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Instrumentation as First Line of Defense for Prediction of Rock Movement in Underground Coal Mines. Dr. Satyendra K. Singh Chief Scientist & Head of Research Group , Mining Method & Design Simulation Central Institute of Mining & Fuel Research, Dhanbad. NEED OF INSTRUMENTATION.
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Instrumentation as First Line of Defense for Prediction of Rock Movement in Underground Coal Mines Dr. Satyendra K. Singh Chief Scientist & Head of Research Group , Mining Method & Design Simulation Central Institute of Mining & Fuel Research, Dhanbad
NEED OF INSTRUMENTATION • Today instrumentation is a basic need for prediction of rock strata behavior. • Many instruments: FOR STRESS & DEFORMATION MEASUREMENT, used for ground control measures and monitoring of strata movement. • Geotechnical Problems in underground coalmines and tunnels as the overlying rock above the immediate : After excavation process, overlying bedded rocks get associated with strata movement if proper support system has not been provided. • In underground mines and tunnels roof fall is the major reason of casualties. By instrumentation it gets much easier to monitor the rock movement, which enables management to reinforce their support system, if analysis of instrmentation-data so suggest, or else reduce the support or increase the width of excavation. • Continuous monitoring of strata helps to envisage the likely excessive rock movement and stress, which may create big problem during excavation.
Objectives Of Instrumentation: • Monitoring rock behavior • Movement of strata in immediate roof • To evaluate ground stability at working areas in experimental panel. • To assess the efficacy of the proposed support system, subsequently to modify the support design, as and when required; and finally, • To establish a support design norm-using roof bolts in similar geomining environment.
Different Instruments: Used In Mines to study Strata Movement • Bore Hole Extensometer • Load Cell • Stress Meter • Tale tale • Remote Convergence Indicator • Instrumented Rock Bolts • Auto-warning Tale tale • Indicator props
Bore Hole Extensometer: • Borehole extensometer are used to measure movements of soil and rock along the axis of a bore hole. Typically borehole extensometer is applied for the following purposes • Monitoring settlement in foundations. • Monitoring subsidence above tunnels and mines. • Monitoring heave in excavations. • Monitoring the stability of tunnels and other underground openings. • Monitoring deformation in abutments and walls. • Monitoring movements in rock slides.
x x x x x x x x x x x Groutable bore hole anchors at different horizons Grout Measuring Rods under PBC pipe Flanges 1 Protective cap Fig no. 1: Borehole extensometer (Jeremic et. al 1987)
Operation • Components of a rod extensometer include anchors, rods with protective tubing, and a reference head. The anchors, with rods attached, are installed in the drill hole. The rods span the distance between the anchors and the reference head, which is installed at the borehole collar. Measurements are obtained at the reference head with a sensor or a depth micrometer, either of which measures the distance between the top (near) end of the rod and a reference surface. • A change in this distance, found by comparing the current measurement to the initial measurement, indicates that movement has occurred. Movement may be referenced to an anchor that is installed in the roof strata or to the reference head, which can be surveyed. The resulting displacement data can be used to determine the zone, rate, and acceleration of movements, or to calculate strain.
Multipoint Monitoring • The rod extensometer can monitor up to six points along the borehole. The number of monitored points is limited by the size of the borehole, the type of anchor used, the diameter of the rods, and the amount of tubing required for grouting and activating anchors. • A two-point extensometer typically requires a 60 mm (2.4 in) borehole; a four-point extensometer requires a 76 mm (3 in) borehole; and a six-point extensometer requires a 96 mm (3.8 in) borehole. Borehole size for packer anchors depends on number of anchors. • The anchors are grouted at different heights in the roof rock. These anchors are connected through steel rod passing to the reference head at the outer end of the bore hole. • Anchor rods are covered with PVC tubing, which keeps them free from grouting and allows them to move freely. • Sensor is fitted at the collar of the bore hole. The extensometer is grouted in the bore hole with cement slurry fixing the anchor in the rock leaving rod movement free as they are inside the PVC pipe.
Load Cell • Vibrating wire type load cells are used for measuring load in foundation anchors, rock bolts, tiebacks, cables or struts. They are also installed for long term performance monitoring of different kind of anchor systems and compressive load measurement between structural members i.e. tunnel supports or at the junction between a beam and the top of a pile • They are available in capacities ranging from 5 tf to 200 tf with a central cylindrical hole. Higher capacity load cells and those having an internal diameter different from the standard specified range are available on request. • The center hole load cell comprises of a cylinder of high strength martensitic stainless steel. Load cell generally uses three vibrating wire strain gages mounted at 120o to each other to minimize the effect of uneven and eccentric loading. Load applied to the cell is measured by the vibrating wire strain gages using any standard digital read-out unit. The average of the three readings is taken.
Load Cell • The vibrating wire and coil magnet assembly are mounted on a stainless steel columnar element. The assembly is enclosed in a stainless steel body which is electron beam welded to the element through two stainless steel diaphragms welded at the top and bottom. • This results in a vacuum of around 1/1000 torr inside the sensor resulting in it becoming immune to the effect of any ingress of water or chemicals in the environment. The sectional area of the columnar element is varied in the different capacity load cells to give approximately the same frequency range for a variation of zero to full load.
VIBRATING WIRE TYPE CENTRE HOLE LOAD CELL Important features:
Stress Meter • Features • The vibrating wire technology has unique advantage of providing a frequency based output signal, which can travel unaffected over very long distance. • High stability in inferior environment. • Direct measurement of stress change. • Water proof & corrosion resistance • Virtually insensitive to the temperature changes. • Stable frequency output signal. • High Stress and Load Sensitivity.
Applications • Measurement of bore hole stress in hard and soft rocks. • Monitoring of stress in retaining columns and load • bearing walls of mines. • Measurement and monitoring of stress in applications connected with disposal of nuclear waste in deep • abandoned mines. • Stress in the rock walls of unlined caverns.
STRATA MOVEMENT INDICATORS- (Telltales) Visual Telltales (Single, Dual and Triple Height) RMT's range of telltales are low cost, easily installed safety monitoring devices for indicating tunnel stability. The visual dual height telltale has become the industry standard. The great advantage of the dual height indicator is that it allows movement above and below the rockbolted horizon to be distinguished at a glance. This provides underground workers with instant information on roof condition and operational management with roof deformation history and advance warning of remedial action requirements. In many countries, RMT telltales are installed and read on a routine basis to provide a reliable roof control management system. RMT telltales are installed as mandatory safety devices in UK coal mines. The system is patented in the UK, USA, Australia, China, Poland, and Canada.
Dual Height Telltale Dual height telltales are designed for reinforced rock, providing a differential strata-movement indication based on patented concentric indicators. The two sets of reflective colored bands indicate roof movement within and above the reinforced height. The millimeter scales also allow accurate monitoring of movement trends. Triple height telltales are designed for use where two different lengths of reinforcement are installed. Windows software, 'Telltale for Windows', is available from RMT for recording, presentation and analysis of telltale data. Telltales can be supplied with copper or plastic reference tubes including a water-diverting version for wet conditions. Stainless steel spring-anchors and multi-strand suspension cables are used throughout for corrosion protection. A standard range color-codes and wire-lengths is stocked - others are available to order.
Digital Tape Extensometer • Features • Can be used in any type of environmental conditions. • It is portable, of low cost & very light weight, hence no problem in its mobility. • Reliable readings with accuracy of ± 0.01 mm. • Measurement from 1 m to 50 m. • Measuring tape is made up of good quality steel. • Knurled knob is provided for easy tension adjustment.
Application • The tape extensometer is used to detect and monitor changes in the distance between two reference points. Typical applications include: • Monitoring convergence of tunnel walls. • Monitoring deformations in underground openings. • Monitoring displacement of retaining structures, bridge supports, and other structures. • Mine roof sag • Tunnel closure • Displacement of unstable stopes & structures. • To determine radial movement and convergence of tunnels, shafts and linings. • To study the effectiveness of the roof of a mine or underground cavity and monitor its behavior during the excavation operation. • To determine the stability of concrete structures and buildings.
