Instrumentation for ground support design
Ground Support Optimization
Rock movement is often the result of stresses returning to its equilibrium after an excavation has been made or from the mobilization of the rock mass along a discontinuity. A measurement of how much movement is occurring and where it is occurring is critical in determining the necessary support or control capacity and pattern for rock reinforcement. Hutchinson and Diederichs (1996) have written a book extensively on the cable bolting cycle. The book states that an assessment of rock reinforcement demand is able to identify potential failure modes and the locations where support is necessary to maintain stability. This assessment should be based on:
• Key rock mass properties such as initial stress, strength, stiffness and structural integrity.
• Expected disturbing influences such as stress change and gravity loading.
Instrumented rock reinforcement systems are able to provide these details on a much more detailed scale as part of the verification process shown in the iterative cable bolt cycle in Figure 1. Instrumented rock reinforcement systems are also able to reduce the large costs associated with ground support at underground mines by monitoring and testing the efficiency of a support pattern.
Ground Support Assessment
There are two possible methods for support design verification: visual and field monitoring. Visual verification entails recognizing the cable bolt behavior based on the load applied and the appearance of the rock mass. A visual inspection of the cable bolts are able to give a crude measurement of the rock movement based on the length of the cable bolt strand exposed. The load compared to the bolt capacity can be evaluated by the bolt plate condition and the condition of the cable as shown in Figure 2.
Field monitoring and quantitative verification requires the use of visual data for example data from a borehole camera, numerical modeling and accurate measurements taken from geotechnical instruments such as instrumented cable bolts and borehole extensometers.
Instruments for Ground Support Verification
Quantitative verification is dependent on the ability to obtain a direct measure of the performance of the support in its “as placed” state. Many ground support arrays are over designed, and cables placed at a much greater depth than actually required since optimum cable length is impossible to calculate numerically without site-specific information. Optimization and verification often lead to a reduction in support density and length. Instruments that measure deformation, load and strain can be used to achieve optimization. However the success of an instrumentation system relies on numerous factors prior to installation.
Instrumentation Selection Process
Ralph Peck stresses the importance of proper instrument selection in his book where he states, “Every instrument installed on a project should be selected and placed to assist in answering a specific question”. The use of instrumentation in the field should begin with defining the objective and end with planning how the measurement data will be implemented (Dunnicliff 1988), this is critical for success. Dunniclff (1988) outlines the process for implementing an instrumentation program in the followings steps:
1. Define the project conditions
This entails understanding the rock mechanics, geology and general site conditions. In ground support verification this means identifying what naturally occurring discontinuities or induced fractures are causing and controlling the instability of the rock mass adjacent to the excavation, while remembering that the behavior of jointed rock is characterized by the nature and disposition of the discontinuities. Also that it is the discontinuities closest to the excavation that defines the surface and influence instability.
2. Define the geotechnical questions that need to be answered.
In ground support verification, the instrumentation should provide answers to whether or not the support is working and is the ground support sufficient.
3. Define the purpose of the instrumentation.
Each instrument selected should have a specific purpose. Dunnicliff (1988) recommends that unless a valid reason can be given for a specific instrument then it should be removed. The purpose of the instrument is related to the parameter to be monitored. In ground support verification, monitoring of deformation, load and strain answers questions of cable bolt capacity, density of pattern and length of the bolt. Monitoring parameters and selecting instruments that can identify a cause and effect relationship is extremely beneficial in data interpretation and mitigation of the identified problems.
It is also wise to reiterate that measurements of load and strain are influenced by conditions within a very small zone and are dependent on the local characteristics, thus do not represent conditions one a large scale. However deformation measurements are response to movements within the rock mass on a larger scale. Dunnicliff (1988) states that deformation is generally the most reliable and least ambiguous parameter measurement.
4. Predict the magnitudes of change
There are large assortments of instruments available with varying ranges of accuracy, linearity and sensitivities. These are specifications that are required to select a product and also to gain results that can be used appropriately. Predictions are necessary to supply these specifications. Estimating the maximum possible value or the maximum value of interest will identify the instrument range and an estimate of the minimum value of interest will identify instrument sensitivity and accuracy. In ground support verification, where safety is a large part of its purpose, classifying a range of values that indicate immediate attention or remedial action is necessary. An additional advantage of this process is that it will easily indicate abnormalities in data and the functioning of the instrument.
5. Select Instrument Type
Based on the failure mechanism identified and the values expected, an instrument can now be selected to monitor the parameters chosen.
6. Select Instrument Location
The instrument location is critical, as it should reflect the predicted measurements and behavior. For ground support verification, the locations chosen should be representative of the area of interest and in the particular zone of concern. The locations chosen should provide measurements that can be used for comparative behavior. It is often common practice to select instruments that provide redundancy in measurements, such as an extensometer and an instrumented bolt. The extensometer provides displacement measurements that can be correlated to the displacement measurements by the instrumented bolt.
7. Plan installation, data collection, processing, presentation, and interpretation.
Installation is critical in the success of any instrument. Each instrument often has an ideal borehole diameter and length so that its sensing mechanism can interact with the rock mass effectively. Additionally, hole orientation is important so that instrument conformance is not affected. Poor installation can also damage instrumentation and provide poor results.
Data collection frequency should be related to mining activity, the rate to which readings are changing and to the requirements for data interpretation, for example numerical modelling. A large quantity of data can make interpretation difficult where as too few may causes important events to be missed and prevent timely action from being taken (Dunnicliff, 1988). Frequent readings also help associate trends in the instrument data with discrete mining events (Larson et al., 2000). Thoughts of how the data will be presented to show relationships can also determine data collection frequency. Data intepretation plans are just as critical as this where meaningful conclusions are made.
Common Instruments for ground support verification
The most commonly used instruments in underground applications are those that measure stress and strain; strain is the result of stress and is usually quantified by measuring deformation. In many cases instruments are chosen to work in conjunction with each other so that there is redundancy in the measurements. Three typical instruments selected for ground support verification are multiple borehole extensometers and the instrumented cable bolt. The borehole camera is often used to establish the cause and effect relationship between discontinuities and its influence on the displacement, load and strain measurements.
Borehole imaging requires a radial scan of a borehole wall that is oriented in space such that depth and direction of features can be determined. The extent of borehole coverage and
This camera produces a 360 degree scan of the borehole. From the scans, joint location, orientation and condition can be assessed and with software can reproduce the geological structure as shown in Figure 5. When the borehole camera is used in conjunction with an extensometer, the displacement associated with each joint can be determined to further identify cause and effect relationships. When used for ground support verification, athe borehole should be located with in the zone of concern and kept open for frequent analysis associated with the measurements taken at the extensometer or instrumented cable bolt.
An extensometer is used to measure displacement. It monitors the changing distance between two or more points along the axis of a borehole and consists of an instrument head, usually anchored at the hole collar and a number of fixed in hole anchors as shown in figure 6. When the rock moves, the component parallel to the borehole direction causes a change in length between the other individual anchors as well as between the anchor and the instrument head. The instrument head consists of a transducer, which can be mechanical or electrical. A multiple borehole extensometer is used to monitor the deformation or strain along the axis of an appropriately oriented borehole. Potential failure zones can be identified and surface spalling can be separated from deep rock movements. Figure 6 shows a common extensometer setup oriented perpendicular to the cracks in the rock mass. The orientation of the cracks and their location can be identified with the borehole camera.
Instrumented cable bolt
An instrumented cable bolt consists of a 7-wire standard cable bolt with its king wire replaced to house a sensing mechanism. The sensing mechanism can be in the form on an electrical strain gauge or a mini multiple borehole extensometer housed in a stainless steel tube. The replacement of the king wire with instrumentation reduces the strength capacity of cable by <10% (de Graaf, 1998). In ground support verification, the instrumented cable bolt replaces an ordinary cable bolt in the support array as it is expected to behave in the same manner as the other cable bolts but also track changes in elongation and load experienced within the array.
The cable begins to work when it resists deformation from movement in the rock mass. This occurs as rock mass movement is transferred across the rock grout interface and through the grout to the cable. The instrumented bolt is able to measure both load and displacement. The load on the cable bolt is a function of cable stiffness and elongation and is determined by the following equation:
where F= Load (kN), k = cable stiffness, ε = elongation (m).
Displacement of the discontinuities causes strain with in the cable producing elongation along the axis of the cable. Elongation output is a function of the sensing mechanism. For the SMART cable bolt designed by Mine Design Technologies the stainless steel replacement king wire houses a mini multiple borehole extensometer, shown in Figure 6 and the potentiometric transducer outputs elongation along the cable in volts. This data then has to be converted to meters often with software sold by the manufacturer.
When opting for an instrumented cable bolt such as the SMART cable, the number of anchor points of the extensometer must be chosen in accordance with the objectives of the experiment, in addition to the discontinuities that they are expected to monitor. The locations of the anchor points have a direct effect on the measurements taken.
Data Analysis and Design Application
The data that is recorded and retrieved from the extensometer, instrumented cable bolt and borehole camera enable current support designs to be verified or optimized by assessing the load capacity, cable length and spacing. The extensometer provides a measurement of displacement and its location along the length of the borehole. When used in conjunction with an instrumented cable bolt, the displacement readings from both instruments should be similar. The instrumented cable bolt also provides an indication and location of where the cable takes load along the axis of the cable. Both load and displacement measurements can be used to evaluate stability based on the rate of displacement and its associated load. The borehole camera images are able to correlate the movement and load with specific joints and joint characteristics that can be used to further understand the rock behavior.
The multiple point borehole extensometer was placed amongst SMART cable bolts and Figure 9 represents the displacement recorded by the cable in the same zone. The displacement measurements are very similar and confirm the proper operation of the instrumented bolt and the readings. These graphs enable a rate of displacement to be calculated and assessed. The rate of displacement indicates whether stability has been achieved or is decreasing. In this particular example, at mining step “6e-f”, the instruments were damaged by a nearby blast and stopped working; they do not indicate an increase in displacement.
The instrumented cable bolt experiences load in two ways: axial and shear. The axial load is a response to the increased displacement of the rock along the axis of the cable bolt. The load produced by shearing is a response to the lateral movement of the rock perpendicular to its axis. Load measurements are critical in assessing the support capacity and safety factor of the support design. An increased safety factor is often designed for long-term support in large developments such as garages. A standard cable bolt can withstand a maximum load of 25kN; in monitoring the load a cable experiences, the load capacity of the support and its effectiveness can be evaluated. In cases where low load measurements are recorded but the ground has not yielded, spacing within the pattern can be increased.
Additional to the displacement measurements, the load experienced by the cable is recorded with respect to its location along the bolt and used to identify optimal cable length. Figure 10 shows an installed SMART cable bolt in the hanging wall of a drift at Barricks Bousquet Mine. The load profile shows that maximum load on the cable occurs less that 6m in from the wall consistently; the current cable length is 10m. A reduction in cable bolt length was recommended in this particular area from 10m to 6m as justified by the measurements.
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