Practical experiences of instrumentation and monitoring for depillaring

The challenging nature of strata dynamics in and around goaf edge of a depillaring operation warrants continuous vigilance over the dynamics for a safe and efficient working. Depending upon the nature of the working and surrounding rock mass, a variety of geo-technical instruments are, generally, deployed for the vigilance. Selection of positions and installation of these instruments involve good understanding of behavoiur of the associated rock mass, but continuous monitoring of their observations becomes an important issue. In a conventional depillaring working, most of the observations are monitored manually, which may lead to safety threat and unsafe design of the mining structures. Some of the observations of a data-logger based monitoring are proved to be very useful, but the involved financial constraints have restricted its large scale application. Simple incorporation of an electronic continuity circuit in the single point borehole extensometer is known as auto warning tell-tale (AWTT), which has enabled to watch a given value of the roof lowering continuously. This ability of AWTT has proved to be an advantage while depillaring: especially mechanized depillaring. However, it requires a vital input about the rock mass behaviour: the limit of strata deformation to trigger and flashing the alarm light. Explaining the role of design and safety instruments for given conditions of a depillaring panel, this paper provides experience based examples to choose type and positions of the instruments for the monitoring. Briefly discussing the role of combined instrument approach to compensate the error in estimating the rock mass behaviour at the time of instrumentation, the role of continuous monitoring is illustrated with the help of some field measurements in depillaring panels. With the help of a good input of the estimation of strata characteristics in advance, examples of successful use of AWTTs are presented to show its ability to work as a safety instrument. Experiences and measurements found that the AWTT works well under moderate roof strata. Apart from the nature of overlying strata, the field monitoring indicated a need of supervision in observations of this instrument if the working is influenced by barrier effect, left-our pillars/fenders inside the goaf or by a geological structure. Requirement of other supporting instruments are also mentioned on the basis of the noticed problems of this single point measurement below extreme strata conditions (strong and massive or weak and laminated roof strata).