strata monitoring Instrumentation


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ground movement monitoring , different instruments with their principles and analysis in brief used in underground coal mines and open cast mines

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strata monitoring Instrumentation

  1. 1. GROUND MOVEMENT MONITORING UNDERGROUND & OPENCAST MINES U.Siva Sankar Sr. Under Manager Project Planning Singareni Collieries Company Ltd E-Mail or Visit at: GROUND MOVEMENT MONITORINGMonitoring of ground conditions in mines may be carried out for a variety of reasons, including Obtaining data needed for mine design, such as rock mass deformability or rock stresses; Verifying design data and assumptions, thereby allowing calibration of computer models and Adjustment of mining methods to improve stability; Assessing the effectiveness of existing ground support and Possibly directing the installation of additional support; and Warning of potential ground failures. Monitoring of ground conditions can be done either visually or with the help of specialized instruments. Surface and underground inspections must be done carefully and with the assistance of high-intensity inspection lights if necessary; miners, supervisors, engineers and geologists all have an important role to play in carrying out regular inspections. 1
  2. 2. GROUND MOVEMENT MONITORING Various modes of instrument operation exist, namely: Mechanical: often provide the simplest, cheapest and most reliable methods of detection, transmission and readout. Mechanical movement detectors use a steel rod or tape, fixed to the rock at one end, and in contact with a dial gauge or electrical system at the other. The main disadvantage of mechanical systems is that they do not lend themselves to remote reading or to continuous recording. Optical: used in conventional, precise and photogrammetric surveying methods of establishing excavation profiles, measuring movements of excavation boundaries and monitoring surface subsidence. GROUND MOVEMENT MONITORINGHydraulic and Pneumatic: diaphragm transducers that are used for measuring water pressures, support loads and so forth. The quantity measured is a fluid pressure which acts on one side of a flexible diaphragm made of a metal, rubber or plastic.Electrical: the most common instrument mode used in mines, although mechanical systems still find widespread use in displacement monitoring. Electrical systems operate on one of three principles, electric resistance strain gauge, vibrating wire and self-inductance.Most commonly monitored variables include Movement (using surveying methods, surface devices such as crack gauges and tape extensometers, borehole devices such as rod extensometers or inclinometers); Rock stresses (absolute stress or stress change from borehole devices); Pressure, Load and Strain on ground support devices (e.g., load cells); Seismic events and Blast vibrations. 2
  3. 3. GROUND MOVEMENT MONITORING out of the two principle monitoring technique available, i.e, Deformation/strain and load/stress, the displacement i.e, and load have proven to be more useful, can be measured directly and monitored continuously with a relative ease. Stress/strain, on the other hand, must be determined indirectly from other measurements and is difficult to monitor continuously. Also, displacement measurements provide information on overall movement of the rock mass within the measured distance and thus do not display a large variability such is apparent when a quantity is measured at one point, such as stress or pressure (Bieniawski,1984). GROUND MOVEMENT MONITORING Most electronic instrumentation measurement methods consist of three components: a transducer, a data acquisition system, and a linkage between these two components. A transducer is a component that converts a physical change into a corresponding electrical output signal. Data acquisition systems range from simple portable readout units to complex automatic systems.Generally four types of sensors are used for all strata monitoring instruments; which are based on the following four principles; Linear variable differential transformer (LVDT) Strain gauge Rheostat/variable resistance Vibrating wire 3
  4. 4. GROUND MOVEMENT MONITORING Out of these four types of sensors, the first three types of sensors provide analogue information while the last one sends only frequency which is a fixed value like digital signal. It is very difficult to carry the analogue signals of the first three types of sensors to a distant safe place without noise and distortion. Frequency signal is exceptionally immune from cable effects including length (several kilometers), splicing , resistance, noise pickup and moisture The features of vibrating wire sensors makes it superior to other sensors because it always desirable to monitor instruments from distant and safe place during underground mining. The ingredients of a successful monitoring programme should incorporate, among incorporate, others, the following : Clear and specified objective; Properly designed instrumentation program; Installation of instruments by skilled personnel; Adequate data recording frequency; and Proper analysis of collected data. Underground Strata Monitoring Roof to Floor Convergence Measurement Telescopic Convergence RodPrinciple : It is a simple instrument consisting of a graduated rod fitted in a pipe.It has a least count of 0.5 to 1 mm, and the telescopic movement is for a length of2 to 4 m. The measuring points ("reference stations") are metal rods grouted inthe roof and floor.Procedure : Measurements are taken by simply stretching the telescopic rodbetween the reference points, and reading the graduations on the rod. Theseindicators are useful for understanding the roof to floor closure in the advancegalleries at various stages of extraction. 4
  5. 5. Underground Strata Monitoring Roof to Floor Convergence MeasurementAnchors placed in Roof and floor vertically in a straight lineDistance between anchors is measured with convergence rodSometimes hooks are placed in the both sides of the galleryin the pillars to facilitate measurement of roof convergenceas well as floor heaving Chart Type Convergence Recorder Telescopic Convergence Rod Underground Strata Monitoring Roof to Floor Convergence Measurement Roof For monitoring convergence in goaf Sensor – Vibrating wire type or Potentiometric type Telescopic tube Needs laying of wire from instrument location to measurement site Sensor Spring Spring protective Fails immediately with the failure of immediate cover roof Charging The RCI consists of two telescopic tubes whose ends are Readout unit Display On/Off fixed in the roof and the floor. It has a potentiometer Floor connected to the top sliding telescopic tube, and a slider fixed to the bottom tube. Due to the movement of the roof, the slider moves on the resistance coil, resulting in Remote Convergence change in resistance, which is directly proportional to Indicator the amount of convergence or divergence. The change in resistance is measured with the help of a battery- operated read out unit. The RCIs were installed at the junctions in the middle of the panel. 5
  6. 6. Underground Strata Monitoring Convergence Measurement Anchor Tape Extensometer Can be used for the measurement of convergence just like telescopic convergence rod Also used for surface profile movement measurement of roadwaysUnderground Strata Monitoring Tell tale Extensometer Tell-tales are a low cost, easily installed monitoring device which will provide a Continuous Visual Indication of the roof Conditions For monitoring Immediate Roof convergence Bed separation between layers Progressive failure height of the strata Ensure efficacy of bolting and influence of extraction or development Fixed only in the roof of the gallery Mechanical type Manual reading of deformation Maximum height can be fixed is 10 m or longer 6
  7. 7. Underground Strata Monitoring Spring type Tell tale Extensometer Multi wire Telltale Multi - Wire OTHER TYPES OF TELLTALE: Dual Height tell tale Cablebolting Triple Height 7
  8. 8. Underground Strata Monitoring Dual Height Tell tale ExtensometerUnderground Strata Monitoring Dual Height Tell tale Extensometer 8
  9. 9. Safety Monitoring Colour Graduation A + B = Total movement A Green Red Yellow 20mm 0mm 60mm 40mm B Green Red Yellow 20mm 55mm 20mm 40mm B Green 0mm TOTAL 115mm 40mm 80mm 60mm TOTAL 20mm B BB = movement above bolted height 10 A 20 30 10 40 20A = movement within 50 30 60 bolted height 40 70 50 30 A 60 40 10 30 70 50 20 40 60 30 10 50 20 70 60 40 30 10 50 70 40 B 60 70Underground Strata Monitoring ACTION LEVELS GREEN 0 - 25 mm YELLOW 25 - 50 mm RED 50 mm + 9
  10. 10. Underground Strata Monitoring ACTIONS ACTIONS GREEN No action required, continue routine monitoring YELLOW Identify height of softening [HOS]. If HOS is above bolts, install longer reinforcement. If below top of bolts, install additional standard length reinforcement to increase density of support. RED Restrict access. Investigate. Install longer reinforcementUnderground Strata Monitoring Single point Rotary Tell tale Extensometer 10
  11. 11. Rotary TelltaleThe Rotary design has been developed to give a resolution of 1mmThis accuracy is important at sites where roof deformation levels are generally low How a rotary tell tale works o. EN NC 0 1 RE FE 2 RE 3 4 25 5 4 4 4 24 6 23 23 23 23 7 22 22 8 2 21 21 2 9 20 RMT 10 10 19 1 1 1 Rock Mechanics Technology 11 18 17 12 13 16 1 1 1 14 15 15 15 15 4 4 4 11
  12. 12. Auto Warning Telltale RMT’s Auto-Warning Telltale has been designed to provide additional instantaneous warning of movement occurring in a rockbolted excavation. This is of particular value in dynamic mining situations, such as pillar extraction operations, where workman and equipment are operating close to a developing goaf. The LED is configured to flash when the B indicator shows greater than 25mm of roof deformation. The trigger level on the B indicator is factory set and cannot be adjusted by the user, but alternative trigger levels can be factory set on request. Underground Strata MonitoringConsists of multiple anchors MULTI POINT EXTENSOMETERCan be fixed in roof to monitor Bed separation between layers Progressive failure height of the strata Magnitude and locations of strata movements Ensure efficacy of bolting and influence of extractionor developmentCan be fixed in the side walls of the pillar or gallery tomonitor Pillar spalling or sloughing and cracks development inpillars Can be fixed in the roof either from surface or from Types:upper seam(300m) or from the working seam (24m)tomonitor strata dilation as well as caving behaviour of Sonic proberoof rocks MagneticCan be monitored remotely Mechanical fixing reading withInstrument fixed in the roof from working seams fails the use of VW or resistance orimmediately after immediate roof fails magnetic anchors 12
  13. 13. Underground Strata Monitoring Sonic Probe Extensometer AnchorUnderground Strata Monitoring Principle of Sonic Probe Extensometer The principle of the sonic probe relies on the magnetostrictive properties of the probe material. An electric pulse in the head of Features the probe drives current up the Quick, easy installation and length of the wand. readout Interaction with the field Portable, reusable sonic probe and produced by the toroid magnet sensor induces an ultra sonic signal that Remote electronic readout travels back to the head in the Reads inter-anchor strains directly wave guide. Up to 20 anchors in one borehole This acoustic signal is converted High accuracy and sensitivity into an electrical signal and the time between pulses resolves the differences in position of the anchors as the speed of sound in the wave guide is known. 13
  14. 14. Underground Strata Monitoring Magnetic Extensometer Magnetic anchors Roof layers ProbeRead-out Unit Scale Fig: Disc & Spider magnets Underground Strata Monitoring Principle of Magnetic Extensometer Ring magnets positioned on a central FEATURES access tube are fixed in the ground at Reliable, accurate and simple locations where displacement is to be to read. monitored. Any number of points can be A probe incorporating a Reed switch is monitored. made to travel within the access tube to Probe is portable and can be sense the position of magnets outside the used at many locations; only access tube. The Reed switch closes on the access entering a magnetic field and activates a tubes and magnets are buzzer or an indicator light permanently installed. in the signal receiving instrument. Options available on type of The cable of the probe is marked at probes, access tubes and suitable intervals for measuring the magnet assemblies. location of each magnet from the end of Low cost, rugged and easy to the access tube. install. 14
  15. 15. Underground Strata Monitoring Multipoint ExtensometersUnderground Strata Monitoring 15
  16. 16. Typical Extensometer Results 7 6 5 4 3 2 1 0 0 20 30 40 50 60 mmTypical Extensometer Results 7 7 6 6 Displacement (mm) Strain (mm/m) 5 5 10mm/m (m) 4 (m) 4 (1% strain) LOW 3 3 2 2 1 1 0 0 70 20 40 60 80 70 60 20 70 40 80 60 (mm/m) (mm) 6 6 5 5 (m) 4 (m) 4 MEDIUM 3 3 2 2 1 1 0 70 0 70 20 40 60 80 60 20 70 40 80 60 (mm) (mm/m) 6 6 5 5 HIGH (m) 4 (m) 4 3 3 2 2 1 1 0 0 0 20 40 60 80 60 0 20 70 40 80 60 (mm) (mm/m) 16
  17. 17. Underground Strata Monitoring Vibrating Wire Load cells The load cell is a transducer working on vibrating-wire principle. It has three stretched wires housed in a metal cylinder, which are plucked by anelectric pulse of high energy. Changes in the load exerted on the cell cause changes in the length of the wire,resulting in variations of frequency of vibration of the wire. As the load increases the frequency decreases accordingly and vice-versa. This frequency is measured by a digital read-out unit, and is converted into loadusing calibration charts. Efficacy and adequacy of the support system can beinferred on the basis of these load cells.Underground Strata Monitoring ROCKBOLT LOAD CELL The load cell is a transducer working on vibrating-wire principle. The load cells should be installed under the roof bolts using specially prepared steel seating arrangement . The load cells should be installed at the junctions to understand the change in load over the roof bolt during the operation. 17
  18. 18. Underground Strata Monitoring Solid Load Cell Annular Load cell Vibrating Wire Load Cell Mechanical Load Cell Load cell consists of an elastic disc element sandwiched between two plates. The disc deflects under load and changes distance between plates. The deflection is measured with a dial gauge or suitable electronic transduced. It has limited application due to nonlinear calibration curve and restricted application 18
  19. 19. Underground Strata Monitoring Hydraulic Load Cell Hydraulic Load Cell (Annular)Underground Strata Monitoring Strain Gauge Load Cell 19
  20. 20. Underground Strata Monitoring Stress CellPrinciple : These are used for measuring unidirectional stress change in the pillar.It consists essentially of a wire tensioned across a steel cylinder (38 mm diameter).As the stress within the rock/coal changes, the cylinder deforms, causing tensionin the wire to change.Procedure : A bore hole of 38 mm diameter is required for installing the stressmeters, preferably at mid height of the pillar either horizontally or slightlyrising/dipping according to dip of the seam. The stress meter along with wedge andplaten assembly is set in the borehole with the help of special installation tools, ata depth of about 4 to 9 m. Underground Strata Monitoring Installation Coal Pillar 20
  21. 21. Underground Strata Monitoring STRESS CELL – 3 DIMENSIONAL MEASUREMENT Stress cell consists of a soft inflatable membrane with 18 electrical resistance strain gauges mounted flush on its outer surface. These gauges are glued to the surface of the borehole to directly measure any strain changes that occur in the rock.Underground Strata Monitoring Earth Pressure CellThe pressure cell is also based on the principle of vibrating-wire technology.It is constructed from two stainless steel plates welded together around the periphery so asto leave a narrow space between them.This space is completely filled with de-aired hydraulic oil (Glycol).It is connected to a vibrating-wire pressure transducer where the oil pressure is convertedto an electrical signal which is transmitted through a signal cable to the readout unit.The vibrating-wire based pressure cell has the advantage of long-term stability and remotemonitoring with cables. 21
  22. 22. Underground Monitoring Borehole Pressure Cells Underground Strata Monitoring Pillar strain meterTo measure the deformation of the pillar, electrical strain meters (“Pillar Strain Meters”) areused.In this instrument, there is a steel/brass telescopic rod inside a larger diameter steel/brasstube anchor fixed in the pillar, and the other end is fixed in the anchor at 1 m below it. Thebottom telescopic pipe has a potentiometer (a resistance coil, rheostat) or Vibrating wiresensor attached to it. 22
  23. 23. Underground Strata Monitoring Crack meter •Monitoring joints for unexpected movement to provide early warning of performance problems. •Monitoring joints and cracks in structures that may be affected by nearby excavation and construction activities. •Monitoring cracks in structures that have experienced seismic activity. Underground Strata Monitoring ROCKBOLT STRAIN GAUGEStrain gauged rock bolts can be used to measure the distribution of bolt loads for designpurposes.Bolt loads can be monitored along each bolt by the use of multiple strain gauges.Multiple pairs of strain gauges are embedded at regular spacings along each bolt enablingthe axial loads and bending moments to be measured along its full length. strain gauged rockbolts provide a valuable tool for monitoring support system performanceagainst design specification. • Axial loads and strains • Bending loads and strain difference • High strain zones from contour plots 23
  24. 24. Underground Strata Monitoring ROCKBOLT STRAIN GAUGE Rockbolt strain gauge can be fixed in roof, sides, surface slopes, embankments, etc. Wireless local reading or remote hard-wire reading up to two kilometres away are available. Normally, for the 22 mm diameter bolts, the bolt loads should not exceed 25 t (250 kN), which is the yield strength of the bolt steel.Underground Strata Monitoring ROCKBOLT STRAIN GAUGE 24
  25. 25. Underground Strata Monitoring Shear Strip Shear strip consists of a stainless steel flat bar (25mmx10mm) with foil strain gauges bonded at 50mm intervals on opposite sides of the bar. Change in strain gauge resistance are used to determine compressive and tensile strains along the bar. Processing of both strains allows determination of shear planes to with in 50mm.Underground Strata Monitoring The entire assembly is housed in protective outer plastic tubing to provide mechanical protection as well as water proofing The entire assembly is grouted within a 55 to 60mm borehole Readout cables are routed from the end of the shear strip to remote reading location and readings taken manually using strain bridge monitor 25
  26. 26. Underground Strata Monitoring Continuous Pressure Chart Recorder The resistance or pressures of the powered support are monitored by a pressure recorder consisting of bourdon tubes in phosphor bronze rolled in concentric circles for pressure sensing and a mechanically wound clock that rotates the recording chart once in 24 hours. When the hydraulic lines of the support are connected to the bourdon tube, the hydraulic pressure will deflect a lever arm that is fixed at the tail end of the tube. The amount of deflection is transmitted to the recording pin as pressure and recorded in the rotating chart. It is essential that the writing device in the recording pin maintain clean line plotting and pose no smearing potential. S.No INSTRUMENT PARAMETER MEASURED ACCURACY 1. Telescopic Roof to floor Convergence 0.5 to 1 mm Convergence Indicator 2. Remote Convergence Roof to Floor Convergence 0.5mm Indicator 3. Load cell load over the support or 0.001 Tonne Rockbolt load 4. Stress Cells Change in stress over pillar or 0.5% of full 3D stress Scale 5. Earth Pressure Cell or Stress over pillar 1% Borehole Pressure Cell 6. Rockbolt Strain gauge Load profile of Rockbolt 7. Tell tale extensometer Convergence & Bed Separation 1mm in the immediate roof 8. Multi Point Borehole Bed separation, Caving ± 0.1mm Extensometer behaviour (Goaf) (Goaf) (Sonic or Magnetic) 9. Pillar Strain meter or Deformation of pillar or crack 0.5mm Crack meter growth 10. Continuous Pressure Monitoring leg pressure of 1.5% Chart Recorder chock shield 26
  27. 27. Underground Strata Monitoring Grouted anchor Borehole Extension rod (non-grouted) Sensor box Reference peg in roof Telescopic convergence rod Borehole extensometer Stress Load cell meter Reference peg in floor Remote convergence indicator Remote measuring instruments Manual instruments in and around splitting and slicing in development galleries GENERAL INSTRUMENT LAYOUT IN WORKING PANELUnderground Strata Monitoring GENERAL INSTRUMENT LAYOUT IN WORKING PANEL 27
  29. 29. INSTRUMENTATION LAYOUT IN LONGWALL PANELOpencast Slope Monitoring Types of Instrumentation for Ground Monitoring Suitable Monitoring Instruments Measurement Surface deformation Surveying methods, Crack gages, Tilt meters, Multi- Multi- point liquid level gauges Sub surface Inclinometers, Fixed borehole extensometers, slope deformation extensometers, shear plane indicators, multiple deflect meters, in-place inclinometers, combined in- inclinometer-piezometer system, Acoustic emission inclinometer- monitoring Ground water pressure Single or multi-point piezometers, multi- piezometers, combined piezometer and inclinometer systems 29
  30. 30. Opencast Slope Monitoring SLOPE MONITORING INSTRUMENTS Extensometers Time domain reflectometry (TDR) Inclinometers Piezometers Crack Meters Fig: slope with Extensometer Extensometers Borehole extensometers consists of tensioned rods anchored at different points in a borehole Changes in the distance between the anchor and the rod head provides the displacement information for the rockOpencast Slope Monitoring Time Domain Reflectometry * lower installation costs * no limits on hole depth * immediate determination of movement * remote data acquisition capability In TDR, a cable tester sends a voltage pulse waveform down a cable grouted in a borehole, If the pulse encounters a change in the characteristic impedance of the cable, it is reflected. This can be caused by a crimp, a kink, the presence of water, or a break in the cable. The cable tester compares the returned pulse with the emitted pulse, and determines the reflection coefficient of the cable at that point. The change in impedance with time corresponds qualitatively to the rate of ground movement. 30
  31. 31. Opencast Slope Monitoring Inclinometers Monitoring slopes and landslides to detect zones of movement Monitoring dams, dam abutments, and upstream slopes. Monitoring the effects of tunneling operationsOpencast Slope Monitoring Piezometers • Vibrating wire • Pneumatic • Standpipe piezometers 31
  32. 32. Opencast Slope Monitoring Crack Meters Crack meters can be very useful tools in the early detection of deforming mass movements. These devices measure the displacement between two points on the surface that are exhibiting signs of separation. 32