Site investigation and rock mass characterization

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Contents

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

(thumbnail)
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].

(thumbnail)
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].

(thumbnail)
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.

(thumbnail)
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]:

(thumbnail)
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]:

(thumbnail)
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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