Difference between revisions of "Site investigation and rock mass characterization"

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[[File:wikiwiki.png|thumb|right|middle|900px|'''Figure 1'''<ref name="kinnon" />]]
 
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=='''Introduction'''==
 
=='''Introduction'''==
There are three aspects to a mine design process: data collection and rock mass characterization, analysis and design <ref name="kinnon" />. Through data collection and rock mass characterization, the geological environment of the rock mass is identified so that the proper analysis techniques may be utilized to design the mine <ref name="kinnon" />. The geological environment of a rock mass is therefore key to understand as 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 with various characteristics <ref name="kinnon" />. Therefore, an important aspect to underground mine design is ensuring the stability of rock masses <ref name="kinnon" />. To determine stability, the strength of the different rock mass components must be identified <ref name="kinnon" />. The major components of a rock mass that affect mine stability are: intact rock, joints and faults <ref name="kinnon" />. Through site investigation and rock mass characterization, numbers can be assigned to “measurements” of the geological environment that are later used to carry out design calculations <ref name="kinnon" />.
 
  +
It is important to conduct a site investigation of the geological environment in which a mine may be developed so that its conditions may be predicted <ref name="niosh" />. 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 <ref name="kinnon" />. 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.
  +
  +
   
 
=='''Intact Rock'''==
 
=='''Intact Rock'''==
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====The Rock Quality Designation (RQD) Index====
 
====The Rock Quality Designation (RQD) Index====
   
The RQD index was created to quantitatively assess the integrity of a rock mass based solely upon the degree of intactness and physical dimensions of recovered drill core specimens <ref name="archibald" />. The index may also be used to help select appropriate rock support for the measured rock type, which is based upon historical data <ref name="archibald" />. It also has an important influence on the RMR and Q values of rock mass classifications <ref name="science" />. The RQD index can be assessed in the field at the time of the initial core recovery, during the early stages of core logging in the field or at any later time <ref name="archibald" />.
 
  +
The RQD index was proposed by Deere in 1963 <ref name="lucian" />. This index introduces a technique that provides a quantitative estimate of the quality of a rock mass based of off drill cores <ref name="lucian" />. It measures the degree of fractures and jointing, thereby associating rock masses with a percentage <ref name="lucian" />. The RQD index also has an important influence on the RMR and Q values of rock mass classifications <ref name="science" />.
   
   
 
;Determining the RQD
 
;Determining the RQD
The RQD value is the percentage of core intact pieces of length greater or equal to 100 mm of the total length of core recovered <ref name="archibald" />. The core that is to be evaluated is suggested to be at least 54 mm in diameter and at least 2.0 m in length <ref name="archibald" />. Based upon core observations, the RQD index is calculated using the following equation <ref name="archibald" />:
+
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 <ref name="deere" />. Using the drill core, the RQD index is calculated using [[:File:eq1.png|Equation 1]] <ref name="deere" />:
   
[[File:wiki112.png|centre]]
+
[[File:eq1.png|centre]]
   
Based on the final RQD value, the structural integrity or engineering quality of the rock is determined using the following table <ref name="archibald" />:
+
Based on the RQD value of a drill core, the quality of the rock can be determined using the [[:File:rock mass quality.png|following table]]<ref name="lucian" />:
   
[[File:wiki113.png|centre]]
+
[[File:rock mass quality.png|centre]]
   
The proper technique for carrying out an RQD measurement is illustrated in [[:File:wikiagain.png|Figure 2.]]
 
  +
When core is unavailable, Equation 1 may not be, so in 1982, Palmström suggested that [[:File:eq2.png|Equation 2]] be used <ref name="science" />. 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 <ref name="science" />:
   
[[File:wikiagain.png|centre|frame|'''Figure 2:'''Technique for determining the RQD value of a core <ref name="archibald" />.|300x300px]]
 
  +
[[File:eq2.png|centre]]
   
When no core is available, but discontinuity traces are visible in surface exposures or exploration adits, the RQD may be estimated using the volumetric joint count <ref name="archibald" /><ref name="science" />:
 
  +
It was found that there is a linear relationship between the RQD and the joint frequency, ''&lambda;'', therefore it may be estimated using [[:File:eq3.png|Equation 3]] when ''&lambda;'' is within the range of 6 to 16 <ref name="hud" />:
   
[[File:wiki115.png|centre]]
+
[[File:eq3.png|centre]]
   
If the mean spacing is known between joints, then the RQD may also be determined using the following equation, where ''t'' represented the threshold value or the smallest mapping space <ref name="mc" />:
+
To address the lack of sensitivity for large spacing values between joints, [[:File:eq4.png|Equation 4]] may be used, where a threshold value, ''t '', is considered <ref name="hud" />:
   
[[File:wiki116.png|centre]]
+
[[File:eq4.png|centre]]
 
If a minimum spacing value of 0.1m is taken, then the relationship may be simplified to <ref name="mc" />:
 
 
[[File:wiki117.png|centre]]
 
   
 
The mean RQD value can then be used in the calculation of the Q-value <ref name="norw" />. The RQD value will be a percentage between 0 and 100 <ref name="norw" />. 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 <ref name="norw" />. Several readings of the RQD should be taken along surfaces of different orientation, if possible, perpendicular to each other <ref name="norw" />.
 
The mean RQD value can then be used in the calculation of the Q-value <ref name="norw" />. The RQD value will be a percentage between 0 and 100 <ref name="norw" />. 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 <ref name="norw" />. Several readings of the RQD should be taken along surfaces of different orientation, if possible, perpendicular to each other <ref name="norw" />.
Line 40: Line 42:
   
 
;Limitations of RQD Index
 
;Limitations of RQD Index
The RQD index does not give any information on core pieces less than 10cm in length, thereby giving the RQD index a value of 0 <ref name="lucian" />. It also gives incorrect values when joints contain thin clay fillings or weathered material <ref name="lucian" />.
 
  +
  +
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 <ref name="lucian" />.
  +
   
 
===<u>Strength Measurement</u>===
 
===<u>Strength Measurement</u>===
  +
[[File:becc1.png|right|thumb|'''Figure 1:''' Uniaxial compressive strength laboratory test <ref name="sp" />.]]
  +
Rock mass strength is extremely important for mine design in order to correctly understand stresses and modes of failure in a rock mass <ref name="ed"/>. Knowledge of these stresses increases safety in underground mines by correctly designing to conditions and can also reduce operating costs <ref name="ed"/>. Joints are often found in rock masses and therefore it is important to determine their effect on the intact rock strength in order to correctly identify stress conditions. The strength of intact rock can be determined through suggested methods set by the International Society of Rock Mechanics including: (1) laboratory uniaxial compressive strength tests, (2) the point load strength test, and (3) the Schmidt hammer test <ref name="aj"/>.
   
Intact rock strength can be estimated using in lab compression test, in lab point load tests, schmidt hammer tests and hardness tests <ref name="kinnon" />. 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 <ref name="kinnon" />:
 
[[File:wiki7.png|centre]]
 
[[File:wiki8.png|thumb|left|frame|'''Figure 3:'''Uniaxial compression test results <ref name="kinnon" />.]]
 
;Laboratory Compression Tests
 
A laboratory compression test is the most costly for estimating intact rock strength properties, but typically the most accurate <ref name="kinnon" />. 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 <ref name="kinnon" />. Uniaxial and triaxial compression tests are standard tests in practice and direct shear tests are more common for joint surfaces <ref name="kinnon" />.
 
[[File:wiki110.png|thumb|'''Figure 4:'''Schmidt hammer UCS conversion chart <ref name="kinnon" />.]]
 
:::'''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 <ref name="kinnon" />. The core length to diameter ratio is typically 2.5:1, with a 2” diameter <ref name="kinnon" />. The most important results uniaxial and triaxial compression tests yield are peak strength, Young’s modulus and Poisson’s ratio, as displayed in [[:File:wiki8.png|Figure 3.]]
 
   
;Point Load Test
 
  +
====Uniaxial Compressive Strength Test ====
Point load tests are not as accurate as uniaxial and triaxial compression tests, however they are inexpensive and quick to complete <ref name="kinnon" />. Irregular core or rock sizes are able to be accommodated by the point load test <ref name="kinnon" />. Many samples of rock or core should be tested due to scatter of results <ref name="kinnon" />.
 
  +
The uniaxial compressive strength test is relatively simple in procedure, however has expensive costs corresponding to laboratory use and specific sample preparation <ref name="aj"/>. Samples, often drill cores, are carefully cut into specific dimensions and loaded axially until failure occurs<ref name="sp"/>. [[:File:becc1.png|Figure 1]] displays the typical set up of the laboratory experiment <ref name="sp"/>. The uniaxial compressive strength test is quite accurate, though it is recommended by the American Society for Testing and Materials to test at least ten core samples <ref name="aj"/>.
   
  +
[[File:becc2.png|left|thumb|'''Figure 2:''' Point load strength test apparatus <ref name="cm" />.]]
   
   
;Schmidt Hammer
 
  +
====Point Load Test ====
The Schmidt hammer test is extremely fast and can be easily completed in the field <ref name="kinnon" />. The test assess strength based on the rebound of a spring-loaded hammer <ref name="kinnon" />. Because it is so easy to use, the Schmidt hammer test can quickly collect a lot of data with reasonable consistency <ref name="kinnon" />. [[:File:wiki110.png|Figure 4]] displays the chart used to convert Schmidt hammer readings to uniaxial compression strength, UCS <ref name="kinnon" />.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 <ref name="kinnon" />.
 
  +
The point load test serves as a more economical option to determine the uniaxial compressive strength of rock compared to the uniaxial compressive strength test <ref name="cm"/>. The test procedure consists of loading a drill core sample across its diameter, displayed in [[:File:becc2.png|Figure 2]] <ref name="aj"/>. The point load strength index (Is) can be calculated using [[:File:bim1.png|Equation 5]], where P and d represent the applied load and distance between the loading points respectively <ref name="aj"/>:
   
;Hardness Test
 
  +
[[File:bim1.png|centre]]
The hardness test is another test that can be easily and quickly executed in the field <ref name="kinnon" />. Using [[:File:hardnesstest.png|Figure 5,]] core or outcrops are categorized by hardness with corresponding uniaxial compressive strength and point load index values <ref name="hoek" />. The strength categories correspond to the rock mass rating, RMR, system <ref name="kinnon" />.
 
[[File:hardnesstest.png|thumb|centre|frame|'''Figure 5:'''Hardness test <ref name="hoek" />.|300x300px]]
 
   
=='''Joints'''==
 
   
===<u>Site Investigation</u>===
 
   
====Joint Mapping Methods====
 
   
Various methods are used to map joints. Some of the more popular methods are <ref name="kinnon" />:
 
  +
[[:File:bim2.png|Equation 6]] serves for drill core samples that are 50mm in diameter. If the sample is not this size, it must be corrected in order to obtain a correct strength determination <ref name="aj"/>. For hard rocks, it was found in studies by Broch and Franklin (1972) that the UCS of the rock could be expressed using [[:File:bim2.png|Equation 6]] (the point load strength index), where K represents the conversion factor <ref name="cm"/>:
 
  +
[[File:bim2.png|centre]]
   
;:Scanlines
 
  +
It should be noted while a K value of 24 is typically universally accepted, studies have shown variation in conversion factors for different rock types, ranging from 16 to 24 <ref name="cm"/>.
: This is the most common method <ref name="kinnon" />. It consists of stretching a surveyors tape (typically 30m in length) in a straight line along an exposed rock face <ref name="kinnon" />. All the discontinuities intersecting the tape should be noted and the projection of where they intersect the tape should be recorded <ref name="kinnon" />. It is important to distinguishing blast-induced fractures from joints as they should be excluded from your data collection <ref name="kinnon" />.
 
   
[[File:wiki3.png|thumb|'''Figure 6:'''Example of onsite scanline measurements <ref name="kinnon" />.]]
+
[[File:bec1.png|left|thumb|'''Figure 3:'''Schmidt Hammer<ref name="ulu" />.]]
   
;:Cell Mapping
 
  +
====Schmidt Hammer Method====
: 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 <ref name="sme" />. The fracture sets within the mapped surfaces are visually identified <ref name="sme" />.
 
  +
The Schmidt Hammer method is an easy field test to estimate the UCS of intact rock. It utilizes a spring loaded plunger that rebounds off of a surface for a reading, which is then converted to a compressive strength using Miller’s (1965) method <ref name="bar"/>. Schmidt hammer models come in types L and N; L models apply 0.735 Nm while type N applies 0.245 Nm <ref name="tor"/>. [[:File:bec2.png|Figure 4]] can be used to convert the reading from the Schmidt Hammer test to UCS.
   
;:Core Logging
 
  +
[[File:bec2.png|thumb|'''Figure 4:'''Schmidt Hammer UCS Conversion Chart <ref name="hek" />.|300x300px]]
: Sometimes information about the rock mass may only be acquired from core logging <ref name="kinnon" />. However, determining joint orientation from the core is difficult and time consuming, therefore it is seldom carried out <ref name="kinnon" />.
 
   
;:Laser Imaging
 
  +
This method is acceptable for use, however, a better correlation was found by Miller using [[:File:bim7.png|Equation 7]], in which the dry density of the rock is multiplied by the reading from the test <ref name="bar"/>. "&sigma;c", "Y", and "R" represent the uniaxial compressive strength of the rock, dry density of the rock (kN/m2), and rebound number respectively.
: Laser image acquisition allows for vast amounts of data to be quickly collected, however appropriate analysis software has not yet been developed in order to fully realize the advantage of this technology <ref name="kinnon" />. Therefore, it is recommended that the manual and laser image acquired joint measurements be compared, thereby determining the validity of the new technology for its next uses <ref name="kinnon" />. An example of such technology is 3D point cloud imaging <ref name="kinnon" />. This software allows for the scanned inage to be fully rotated <ref name="kinnon" />. This is possible due to the millions of points in space on the X, Y, and Z planes that make up the point cloud and the intensity they associate with each point <ref name="kinnon" />.
 
  +
[[File:bim7.png|centre]]
   
;:Borehole Logging
 
  +
This test can also be used to test the strength of joints if the test is done on an open joint face.
: This is a common technique to use onsite. The details of this technique can be found on the [http://minewiki.engineering.queensu.ca/mediawiki/index.php/Rock_mass_characterization_using_geophysical_methods ''Rockmass Characterization using Geophysical Methods'' wiki page.]
 
  +
Due to the manual nature of the test, there are certain details of the testing that should be made aware for accurate results.
  +
In terms of orientation, the hammer should always be perpendicular to the surface of the rock or joint face <ref name="ulu"/>. If it is not perpendicular, the tip of the plunger could slide or chip the material and result in an incorrect reading <ref name="ulu"/>. Additionally, the reading will be at a minimum when the hammer is used vertically downwards and at a maximum when used upwards due to gravity <ref name="bar"/>.
  +
If the test is being performed on a sample rock only (i.e. core or rock that can be easily moved), it is important to anchor the sample so the hammer does not displace the rock when the test is performed <ref name="bar"/>. Displacement of the rock will not yield a correct reading <ref name="bar"/>.
   
  +
====Additional Rock Mass Strength Techniques====
  +
Seismic techniques have also been tested in Canadian mines to suggest characterization of rock mass behaviour <ref name="tuc"/>. Low seismological techniques that include the use of microseismic waveforms to experiment the calculation of stress orientations and changes of stress in rock <ref name="tuc"/>. Studies in Canadian mines have shown that seismic techniques can aid in defining principal stresses and orientations as well as local stress influences around failures <ref name="tuc"/>.
   
====Determining the Orientation of Mapped Joints====
 
   
Joint orientation is also measured at this time, usually via dip and direction, or less frequently, via strike and dip <ref name="kinnon" />.The orientation may be taken using various tools <ref name="kinnon" />.The following are some of the more common tools <ref name="kinnon" />:
 
  +
=='''Joints'''==
   
[[File:wiki4.png|thumb|'''Figure 7:''' Example of a clino ruler <ref name="kinnon" />.]]
 
  +
===<u>Site Investigation</u>===
;:Freiberg Compass
 
: This compass was designed to measure orientation using dip and dip direction <ref name="kinnon" />. The lid of the instrument tilts to fit along the dip of the joint surface so that the body of the compass does not interfere with the registration <ref name="kinnon" />. It also has spirit levels built into it to ensure that the body of the compass is held horizontally when taking the measurements <ref name="kinnon" />.
 
   
;:Brunton Compass
 
: This compass measures orientation using strike and dip <ref name="robert" />.It consists of three basic instruments: a compass, clinometer and hand level <ref name="robert" />. 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 <ref name="robert" />.
 
   
;:Clino Ruler
 
: In some locations, a compass may not be viable as there may be magnetic interference <ref name="kinnon" />. Therefore, this ruler was designed to measure the angle between two sections on the face of the rock mass <ref name="kinnon" />. However, the orientation of the strike must always be measured relative to some known orientation, such as the strike of the drift <ref name="kinnon" />. This device also allows for easy dip measurements <ref name="kinnon" />. An example of this site tool is illustrated in the [[Media:File:wiki4.png|figure]].
 
   
   
   
====Data Collection Method====
+
====Joint Mapping Methods====
   
All of the information that had been collected on site must be recorded in order to determine information, such as rock mass classification, for future design references <ref name="kinnon" />. Therefore, joint mapping sheets are provided by every company to document certain joint characteristics, such as the orientation, length and spacing <ref name="kinnon" />. Every company provides their own mapping sheets because there is no such thing as a single industry standard mapping sheet <ref name="kinnon" />. [[:File:wiki5.png|Figure 8]] and [[:File:wiki6.png|Figure 9]] illustrate what a joint mapping sheet may look like, as shown, a legend of the abbreviations used within the mapping sheet must be included <ref name="kinnon" />.
 
  +
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 <ref name="kinnon" />:
  +
  +
[[File:im1.png|thumb|'''Figure 5:'''Example of onsite scanline measurements <ref name="nor" />.|300x300px]]
   
<gallery widths=550px caption="Figure 8 and Figure 9:Example of a Joint Mapping Sheet">
 
  +
;:(1) Survey Line Tape
File:wiki5.png
 
  +
: 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) <ref name="nor" />. The idea behind this technique is to measure all of the joint sets that intersect the scanline <ref name="nor" />. Using this method, the local condition and orientation, trend, and plunge of the rock mass are identified <ref name="nor" />. An example of this method is illustrated in [[:File:im2.png|Figure 5]].
File:wiki6.png
 
</gallery>
 
   
  +
;:(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 <ref name="sme" />. The fracture sets within the mapped surfaces are visually identified <ref name="sme" />.
   
[[File:wiki1.png|right|frame|'''Figure 10:'''Effect of multiple discontinuities on rock masses <ref name="kinnon" />.|200x200px]]
 
  +
;:(3) Borehole Logging
===<u>Strength Measurement</u>===
 
  +
: The details of this technique can be found on the [http://minewiki.engineering.queensu.ca/mediawiki/index.php/Rock_mass_characterization_using_geophysical_methods ''Rockmass Characterization using Geophysical Methods'' wiki page.]
   
Joints have a critical effect on stability; they are the weakest part of rock masses which has to be understood for correct mine design <ref name="kinnon" />. [[:File:wiki1.png|Figure 10]] displays the effect that discontinuities have on the strength of the rock. Characteristics of joints include dip/dip direction, spacing, persistence, roughness and aperature <ref name="kinnon" />.
 
  +
;:(4) Laser Scanning
  +
: This method has the potential to bring numerous benefits to the mining industry <ref name="afr" />. 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 <ref name="afr" />. An example of such technology is a 3D point cloud imaging system <ref name="jak" />. This is a system that uses mobile stereo and monocular cameras to obtain multiple images that create realistic models of the environment being analyzed <ref name="jak" />.
   
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 <ref name="kinnon" />. Therefore, joint shear strength can be estimated using laboratory tests, field experiments or empirically based on the characteristics of the joints <ref name="kinnon" />. The following equation is used, where ''JRC'',''JCS'' and ''&Phi;r'' are the joint roughness coefficient, joint wall compressive strength and residual friction angle respectively <ref name="kinnon" />.
 
   
[[File:wiki2.png|centre]]
 
   
The joint roughness coefficient is estimated using sample profiles <ref name="kinnon" />. The joint wall compressive strength can be estimated using the Schmidt hammer test <ref name="kinnon" />. The residual friction angle can be estimated using a tilt test, however it should not exceed about 50&deg; and if there is not enough sufficient information to make an estimation then 30&deg; is a reasonable estimate <ref name="kinnon" />.
 
  +
====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 <ref name="kinnon" />:
   
  +
[[File:im2.png|left|thumb|'''Figure 6:''' Example of a clino ruler <ref name="min" />.|200x200px]]
   
=='''Faults'''==
 
  +
;:(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 <ref name="fpm" />. Interestingly enough, the compass’ casing may be used to determine the inclination of a dip by placing it sideways on an exposed rock face <ref name="fpm" />.
   
Faults are also an important form of discontinuity within rock masses <ref name="kinnon" />. They typically extend over much larger distances than joints and can therefore affect much larger volumes of rock masses due to the extent of their displacement <ref name="kinnon" />. This displacement usually causes fault surfaces to be damaged and therefore weaker than joint surfaces. Unlike joints, damage zones may appear on either sides of the fault <ref name="kinnon" />.
 
  +
;:(2) Brunton Compass
  +
: This compass measures orientation using strike and dip <ref name="robert" />.It consists of three basic instruments: a compass, clinometer and hand level <ref name="robert" />. 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 <ref name="robert" />.
   
===<u>Site Investigation</u>===
 
  +
;:(3) Clino Ruler
  +
: This ruler is popularly used underground <ref name="min" />. It is used to measure the dip and slope of angles, such as those between joints <ref name="min" />. An example of this site tool is illustrated in [[:File:im2.png|Figure 6]].
   
Since faults show the same discontinuities as joints, but on a much larger scale, they can be identified, or parts of the fault can be identified, within rock masses using the same techniques as mentioned above for joints.
 
   
===<u>Strength Measurement</u>===
 
   
At the engineering scale, there is no method for estimating, or even characterizing, fault strength <ref name="kinnon" />.
 
   
   
   
=='''Application of Collected Site and Rock Mass Characterization Data'''==
 
   
Rock mass characteristic systems are processes that help classify the quality and indices of rocks <ref name="archibald" />. These rock classification systems are useful in determining specific properties of rock materials that may be used to aid in the safe design of rock structures and stable excavations <ref name="archibald" />. They compare site data against documented data from other existing operations, thereby providing some measure of comparison between rock types and similar excavation structures <ref name="archibald" />. This permits qualitative judgment of structural properties based upon historical observations made by others <ref name="archibald" />. Typical tests require considerable time, effort and cost making these systems favorable and widely used in the field <ref name="archibald" />.
 
  +
===<u>Strength Measurement</u>===
   
  +
Rock joints are defined as discontinuities in geology that are present in a majority of rock masses near the surface <ref name="bar"/>. These discontinuities affect the shear strength and characteristics of the surrounding rock, with the effective normal stress acting across the joint being the property most affecting <ref name="bar"/>.
  +
Joint strength measurement methods include empirical calculations as well as laboratory and field tests. The empirical calculation of shear strength is shown in [[:File:bim8.png|Equation 8]] where "JRC", "JCS", and "&Phi;r" represent the joint roughness coefficient, joint wall compressive strength, and residual friction angle respectively <ref name="bart"/>:
  +
[[File:bim8.png|centre]]
  +
This equation estimates the shear strength of the joints <ref name="bar"/>. Index tests can be used to determine the constants, and if shear strength tests have been performed, T and &sigma;n are known <ref name="bar"/>. The residual friction angle &Phi;r can be estimated <ref name="bar"/>. Laboratory tests can correctly estimate intact strength through point load test on rock core <ref name="bar"/>. The equation can also be used for curve fitting to and extrapolating peak shear strength data as well as the prediction of peak shear strength <ref name="bar"/>.
  +
The joint wall compressive strength (JCS) is a very important factor contributing to the overall strength of the joint; the controlling component of the strength and deformation properties of the rock mass are the thin layers of rock that lie next to the joint walls <ref name="bar"/>. Joint weathering has an impact on shear strength. If the joints are completely weathered, the JCS will be equivalent to the UCS of the unweathered rock <ref name="bart"/>. However, rock joint walls that are moderately weathered will have a JCS lower than the UCS <ref name="bart"/>. Rock that is permeable will also typically have a lower JCS <ref name="tor"/>.
   
The more commonly utilized rock mass classification systems are the rock mass rating, RMR, the rock quality designation, RQD, index and the NGI tunneling quality index, Q, system <ref name="science" />. Geological, geometric and design/engineering parameters are all incorporated into these systems in determining the quantitative values of rock mass qualities <ref name="science" />.
 
  +
=='''Faults'''==
  +
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 <ref name="jbp" />. 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 <ref name="jbp" />.
   
  +
===<u>Site Investigation</u>===
   
The difference between the systems lies in the different weightings given to similar parameters and in the use of distinct parameters <ref name="science" />. For instance, the RMR system uses compressive strength directly while the Q index only considers strength as it relates to in situ stress in competent rock <ref name="science" />. It is important to recognize that the classification methods only provide preliminary assistance for inferring rock mass stability or integrity <ref name="archibald" />. Therefore, when using any one of these rock mass characteristic systems, one must take caution to solely apply these systems for preliminary estimation of ground stability conditions <ref name="archibald" />.
 
  +
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.
   
  +
===<u>Strength Measurement</u>===
   
  +
There is no method for estimating, or even characterizing, fault strength <ref name="kinnon" />.
   
===<u>The NGI Tunneling Quality Index (Q-Value)</u>===
 
   
====Determining the Q-Value====
 
   
The Q-value is calculated using 6 factors within the following equation, where ''RQD'',''Jn '', ''Jr'', ''Ja'', ''Jw'' and ''SRF'' represent the rock quality designation, joint set number (number of joint sets existing), joint roughness number (frictional character), joint alteration number (infilling character), joint water reduction factor (effect of water) and stress reduction factor (effect of weakness zones) respectively <ref name="archibald" />:
 
  +
=='''Applying Collected Site and Rock Mass Characterization Data'''==
   
[[File:wiki119.png|centre]]
 
  +
By characterizing a rock mass, it may then be classified to help generalize and understand its behaviour and relationship to other rock masses <ref name="ben" />. 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 <ref name="science" />. Geological, geometric and design/engineering parameters are all incorporated into these systems for determining the quantitative values of rock mass qualities <ref name="science" />. However, each classification system varies in parameters, therefore, it is important to cross reference systems in order to develop a simplified and site-related rock mass classification system <ref name="science" /><ref name="bien" />.
   
The individual parameters are determined during geological mapping of underground excavations, surface or by core logging using tables that give numerical values to a described situation <ref name="norw" />. When analyzing the three-paired expressions within the equation, they describe the stability in underground openings, where <ref name="norw" />:
 
   
[[File:wiki120.png]]
 
  +
===<u>The NGI Tunneling Quality Index (Q-Value)</u>===
   
The Q-value should lie somewhere between 0.001 and 1000 <ref name="norw" />. When this is not the case, use these values even if your values are higher or lower than these by extreme combinations of parameters <ref name="norw" />.
 
  +
====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 <ref name="science" /><ref name="hart" />. It is calculated using 6 parameters, as represented in [[:File:bim9.png|Equation 9]] where, ''RQD'',''Jn '', ''Jr'', ''Ja'', ''Jw'' and ''SRF'' represent the rock quality designation, joint set number, joint roughness number, joint alteration number, joint water reduction factor, and stress reduction factor respectively <ref name="hart" />:
   
The “Q” value can be used to estimate both an equivalent excavation dimension (maximum dimension of excavation capable of standing safely) and estimate the support requirements for excavations <ref name="archibald" />.
 
  +
[[File:bim9.png|centre]]
  +
  +
The individual parameters are determined during geological mapping or by core logging <ref name="norw" />. 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 <ref name="hart" />. The Q-value should lie somewhere between 0.001 and 1000 <ref name="norw" />.
   
 
====Applications of the Q-Value====
 
====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 <ref name="norw" />. It provides qualitative estimation of near surface tunneling stability conditions in terms of numerical “Q” indices, which range between 0.001 and 1000 <ref name="archibald" />. Therefore, the Q-value is most precise when mapped in underground openings <ref name="norw" />. The value depends on the geometry of the underground opening and is therefore not an independent characterization of the rock mass <ref name="norw" />. 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 <ref name="norw" />. Therefore, one may use this system as a guideline in rock support design decisions and for documentation of rock mass quality <ref name="norw" />. High values indicate a good stability and low values indicate a poor stability <ref name="norw" />.
+
The Q-system is a classification system for jointed rock masses, where it is most precise when mapped in underground openings to determine their stability <ref name="norw" />. 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 <ref name="norw" />. Therefore, one may use this system as a guideline in rock support design decisions and for documentation of rock mass quality <ref name="norw" />. High values indicate a good stability and low values indicate a poor stability <ref name="norw" />.
   
 
====Limitations of the Q-Value====
 
====Limitations of the Q-Value====
   
The Majority of the case histories used to derive the Q-system were from hard and jointed rocks <ref name="norw" />. 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 <ref name="norw" />. The system is empirical with regards to rock support making the rock support recommendations quite conservative <ref name="norw" />.
+
The majority of the case histories used to derive the Q-system were based on hard and jointed rocks <ref name="norw" />. 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 <ref name="norw" />. The system is empirical with regards to rock support making the rock support recommendations quite conservative <ref name="norw" />.
 
 
   
 
===<u>Rock Mass Rating (RMR) System</u>===
 
===<u>Rock Mass Rating (RMR) System</u>===
Line 186: Line 186:
 
====Determining the RMR Value====
 
====Determining the RMR Value====
   
The RMR system utilizes five factors, plus a joint or discontinuity orientation factor, which include; Rock Quality Designation (RQD), joint/discontinuity spacing, joint/discontinuity condition, groundwater condition, and a rating adjustment for joint/discontinuity orientations <ref name="archibald" />.
+
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 <ref name="hart" />.
   
For each factor, a measure is first made so that a rating value may be assigned <ref name="archibald" />. Factor values are then summed to provide a number ranging between 0 and 100 <ref name="archibald" />. Within the RMR summation, a “penalty factor” rating is provided so that unstable or poor orientation factors may be adjusted <ref name="archibald" />.
 
  +
When applying the RMR system, the rock mass is divided into a number of structural regions and each region is classified separately <ref name="science" />. The boundaries of the structural regions usually correspond with major structural features, such as a fault <ref name="science" />. Each parameter is then assigned a rating value (based off of rock mass analysis) so that by summing them up, an RMR value anywhere between 0 and 100 may be produced <ref name="archibald" />.
   
Based on derived rating values from the period 1973-1989 by Bieniawski, tables and charts providing estimation of support requirements and unsupported “stand-up” time for excavations were developed <ref name="archibald" />. [[:File:wiki118.png|Figure 11]] is an example of such a table <ref name="archibald" />. When applying the RMR system, the rock mass is divided into a number of structural regions and each region is classified separately <ref name="science" />. The boundaries of the structural regions usually correspond with major structural features, such as a fault <ref name="science" />.
 
  +
Based on the RMR value, support systems may be put into place. [http://minewiki.engineering.queensu.ca/mediawiki/index.php/Ground_support The '' Ground Support''] wiki page provides a table that outlines the various excavation support requirements for specific RMR values.
 
[[File:wiki118.png|centre|frame|'''Figure 11:'''Table providing estimations for excavation support requirements <ref name="archibald" />.]]
 
   
 
====Applications of the RMR System====
 
====Applications of the RMR System====
   
The RMR system was developed at the Norwegian Geotechnical Institute to permit preliminary estimation of excavation dimensions based upon observed rock mass characteristics <ref name="archibald" />. It is useful for providing more details about the discontinuities within the rock mass and producing a qualitative assessment of the structural integrity of proposed excavations <ref name="archibald" /><ref name="mc" />. It also provides “hard data” with which to accomplish engineering design calculations <ref name="archibald" />. A notable feature of the system is that some of the input data may be additionally used in later stages of mine design work, thereby refining the structural assessment of excavations <ref name="archibald" />.
 
  +
  +
The RMR system was developed by Bieniawski between the years of 1973 and 1972 and is also referred to as ‘’Geomechanics Classification’’ <ref name="bien" />. However, over the years with the availability of more case studies, the system had been modified so that the system met international and procedural standards, as well as ensuring that the system was able to interpret the influence of various rock masses on rock stability <ref name="pak" /><ref name="bien" />. Since there are several versions of the RMR system, one must understand which version suites their purposes the best <ref name="science" />. There are five goals of this system, to: (1) identify the parameter that most impacts rock mass behaviour, (2) identify the various classes of quality within a rock mass, (3) understanding the characteristics of each class, (4) being able to use the data from the system for engineering purposes, and (5) allow for a basis of communication understood by those involved in engineering design <ref name="bien" />.
  +
  +
This system has been used in various engineering projects, such as for tunnel, mine, and chamber foundations due to its ease of use and data collection (using borehole data or underground mapping) <ref name="bien" />. It may also be incorporated into theoretical concepts, such as Unal, and Hoek-Brown <ref name="bien" />.
   
 
====Limitations of the RMR System====
 
====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 <ref name="science" />. The RMR system was originally based upon case histories drawn from civil engineering, which was seen as conservative within the mining industry <ref name="science" />. 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 <ref name="science" />. 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 <ref name="science" />. Because of its relevance to mining, the MRMR recommends a set of supports that should be considered based on the resulting MRMR value <ref name="science" />. One must note that, many of the case histories upon which the MRMR was derived, were caving operations <ref name="science" />. 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 <ref name="science" />.
+
The RMR system was originally based upon case histories drawn from civil engineering, which was seen as conservative within the mining industry <ref name="science" />. 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 <ref name="science" />. This MRMR system takes the basic RMR values and uses an adjustment factor to account for insitu and induced stresses, stress changes, and the effects of blasting and weathering <ref name="science" /><ref name="lau" />. Because of its relevance to mining, the MRMR recommends a set of supports that should be considered based on the resulting MRMR value <ref name="science" />. One must note that, many of the case histories upon which the MRMR was derived, were caving operations <ref name="science" />.
   
===GSI===
 
  +
This topic has been covered on [http://minewiki.engineering.queensu.ca/mediawiki/index.php/The_geological_model ''The Geological model'' wiki page.]
 
  +
===<u>Geological Strength Index (GSI)</u>===
  +
This topic has been covered on the [http://minewiki.engineering.queensu.ca/mediawiki/index.php/The_geological_model '' Geological Model'' wiki page.]
   
 
=='''References'''==
 
=='''References'''==
Line 210: Line 212:
 
<ref name="sme">K. Charles A., "Slope Stability," in SME Mining Engineering Handbook, 2011, pp. 495-527.</ref>
 
<ref name="sme">K. Charles A., "Slope Stability," in SME Mining Engineering Handbook, 2011, pp. 495-527.</ref>
 
<ref name="robert">C. Robert R., "Using the Compass, Clinometer, and Hand Level," in Manual of Field Geology, Wiley, 1962, pp. 20-48.</ref>
 
<ref name="robert">C. Robert R., "Using the Compass, Clinometer, and Hand Level," in Manual of Field Geology, Wiley, 1962, pp. 20-48.</ref>
<ref name="hoek">E. Hoek, "Rock Mass Classification," in Practical Rock Engineering, 2007.</ref>
 
 
<ref name="archibald">D. J. F. Archibald, "MINE 325: Applied Rock Mechanics," Queen's Mining Engineering, Kingston, 2012.</ref>
 
<ref name="archibald">D. J. F. Archibald, "MINE 325: Applied Rock Mechanics," Queen's Mining Engineering, Kingston, 2012.</ref>
<ref name="science">Rocscience, "Rock Mass Classification," [Online]. Available: https://www.rocscience.com/hoek/corner/3_Rock_mass_classification.pdf.</ref>
+
<ref name="science">Rocscience, "Rock Mass Classification," [Online]. Available: https://www.rocscience.com/hoek/corner/3_Rock_mass_classification.pdf.[Accessed 24 February 2015]</ref>
<ref name="mc">S. McKinnon, "Site Investigation," in MINE 469/823, Kingston, Queen`s Mining Engineering, 2014, pp. 1-66.</ref>
+
<ref name="lucian">Wangwe E. M., Lucian C., "The Usefulness of Rock Quality Designation (RQD) in Determining Strength of the Rock," International Refereed Journal of Engineering and Science (IRJES), vol. 2, no. 9, pp. 36-40, 2013.</ref>
<ref name="lucian">W. E. M. Lucian C., "The Usefuleness of Rock Quality Designation (RQD) in Determining Strength of the Rock," International Refereed Journal of Enginereing and Science (IRJES), vol. 2, no. 9, pp. 36-40, 2013.</ref>
+
<ref name="norw">Norwegian Geotechnical Institute, "Using the Q-System: Rock Mass Classification and Rock Design," 2013.</ref>
<ref name="norw">Norwegian Geotechnical Institute, "Uisng the Q-System: Rock Mass Classification and Rock Design," 2013.</ref>
+
<ref name="deere">D. U. Deere and D. W. Deere, "The Rock Quality Designation (RQD) Index in Practice," in Rock Classification Systems for Engineering Purposes, L. Kirkaldie, Ed., American Society for Tseting and Materials, 1988, pp. 91-101.</ref>
  +
<ref name="hud">J. A. Hudson and J. P. Harrison, "Geometrical Properties of Discontinuities," in Engineering Rock Mechanics: An Introduction to the Principles, Elsevier Ltd., 1997-2005, pp. 118-121.</ref>
  +
<ref name="nor">M. Noroozi, R. Kakaie and S. E. Jalali, "3D Geomaterical-Stochastical Modeling of Rock Mass Joint Networks: Case Study of the Right Bank of Rudbar Lorestan Dam Plant," Journal of Geology and Mining Research, vol. 7, no. 1, pp. 1-10, 29 January 2015.</ref>
  +
<ref name="jak">R. H. Jakola, D. O. Parry, P. Jasiobedzki and S. Y. S. Se, 3D Imaging System, 2010, pp. 1-20.</ref>
  +
<ref name="afr">D. C. Anderson and J. W. Van der Merwe, "Applications and Benefits of 3D Laser Scanning for the Mining Industry," The South African Institute Of Mining and Metallurgy, pp. 501-518, 2012.</ref>
  +
<ref name="min">Miners Incorporated, "Clinometer, Rascal Rule, (24 Inch)," [Online]. Available: http://www.minerox.com/product-p/ax926.htm. [Accessed 24 February 2015].</ref>
  +
<ref name="fpm">FPM Holding GmbH, "Geologist's Compass: Operating Manual," [Online]. Available: http://www.fpm.de/downloads/GeologistCompass_eng.pdf. [Accessed 24 February 2015].</ref>
  +
<ref name="jbp">J.-P. Burg, "Faults," in Structural Geology and Tectonics, 2015, pp. 93-127.</ref>
  +
<ref name="hart">W. Hartman and M. F. Handley, "The Application of the Q-Tunnelling Quality Index to Rock Mass Assessment at Impala Platinum Mine," The Journal of The South African Institute of Mining and Metallurgy, pp. 155-166, April 2002.</ref>
  +
<ref name="pak">D. Milne, J. Hadjigeorgiou and R. Pakalnis, "Rock Mass Characterization for Underground Hard Rock Mines".</ref>
  +
<ref name="ed">C. Edelbro, "Strength of hard rock masses," Lulea University of Technology, 2006. </ref>
  +
<ref name="bar">V. C. N. Barton, "The Shear Strength of Rock Joints in Theory and Practice," Springer-Verlag, 1976. </ref>
  +
<ref name="bart">N. Barton, "The Shear Strength of Rock and Rock Joints," Rock Mechanics Review, vol. 13, p. 25, 1976. </ref>
  +
<ref name="tor">M. A. M. J. S.R. Torabi, "Application of Schmidt rebound number for estimating rock strength under specific geological conditions," Journal of Mining & Environment, vol. 1, no. 2, p. 8, 2010. </ref>
  +
<ref name="ulu">R. Ulusay, ISRM Suggested Methods for Rock Characterization, Testing and Monitoring, Springer, 2014. </ref>
  +
<ref name="hek">E. Hoek, "Shear strength of rock discontinuities," in Practical Rock Engineering, 2007. </ref>
  +
<ref name="tuc">T. U. C-I. Trifu, "Characterization of rock mass behaviour using mining induced microseismicity," CIM, vol. 90, no. 1013, p. 7. </ref>
  +
<ref name="bien">Z. T. Bieniawski, "The Rock Mass Rating (RMR) System (Geomechanics Classification) in Engineering Practice," in Rock Classification Systems for Engineering Purposes, L. Kirkaldie, Ed., American Society for Tseting and Materials, 1988, pp. 17-34. </ref>
  +
<ref name="lau">D. H. Laubscher and J. Jakubec, "The MRMR Rock Mass Classification for Jointed Rock Masses," Bushman's River Mouth and SRK Consulting (Canada). </ref>
  +
<ref name="ben"> H. J. Pincus, "Opening Remarks," in Rock Classification Systems for Engineering Purposes, L. Kirkaldie, Ed., American Society for Tseting and Materials, 1988, pp. 1-3. </ref>
  +
<ref name="niosh"> National Institute for Occupational Health and Safety, "Proceedings of the International Workshop on Rock Mass Classification in Underground Mining," 2007. </ref>
  +
<ref name="aj"> A. S. J.S. Cargill, "Evaluation of Empirical Methods for Measuring the Uniaxial Compressive Strength of Rock," International Journal of Rock Mechanics and Mining Sciences and Geomechanics Abstracts, vol. 27, no. 6, p. 9, 1990. </ref>
  +
<ref name="sp"> SP Technical Research Institute of Sweden, "Uniaxial compression tests," [Online]. Available: http://www.sp.se/en/index/services/rockmechanicaltesting/uniaxial/Sidor/default.aspx. [Accessed 2 March 2015]. </ref>
  +
<ref name="cm"> C. M. John Rusnak, "Using the point load test to determine the uniaxial compressive strength of coal measure rock," in Proceedings of the 19th International Conference on Ground Control in Mining, Morgantown, WV, 2000. </ref>
  +
  +
 
</references>
 
</references>

Latest revision as of 15:38, 3 March 2015


Introduction

It is important to conduct a site investigation of the geological environment in which a mine may be developed so that its conditions may be predicted [1]. 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 [2]. 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.


Intact Rock

Site Investigation

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 [3]. This index introduces a technique that provides a quantitative estimate of the quality of a rock mass based of off drill cores [3]. It measures the degree of fractures and jointing, thereby associating rock masses with a percentage [3]. The RQD index also has an important influence on the RMR and Q values of rock mass classifications [4].


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 [5]. Using the drill core, the RQD index is calculated using Equation 1 [5]:

Eq1.png

Based on the RQD value of a drill core, the quality of the rock can be determined using the following table[3]:

Rock mass quality.png

When core is unavailable, Equation 1 may not be, so in 1982, Palmström suggested that Equation 2 be used [4]. 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 [4]:

Eq2.png

It was found that there is a linear relationship between the RQD and the joint frequency, λ, therefore it may be estimated using Equation 3 when λ is within the range of 6 to 16 [6]:

Eq3.png

To address the lack of sensitivity for large spacing values between joints, Equation 4 may be used, where a threshold value, t , is considered [6]:

Eq4.png

The mean RQD value can then be used in the calculation of the Q-value [7]. The RQD value will be a percentage between 0 and 100 [7]. 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 [7]. Several readings of the RQD should be taken along surfaces of different orientation, if possible, perpendicular to each other [7].


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 [3].


Strength Measurement

Figure 1: Uniaxial compressive strength laboratory test [8].

Rock mass strength is extremely important for mine design in order to correctly understand stresses and modes of failure in a rock mass [9]. Knowledge of these stresses increases safety in underground mines by correctly designing to conditions and can also reduce operating costs [9]. Joints are often found in rock masses and therefore it is important to determine their effect on the intact rock strength in order to correctly identify stress conditions. The strength of intact rock can be determined through suggested methods set by the International Society of Rock Mechanics including: (1) laboratory uniaxial compressive strength tests, (2) the point load strength test, and (3) the Schmidt hammer test [10].


Uniaxial Compressive Strength Test

The uniaxial compressive strength test is relatively simple in procedure, however has expensive costs corresponding to laboratory use and specific sample preparation [10]. Samples, often drill cores, are carefully cut into specific dimensions and loaded axially until failure occurs[8]. Figure 1 displays the typical set up of the laboratory experiment [8]. The uniaxial compressive strength test is quite accurate, though it is recommended by the American Society for Testing and Materials to test at least ten core samples [10].

Figure 2: Point load strength test apparatus [11].


Point Load Test

The point load test serves as a more economical option to determine the uniaxial compressive strength of rock compared to the uniaxial compressive strength test [11]. The test procedure consists of loading a drill core sample across its diameter, displayed in Figure 2 [10]. The point load strength index (Is) can be calculated using Equation 5, where P and d represent the applied load and distance between the loading points respectively [10]:

Bim1.png



Equation 6 serves for drill core samples that are 50mm in diameter. If the sample is not this size, it must be corrected in order to obtain a correct strength determination [10]. For hard rocks, it was found in studies by Broch and Franklin (1972) that the UCS of the rock could be expressed using Equation 6 (the point load strength index), where K represents the conversion factor [11]:

Bim2.png

It should be noted while a K value of 24 is typically universally accepted, studies have shown variation in conversion factors for different rock types, ranging from 16 to 24 [11].

Figure 3:Schmidt Hammer[12].

Schmidt Hammer Method

The Schmidt Hammer method is an easy field test to estimate the UCS of intact rock. It utilizes a spring loaded plunger that rebounds off of a surface for a reading, which is then converted to a compressive strength using Miller’s (1965) method [13]. Schmidt hammer models come in types L and N; L models apply 0.735 Nm while type N applies 0.245 Nm [14]. Figure 4 can be used to convert the reading from the Schmidt Hammer test to UCS.

Figure 4:Schmidt Hammer UCS Conversion Chart [15].

This method is acceptable for use, however, a better correlation was found by Miller using Equation 7, in which the dry density of the rock is multiplied by the reading from the test [13]. "σc", "Y", and "R" represent the uniaxial compressive strength of the rock, dry density of the rock (kN/m2), and rebound number respectively.

Bim7.png

This test can also be used to test the strength of joints if the test is done on an open joint face. Due to the manual nature of the test, there are certain details of the testing that should be made aware for accurate results. In terms of orientation, the hammer should always be perpendicular to the surface of the rock or joint face [12]. If it is not perpendicular, the tip of the plunger could slide or chip the material and result in an incorrect reading [12]. Additionally, the reading will be at a minimum when the hammer is used vertically downwards and at a maximum when used upwards due to gravity [13]. If the test is being performed on a sample rock only (i.e. core or rock that can be easily moved), it is important to anchor the sample so the hammer does not displace the rock when the test is performed [13]. Displacement of the rock will not yield a correct reading [13].

Additional Rock Mass Strength Techniques

Seismic techniques have also been tested in Canadian mines to suggest characterization of rock mass behaviour [16]. Low seismological techniques that include the use of microseismic waveforms to experiment the calculation of stress orientations and changes of stress in rock [16]. Studies in Canadian mines have shown that seismic techniques can aid in defining principal stresses and orientations as well as local stress influences around failures [16].


Joints

Site Investigation

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 [2]:

Figure 5:Example of onsite scanline measurements [17].
(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) [17]. The idea behind this technique is to measure all of the joint sets that intersect the scanline [17]. Using this method, the local condition and orientation, trend, and plunge of the rock mass are identified [17]. An example of this method is illustrated in Figure 5.
(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 [18]. The fracture sets within the mapped surfaces are visually identified [18].
(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 [19]. 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 [19]. An example of such technology is a 3D point cloud imaging system [20]. This is a system that uses mobile stereo and monocular cameras to obtain multiple images that create realistic models of the environment being analyzed [20].


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 [2]:

Figure 6: Example of a clino ruler [21].
(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 [22]. Interestingly enough, the compass’ casing may be used to determine the inclination of a dip by placing it sideways on an exposed rock face [22].
(2) Brunton Compass
This compass measures orientation using strike and dip [23].It consists of three basic instruments: a compass, clinometer and hand level [23]. 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 [23].
(3) Clino Ruler
This ruler is popularly used underground [21]. It is used to measure the dip and slope of angles, such as those between joints [21]. An example of this site tool is illustrated in Figure 6.




Strength Measurement

Rock joints are defined as discontinuities in geology that are present in a majority of rock masses near the surface [13]. These discontinuities affect the shear strength and characteristics of the surrounding rock, with the effective normal stress acting across the joint being the property most affecting [13]. Joint strength measurement methods include empirical calculations as well as laboratory and field tests. The empirical calculation of shear strength is shown in Equation 8 where "JRC", "JCS", and "Φr" represent the joint roughness coefficient, joint wall compressive strength, and residual friction angle respectively [24]:

Bim8.png

This equation estimates the shear strength of the joints [13]. Index tests can be used to determine the constants, and if shear strength tests have been performed, T and σn are known [13]. The residual friction angle Φr can be estimated [13]. Laboratory tests can correctly estimate intact strength through point load test on rock core [13]. The equation can also be used for curve fitting to and extrapolating peak shear strength data as well as the prediction of peak shear strength [13]. The joint wall compressive strength (JCS) is a very important factor contributing to the overall strength of the joint; the controlling component of the strength and deformation properties of the rock mass are the thin layers of rock that lie next to the joint walls [13]. Joint weathering has an impact on shear strength. If the joints are completely weathered, the JCS will be equivalent to the UCS of the unweathered rock [24]. However, rock joint walls that are moderately weathered will have a JCS lower than the UCS [24]. Rock that is permeable will also typically have a lower JCS [14].

Faults

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 [25]. 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 [25].

Site Investigation

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.

Strength Measurement

There is no method for estimating, or even characterizing, fault strength [2].


Applying Collected Site and Rock Mass Characterization Data

By characterizing a rock mass, it may then be classified to help generalize and understand its behaviour and relationship to other rock masses [26]. 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 [4]. Geological, geometric and design/engineering parameters are all incorporated into these systems for determining the quantitative values of rock mass qualities [4]. However, each classification system varies in parameters, therefore, it is important to cross reference systems in order to develop a simplified and site-related rock mass classification system [4][27].


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 [4][28]. It is calculated using 6 parameters, as represented in Equation 9 where, RQD,Jn , Jr, Ja, Jw and SRF represent the rock quality designation, joint set number, joint roughness number, joint alteration number, joint water reduction factor, and stress reduction factor respectively [28]:

Bim9.png

The individual parameters are determined during geological mapping or by core logging [7]. 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 [28]. The Q-value should lie somewhere between 0.001 and 1000 [7].

Applications of the Q-Value

The Q-system is a classification system for jointed rock masses, where it is most precise when mapped in underground openings to determine their stability [7]. 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 [7]. Therefore, one may use this system as a guideline in rock support design decisions and for documentation of rock mass quality [7]. High values indicate a good stability and low values indicate a poor stability [7].

Limitations of the Q-Value

The majority of the case histories used to derive the Q-system were based on hard and jointed rocks [7]. 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 [7]. The system is empirical with regards to rock support making the rock support recommendations quite conservative [7].

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 [28].

When applying the RMR system, the rock mass is divided into a number of structural regions and each region is classified separately [4]. The boundaries of the structural regions usually correspond with major structural features, such as a fault [4]. Each parameter is then assigned a rating value (based off of rock mass analysis) so that by summing them up, an RMR value anywhere between 0 and 100 may be produced [29].

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 developed by Bieniawski between the years of 1973 and 1972 and is also referred to as ‘’Geomechanics Classification’’ [27]. However, over the years with the availability of more case studies, the system had been modified so that the system met international and procedural standards, as well as ensuring that the system was able to interpret the influence of various rock masses on rock stability [30][27]. Since there are several versions of the RMR system, one must understand which version suites their purposes the best [4]. There are five goals of this system, to: (1) identify the parameter that most impacts rock mass behaviour, (2) identify the various classes of quality within a rock mass, (3) understanding the characteristics of each class, (4) being able to use the data from the system for engineering purposes, and (5) allow for a basis of communication understood by those involved in engineering design [27].

This system has been used in various engineering projects, such as for tunnel, mine, and chamber foundations due to its ease of use and data collection (using borehole data or underground mapping) [27]. It may also be incorporated into theoretical concepts, such as Unal, and Hoek-Brown [27].

Limitations of the RMR System

The RMR system was originally based upon case histories drawn from civil engineering, which was seen as conservative within the mining industry [4]. 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 [4]. This MRMR system takes the basic RMR values and uses an adjustment factor to account for insitu and induced stresses, stress changes, and the effects of blasting and weathering [4][31]. Because of its relevance to mining, the MRMR recommends a set of supports that should be considered based on the resulting MRMR value [4]. One must note that, many of the case histories upon which the MRMR was derived, were caving operations [4].


Geological Strength Index (GSI)

This topic has been covered on the Geological Model wiki page.

References

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