ABSTRACT

An issue of measurement and analysis of strata control parameters during underground coal mining (bord and pillar) in India is discussed. Requirement of different instruments along with their suitability to monitor the strata control parameters under varying geo-mining conditions of different coal mines is explained. On the basis of field experiences, it is observed that the type of instrument to be placed in and around a depillaring face is more or less dependant upon the geo-mining conditions of the site. Field investigations showed that the continuous monitoring (in time) of these instruments, preferably with the help of a data logger, can provide important information about performance of mining structures (bord and pillars) during caving of a competent overlying roof stratum. However, in absence of the data logger, behaviour of overlying strata is visualised through observations of a number of instruments under Combined- Instruments-Approach (CIA). Some results of CIA based field measurements are presented to demonstrate the role of in situ instrumentation and monitoring for safety of the underground coal mining.

1 INTRODUCTION

Monitoring and measurements of different strata control parameters duringundergroundminingis,generally, doneforsafetyofworking,fieldvalidationofadesign and performanceevaluationofatechnologyunder changed conditionsofsite. These measurements are relatively more difficult for underground mining of coal8 due tofragilenatureoftheformations. As per requirement, it is always better to be continuous in space and time during such measurements. But it is practically impossible to be continuous in space for an underground measurement. Depending upon dimension of the void and nature ofoverlyingroofrockmass,differentpoints/ stations are selected in and around an underground panel onthebasisofexperienceofthesite. Development of stressandstrainatthesepointsismonitoredthrough embedded electro-magneticsensorswithprogressofthe mining. Even for such discrete measurement in space, it is alwaysbettertokeeplarge/densenumberofthe measuring points/stationsinthepanel. The number of such observational points/stations in a panel is optimized through experienceandanadvanceassessmentofthe rock mass behaviour.

Again, it is difficult to have any "typical" mining environment,whichcanbeusedasabenchmarkto decidethequalityandquantityoftheinstrumentstobe deployed forthestratacontrolinvestigation. However, from stabilitypointofview,undergroundcoalmining involves threetypesofstructures,termedas:Long, Medium and Short term stable structures. The sophistication andremotenessofthemonitoring instruments increasewithdecreaseinstabilityofthe structure. Instruments placed to monitor performance of structuresofthefirsttwocategoriesneednottobeof remote typeastheareaaroundthemremainsmostly accessible andsafe. Further the required time interval between two consecutive observations

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