Most Used Rock Mass Classifications for Underground Opening Al-Jbori A’ssim and Zhang

Problem statement: Rock mass characterization is an integral part of r ock engineering practice. The empirical design methods based on roc k mass classifications systems provide quick assessments of the support requirements for undergr ound excavations at any stage of a project, even if the available geotechnical data are limited. The un derground excavation industry tends to lean on empirical approaches such as rock mass classificati on methods, which provide a rapid means of assessing rock mass quality and support requirement s. Approach: There were several classifications systems used in underground construction design. Th is study reviewed and summarized the must used classification methods in the mining and tunneling systems. Results: The method of this research was collected of the underground excavations classifica t ons method with its parameters calculations procedures for each one, trying to find the simples t, less costs and more efficient method. Conclusion: The study concluded with reference to errors that m y arise in particular conditions and the choice of rock mass classification depend o n the sensitivity of the projects, costs and the efficient.


INTRODUCTION
In underground excavation engineering, rock mass classification methods have always played an important role, particularly in predicting support requirements for excavations in rock. Based on experience in broadly similar ground conditions elsewhere in previous projects, these methods relate rock mass conditions to support requirements and construction procedures in new projects. In contrast the rational or theoretical approach to underground excavation design uses explicit models representing the behavior of rock masses developed based on the principles of the mechanics of materials. The application of this approach requires access to accurate information on the rock mass properties, groundwater conditions and in situ stress condition and is often time consuming and costly. While both approaches serve the same purpose, the classification methods are used when there is insufficient information to establish an explicit model or when time and cost limitations prevent the use of other models. This means in underground excavation engineering these are primarily found in two applications: • Before the commencement of construction when geological, geotechnical and construction data are limited, but time is not strictly limited. At this stage the main applications are for detailed planning and the design of initial support, determination of construction procedure and preliminary design of final support • During construction when detailed information on the rock mass can be readily obtained by observations or simple tests, but time is limited due to contractual obligations and project completion deadlines. The main applications at this stage are for the determination and adaptation of initial support details, determination or confirmation of construction procedure and detailed design of the final support In order to be efficient and reliable at both stages of these applications, as noted by Einstein et al. (1979), a rock mass classification method should: • Be easily applicable and robust • Use easily determinable input parameters • Accurately represent rock mass behavior • Avoid subjectivity • Ensure safety and economy

MATERIALS AND METHODS
Methods that have been used in designing support for underground opening in rock: Rock Quality Designation (RQD): The RQD is a core recovery percentage that is indirectly based on the number of fractures and the amount of softening in the rock mass that is observed from the drill cores. Only the intact pieces with a length longer than 100 mm (4 in.) are summed and divided by the total length of the core run: langth of core piesces 10 cm RQD 100(%) Totalcorelength It is used as a standard parameter in drill core logging and its greatest value is perhaps its simplicity and quick determination and also that it is inexpensive. RQD is to be seen as an index of rock quality where problematic rock that is highly weathered, soft, fractured, sheared and jointed is counted in complement to the rock mass (Deere and Deere, 1988).

Direct method (core logs available):
The procedure for measuring RQD directly is illustrated in Fig. 1. The recommended procedure of measuring the core length is to measure it along the centerline. Core breaks caused by the drilling process should be fitted together and counted as one piece. All the artificial fractures should be ignored while counting the core length for RQD, even if they pass the requisite 100 mm length.  (Deere and Deere, 1988)  For RQD determination, the International Society for Rock Mechanics (ISRM) recommends a core size of at least 54.7 mm. According to Deere and Deere (1988), the recommended run length for calculating RQD is based on the actual drilling-run length used in the field, preferably no greater than 1.5 m. The ISRM Commission on Standardization of Laboratory and Field Tests recommends RQD-calculations using variable "run lengths" to separate individual beds, structural domains and weakness zones, so as to indicate any inherent variability and provide a more accurate picture of the location and width of zones with low RQD values. The relationship between the numerical value of RQD and the engineering quality of the rock mass is given in Table 1.
Indirect method (no core logs are available): In situ estimations of RQD was in 1973 suggested to be carried out using the following equation (Afrouz, 1992): where, D v is the total number of discontinuities per cubic meter of rock mass. The plane of discontinuities is not perpendicular to the direction of maximum principal stress. The constants A, B, x, y are related to the above noted factors in such a way that A x is 105-120 and By is 2-12. Priest and Hudson found that an estimate of RQD could be obtained from joint spacing (λ joints/m) measurements made on an exposure by using (Brady and Brown, 2004): Though RQD is dependent on the borehole orientation. In principle, it is based on the measurement of the angle between each joint and the surface or the drill hole. The weighted Joint density (wJd) is for measurements on rock surfaces: Rock Structure Rating (RSR): The Rock Structure Rating (RSR) introduced numerical ratings of the rock mass properties and was a precursor to the two most used classification systems today (the RMR and the Qsystem). The RSR value is a numerical value in the interval of 0-100 and is the sum of weighted numerical values determined by three parameters. The three parameters are called A, B and C. Parameter A is said to combine the generic rock type with an index value for rock strength along with the general type of structure in the studied rock mass. Parameter B relates the joint pattern with respect to the direction of drive. Parameter C considers the overall rock quality with respect to parameters A and B and also the degree of joint weathering and alteration and the amount of water inflow. The US Bureau of Mines (Skinner, 1988) developed the RSR system further and selected six possible factors as being the most essential for prediction of the support requirements. By using only six factors they tried to make a method that is simple and easy to use. The six factors are: Higher RSR value requires less support under normal tunneling conditions.

Rock Mass Rating (RMR):
The reasons for using RMR are, according to Bieniawski (1989), the ease of use and the versatility in engineering practice. It should be observed that the RMR-system is calibrated using experiences from coalmines, civil engineering excavations and tunnels at shallow depths. The RMR system uses the following six parameters, whose ratings are added to obtain a total RMR-value: • Uniaxial compressive strength of intact rock material • Rock Quality Designation (RQD) • Joint or discontinuity spacing • Joint condition • Ground water condition • Joint orientation Each of these parameters is given a rating that symbolizes the rock quality description All the ratings are algebraically summarized for the five first given parameters and can be adjusted depending on the joint and tunnel orientation by the sixth parameter as shown in the following equations:

RMR
= RMR basic + adjustment for joint orientation RMR basic = ∑parameters (i + ii + iii + iv + v) The final RMR values are grouped into five rock mass classes, where the rock mass classes are in groups of twenty ratings each Table 2.   (Bieniawski, 1978) Parameter/properties of rock mass Rock mass rating (rock class) The rock mass Quality (Q)-system: Barton (1988) first introduced the rock tunneling Quality Index, the Qsystem in 1974. The original Q-system (Barton et al., 1974) uses the following six parameters: RQD, Number of joint sets, Joint roughness, Joint alteration, Joint water conditions and Stress factor. The fundamental geotechnical parameters are, according to Barton (1988), block size, minimum inter-block shear strength and active stress. These fundamental geotechnical parameters are represented by the following ratios (Barton, 2002): The rock mass quality is defined as (Barton et al., 1974): Where: RQD = Deere's Rock Quality Designation ≥10 J n = Joint set number J r = Joint roughness number (of least favorable discontinuity or joint set) J a = Joint alteration number (of least favorable discontinuity or joint set) J w = Joint water and pressure reduction factor SRF = Stress reduction factor-rating for faulting, strength/stress ratios in hard massive rocks and squeezing and swelling rock Use of the Q-system is specifically recommended for tunnels and caverns with an arched roof. The rock mass has been classified into nine categories based on the Q value, as can be seen in Table 3. The range of Q values varies between 0.001 and 1000.
To relate the tunneling Quality index (Q) to the behavior and support requirements of an underground excavation a term called the equivalent Dimension (D c ) was defined: c Exca var ion span,diameter or heighr (m) D Excavation support ratio = The Excavation Support Ratio (ESR) was determined from investigations of the relation between existing maximum unsupported excavation span (SPAN) and Q around an excavation standing up for more than 10 years. The following relationship was defined: Barton et al. (1976) gives suggested values for ESR according to Table 4. The Q-system has been modified due to changes in the stress reduction factor (Grimstad and Barton, 1993) and they presented an updated Q-support chart for the new supporting methods, Fig. 3.

Mining Rock Mass Rating (MRMR):
The MRMRsystem takes into account the same parameters as the basic RMR-value. The MRMR is determined by the rating of intact rock strength, RQD, joint spacing and joint condition. The range of MRMR lies, as the RMRsystem, between zero and 100, values that are stated to cover all variations in jointed rock masses from very poor to very good. The rating system is divided into five classes and ten sub-classes. The five classes rates between 0-20 points and the subclasses with a 10-point rating. Laubscher (1984) presented a relation between MRMR and the in situ rock mass strength as: The Unified Rock Classification System (URCS) dates from 1975 and is used by the US Forest Service for design of forest access roads (Williamson, 1984). The URCS consists of four fundamental physical properties: • Degree of weathering • Estimated strength • Discontinuities or directional weaknesses • Unit weight or density    (Grimstad and Barton, 1993)   Each of these four properties consists of five categories ranging from A through E, which represents the design limiting conditions of each of the basic elements of the system. Rock material designated AAAA will require the least design evaluation while EEEE will require the most. The URCS is used for making rapid initial assessments of rock conditions, using simple field equipment and relate those to design. According to Williamson (1984): "The equipment used for the field tests and observations is simple and available: One's fingers, a 10 power hand lens, a 1pound (0.5 kg) ball peen hammer, a spring-loaded "fish" scale of the 10-pound (5 kg) range and a bucket of water. Fingers are used in determining the degree of weathering and the lower range of strength. The hand lens is used in defining the degree of weathering. The ball peen hammer is used to estimate the range of unconfined compressive strength from impact reaction. The spring-loaded scale and bucket of water are used to measure the weight of samples for determining apparent specific gravity. "According to Williamson (1984), the density or unit weight is one of the most useful and reliable parameter for determining rock quality.

Basic Geotechnical Description (BGD): A Basic
Geotechnical Description of Rock Masses (BGD) was established in 1981 by ISRM. The intent was to characterize the various zones that constitute a rock mass, in a simplified form. The rock mass should be divided into geotechnical units and zones before applying the BGD. The representative BGD-value for each zone is determined by: • The rock name, with a simplified geological description such as geologic structure, color, texture and degree of weathering • Two structural characteristics of the rock mass: The layer thickness and fracture intercept, Table 5 and 6 • Two mechanical characteristics; the uniaxial compressive strength of the rock material and the angle of joint friction, Table 7 and 8 This classification of BGD results in that each zone is characterized by its rock name followed by the interval symbols    (Stille et al., 1982) 1 or 2 joint sets   Stille et al. (1982), is a modification of the RMR-system, as it includes the first five parameters of RMRbasic. The loading conditions and initial stress field are not considered which means that the RMS is a strength classification. In addition to the RMRbasic value, every combination of three different types of joint sets and two different types of joints is rated as can be seen in Table 9.
The sum of the RMRbasic and the rating reduction, due to the number of joint sets, is the RMS-value for the rock mass. Using the RMS-value, the rock mass strength can be estimated according to Table 10.

Modified Basic RMR system (MBR):
The Modified Basic RMR system (MBR) is a modified RMR for mining applications and therefore uses many of the same input parameters. The data base values of MBR range from 20 to almost 70. The studied depths varied from about 213 m to over 610 m (Kendorski et al., 1983). The final mining, FMBR, which is used to obtain permanent drift support recommendations, can be expressed as: Where: DC = The adjustment rating for the distance to cave line PS = The block/panel size adjustment S = The adjustment for orientation of major structures, dependent on their width, dip and distance and the Adjusted MBR (AMBR) is expressed by: where, the Modified Basic RMR (MBR) is dependent on the strength of intact rock, discontinuity density (RQD, spacing), discontinuity condition and groundwater condition, A B is the adjustment due to used blasting method and its blasting damage, A S is the induced stress adjustment and A O is the adjustment for fracture orientation.
Simplified Rock Mass Rating (SRMR) system for mine tunnel support Brook and Dharmaratne (1985): Since Brook and Dharmaratne (1985) thought that the joint spacing ratios were mysteriously obtained in the MRMR system and since they preferred a simplified system that does not need the RQD-value, the SRMR was developed. The simplified rock mass rating has three major components the intact rock strength, joint spacing and joint type. The final rating is based on the three major components, together with groundwater consideration, (Table 11).

RESULTE AND DISCUSSION
Ramamurthy and Arora classification: Ramamurthy and Arora (1993) suggested a classification for intact rock and jointed rocks based on their compressive strengths and modulus values in unconfined state. This classification is based on the modulus ratio (M rj ) of a linear stress-strain condition, which is stated as: where, subscript j refers to jointed rock. E t is the tangent modulus at 50% of the failure stress.   To estimate the rock strength and modulus ratio one has to determine the joint factor. The joint factor represents a factor of weakness in the rock mass due to the influence of the joint systems. This resulted in the following: σ cj is the jointed rock strength, whose description is stated in Table 12. Since the σ cj and E tj are known, the modulus ratio can be estimated and classified according to Table 13.
The rock (mass) is described by two letters, for instance CD means that the rock has moderate compressive strength in the range of 50-100 MPa, with a low modulus ratio of 50-100.

Geological Strength Index (GSI):
This GSI estimates the reduction in rock mass strength for different geological conditions. There are three ways of calculating the GSI: • By using the rock mass rating for better quality rock masses (GSI>25) • By using the Q-system • By using their own GSI-classification

Rock mass Number (N) and Rock Condition Rating (RCR):
The rock mass number, N and rock condition rating, RCR, are modified versions of the Q-system and RMR-system. The N-system is a stress-free Qsystem, as can be seen by its definition: The RCR and the N-system were proposed to find a relation between the Q-system and the RMR-system. The Q-system and the RMR system are equivalent if the joint orientation and intact rock strength are ignored in the RMR-system and the stress reduction factor is ignored in the Q-system.

Rock Mass index (RMi):
The Rock Mass index, RMi, has been developed to characterize the strength of the rock mass for construction purposes. The main focus of the development of RMi was on the effects of the defects in a rock mass that reduce the strength of the intact rock. The RMi is linked to the material and represents only the inherent properties of a rock mass. The input parameters in a general strength characterization of a rock mass are selected as: • The size of the blocks delineated by jointsmeasured as block volume • The strength of the block material-as uniaxial compressive strength • The shear strength of the block faces-measured as friction angle • The size and termination of the joints-measured as length and continuity The RMi is principally the reduced rock strength caused by jointing and is expressed as: where, JP is the jointing parameter, which is a reduction factor representing the block size and the condition of its faces as represented by their friction properties and the size of the joints. The value of JP varies from almost 0 for crushed rocks to 1 for intact rock. Its value is found by combining the block size and the joint conditions. An overview of the parameters applied in RMi can be seen in Fig. 4.
The joint condition factor jC represents the interblock frictional properties and is expressed as: jR js.jw jC jL. jL jA jA where, jL is the size factor representing the influence of the size and termination of the joint. The joint size factor (jL) is chosen as larger joints have a markedly stronger impact on the behavior of a rock mass than smaller joints have. The roughness factor (jR) represents the unevenness of the joint surface which consists of: • The smoothness (js) of the joint surface • The waviness (jw) or planarity of the joint wall The alteration factor (jA) expresses the characteristics of the joint: • The strength of the rock wall • The thickness and strength of a possible filling The factors jR and jA are similar to Jr and Ja in the Q-system. JP is given by the following expression:

CONCLUSION
Rock mass classification is one of the only approaches for estimating large-scale rock mass properties. In the underground industry the classifications systems forms the basis of many empirical design methods, as well as the basis of failure criteria used in many numerical modeling programs. Practitioners should be aware that classification and design systems are evolving and that old versions of classification systems are not always compatible with new design approaches. Care must be taken when using classification systems with empirical design methods. The user must be sure that the classification system used matches the approach taken for the development of the empirical design method. Under these circumstances, for purposes of continuity, it is sometimes necessary to continue using an earlier version. Design methods which do not rely on case histories or past experience, do not have the same constraints. This approach to classification is warranted in complex mining situations. Serious errors can result if these simplified classification systems are applied to the empirical civil tunnel design approaches such as the Q support graph. Despite their limitations, the reviewed classification systems are still in use as they provide an invaluable reference to past experience.