Site investigation and rock mass characterization
- 1 Introduction
- 2 Intact Rock
- 3 Joints
- 4 Faults
- 5 Application of Collected Site and Rock Mass Characterization Data
- 5.1 The NGI Tunneling Quality Index (Q-Value)
- 5.2 Rock Mass Rating (RMR) System
- 5.3 Geological Strength Index (GSI)
- 6 References
The geological environment of a rock mass is important to understand when proposing to develop a mine because it indicates the type of rock behaviour that could be anticipated by disturbing the rock mass. This, however, can be quite complicated as a rock mass is made up of intact rock regions that are separated by discontinuities that affect rock stability, such as joints and faults . Therefore, this wiki helps determine: (1) how to identify these three characteristics on site, (2) how to determine their strength for rock mass stability analysis, and (3) how to use the collected on site information to classify the rock mass.
Identifying intact rock requires drill core specimens, which are then analyzed using the rock quality designation, RQD, index.
The Rock Quality Designation (RQD) Index
The RQD index was proposed by Deere in 1963 . This index introduces a technique that provides a quantitative estimate of the quality of a rock mass based of off drill cores . It measures the degree of fractures and jointing, thereby associating rock masses with a percentage . The RQD index also has an important influence on the RMR and Q values of rock mass classifications .
- Determining the RQD
When recording RQD measurements, any core size of 36.5 mm to 85 mm could be used, however the optimal core size to use would be 47.5 mm . Using the drill core, the RQD index is calculated using Equation 1 :
When core is unavailable, Equation 1 may not be, so in 1982, Palmström suggested that Equation 2 be used . Therefore, using visible discontinuities within the rock mass surface, the RQD may be calculated using the following equation, where Jv represents the sum of the joints per unit length for all joint sets :
The mean RQD value can then be used in the calculation of the Q-value . The RQD value will be a percentage between 0 and 100 . If you use a value of 0, then the RQD value will cause the Q-value to be 0, therefore when calculating the Q-value, all RQD values between 0 and 10 are increased to 10 . Several readings of the RQD should be taken along surfaces of different orientation, if possible, perpendicular to each other .
- Limitations of RQD Index
There are three main limitations to the RQD index: (1) it fails to provide information on core pieces less than 10cm in length, thereby giving the RQD index a value of 0, (2) it outputs incorrect values when joints contain thin clay fillings or weathered material, and (3) it does not account for joint orientation .
Intact rock strength can be estimated using in lab compression test, in lab point load tests, schmidt hammer tests and hardness tests . These tests can be ranked from lowest to highest in terms of accuracy and cost, with laboratory compression tests being the highest and hardness tests being the lowest :
- Laboratory Compression Tests
A laboratory compression test is the most costly for estimating intact rock strength properties, but typically the most accurate . It requires careful sample preparation and though it may not be used in the early stages of mine design, it is required for any design analysis after the pre-feasibility stage of the project . Uniaxial and triaxial compression tests are standard tests in practice and direct shear tests are more common for joint surfaces .
- Uniaxial and Triaxial Compression Tests
- Uniaxial and triaxial compression tests are standard tests in industry, in which uniaxial and triaxial loading is induced on a core sample . The core length to diameter ratio is typically 2.5:1, with a 2” diameter . The most important results uniaxial and triaxial compression tests yield are peak strength, Young’s modulus and Poisson’s ratio, as displayed in Figure 1.
- Point Load Test
Point load tests are not as accurate as uniaxial and triaxial compression tests, however they are inexpensive and quick to complete . Irregular core or rock sizes are able to be accommodated by the point load test . Many samples of rock or core should be tested due to scatter of results .
- Schmidt Hammer
The Schmidt hammer test is extremely fast and can be easily completed in the field . The test assess strength based on the rebound of a spring-loaded hammer . Because it is so easy to use, the Schmidt hammer test can quickly collect a lot of data with reasonable consistency . Figure 2 displays the chart used to convert Schmidt hammer readings to uniaxial compression strength, UCS .However, in order to use the chart, the density of the rock must be known, if it is unknown, then a good estimate is 2700 kg/m3 .
- Hardness Test
The hardness test is another test that can be easily and quickly executed in the field . Using Figure 3, core or outcrops are categorized by hardness with corresponding uniaxial compressive strength and point load index values . The strength categories correspond to the rock mass rating, RMR, system .
Joint Mapping Methods
Various methods are used to map joints, but some of the more popular methods are: (1) Survey Line Tape, (2) cell mapping, (3) borehole logging, and (4) laser imaging :
- (1) Survey Line Tape
- This method requires the use of a 20 m to 30 m survey line tape (or scanline) that is placed along an exposed face of rock mass (preferably a clean and planar rock face with the steepest dip) . The idea behind this technique is to measure all of the joint sets that intersect the scanline . Using this method, the local condition and orientation, trend, and plunge of the rock mass are identified . An example of this method is illustrated in Figure 4.
- (2) Cell Mapping
- This method is quite simple. In this method, mapping surfaces are divided into cells, where typically the width and height of the cells are equal . The fracture sets within the mapped surfaces are visually identified .
- (3) Borehole Logging
- The details of this technique can be found on the Rockmass Characterization using Geophysical Methods wiki page.
- (4) Laser Scanning
- This method has the potential to bring numerous benefits to the mining industry . Some of these benefits include: (1) improved efficiency of surveying large areas, (2) improved safety because less personnel are required, and (3) quality control and assurance of underground installations and project deviations . An example of such technology is a 3D point cloud imaging system . This is a system that uses mobile stereo and monocular cameras to obtain multiple images that create realistic models of the environment being analyzed .
Determining the Orientation of Mapped Joints
Various tools may be used to identify joint orientation, but some of the more common tools are: (1) Freiberg Compass, (2) Brunton Compass, and (3) Clino Ruler :
- (1) Freiberg Compass
- This compass measures strike directions, down dips and angles of pitch, fault interference areas, anticlinal axes, and dipping angles of areal and linear geological elements . Interestingly enough, the compass’ casing may be used to determine the inclination of a dip by placing it sideways on an exposed rock face .
- (2) Brunton Compass
- This compass measures orientation using strike and dip .It consists of three basic instruments: a compass, clinometer and hand level . Therefore, this compass can be used to measure magnetic bearing, vertical inclination of planes and it can be used for line surveying at a hand level .
- (3) Clino Ruler
- This ruler is popularly used underground . It is used to measure the dip and slope of angles, such as those between joints . An example of this site tool is illustrated in Figure 5.
Joints have a critical effect on stability; they are the weakest part of rock masses which has to be understood for correct mine design . Figure 6 displays the effect that discontinuities have on the strength of the rock. Characteristics of joints include dip/dip direction, spacing, persistence, roughness and aperature .
Factors influencing the strength of joints include roughness, strength of joint walls, type and strength of infilling material, confinement, and the base friction strength of the rock . Therefore, joint shear strength can be estimated using laboratory tests, field experiments or empirically based on the characteristics of the joints . Equation 5 is used, where JRC,JCS and Φr are the joint roughness coefficient, joint wall compressive strength and residual friction angle respectively .
The joint roughness coefficient is estimated using sample profiles . The joint wall compressive strength can be estimated using the Schmidt hammer test . The residual friction angle can be estimated using a tilt test, however it should not exceed about 50° and if there is not enough sufficient information to make an estimation then 30° is a reasonable estimate .
A fault is identified when the rock on both sides of a plane have moved relative to each other, whereas joints do not have displacement parallel to a plane . Therefore, faults are much larger discontinuities that occur at every scale within a rock mass, so they must also be considered when analyzing a rock mass .
Since faults show the same discontinuities as joints, they can be identified, or parts of the fault can be identified, within rock masses using the same techniques as mentioned above for joints.
There is no method for estimating, or even characterizing, fault strength .
Application of Collected Site and Rock Mass Characterization Data
By characterizing a rock mass, the quality of a rock mass may be defined using rock mass classification systems. The more commonly utilized rock mass classification systems are: (1) the rock mass rating, RMR, (2) the rock quality designation, RQD, (3)the index and the NGI tunneling quality index, Q, system, and (4) the geological strength index, GSI . Geological, geometric and design/engineering parameters are all incorporated into these systems in determining the quantitative values of rock mass qualities . However, each classification system varies in parameters, therefore, it is important to cross reference classification findings with at least one other system before commencing with a project .
The NGI Tunneling Quality Index (Q-Value)
Determining the Q-Value
Barton et al of the Norwegian Geotechnical Institute proposed this Q-value system in 1974 for determining rock mass characteristics and tunnel support requirements . It is calculated using 6 parameters, as represented in Equation 6 where, RQD,Jn , Jr, Ja, Jw and SRF represent the (1) rock quality designation, (2) joint set number, (3) joint roughness number, (4) joint alteration number, (5) joint water reduction factor, and (6) stress reduction factor respectively :
The individual parameters are determined during geological mapping or by core logging . When analyzing the three-paired expressions within the equation, they identify that the Q-value is a measure of: (1) block size, (2) inter-block shear strength, and (3) active stress . The Q-value should lie somewhere between 0.001 and 1000 . When this is not the case, use these values even if your values are higher or lower than these by extreme combinations of parameters .
Applications of the Q-Value
The Q-system is a classification system for jointed rock masses with respect to the stability of underground openings based on the estimation of six rock mass parameters . The Q-value is most precise when mapped in underground openings . The value depends on the geometry of the underground opening and is therefore not an independent characterization of the rock mass . The Q value gives a description of the rock mass quality, thereby relating to different types of permanent supports through the use of a schematic support chart . Therefore, one may use this system as a guideline in rock support design decisions and for documentation of rock mass quality . High values indicate a good stability and low values indicate a poor stability .
Limitations of the Q-Value
The Majority of the case histories used to derive the Q-system were from hard and jointed rocks . Therefore one must use caution when applying this system to weak rocks with few or no joints and should consider using other methods in addition to the Q system for support design . The system is empirical with regards to rock support making the rock support recommendations quite conservative .
Rock Mass Rating (RMR) System
Determining the RMR Value
The RMR system utilizes six parameters: (1) Rock Quality Designation (RQD), (2) spacing of discontinuities, (3) condition of discontinuities, (4) orientation of discontinuities, (5) groundwater conditions, and (6) uniaxial compressive strength of the rock mass .
When applying the RMR system, the rock mass is divided into a number of structural regions and each region is classified separately . The boundaries of the structural regions usually correspond with major structural features, such as a fault . Each parameter is then assigned a rating value (based off of rock mass analysis) so that by summing them all up, an RMR value will be produced that is represented by a value within the range of 0 to 100 .
Based on the RMR value, support systems may be put into place. The Ground Support wiki page provides a table that outlines the various excavation support requirements for specific RMR values.
Applications of the RMR System
The RMR system was originally proposed by Bieniawski in 1973 . However, overtime the system has evolved to being able to interpret the influence of various rock masses on rock stability . It is useful for providing details about rock mass discontinuities and producing a qualitative assessment of structural integrity for proposed excavations .
Limitations of the RMR System
There are several versions of the RMR system, some which date back as far as 1973, which means one must use caution when applying this rock mass classification system . The RMR system was originally based upon case histories drawn from civil engineering, which was seen as conservative within the mining industry . Therefore, several modifications were proposed in order to make the classification more relevant to mining applications, thereby creating the Modified Rock Mass Rating (MRMR) system . This MRMR system takes the basic RMR values and adjusts them to account for insitu and induced stresses, stress changes and the effects of blasting and weathering . Because of its relevance to mining, the MRMR recommends a set of supports that should be considered based on the resulting MRMR value . One must note that, many of the case histories upon which the MRMR was derived, were caving operations . Originally, block caving in asbestos mines in Africa formed the basis for the modifications but, subsequently, other case histories from around the world have been added to the database .
Geological Strength Index (GSI)
This topic has been covered on the Geological Model wiki page.
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