Classification of geological and engineering properties in weak rock: a case study of a tunnel in a fault zone in southeastern Korea

Given that the rock mass classifications of weak rocks observed in tunnels are evaluated only as ‘poor rock mass’ or grades IV–V using existing rock mass classification methods, a new scheme is needed that would better distinguish the various geological properties of weak rocks. In this study, geological and engineering properties of weak rocks were classified based on the analysis results of a total of 55 faces in a tunnel that passes through a fault zone in southeastern South Korea. Geological observations, point load tests, and Schmidt hammer tests were conducted to analyze the properties of the weak rocks. Through this analysis, the weak rocks were classified into two weathering types: ‘disintegrated rocks’ and ‘decomposed rocks’. This division is based on the tectonic mechanisms and parameters necessary to determine the engineering geological properties of individual rock-weathering types according to their geological properties. The ‘disintegrated rocks’ have been physically weathered by brittle deformation and are classified into three subtypes using the rock quality designation (RQD) and joint set number (J_n), which can characterize the discontinuity properties of the rocks. Point load tests were conducted on these individual subtypes to calculate the point load index (Is₍₅₀₎). The resultant values are clearly related to the assigned three subtypes. The ‘decomposed rocks’ have been chemically weathered by ductile deformation and are classified into five subtypes based on the results of Schmidt hammer tests conducted on faces and Is₍₅₀₎ values calculated using the average of the Schmidt hammer values. The geological properties of the individual subtypes of the ‘disintegrated rocks’ and ‘decomposed rocks’ are clearly related to their engineering properties. Based on these results, rock mass classification diagrams are proposed that can be simply and easily applied to weak rocks.

     

Multifactorial fuzzy approach to the penetrability classification of TBM in hard rock conditions

Rate of penetration of a tunnel boring machine in a hard rock environment is generally a key parameter which expresses the ease or difficulty with which the rock mass can be excavated. In this paper, the penetrability of TBM in hard rock conditions was investigated with the developed fuzzy classification system. TBM penetration rate and rock properties (such as Uniaxial Compressive Strength (UCS), Brazilian Tensile Strength (BTS), rock brittleness/toughness, Average Distance between Planes of Weakness (DPW) and orientation of discontinuities in rock mass) were evaluated by using the multifactorial fuzzy approach which is a special case of multiple objective multifactorial decision making for the penetrability classification of TBM in hard rock conditions. Using the decision function, the penetrating performance of TBM was classified into three categories; Good, Medium and Poor. Eventually, it is possible to evaluate the penetrability and determine the advance rate for new conditions by carrying out the proposed rock properties tests and using the developed fuzzy classification system.     

A geo-engineering classification for rocks and rock masses

In this article, an attempt is made to assess the reliability of predicting the uniaxial compressive strength and the corresponding modulus of a rock mass by current approaches. These two basic engineering properties, when estimated from rock mass rating (RMR), Q and geological strength index (GSI), indicate hardly any change in the modulus ratio with the change in the quality of the rock mass from very good to very poor. However, the modulus ratio obtained from the relations involving the joint factor, J_f, indicate a definite decrease in the modulus ratio with a decrease in the quality of the rock mass. The strength and modulus in the unconfined and confined states, the modulus ratio and failure strain in the unconfined case were linked to J_f in earlier publications based on a large experimental database. Some of these relations were adopted to verify the response of jointed test specimens, the response of the rock mass during excavations for mining and civil underground chambers, in establishing ground reaction curves including the extent of the broken zone, and the bearing capacity of shallow foundations.The joint factor is now linked to RMR, Q and GSI. The prediction of compressive strength and modulus of the rock mass appears to be more suitable. For classifying the rock, based on these properties, the Deere and Miller engineering classification, applicable to intact rocks, has been suitably modified and adopted. The results of different modes of failure of jointed specimens establish definite trends of changes in the modulus ratio originating from the intact rock value on the modified Deere and Miller plot. A geo-engineering classification is evolved by considering strength, modulus, quantifiable weathering index and lithological aspects of the rock.     

Strength classification of rock material based on textural properties

Rock, as a construction material, has great importance during the construction and service phases in a rock environment. The classification of rock materials based on their strength behavior provides a simple and fast solution to determine the type and application of support system as well as the method for opening underground structures. Intact rock materials are generally classified with regard to the strength, such as uniaxial compressive and point load strength. Rock texture, which consists of grains and matrix, directly affects the strength. The relation between the textural and mechanical properties of rock materials has been investigated, and rock texture was quantified from the texture coefficient (TC). The coefficient can be used to put a number on rock textures with experimental studies carried out on thin sections of rock material using image analysis. The main scope of this research is to classify the rock material according to its TC values based on the binary and fuzzy domain. In this study, TC is divided into five classes from very low to very high, and a fuzzy model is proposed to predict the uniaxial compressive strength from TC. A dataset is prepared to construct an objective study with 12 litho-type rock materials from 19 locations in Turkey. The binary and fuzzy classification as well as fuzzy model for the prediction of compressive strength is also applied to the dataset to illustrate the use of the proposed classification and model for underground construction in rock engineering. The model is applied to determine the intact rock material’s rating in rock mass rating classification (RMR) from the proposed classification as well as from the fuzzy model. The results of the example encourage the application of the proposed methods, especially for pre-feasibility studies of rock engineering projects.     

模糊数学在岩石工程分类中的应用

由于岩石的物理-力学参数的不确定性的特征,本文基于模糊数学的概念,提出了一个岩石工程分类的新方法。首先把岩石分为最好岩石、好的岩石等k类,每一类分别对应一模糊集（λ_i,i=1,2,……k）。对一项实际工程所涉及的岩石,先就抗压强度、地下水等n个方面作单因素评价,并统一转换成分数,获得n个模糊集。再拟定m组权数,说明以上各个因素在综合评价中所占比例的各种可能性,因此对每一组权组即可得一综合评价。最后得出岩石的工程分类。     

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.     

Evaluation of empirical estimation of uniaxial compressive strength of rock using measurements from index and physical tests

The uniaxial compressive strength(UCS) of rock is an important parameter required for design and analysis of rock structures,and rock mass classification.Uniaxial compression test is the direct method to obtain the UCS values.However,these tests are generally tedious,time-consuming,expensive,and sometimes impossible to perform due to difficult rock conditions.Therefore,several empirical equations have been developed to estimate the UCS from results of index and physical tests of rock.Nevertheless,numerous empirical models available in the literature often make it difficult for mining engineers to decide which empirical equation provides the most reliable estimate of UCS.This study evaluates estimation of UCS of rocks from several empirical equations.The study uses data of point load strength(Is(50)),Schmidt rebound hardness(SRH),block punch index(BPI),effective porosity(n) and density(ρ)as inputs to empirically estimate the UCS.The estimated UCS values from empirical equations are compared with experimentally obtained or measured UCS values,using statistical analyses.It shows that the reliability of UCS estimated from empirical equations depends on the quality of data used to develop the equations,type of input data used in the equations,and the quality of input data from index or physical tests.The results show that the point load strength(Is(50)) is the most reliable index for estimating UCS among the five types of tests evaluated.Because of type-specific nature of rock,restricting the use of empirical equations to the similar rock types for which they are developed is one of the measures to ensure satisfactory prediction performance of empirical equations.     

Classification of black decorative stones from Warangal District, Andhra Pradesh, India

Despite abundant usage of Indian black decorative stones, no systematic approach has been made to study their properties. In this paper an attempt has been made to study the geoparametrical and geotechnical properties of the stones from ten active quarries in the Dornakal-Balpao-Venkatayapalem, Warangal district, Andhra Pradesh, India, including colour, hardness, texture, grain size, composition, macro- and micro-joints, specific gravity, dry density, moisture content, water absorption, porosity, uniaxial compressive strength (compressive strength), tensile strength, shear strength, rebound hardness number, point load strength index and rock hardness. Regression analyses were undertaken; the correlation coefficient obtained from ranged from 0.80–0.96. On the basis of these results, a new classification system for black decorative stone is proposed with four classes ranging from I (very good) to IV (poor). It is appreciated that the classification is based on limited data but it provides an important advance with implications for improving the present ad hoc, often wasteful quarrying philosophy in India.     

Modified Mohr–Coulomb criterion for non-linear triaxial and polyaxial strength of intact rocks

Triaxial or polyaxial strength of rocks is required while analysing many civil and mining engineering structures in rocks. Mohr–Coulomb criterion is the most widely used strength criterion in rock engineering problems. In its present form the criterion suffers from two major limitations. Firstly, it represents the strength of rock as a linear function of confining pressure. Secondly, the effect of intermediate principal stress is not considered by this criterion. In the present study, this criterion is modified to take into account the non-linearity and effect of intermediate principal stress on strength behaviour. Barton's [1] critical state concept for rocks has been employed for this purpose. The applicability of the proposed simple non-linear triaxial and polyaxial strength criteria has been verified by applying them to experimental results for the intact isotropic rock material available from literature and comparing the prediction with the other popular criteria in vogue. The agreement has been found to be excellent. The applicability of the concept to jointed rocks will be discussed in separate publication.     

Observations and classification of roof strata behaviour over longwall coal mining panels in India

A study has been made of a number of mechanized longwall panels in various coal mines in India, concentrating on the geology, physico-mechanical rock properties and the behaviour of the coal measure roof rocks during mining. The research has highlighted the splitting and caving characteristics of the strata rocks and enabled the development of a roof-rock classification system. Roof rocks are classified into six categories based on the: (a) mean weighted uniaxial compressive strength; (b) RQD; (c) (i) type of rocks, (ii) presence of cracks, fissile beds, splitting, etc., (iii) presence of water; and (d) thickness of bed layer. The behaviour of layers and composite layers in the immediate roof rock mass is also enumerated. Relations between the rock strength properties and values for the bulking factor of the failed rock are included. The concepts of a ‘weighting zone’ and ‘caving zone’ are proposed. The paper also clarifies the occurrence of different types of failure, such as periodic falls. All these findings should be helpful during calculation of the powered support capacity.     

Applying rock mass classifications to carbonate rocks for engineering purposes with a new approach using the rock engineering system

Classical rock mass classification systems are not applicable to carbonate rocks, especially when these are affected by karst processes. Their applications to such settings could therefore result in outcomes not representative of the real stress–strain behavior. In this study, we propose a new classification of carbonate rock masses for engineering purposes, by adapting the rock engineering system (RES) method by Hudson for fractured and karstified rock masses, in order to highlight the problems of implementation of geomechanical models to carbonate rocks. This new approach allows a less rigid classification for carbonate rock masses, taking into account the local properties of the outcrops, the site conditions and the type of engineering work as well.     

TBM performance estimation using rock mass classifications

Three tunnels for hydraulic purposes were excavated by tunnel-boring machines (TBM) in mostly hard metamorphic rocks in Northern Italy. A total of 14 km of tunnel was surveyed almost continually, yielding over 700 sets of data featuring rock mass characteristics and TBM performance. The empirical relations between rock mass rating and penetration rate clearly show that TBM performance reaches a maximum in the rock mass rating (RMR) range 40–70 while slower penetration is experienced in both too bad and too good rock masses. However, as different rocks gives different penetrations for the same RMR, the use of Bieniawski's classification for predictive purpose is only possible provided one uses a normalized RMR index with reference to the basic factors affecting TBM tunneling. Comparison of actual penetrations with those predicted by the Innaurato and Barton models shows poor agreement, thus highlighting the difficulties involved in TBM performance prediction.     

The Sumela Monastery slope in Maçka,Trabzon, Northeast Turkey: rock mass properties and stability assessment

In 2001, rock falls occurred on the southern slope of the Sumela Monastery, which was built on a cliff in Trabzon, north east Turkey, and is visited by many tourists. Considering the steepness of the slope and difficulty of access, rock classifications were made based on seismic wave velocity, rock mass quality (Q), rock quality designation (RQD) and rock mass rating (RMR). The results indicated extremely poor to very poor rock in the cracked, fractured and weathered parts and poor to fair rock in the intact parts of the monastery slope. As a consequence, rock fall, slide and rolling may occur from time to time. These constitute a hazard to the facilities of the monastery and access paths. It is recommended that cracked areas are strengthened with cement-based materials, that accumulated rocks are removed, and that retaining walls are constructed on solid rock around the unstable rock blocks.     

Assessing the ability of rock masses to support block breakage at the TBM cutter face

In order for tunnel boring machines to efficiently cut or break rock, it is necessary that the block of rock in contact with the cutter be adequately supported by the surrounding rock mass. This support is provided by the interlocking of blocks and the friction of the surfaces. If blocks are inadequately supported or become free without breakage the result can be jamming at the TBM face. Such blocky ground conditions are typically assessed according to the spacing and orientation of discontinuities (including joints) within the rock mass, typically using a rock mass classification system. In laboratory tests on cuttability or abrasivity of rocks, test samples are typically supported securely in a frame or jig. Numerical models of rock breakage also assume boundary conditions in which the sample is completely supported. Therefore the applicability of the results from laboratory and numerical studies depends on the same degree of support of blocks in the ground. The conditions required to adequately support a block for breakage are investigated and related to rock mass parameters, in particular, the three-dimensional patterns of discontinuities. A rock mass can be capable of providing adequate support to a block of rock such that the cuttability is adequately described by conventional methods. However, there are some rock mass conditions where support of blocks is not well developed, potentially resulting in otherwise unexpected poor TBM progress or jamming of TBM with loose blocks. Three-dimensional discontinuity patterns can be assessed using stereographic methods or borehole (α–β) methods. It is proposed that problematic conditions may occur where: two or more oblique (α between 20° and 70°) discontinuity sets are present (and over-represented relative to a uniform distribution); one or more of these discontinuity sets are dipping into the opening (β = 180° ± 90°) and additional discontinuities (in sets or randomly oriented) are present to form complete tetrahedral wedge blocks.     

General report,session I: Engineering properties of carbonate rocks

After reviewing the geological classification of carbonate rocks, a method of describing carbonate rocks for engineering purposes is developed following the recommendations set out by the I.A.E.G. Mapping Commission. Particular attention is paid to the details of the weathering profile developed on carbonate rocks. The engineering classification of carbonate rocks is based on both the engineering behaviour of the rock as a material and in the mass. Physical properties, including strength and deformation characteristics, are used to determine a modulus ratio for a wide range of limestones including the English chalk. In addition to rock material properties, and engineering classification of the rock mass has to take account of structure, discontinuities, and details of the weathering profile. Assessment of rock mass properties requiresin situ testing to determine deformation characteristics. If the usually limited amount of data on such quantitativein situ properties is related to other measures of rock quality, then engineering geological mapping can be used to extend the applicability of the limitedin situ data to a whole site. RQD has been related to the compressibility of the rock mass, and a rock mass factor has been proposed that links the deformability of the rock material to that of the rock mass. Rock mass classifications for large underground openings may be used with limestones. Engineering problems associated with carbonate rocks are briefly reviewed, as are the properties of limestones as a building stone and as aggregate.     

Experimental assessment of long-term durability of some weak rocks

Slake durability tests, point load strength tests and X-ray diffraction analyses were carried out on thirteen rock types in an attempt to correlate the durability of the rocks with their strengths and mineral compositions. A concept is proposed to describe the rock degradation characteristics from the results of slake durability test cycles. A new classification system is introduced for rock durability, which allows the prediction of the strength of a rock when it has been affected by the weathering process. The aim of the study was to predict the influence of the weathering process (simulated by wetting and drying and heating and cooling) on the durability and strength of the volcanic, metamorphic and sedimentary rocks outcropping in eastern Thailand. Field studies were carried out in an attempt to relate the heat absorption of the rocks in situ to that measured on the laboratory specimens.     

Latest progress of soft rock mechanics and engineering in China

The progress of soft rock mechanics and associated technology in China is basically accompanied by the development of mining engineering and the increasing disasters of large rock deformation during construction of underground engineering.In this regard,Chinese scholars proposed various concepts and classification methods for soft rocks in terms of engineering practices.The large deformation mechanism of engineering soft rocks is to be understood through numerous experiments;and thus a coupled support theory for soft rock roadways is established,followed by the development of a new support material,i.e.the constant resistance and large deformation bolt/anchor with negative Poisson’s ratio effect,and associated control technology.Field results show that large deformation problems related to numbers of engineering cases can be well addressed with this new technology,an effective way for similar soft rock deformation control.     

Applicability of the geological strength index (GSI) classification for very weak and sheared rock masses. The case of the Athens Schist Formation

 The Athens Schist Formation includes a wide variety of metasedimentary rocks, varying from strong or medium strong rocks such as sericite metasandstone, limestone, greywacke, sericite schist through to weak rocks such as metasiltstone, clayey and silty shale and phyllite. The overall rock mass is highly heterogeneous and anisotropic owing to the combined effect of advanced weathering and severe tectonic stressing that gave rise to intense folding and shearing followed by extensional faulting, which resulted in highly weathered rock masses and numerous shear and/or mylonite zones with distinct downgraded engineering properties. This paper is focused on the applicability of the GSI classification system to these highly heterogeneous rock masses and proposes an extension of the GSI system to account for the foliated or laminated weak rocks in the lower range of its applicability. Received: 5 March 1998 · Accepted: 13 July 1998     

Development of a new classification system for assessing of rock mass drillability index (RDi)

The drilling process and its results are affected by various parameters of the rock material and rock mass. The effects of rock material have been emphasized in various studies; however lack of perfect knowledge of rock mass structural parameters may lead to unpredictable results. This paper presents a new classification system for specifying the rock mass drillability index (RDi). For this purpose, six parameters of the rock mass, including texture and grain size, Mohs hardness, uniaxial compressive strength (UCS), joint spacing, joint filling (aperture) and joint dipping have been investigated by physical modeling and rated. Physical modeling in particular has been used for investigating the effects of joint characteristics on drilling rate. In the proposed RDi system, each rock mass is assigned a rating from 7 to 100, with a higher rating corresponding greater ease of drilling. Based on the RDi rating, the drilling rate may be classified into five modes: slow, slow-medium, medium, medium-fast, and fast.