We exceed the extensional strain limits and start to see acoustic emission(AE) when tangential stress σθ≈ 0.4σc,due to simple arithmetic.The combination of 2D theoretical FRACOD models and actual tunnelling suggests frequent initiation of failure by 'stable' extensional strain fracturing,but propagation in 'unstable' and therefore dynamic shearing.In the case of very deep tunnels(and 3D physical simulations),compressive stresses may be too high for extensional strain fracturing,and shearing will dominate,both ahead of the face and following the face.When shallower,the concept of 'extensional strain initiation but propagation' in shear is suggested.The various failure modes are richly illustrated,and the inability of conventional continuum modelling is emphasized,unless cohesion weakening and friction mobilization at different strain levels are used to reach a pseudo state of yield,but still considering a continuum.     

An experience of tunnelling in mudstone area in southwestern Taiwan

Many tunnels will be constructed in southwestern Taiwan in the upcoming decade to meet the huge demands of transportation, energy and water infrastructure projects. Mudstone strata cover more than a 1000 km² area, consisting mainly of massive mudstone or alternation of mudstone and sandy layer, exhibiting unfavorable geological and hydrological characteristics, making it extremely difficult for tunnelling. This paper presents the lessons learned from three tunnelling projects in the 1990s in a mudstone area. In addition to discussing the rock behavior in the vicinity of tunnel and its failure patterns, the monitoring data during tunnelling are presented as well. Finally, the effective method of design and construction are recommended for tunnelling in mudstone area.     

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.     

Computing in-situ strength of rock masses based upon RQD and modified joint factor: Using pressure and damage sensitive constitutive relationship

In this study, a new model was presented for computing strength of rock masses based upon in-situ observations of RQD popularly known as rock quality designation. This model links up the rock mass parameters from in-situ investigations with the strength parameters of jointed rocks obtained from laboratory scale experimental observations. Using the constitutive relation, the author derived a pressure and damage sensitive plastic parameter to determine strength of rock masses for varied extents of discontinuity and pressure induced damage. The test results show that plasticity characterized by hardening and softening inclusive of damage invariably depends upon mean pressure and extent of deformations already experienced by rock masses. The present work explores the test data that reveal the dependence of in-situ strength on incremental joint parameters obtained from the joint number,joint orientation, joint roughness, gouge parameters and water pressure. Substituting the relationship between the RQD and modified joint factor with that between modulus ratio and strength ratio, the model shows successfully that using damage inclusive plastic parameter and RQD provides a relationship for estimating the strength of rock masses. One of the main objectives of this work is to illustrate that the present model is sensitive to plasticity and damage together in estimating in-situ strength of rock masses in foundations, underground excavation and tunnels.     

Influence of degree of interlock on confined strength of jointed hard rock masses

The strength of jointed rock mass is strongly controlled by the degree of interlock between its constituent rock blocks. The degree of interlock constrains the kinematic freedom of individual rock blocks to rotate and slide along the block forming joints. The Hoek–Brown (HB) failure criterion and the geological strength index (GSI) were developed based on experiences from mine slopes and tunneling projects in moderately to poorly interlocked jointed rock masses. It has since then been demonstrated that the approach to estimate the HB strength parameters based on the GSI strength scaling equations (called the ‘GSI strength equations’) tends to underestimate the confined peak strength of highly interlocked jointed rock masses (i.e. GSI > 65), where the rock mass is often non-persistently jointed, and the intact rock blocks are strong and brittle. The estimation of the confined strength of such rock masses is relevant when designing mine pillars and abutments at great depths, where the confining pressure is high enough to prevent block rotation and free sliding on block boundaries. In this article, a grain-based distinct element modeling approach is used to simulate jointed rock masses of various degrees of interlock and to investigate the influences of block shape, joint persistence and joint surface condition on the confined peak strengths. The focus is on non-persistently jointed and blocky (persistently jointed) rock masses, consisting of hard and homogeneous rock blocks devoid of any strength degrading defects such as veins. The results from this investigation confirm that the GSI strength equations underestimate the confined strength of highly interlocked and non-persistently jointed rock masses. Moreover, the GSI strength equations are found to be valid to estimate the confined strength of persistently jointed rock masses with smooth and non-dilatant joint surfaces.     

Geotechnical aspects of rock tunnelling in China

To meet the huge demands of transportation, energy and other infrastructure projects, a large volume of rock tunnelling is being carried out across the country as China embarks on her modernization plan. This paper summaries the geotechnical issues encountered on such projects, including the various modes of rock mass instability and geological hazards. Methods of evaluating rock mass quality and ground conditions as developed in the West are now widely used on tunnelling projects in China. As well Chinese engineers and researchers have also developed geomechanical systems for rock mass classification, failure modes and support requirements. These systems have been proven to be effective in dealing with the complex geologic and tunnelling conditions in China. Also described in the paper are analytical techniques currently used in China for tunnel performance prediction, including canonical analysis, expert systems, block vector analysis and advanced numerical modelling techniques.     

Characterisation and engineering properties of tectonically undisturbed but lithologically varied sedimentary rock masses

Tectonically undisturbed sedimentary rocks deposited in a quiescent shallow marine environment often include a sequence of strata that may present significant lithological variety at the scale of an engineering structure. Such rock masses exhibit engineering properties that are significantly different from tectonically disturbed rock masses of similar composition. For example, molasse consists of a series of tectonically undisturbed sediments of sandstones, conglomerates, siltstones and marls, produced by the erosion of mountain ranges after the final phase of an orogeny. They behave quite differently from flysch which has the same composition but which was tectonically disturbed during the orogeny. The molasses behave as continuous rock masses when they are confined at depth and the bedding planes do not appear as clearly defined discontinuity surfaces. Close to the surface the layering of the formations is discernible and only then similarities may exist with the structure of some types of flysch. Therefore extreme care is necessary in the use of geotechnical classification systems for the selection of design parameters, in order to avoid penalizing the rock mass unnecessarily. A discussion on the use of the geological strength index (GSI) for the characterisation of such rock masses is presented. Two GSI charts are proposed for estimating the mechanical properties of these masses: one mainly for tunnels; and the second for surface excavations. An example is given to illustrate the process of tunnel design in molassic rocks.     

Estimating the geotechnical properties of heterogeneous rock masses such as flysch

The design of tunnels and slopes in heterogeneous rock masses such as flysch presents a major challenge to geologists and engineers. The complex structure of these materials, resulting from their depositional and tectonic history, means that they cannot easily be classified in terms of widely used rock mass classification systems. A methodology for estimating the Geological Strength Index and the rock mass properties for these geological formations is presented in this paper. Electronic Publication     

In situ strength criteria for tunnel design in highly-stressed rock masses

The selection of rock mass strength parameters and the approach to assessing the mechanical and thermal-mechanical rock mass behaviour around tunnels are important aspects of designing underground openings for a spent nuclear fuel waste repository. This paper demonstrates how strength criteria based on in situ observations can be used to design stable underground openings in rock masses subjected to adverse stress conditions. The findings are based upon work conducted at AECL’s Underground Research Laboratory (URL) regarding rock mass strength around tunnels in sparsely fractured granite.     

互层状岩体中群洞开挖稳定性研究和实践

通过群洞开挖的有限元计算和地质力学模型试验研究,论证了群洞开挖的稳定性和开挖方案的可行性。通过现场开挖施工安全监测与理论研究结果的比较,揭示了灰页岩互层岩体中大型群洞开挖过程中的规律。     

A new tunnel inflow classification (TIC) system through sedimentary rock masses

A new classification system with respect to the engineering geological characteristics of rock masses in different geological conditions were presented based on the authors’ experiences and observations. Rock mass composition (RMC), rock type (RT), clay-bearing content (CBC), unconfined compressive strength (UCS) and tunnel depth (TD) were found as the major factors affecting the tunnel inflow. In order to minimize judgmental bias and set up a basic database, data pertaining to these factors were compiled from 33 tunnels project with a total length of about 200 km thoroughly excavated in sedimentary rocks. The classification factors were rated using a combination of the analytic hierarchy process (AHP) and statistical methods. In order to cover all rock mass varieties and lessen the uncertainties, major factors were divided into categories of varying quality. Two statistical criteria were introduced to calculate the weighing of categories. The main advantage of this procedure is its capability of effectively predicting groundwater inflows in a vast variety of geological conditions especially from a single flow pathway such as a brittle fault zone to low permeable rock masses. The proposed classification was applied to the actual rock tunnels. It was revealed that the predicted values were in a good agreement with the actual field measurements and could provide quantitative measures of tunnel inflow. The proposed method could be more feasible for a reliable pre-assessment of groundwater inflows in the future tunnel construction projects under heterogeneous geological conditions. Furthermore, the most important factors as well as their combination are introduced for sedimentary rocks.     

一种确定节理岩体残余强度参数方法的探讨

 岩体的地质强度指标(GSI)集中考虑岩体结构和结构面表面特征2个方面的因素,已被广泛地应用于岩体强度参数与变形参数的计算中。在GSI系统的基础上,采用残余GSI量化评价方法,通过对峰值地质强度指标GSI进行折减,以得到残余地质强度指标 ,据此来计算节理岩体的残余强度参数。首先,结合岩体分类指标法(RMi),在 和 的基础上探讨残余岩块体积 和残余节理条件系数 取值的确定方法;然后,由 和 计算出 ,据广义Hoek-Brown准则计算出节理岩体的残余强度参数,重点对4种典型岩体的残余强度取值进行分析,讨论残余岩体体积对岩体残余强度参数的影响;最后,通过对岩体原位剪切实验数据和一桥墩承台开挖边坡的稳定性反分析,证实基于 指标的节理岩体残余强度参数确定方法的合理性和可靠性,为节理岩体残余强度参数的确定提供一条新的思路。     

Modification of rock mass strength assessment methods and their application in geotechnical engineering

Due to complicated structures and discontinuities in surrounding rock mass, existing empirical failure criteria cannot meet the requirements of engineering practice such as tunnels. To improve estimation accuracy on the strength of rock mass with joints, a modified chart of the Geological Strength Index using Hoek–Brown criteria was further tested to estimate rock mass strength [Lin et al. (2014) Bull Eng Geol Environ 4(73):1245–1258], and, in this paper, new strength estimation equations for jointed rock mass were then modified based on a large dataset obtained from Chinese projects. Here, standard drilling time is first introduced and described in this study, and then used as a parameter to estimate rock strength. Different empirical formulas based on joint density, rock mass classification, Hoek–Brown criteria, and elastic wave velocity are thus used to estimate rock mass strength by using data from the Jiubao tunnel. The results estimated based on different empirical formulas were similar, indicating that the modified assessment method presented in this paper can be used to estimate rock mass strength under certain circumstances. Cross-correlation of different empirical methods provides significant confidence in predicted rock mass strength calculations.     

The Hoeke-Brown failure criterion and GSI-2018 edition

The Hoeke Brown criterion was introduced in 1980 to provide input for the design of underground excavations in rock. The criterion now incorporates both intact rock and discontinuities, such as joints,characterized by the geological strength index(GSI), into a system designed to estimate the mechanical behaviour of typical rock masses encountered in tunnels, slopes and foundations. The strength and deformation properties of intact rock, derived from laboratory tests, are reduced based on the properties of discontinuities in the rock mass. The nonlinear Hoeke Brown criterion for rock masses is widely accepted and has been applied in many projects around the world. While, in general, it has been found to provide satisfactory estimates, there are several questions on the limits of its applicability and on the inaccuracies related to the quality of the input data. This paper introduces relatively few fundamental changes, but it does discuss many of the issues of utilization and presents case histories to demonstrate practical applications of the criterion and the GSI system.     

The Hoek–Brown failure criterion and GSI – 2018 edition

The Hoek–Brown criterion was introduced in 1980 to provide input for the design of underground excavations in rock. The criterion now incorporates both intact rock and discontinuities, such as joints, characterized by the geological strength index (GSI), into a system designed to estimate the mechanical behaviour of typical rock masses encountered in tunnels, slopes and foundations. The strength and deformation properties of intact rock, derived from laboratory tests, are reduced based on the properties of discontinuities in the rock mass. The nonlinear Hoek–Brown criterion for rock masses is widely accepted and has been applied in many projects around the world. While, in general, it has been found to provide satisfactory estimates, there are several questions on the limits of its applicability and on the inaccuracies related to the quality of the input data. This paper introduces relatively few fundamental changes, but it does discuss many of the issues of utilization and presents case histories to demonstrate practical applications of the criterion and the GSI system.     

Damage evolution mechanisms of rock in deep tunnels induced by cut blasting

The method of cut blasting is widely used in tunnelling excavation, which concerns the success in subsequent stop blasting and smooth blasting. Rock masses in deep tunnels are subjected to high in-situ stress, and the mechanisms of damage evolution of rock mass in deep tunnels induced by cut blasting are not well studied. In this paper, a tension and compression-shear damage model is developed, and then is implemented into the commercial software LS-DYNA as a user-defined material model. To validate the newly developed model, the comparison between numerical results and an existing field test is conducted. The effects of free-surface boundary conditions, in-situ stress and the coefficient of lateral pressure on cut blasting are considered in depth. Numerical results indicate that the superstition of stress wave and the reflected tension waves from free surfaces contribute to the damage interconnection near cut holes. The high in-situ stress has the resistance on the radially oriented pressure and the damage extension around cut holes. The coefficients of lateral pressure influence the extending direction of the tensile damage zone.     

A comparative review in regards to estimating bearing capacity in jointed rock masses in northeast Jordan

There are a number of different methods used for estimating the bearing capacity in jointed rock masses. In this paper, the geological strength index (GSI) introduced by Hoek et al. (1995) was used to estimate the bearing capacity of the rock mass via rock mass rating (RMR). An empirical relationship is proposed to estimate the bearing capacity of the rock mass using the GSI-dependent toughness factor (TF). The proposed formula was correlated with bearing capacity equations used in the literature. The regression analyses showed exponential relationships with a high correlation coefficient.     

Generalized crack initiation and crack damage stress thresholds of brittle rock masses near underground excavations

The rock mass failure process is characterized by several distinct deformation stages which include crack initiation, crack propagation and coalescence. It is important to know the stress levels associated with these deformation stages for engineering design and practice.Extensive theoretical, experimental and numerical studies on the failure process of intact rocks exist. It is generally understood that crack initiation starts at 0.3 to 0.5 times the peak uniaxial compressive stress. In confined conditions, the constant-deviatoric stress criterion was found to describe the crack initiation stress level.Here, generalized crack initiation and crack damage thresholds of rock masses are proposed. The crack initiation threshold is defined by σ₁−σ₃=A σ_cm and the crack damage threshold is defined by σ₁−σ₃=B σ_cm for jointed rock masses, where A and B are material constants and σ_cm is the uniaxial compressive strength of the rock masses. For a massive rock mass without joints, σ_cm is equal to σ_cd, the long-term uniaxial strength of intact rock. After examining data from intact rocks and jointed rock masses, it was found that for massive to moderately jointed rock masses, the material constants A and B are in the range of 0.4 to 0.5, 0.8 to 0.9, respectively, and for moderately to highly jointed rock masses, A and B are in the range of 0.5 to 0.6, 0.9 to 1.0, respectively. The generalized crack initiation and crack damage thresholds, when combined with simple linear elastic stress analysis, assist in assessing the rock mass integrity in low confinement conditions, greatly reducing the effort needed to obtain the required material constants for engineering design of underground excavations.