Wireless Sensor Networks based Monitoring of Slope Stability over Old Underground Coal workings

Abstract

Coal is primary energy source in India for generation of electricity and other industrial uses. Though Indian coal industry is one of the oldest in the world, but still India is importing coal for domestic needs. In order to increase the production of coal, many approaches are being used such as shortwall, highwall longwall mining etc. One of them is conversion of the old underground workings where coal is blocked in the form pillars into opencast projects. In the olden days, coal seams were developed using bord & pillar method of working and due to various technical reasons like strata control or fire, these were either partially depillared or not depillared yet and the panels were closed permanently. The coal left out was supposed to be a permanent loss of natural resource. Such panels are being extracted using surface mining methods. But, there are some problems in conversion of underground galleries into opencast projects like the stability of slopes and collapse of partitions due to the moment of Heavy Earth Moving Machinery (HEMM). Factors leading to the stability of old underground workings include material properties of the partition/cover, thickness of cover/partition, movement of machinery, dimensions of old workings and relative position of the underground workings to the mine bench as the bench progresses. These factors may lead to slope failures and cause damage to equipment and loss of human lives. Monitoring of stability of partition and slope is required over old underground galleries. Partition and slope monitoring is generally carried out with conventional methods. In many of the cases, the data is acquired and analyzed in offline. In such type of monitoring, the physical presence of a person is required at the site, the readout units are physically connected to the base units and generally, such monitoring can be done only during daylight. On the other hand, the wireless-based instrumentation like Slope Stability Radar (SSR) can monitor slope moments effectively but these are highly technology-based and very expensive. In order to address this ambiguity and to get real-time data to study the dynamic behavior of workings, a low cost, state of the art Zigbee based Wireless Data Acquisition System (WDAQ) was developed. It consists of sensing unit, Wireless DAQ and base station. Institute of Electrical and Electronics Engineers (IEEE)ii 802.15.4 standard of Zigbee wireless communication network was used for collecting and sending data from monitoring point to base station. Field investigations were carried out in two large opencast mines where conversion of old galleries are being done in southern India. In total, 144 locations were monitored, at each location data was captured for about 5hours to 8hours. A conventional data logger was used with the similar set of sensors for validation of zigbee based WDAQ. The variation between Zigbee based WDAQ and data logger is around 10.27% to 13.30% which says zigbee based WDAQ data is very reliable. Numerical modeling approach was used for simulating field conditions and to assess the influence of geometrical dimensions, rock properties and external load on the stability of old underground coal workings. Geometrical dimensions includes gallery width, gallery height, pillar width, partition thickness, slope angle and berm width. Regression analysis was carried out to evaluate the influence and relative significance of various parameters. Regression analysis results revealed that external load was found to be the most influencing parameter on the stability of old underground galleries. Pillar width and gallery width were found to be second and third most important parameters. Partition thickness was found to be the fourth most influencing parameter. Gallery height, Slope angle and berm width were found to be fifth, sixth and seventh most important parameters respectively considered in this study. Density of sandstone was found to be eighth most important parameter and followed by compressive strength of sandstone. Similarly, density and compressive strength of coal was found to be tenth and eleventh position to influence the stability of old galleries. Further analysis was carried out for developing design guidelines for safe extraction of old galleries. Design guidelines were developed based on FOS which was determined using Mohr-Coulomb theory in numerical modeling studies. In this study, FOS was categorized as unsafe, moderately safe and highly safe. If FOS was more than '2.0', the model considered as highly safe. If FOS was in between '1.5'iii to '2.0', the model considered as moderately safe. If FOS is below '1.5', it was considered as unsafe. Therefore, the design guidelines recommended that the partition thickness with respect to the slope angle for different gallery widths, pillar widths, gallery heights, berm width, rock properties, external load was based on FOS of “highly safe" and “moderately safe". Finally, a user-friendly software was developed to use the guidelines in simple way. The software takes input as geometrical dimensions, rock properties and external load. Optimum partition thickness, slope angle and FOS are output parameters for given input.