strata monitoring Instrumentation

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GROUND MOVEMENT MONITORINGGROUND MOVEMENT MONITORINGUNDERGROUND & OPENCAST MINES UNDERGROUND & OPENCAST MINES

U.SivaU.Siva SankarSankarSr. Under ManagerSr. Under ManagerProject PlanningProject Planning

Singareni Collieries Company LtdSingareni Collieries Company Ltd

EE--Mail :Mail :[email protected]@gmail.comor or [email protected][email protected]

Visit at:Visit at:www.slideshare.net/sankarsulimellawww.slideshare.net/sankarsulimella

Monitoring 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.

GROUND MOVEMENT MONITORING

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Various modes of instrument operationexist, 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.


Hydraulic 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 Strainon ground support devices (e.g., load cells); � Seismic events and Blast vibrations.


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�� out of the two principle monitoring technique available, out of the two principle monitoring technique available, i.ei.e, Deformation/strain and load/stress, the displacement , Deformation/strain and load/stress, the displacement and load have proven to be more useful, can be and load have proven to be more useful, can be measured directly and monitored continuously with a measured directly and monitored continuously with a relative ease. relative ease.

�� Stress/strain, on the other hand, must be determined Stress/strain, on the other hand, must be determined indirectly from other measurements and is difficult to indirectly from other measurements and is difficult to monitor continuously. monitor continuously.

�� Also, displacement measurements provide information Also, displacement measurements provide information on overall movement of the rock mass within the on overall movement of the rock mass within the measured distance and thus do not display a large measured distance and thus do not display a large variability such is apparent when a quantity is measured variability such is apparent when a quantity is measured at one point, such as stress or pressure at one point, such as stress or pressure (Bieniawski,1984).(Bieniawski,1984).


�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

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�� Clear and specified objective;Clear and specified objective;

�� Properly designed instrumentation program;Properly designed instrumentation program;

�� Installation of instruments by skilled personnel;Installation of instruments by skilled personnel;

�� Adequate data recording frequency; andAdequate data recording frequency; and

�� Proper analysis of collected data.Proper analysis of collected data.

The ingredients of a successful monitoring programme should incoThe ingredients of a successful monitoring programme should incorporate, among rporate, among others, the following :others, the following :

� 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 signalsof 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.


Telescopic Convergence Rod

Roof to Floor Convergence Measurement

Principle : 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 of

2 to 4 m. The measuring points ("reference stations") are metal rods grouted in

the roof and floor.

Procedure : Measurements are taken by simply stretching the telescopic rod

between the reference points, and reading the graduations on the rod. These

indicators are useful for understanding the roof to floor closure in the advance

galleries at various stages of extraction.

Underground Strata Monitoring

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Chart Type Convergence Recorder


Telescopic Convergence Rod

Anchors placed in Roof and floor vertically in a straight line

Distance between anchors is measured with convergence rod

Sometimes hooks are placed in the both sides of the gallery in the pillars to facilitate measurement of roof convergence as well as floor heaving


Telescopic tube

Sensor

Spring protectivecover

DisplayReadout unit

Floor

On/Off

Charging

Spring

Roof

Telescopic tube

Sensor

Spring protectivecover

DisplayReadout unit

Floor

On/Off

Charging

Spring

Roof


Remote Convergence Indicator

� For monitoring convergence in goaf

� Sensor – Vibrating wire type or Potentiometric type

� Needs laying of wire from instrument location to measurement site

� Fails immediately with the failure of immediate roof

The RCI consists of two telescopic tubes whose ends are

fixed in the roof and the floor. It has a potentiometer

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

change in resistance, which is directly proportional to

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.


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Convergence Measurement

Tape Extensometer

�Can be used for the measurement of convergence just like telescopic convergence rod

�Also used for surface profile movement measurement of roadways

Anchor


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


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Spring type Tell tale ExtensometerUnderground Strata Monitoring

Multi wire Telltale

Multi - Wire

Triple Height

Dual Height tell tale

Cablebolting

OTHER TYPES OF TELLTALE:

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Dual Height Tell tale Extensometer


Dual Height Tell tale Extensometer


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Safety Monitoring

Colour Graduation

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A + B = Total movement

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A + B = Total movement A Green 20mm

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TOTAL 20mm

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B Green 20mm

TOTAL 40mm

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A = movement within bolted height


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B Green 20mm

TOTAL 60mm

A Yellow 40mm



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B Yellow 40mm

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B Red 55mm

TOTAL 115mm

A Red 60mm

ACTION LEVELSACTION LEVELS

GREEN 0 - 25 mm

YELLOW 25 - 50 mm

RED 50 mm +


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ACTIONSACTIONS

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 ofsupport.

RED Restrict access. Investigate. Install longer reinforcement


Single point Rotary Tell tale ExtensometerUnderground Strata Monitoring

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Rotary Telltale

The Rotary design has been developed to give a reso lution of 1mm

This accuracy is important at sites where roof defo rmation levels are generally low

How a rotary tell tale worksHow a rotary tell tale works

R M TRock Mechanics

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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.

MULTI POINT EXTENSOMETERConsists of multiple anchors

Can 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 extraction or development

Can be fixed in the side walls of the pillar or galleryto monitor

�Pillar spalling or sloughing and cracks development in pillars

Can be fixed in the roof either from surface or from upper seam(300m) or from the working seam (24m)to monitor strata dilation as well as caving behaviour of roof rocks

Can be monitored remotely

Instrument fixed in the roof from working seams fails immediately after immediate roof fails

Types:

� Sonic probe

� Magnetic

� Mechanical fixing reading with the use of VW or resistance or magnetic anchors


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Sonic Probe Extensometer

Anchor


� The principle of the sonic probe relies on the magnetostrictiveproperties of the probe material.

� An electric pulse in the head of the probe drives current up the length of the wand.

� Interaction with the field produced by the toroid magnet induces an ultra sonic signal that travels back to the head in the wave guide.

� This acoustic signal is converted 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.

Features� Quick, easy installation and

readout� Portable, reusable sonic probe and

sensor� Remote electronic readout� Reads inter-anchor strains directly� Up to 20 anchors in one borehole� High accuracy and sensitivity

Principle of Sonic Probe Extensometer


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Roof layers

Read-out UnitScale

Probe

Magnetic anchorsRoof layers

Read-out UnitScale

Probe

Magnetic anchors

Magnetic Extensometer

Fig: Disc & Spider magnets


FEATURES� Reliable, accurate and simple

to read.� Any number of points can be

monitored.� Probe is portable and can be

used at many locations; only the access

� tubes and magnets are permanently installed.

� Options available on type of probes, access tubes and magnet assemblies.

� Low cost, rugged and easy to install.

�Ring magnets positioned on a central access tube are fixed in the ground at locations where displacement is to be monitored.

�A probe incorporating a Reed switch is made to travel within the access tube to sense the position of magnets outside the access tube. The Reed switch closes on entering a magnetic field and activates a buzzer or an indicator light

� in the signal receiving instrument. �The cable of the probe is marked at

suitable intervals for measuring the location of each magnet from the endof the access tube.

Principle of Magnetic Extensometer


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Multipoint Extensometers



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Typical Extensometer ResultsTypical Extensometer Results

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Typical Extensometer ResultsTypical Extensometer Results

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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 an electric 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 load using calibration charts. Efficacy and adequacy of the support system can be inferred on the basis of these load cells.


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.


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Solid Load Cell

Annular Load cellVibrating 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

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Hydraulic Load Cell (Annular)

Hydraulic Load Cell


Strain Gauge Load Cell


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Principle : 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 tension

in the wire to change.

Procedure : A bore hole of 38 mm diameter is required for installing the stress

meters, preferably at mid height of the pillar either horizontally or slightly

rising/dipping according to dip of the seam. The stress meter along with wedge and

platen assembly is set in the borehole with the help of special installation tools, at

a depth of about 4 to 9 m.

Stress Cell


Installation Coal Pillar


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STRESS CELL – 3 DIMENSIONAL MEASUREMENTStress cell consists of a soft inflatable membrane with 18 electrical resistancestrain 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.


Earth Pressure Cell�The 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 as

to 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 converted

to 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 remote

monitoring with cables.


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Borehole Pressure Cells

Underground Moni toring

To measure the deformation of the pillar, electrical strain meters (“Pillar Strain Meters”) are used. In this instrument, there is a steel/brass telescopic rod inside a larger diameter steel/brass tube anchor fixed in the pillar, and the other end is fixed in the anchor at 1 m below it. The bottom telescopic pipe has a potentiometer(a resistance coil, rheostat) or Vibrating wiresensor attached to it.

Pillar strain meter


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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.


• Axial loads and strains• Bending loads and strain difference• High strain zones from contour plots

ROCKBOLT STRAIN GAUGE�Strain gauged rock bolts can be used to measure the distribution of bolt loads for design

purposes. �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 enabling

the axial loads and bending moments to be measured along its full length.� strain gauged rockbolts provide a valuable tool for monitoring support system performance

against design specification.


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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.


ROCKBOLT STRAIN GAUGE


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�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.

Shear Strip


�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


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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.

1%1%Stress over pillarStress over pillarEarth Pressure Cell or Earth Pressure Cell or Borehole Pressure CellBorehole Pressure Cell

5.5.

0.5mm0.5mmRoof to Floor ConvergenceRoof to Floor ConvergenceRemote Convergence Remote Convergence IndicatorIndicator

2.2.

0.5mm0.5mmDeformation of pillar or crack Deformation of pillar or crack growthgrowth

Pillar Strain meter or Pillar Strain meter or Crack meterCrack meter

9.9.

0.5% of full 0.5% of full Scale Scale

Change in stress over pillar or Change in stress over pillar or 3D stress3D stress

Stress CellsStress Cells4.4.

1.5% 1.5% Monitoring leg pressure of Monitoring leg pressure of chock shield chock shield

Continuous Pressure Continuous Pressure Chart Recorder Chart Recorder

10.10.

±± 0.1mm 0.1mm Bed separation, Caving Bed separation, Caving behaviour (behaviour (GoafGoaf) )

Multi Point Borehole Multi Point Borehole Extensometer Extensometer (Sonic or Magnetic)(Sonic or Magnetic)

8.8.

1mm 1mm Convergence & Bed Separation Convergence & Bed Separation in the immediate roof in the immediate roof

Tell tale extensometer Tell tale extensometer 7.7.

Load profile of RockboltLoad profile of RockboltRockbolt Strain gaugeRockbolt Strain gauge6.6.

0.001 0.001 TonneTonneload over the support or load over the support or Rockbolt loadRockbolt load

Load cellLoad cell3.3.

0.5 to 1 mm 0.5 to 1 mm Roof to floor ConvergenceRoof to floor ConvergenceTelescopic Telescopic Convergence Indicator Convergence Indicator

1.1.

ACCURACYACCURACYPARAMETER MEASUREDPARAMETER MEASUREDINSTRUMENTINSTRUMENTS.NoS.No

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Remote convergence indicator

BoreholeExtension rod (non-grouted)

Grouted anchor

Sensor box

Load cell

Telescopic convergence rod

Reference peg in floor

Reference peg in roof

Stressmeter

Remote measuring instruments Manual instrumentsin and around splitting and slicing in development galleries

Borehole extensometer

GENERAL INSTRUMENT LAYOUT IN WORKING PANEL


GENERAL INSTRUMENT LAYOUT IN WORKING PANEL


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INSTRUMENTATION LAYOUT IN BORD & PILLAR PANEL Underground Strata Monitoring

INSTRUMENTATION LAYOUT IN BORD & PILLAR PANEL WORKED WITH CONTINUOUS MINER

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INSTRUMENTATION LAYOUT IN LONGWALL PANEL

Types of Instrumentation for Ground Monitoring

Single or multiSingle or multi--point point piezometerspiezometers, combined , combined piezometerpiezometerand inclinometer systemsand inclinometer systems

Ground water pressureGround water pressure

Inclinometers, Fixed borehole extensometers, slope Inclinometers, Fixed borehole extensometers, slope extensometers, shear plane indicators, multiple extensometers, shear plane indicators, multiple deflect meters, indeflect meters, in--place inclinometers, combined place inclinometers, combined inclinometerinclinometer--piezometerpiezometersystem, Acoustic emission system, Acoustic emission monitoringmonitoring

Sub surface Sub surface deformationdeformation

Surveying methods, Crack gages, Tilt meters, MultiSurveying methods, Crack gages, Tilt meters, Multi--point liquid level gaugespoint liquid level gauges

Surface deformationSurface deformation

Suitable Monitoring InstrumentsSuitable Monitoring InstrumentsMeasurementMeasurement

Opencast Slope Monitoring

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SLOPE MONITORING INSTRUMENTS

Extensometers

Time domain reflectometry (TDR)

Inclinometers

Piezometers

Crack Meters

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 rock

Extensometers

Fig: slope with Extensometer


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.


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Inclinometers

Monitoring slopes and landslides to detect zones of movementMonitoring dams, dam abutments, and upstream slopes. Monitoring the effects of tunneling operations


• Vibrating wire

• Pneumatic

• Standpipe piezometers

PiezometersOpencast Slope Monitoring

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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.


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strata monitoring Instrumentation