Site investigation and rock mass characterization

Figure 1

Introduction

There are three aspects to a mine design process: data collection and rock mass characterization, analysis and design . 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 . 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 . Therefore, an important aspect to underground mine design is ensuring the stability of rock masses . To determine stability, the strength of the different rock mass components must be identified . The major components of a rock mass that affect mine stability are: intact rock, joints and faults . 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 .

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 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 . The index may also be used to help select appropriate rock support for the measured rock type, which is based upon historical data . It also has an important influence on the RMR and Q values of rock mass classifications . 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 .

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 . 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 . Based upon core observations, the RQD index is calculated using the following equation :

Based on the final RQD value, the structural integrity or engineering quality of the rock is determined using the following table :

The proper technique for carrying out an RQD measurement is illustrated in Figure 2.

Figure 2:Technique for determining the RQD value of a core .

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 :

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 :

If a minimum spacing value of 0.1m is taken, then the relationship may be simplified to :

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

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 . It also gives incorrect values when joints contain thin clay fillings or weathered material .

Strength Measurement

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 :

Figure 3:Uniaxial compression test results .

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 .

Figure 4:Schmidt hammer UCS conversion chart .

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 3.

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 4 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 5, 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 .

Figure 5:Hardness test .

Joints

Site Investigation

Joint Mapping Methods

Various methods are used to map joints. Some of the more popular methods are :

Scanlines

This is the most common method . It consists of stretching a surveyors tape (typically 30m in length) in a straight line along an exposed rock face . All the discontinuities intersecting the tape should be noted and the projection of where they intersect the tape should be recorded . It is important to distinguishing blast-induced fractures from joints as they should be excluded from your data collection .

Figure 6:Example of onsite scanline measurements .

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 .

Core Logging

Sometimes information about the rock mass may only be acquired from core logging . However, determining joint orientation from the core is difficult and time consuming, therefore it is seldom carried out .

Laser Imaging

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 . 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 . An example of such technology is 3D point cloud imaging . This software allows for the scanned inage to be fully rotated . 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 .

Borehole Logging

This is a common technique to use onsite. The details of this technique can be found on the Rockmass Characterization using Geophysical Methods wiki page.

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 .The orientation may be taken using various tools .The following are some of the more common tools :

Figure 7: Example of a clino ruler .

Freiberg Compass

This compass was designed to measure orientation using dip and dip direction . 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 . It also has spirit levels built into it to ensure that the body of the compass is held horizontally when taking the measurements .

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 .

Clino Ruler

In some locations, a compass may not be viable as there may be magnetic interference . Therefore, this ruler was designed to measure the angle between two sections on the face of the rock mass . However, the orientation of the strike must always be measured relative to some known orientation, such as the strike of the drift . This device also allows for easy dip measurements . An example of this site tool is illustrated in the figure.

Data Collection Method

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 . Therefore, joint mapping sheets are provided by every company to document certain joint characteristics, such as the orientation, length and spacing . Every company provides their own mapping sheets because there is no such thing as a single industry standard mapping sheet . Figure 8 and 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 .

• Figure 8 and Figure 9:Example of a Joint Mapping Sheet
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Figure 10:Effect of multiple discontinuities on rock masses .

Strength Measurement

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 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 .

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 . The following equation 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 .

Faults

Faults are also an important form of discontinuity within rock masses . 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 . 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 .

Site Investigation

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.

Strength Measurement

At the engineering scale, there is no method for estimating, or even characterizing, fault strength .

Application of Collected Site and Rock Mass Characterization Data

Rock mass characteristic systems are processes that help classify the quality and indices of rocks . 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 . They compare site data against documented data from other existing operations, thereby providing some measure of comparison between rock types and similar excavation structures . This permits qualitative judgment of structural properties based upon historical observations made by others . Typical tests require considerable time, effort and cost making these systems favorable and widely used in the field .

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 . Geological, geometric and design/engineering parameters are all incorporated into these systems in determining the quantitative values of rock mass qualities .

The difference between the systems lies in the different weightings given to similar parameters and in the use of distinct parameters . 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 . It is important to recognize that the classification methods only provide preliminary assistance for inferring rock mass stability or integrity . 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 .

The NGI Tunneling Quality Index (Q-Value)

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 :

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 . When analyzing the three-paired expressions within the equation, they describe the stability in underground openings, where :

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 .

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 .

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 . It provides qualitative estimation of near surface tunneling stability conditions in terms of numerical “Q” indices, which range between 0.001 and 1000 . Therefore, 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 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 .

For each factor, a measure is first made so that a rating value may be assigned . Factor values are then summed to provide a number ranging between 0 and 100 . Within the RMR summation, a “penalty factor” rating is provided so that unstable or poor orientation factors may be adjusted .

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 . Figure 11 is an example of such a table . 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 .

Figure 11:Table providing estimations for excavation support requirements .

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 . 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 . It also provides “hard data” with which to accomplish engineering design calculations . 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 .

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 .

GSI

This topic has been covered on The Geological model wiki page.