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     

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.     

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.     

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.     

Influence of data analysis when exploiting DFN model representation in the application of rock mass classification systems

Discrete fracture network (DFN) models have been proved to be effective tools for the characterisation of rock masses by using statistical distributions to generate realistic three-dimensional (3D) representations of a natural fracture network. The quality of DFN modelling relies on the quality of the field data and their interpretation. In this context, advancements in remote data acquisition have now made it possible to acquire high-quality data potentially not accessible by conventional scanline and window mapping. This paper presents a comparison between aggregate and disaggregate approaches to define fracture sets, and their role with respect to the definition of key input parameters required to generate DFN models. The focal point of the discussion is the characterisation of in situ block size distribution (IBSD) using DFN methods. An application of IBSD is the assessment of rock mass quality through rock mass classification systems such as geological strength index (GSI). As DFN models are becoming an almost integral part of many geotechnical and mining engineering problems, the authors present a method whereby realistic representation of 3D fracture networks and block size analysis are used to estimate GSI ratings, with emphasis on the limitations that exist in rock engineering design when assigning a unique GSI value to spatially variable rock masses.     

Effects of confinement on rock mass modulus: A synthetic rock mass modelling (SRM) study

The main objective of this paper is to examine the influence of the applied confining stress on the rock mass modulus of moderately jointed rocks (well interlocked undisturbed rock mass with blocks formed by three or less intersecting joints). A synthetic rock mass modelling (SRM) approach is employed to determine the mechanical properties of the rock mass. In this approach, the intact body of rock is represented by the discrete element method (DEM)-Voronoi grains with the ability of simulating the initiation and propagation of microcracks within the intact part of the model. The geometry of the pre-existing joints is generated by employing discrete fracture network (DFN) modelling based on field joint data collected from the Brockville Tunnel using LiDAR scanning. The geometrical characteristics of the simulated joints at a representative sample size are first validated against the field data, and then used to measure the rock quality designation (RQD), joint spacing, areal fracture intensity (P₂₁), and block volumes. These geometrical quantities are used to quantitatively determine a representative range of the geological strength index (GSI). The results show that estimating the GSI using the RQD tends to make a closer estimate of the degree of blockiness that leads to GSI values corresponding to those obtained from direct visual observations of the rock mass conditions in the field. The use of joint spacing and block volume in order to quantify the GSI value range for the studied rock mass suggests a lower range compared to that evaluated in situ. Based on numerical modelling results and laboratory data of rock testing reported in the literature, a semi-empirical equation is proposed that relates the rock mass modulus to confinement as a function of the areal fracture intensity and joint stiffness.     

An integrated multiscale approach for characterization of rock masses subjected to tunnel excavation

The design of tunnels must be conducted based on the knowledge of the territory. The longer the structure, the larger the area to be investigated, and the greater the number of surveys and tests to be performed in order to thoroughly examine all the relevant features. Therefore, optimization of the investigation process is strongly required to obtain complete and reliable data for the design of the infrastructure. The fast development of remote sensing technologies and the affordability of their products have contributed to proving their benefits as supports for investigation, encouraging the spreading of automatic or semi-automatic methods for regional scale surveys. Similarly, considering the scale of the rock outcrop, photogrammetric and laser scanner techniques are well-established techniques for representing geometrical features of rock masses, and the benefits of non-contact surveys in terms of safety and time consumption are acknowledged. Unfortunately, in most cases, data obtained at different scales of investigations are only partially integrated or compared, probably due to the missing exchange of knowledge among experts of different fields (e.g. geologists and geotechnical engineers). The authors, after experiencing such a lack of connection among the results of different surveys concerning tunnels, propose a multiscale approach for the optimization of the investigation process, starting from the regional scale, to obtain the data that can be useful not only for planning more detailed surveys in a preliminary phase, but also for making previsions on the discontinuity sets that are present in the rock masses subjected to excavations. A methodological process is proposed and illustrated by means of a case study. Preliminary results are discussed to highlight the potentiality of this method and its limitations.     

Analysis of the behaviour of a yield-control support system in squeezing rock

Recent innovations in yield-control support systems allow to increase the rate of advance when tunnelling in difficult conditions is associated with severely squeezing rock. Such systems which imply the insertion in the lining of highly deformable concrete elements are being adopted successfully in tunnelling projects using conventional excavation methods. The Saint Martin access adit excavated in a Carboniferous Formation along the Base Tunnel of the Lyon-Turin rail line is presented as a case study. Numerical analyses are discussed to compare the results of computed and measured performance of a typical monitored section and to find out possible optimizations of the support system adopted.     

基于区间理论与GSI的岩质边坡稳定可靠性分析方法

针对采用结构面表面特征与岩体结构的定性描述确定岩体地质强度指标GSI存在较强主观性的特点,首先以结构面表面等级SCR与岩体结构等级SR对其进行量化处理,并根据组成SCR与SR的量化指标取值过程中所具有的不确定性,采用区间值表示参数取值,建立岩体量化GSI区间值确定方法;其次,根据Hoek-Brown准则等效岩体抗剪强度参数转换关系,采用区间数学理论建立出岩体力学参数区间值确定方法;然后,通过区间组合法求解岩质边坡安全系数区间值,并在确定安全系数要求值的基础上采用非概率可靠性分析方法进行稳定可靠性评价,进而建立出基于区间理论与GSI的岩质边坡稳定可靠性分析方法;最后,将其应用于分析湖南省柘溪水电站进水口高边坡稳定性,结果表明该方法具有一定的合理性与可行性。在缺乏现场岩体力学试验与大量样本数据的情况下,该方法为岩质边坡从岩体力学参数确定到稳定可靠性评价提供了一条新途径。     

深埋隧道软弱破碎围岩支护技术研究

以赣龙铁路隧道埋深300 m～320 m的Ⅴ级围岩段为背景,采用解析计算、数值计算和现场实测相结合的研究方法对大埋深条件下软弱、破碎围岩隧道的支护技术进行了研究。研究表明:在大埋深条件下,软弱、破碎围岩隧道的支护宜选择以提高围岩强度为主,适当提高支护力为辅的支护方法;利用管式锚杆兼作注浆管,通过注浆提高围岩强度和锚杆锚固力的锚注联合支护技术对大埋深条件下软弱、破碎围岩隧道有较好的支护效果。     

武广客专大断面软弱破碎围岩隧道开挖及支护

就武广客专大断面软弱或破碎围岩隧道的开挖方法和支护措施进行了分析,探讨了适用于大断面不同地质情况低级别围岩隧道的快速开挖和支护技术,指出大断面客专隧道选择开挖方法时应结合具体地质条件,施工技术和现场管理水平决定。     

软岩隧道围岩变形分析及处理

从介绍软岩隧道的基本变形特征出发，系统分析了引起软岩隧道变形的原因，全面讨论了软岩隧道的支护原理、原则与方法，并详细介绍了底鼓问题的防治方法，对软岩地质条件下的施工具有重要的现实意义。     

A new estimation method and an anisotropy index for the deformation modulus of jointed rock masses

The deformation modulus of a rock mass is an important parameter to describe its mechanical behavior.In this study,an analytical method is developed to determine the deformation modulus of jointed rock masses,which considers the mechanical properties of intact rocks and joints based on the superposition principle.Due to incorporating the variations in the orientations and sizes of joint sets,the proposed method is applicable to the rock mass with persistent and parallel joints as well as that with nonpersistent and nonparallel joints.In addition,an anisotropy index AIdmfor the deformation modulus is defined to quantitatively describe the anisotropy of rock masses.The range of AIdmis from 0 to 1,and the more anisotropic the rock mass is,the larger the value of AIdmwill be.To evaluate the proposed method,20 groups of numerical experiments are conducted with the universal distinct element code(UDEC).For each experimental group,the deformation modulus in 24 directions are obtained by UDEC(numerical value)and the proposed method(predicted value),and then the mean error rates are calculated.Note that the mean error rate is the mean value of the error rates of the deformation modulus in 24 directions,where for each direction,the error rate is equal to the ratio of numerical value minus predicted value to the numerical value.The results show that(i)for different experimental groups,the mean error rates vary between 5.06%and 22.03%;(ii)the error rates for the discrete fracture networks(DFNs)with two sets of joints are at the same level as those with one set of joints;and(iii)therefore,the proposed method for estimating the deformation modulus of jointed rock masses is valid.     

锚喷支护技术在滑坡治理中的应用

通过具体工程实例,详细介绍了锚喷支护技术在边坡滑坡地质灾害处理中的应用情况,监测结果说明锚喷支护技术作为煤矿煤泥堆放场地边坡的一种新的防护形式是可行的,从而推广锚喷支护技术的应用.     

岔道1号双连拱隧道围岩压力及支护应力量测

以张石高速公路岔道1号隧道为依托,对其围岩压力及支护应力进行量测分析,得出了双连拱隧道在浅埋偏压情况下围岩压力及支护应力的分布规律,为类似工程项目提供了类比依据。     

Hydraulic properties of fractured rock masses with correlated fracture length and aperture

Permeability of fractured rocks is investigated considering the correlation between distributed fracture aperture and trace length, based on a newly developed correlation equation. The influence of the second moment of the lognormal distribution of apertures on the existence of representative elementary volume (REV), and the possibility of equivalent permeability tensor of the fractured rock mass, is examined by simulating flow through a large number of stochastic discrete fracture network (DFN) models of varying sizes and varying fracture properties.The REV size of the DFN models increases with the increase of the second moment of the lognormal distribution, for both the correlated and uncorrelated cases. The variation of overall permeability between different stochastic realizations is an order of magnitude larger when the aperture and length are correlated than when they are uncorrelated. The mean square error of the directional permeability increases with increasing value of the second moment of the lognormal distribution function, and good fitting to an ellipsis of permeability tensor can only be reached with very large sizes of DFN models, compared with the case of constant fracture aperture, regardless of fracture trace length.     

深部煤层力学参数反演和气体钻井井壁稳定性

如何确定深部煤层的地质力学参数是气体钻井井壁稳定评价的关键。基于量化的地质强度指标GSI和Hoek-Brown强度准则分析了反演深部煤层地质力学参数的方法,结合DB1井氮气钻井,以弹塑性方法分析了侏罗系炭质泥岩和煤层的井壁稳定性。研究表明,塔里木盆地依奇克里克区块侏罗系煤GSI介于45~55,变形模量约在1 560~2 850 MPa之间。井眼钻开后炭质泥岩和煤层井周存在塑性区,炭质泥岩层井周塑性区约为1.37倍井眼半径,煤层井周塑性区约为2.07~2.59倍井眼半径。塑性区应力松弛造成围压减小,导致节理岩体抵抗工程扰动的能力减弱。以工程允许塑性半径等于1.5倍井眼半径为临界稳定条件,井眼钻开后煤层会出现大面积坍塌失稳,不宜采用气体钻井;工程扰动造成炭质泥岩井眼扩径,扩径后井眼仍能保持稳定。     

Fractal behaviour of the microseismic energy associated with immediate rockbursts in deep,hard rock tunnels

Rockbursts in tunnels can be a significant hazard. During the development of immediate rockbursts, a series of fractures is generated which can be monitored using microseismic events. A fractal calculation method has been proposed to study the self-similarity of the energy distribution of microseismic events during the development of immediate rockbursts. The proposed method was used to study the fractal behaviour of the energy distribution of microseismic events during the development of immediate rockbursts that occurred in four deep headrace tunnels and one drainage tunnel at the Jinping II hydropower project, China. The overburden of the studied tunnels, which are excavated in marble, is between 1900 m and 2525 m. The tunnels have a total length of 12.4 km. The results indicated that the energy distribution of microseismic events during the development of immediate rockbursts has a fractal structure. The fractal behaviour of microseismic energy during the development of immediate strainbursts and immediate strain-structure slip rockbursts are distinguishable. The energy fractal dimensions of immediate strainbursts were larger than 0.2 but those of immediate strain-structure slip rockbursts were less than 0.2. For both immediate strainbursts and immediate strain-structure slip rockbursts, if the intensity was higher, the energy fractal dimensions were larger. The daily energy fractal dimension for microseismic events increased as approach of the immediate rockbursts. The energy fractal behaviour was used as a guide to establish a dynamic warning system based on the evolution of microseismic events to reduce the risk of immediate rockbursts during excavation of deep, hard-rock tunnels.