Chapter 3.0 Underground monitoring of roof and support behaviour
One of the most important prerequisite in the design of a support system is to understand the roof and support behaviour in different geotechnical environments. An extensive monitoring programme was therefore undertaken in order to establish the behaviour and the interaction between the support units and the roof. Critical deformations beyond which the roof fails will occur was also investigated.
A total of 29 sites at five collieries were monitored using sonic probe extensometers and in order to cover as much of the roof strata as possible, and avoid losing what could in time turn out to be valuable information, the full string of 21 anchors with the top anchor at approximately 7.3 m was installed at all the monitoring sites.
To process the monitoring data as quickly and efficiently as possibly, a customised program was written as part of this study, culminating in an easy to understand set of graphic results. The basic function of this program is to compare all subsequent sets of readings with the original set and produce displacement-with-time graphs. Various modifications and improvements were introduced to include the option of producing velocity and acceleration graphs to assist with the interpretation of the results.
Underground monitoring procedure
In this monitoring programme sonic probe extensometer is utilised. The sonic probe extensometer system is a sophisticated electronic device. It generates a pulse that travels at the speed of sound, and is able to accurately determine the distance between magnetic fields, set up by magnets which are integral to the extensometer anchors.
The cylindrical magnetic anchors are locked in place at predetermined locations in a borehole and have a plastic tube inserted through their centres. This tube acts as a guide for a flexible probe that is then inserted through the entire string of anchors. The readout unit is connected to the probe and the distances between the magnetic fields are individually displayed and manually recorded.
In order to record all the information relevant to roof strata deformation prior to the installation of any roof support, would necessitate the installation of instrumentation a few metres ahead of the face. Since this is clearly not possible the next best scenario is to install the instrumentation at the face. However, due to practicalities such as not working under unsupported roof and the limitations on how close the roofbolters can get to the face, it is not usually possible to drill closer than about 0.5 m from the face. This results in the monitoring hole being in or close to the last row of support. Drill bit sizes, resin quantities and support types and lengths were also monitored. In the underground situation the quality of roof support installation is dependent on a number of factors. With resin bonded bolts the bond length and quality are dependent on the actual average hole diameter, the overdrilling of holes and deviations from the recommended resin spin and hold times. It was not practical or possible to monitor or control the support installation at the monitoring sites. The support performance monitored is therefore a true representation of the support systems as installed underground and includes any effects linked to imperfections in the installation of the support.
At the monitoring site, close to the face, and situated in the middle of the advancing roadway, an 8.0 m deep hole was drilled vertically with a roofbolter into the roof and reamed out to 50 mm in diameter to accommodate the sonic probe magnetic anchors. Although most of the drilling process was carried out with water flushing, the final reaming of the hole is done dry, as the modified custom made reaming bits cannot accommodate water channelling. The hole was cleaned by inserting a water hose to the top or by spinning one of the smaller drill bits up the hole with the water switched on. A petroscope was then inserted into the hole and the lower 2.5 m was examined to detect the presence of any open laminations or fractures. However, final reaming of the holes to enlarge the hole by a few millimetres was carried out dry. During this process, the moisture left in the hole by the original wet drilling mixed with the powdered coal duff and form a paste that was then smeared into any openings by the reaming process. Therefore, the petroscope monitoring of the holes did not result in reliable information and was taken out of the monitoring programme.
A full string of 21 anchors was then installed in each hole at predetermined intervals (approximately 250 mm apart) using a set of installation rods. The top anchor, the first to be installed, is placed at approximately 7.3 m. An extra anchor that does not have a magnet fitted is installed in front of the last anchor, a short distance from the collar of the hole. This is a prerequisite in a vertical hole and is used to suspend the sonic probe to prevent it moving during the reading process.
Depending on the mining method and speed of face advance, the time lapse between further sets of readings varied from hours to days apart. In a typical development section underground three or four sites close to the centre of the panel were monitored. Where possible, the sites included both roadways and intersections to be able to evaluate and compare the strata behaviour and support performance in the two different locations. Prior to any development of the intersection taking place, the instrumented hole was positioned at the face so as to be as close as possible to the centre of the proposed intersection.
Survey levelling was used in conjunction with the sonic probe to assist in assessing the accuracy of the probe. The relative displacement measured between points anchored at 0.1 m in from the roof skin and at an elevation of approximately 1.8 m should ideally be compared against displacements measured between anchors at similar elevations by the sonic probe. However, at most of the monitoring sites where levelling was implemented, all the roof displacements took place within 1.8 m of the immediate roof. The levelling results have therefore been compared with the “total relaxation” measured by the sonic probe. The total relaxation is the overall displacement between a stable elevation in the roof and the anchor closest to the roof skin. In the five cases (Colliery D area 2) where displacements occurred up to 2.5 m into the roof, a note concerning the comparative probe and levelling displacement values has been included in the appropriate figures. These values have also been included in Table 3-1 where direct comparisons can be made between the sonic probe results and all the sites where back up levelling was successfully implemented.
In some cases it was not possible to make use of the survey levelling backup system due to factors such as the dip of the seam and the mining method and sequence. Levelling monitoring points that were damaged during the monitoring period were excluded from the results. Levelling backup was successfully implemented at approximately half the monitoring sites. The survey levelling results were included in the sonic probe displacement graphs. In excess of 90 per cent of the cases, the levelling results recorded similar or higher values than those of the sonic probe. A higher value levelling result is perfectly acceptable since the levelling skin anchor is usually about 0.1 m closer to the roof skin than the lowest sonic probe anchor, which is usually placed 0.2 m into the roof. Any displacement that occurs between their respective elevations would only be recorded by the levelling results.
Processing of information
The initial readings were taken as soon as the installation was completed. These comprise a minimum of three sets which were screened for any obvious anomalies or booking errors. They were then entered into the program where they were averaged, and the calculations carried out to produce the graphic results necessary for interpretation. All the subsequent sets of readings were treated in a similar manner with the program comparing them to the first (datum) set of readings from which the displacements were calculated.
The original displacement graphs included all the anchors in the hole up to the 7.3 m elevation. However since the main focus of the investigation was in the vicinity of the support horizon all the support performance graphs have been cropped at the 2.5 m elevation. This does not infer that displacements above the 2.5 m elevation were discarded or ignored.
Included alongside the 2.5 m vertical axis on each graph is a shaded block representing the section of strata column under investigation. The patterns within the block represent the approximate location of the different strata types, typically sandstone, shale and coal. These patterns are included and labelled in Figure 3-1. The stratigraphic column included with each individual displacement graph is representative of the area under investigation.
Although in some cases as many as 15 site visits were carried out and sonic probe readings taken, individual composite graphs have been limited to a maximum of five sets of readings for reasons of clarity.
In order to present the results of the individual site investigations in a simple and efficient manner, a graphic classification system was used. An explanation of this system and the relevance of other information included with it are given in Figure 3-1.
Although the displacements usually start at the roof skin and are evident for some distance into the roof, the section of the strata column under investigation does not extend down to the roof skin. The reason for is that the bottom magnetic anchor of the anchor string has to be approximately 0.2 m into the roof to allow the dummy anchor, used as a suspension point for the sonic probe, to be installed in front of it.
The displacements recorded by the final set of sonic probe readings taken at a particular site are transferred to the strata column. Here they are shown as individual lines approximately midway between the anchors from which each relative displacement value was calculated.
In order to establish a uniform approach to assist in simplifying the interpretations, the following criteria were introduced:
- Only readings outside the accepted error band were accepted.
- Differential displacements between adjacent anchors had to exceed 0.5 mm to be considered, except in the case of a trend involving three or more anchors where displacements down to 0.25 mm were included.
Displacements of 0.25 mm and larger are therefore represented by a line. In order to emphasis the different magnitudes of the various displacement zones, each line has been designated an appropriate thickness proportional to the value. These lines represent the total displacement recorded within the zone (between the two anchors) and do not infer that all the displacement took place at one particular elevation or parting plane; they are primarily an indication of relative magnitudes.
In Figure 3-1, to assist in explaining this concept, the anchor string showing individual anchor elevations is included. Alongside each displacement line the individual displacement values have been recorded. Where no displacement was observed, a zero value (0.0) is evident, as is the lack of a displacement line. The method used to indicate a negative displacement is also indicated. The anchor string and displacement values are included in Figure 3-1 primarily to assist with the explanation. They are not recorded in the graphic presentations of the individual monitoring site figures, as this information is already present in a slightly different form in the sonic probe graph.
To assist in assessing the effectiveness of the various roof support systems, a single support member is also included as part of the shaded strata column block alongside each sonic probe graph. The length of both the support member and the anchoring mechanism is drawn in at the same scale as the vertical axis of the sonic probe graph. A partial column resin anchored bolt is shown in Figure 3-1.
The roof displacements measured by the sonic probe are superimposed on the relevant roof support member for comparison purposes. This does not necessarily infer that these displacements are occurring in or at the support tendon hole, particularly where the hole is full of resin. The sonic probe hole varied between 0.3 to 1.0 m away from the closest support tendon hole.
The anchor height above which no displacements were recorded in a strata column is indicated as the ‘stable elevation’. In cases where some doubt exists it may be referred to as the ‘estimated stable elevation’. The ‘total relaxation’ value indicates the overall displacement between the stable elevation and the bottom anchor in the string.
Included with each displacement graph is a list of notes covering the monitoring site position, layout and mining method as well as a description of the roof strata and support system installed.
Table of Contents
List of Figures
List of Tables
Chapter 1.0 Introduction
1.2 Objectives and scope of research.
1.3 Outline of the thesis
Chapter 2.0 Literature review
2.2 Types of roof bolts
2.2.1 Mechanical coupled roof bolts
2.2.2 Resin point anchors
2.2.3 Full-column single-resin-type bolts
2.2.4 Full-column slow/fast-resin combination bolts
2.2.5 Friction rock stabilisers
2.2.6 Wooden dowels and fibreglass dowels
2.2.7 Spin-to-stall system
2.2.8 Current guidelines for the selection of roof bolt type
2.3 Theories of roof bolting support
2.3.1 Simple skin support.
2.3.2 Suspension mechanism
2.3.3 Beam-building mechanism.
2.4 Roof bolting design
2.4.1 Analytical methods.
2.4.2 Field testing
2.4.3 Numerical modelling
2.4.4 Roof support design based on geotechnical classification
2.4.5 Physical modelling
2.4.6 Probabilistic methods
2.5 Geometric parameters
2.5.1 Bolt length
2.5.2 Bolt diameter
2.5.3 Bolt pattern
2.5.4 Annulus size
2.6 Tensioned versus non-tensioned bolts
2.7 Stiffness of roof support
2.8 Intersection support
2.9 Discussion and conclusions
Chapter 3.0 Underground monitoring of roof and support behaviour
3.2 Underground monitoring procedure
3.3 Processing of information
3.4 Colliery ‘A’
3.5 Colliery ‘B
3.6 Colliery ‘C
3.7 Colliery ‘D’
3.8 Colliery ‘E ’
3.9 Analysis of underground field measurements
3.10 Roadway widening
Chapter 4.0 Effect of cut-out distance on roof performance
4.2 Research conducted
4.3 Underground monitoring
4.4 Colliery ‘A’
4.5 Colliery ‘B’
4.6 Colliery ‘C’
4.7 Colliery ‘D’
4.8 Colliery ‘E’
4.9 Colliery ‘F’
4.10 Analysis of underground monitoring results
4.11 Investigation of trends using numerical modelling
Chapter 5.0 Evaluation of geotechnical classification techniques to design coal mine roofs
5.2 Coal Mine Roof Rating (CMRR)
5.3 Rating systems being used in South African collieries
5.4 Geotechnical testing at different collieries
5.5 Application of proactive systems
5.6 Conclusions and recommendations
Chapter 6.0 Evaluation of roof bolting systems in South Africa
6.2 Specifications for roofbolters
6.3 Performance of roof bolts
6.4 Performance of resin
6.5 Specifications for bolt and resin
6.6 Effect of bit, annulus and rock type
6.7 Quality control procedures for support elements
Chapter 7.0 Roof support design methodology
7.2 Support design based on a probabilistic approach
7.3 Roof behaviour and failure mechanism
7.4 Roof bolting mechanisms
7.5 Determination of stability of the immediate layer between the roof bolts .
7.6 Probability density functions of design parameters and random selection.
7.7 Support design methodology
7.8 Application of the probabilistic design approach to a case study
Chapter 8.0 Conclusions and recommendations
8.2 Recommendations for future research
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