Mining Publication: Ground Stress in Mining (Part 1): Measurements and Observations at Two Western U.S. Longwall Mines

Researchers from the National Institute for Occupational Safety and Health (NIOSH) have been conducting research with the goal of gaining a better understanding of ground stress redistribution resulting from mining. Part 1 of this research, reported here, involves representing stress redistribution using empirical equations or numerical models that are calibrated to observations and measurements. In Part 2 of this research, numerical models are used to calculate ground stress in mines.

NIOSH researchers compiled data sets from two coal mines in western Colorado, herein referred to as Mine A and Mine B. These data sets include observations and measurements as successive, adjacent panels of coal were retreated at each mine. The data sets specifically include general observations of ground behavior, observations from ground condition surveys, measurements of pressure change with borehole pressure cells (BPCs), measurements of load and closure of instrumented support Cans, manual and datalogger-monitored measurements of roof-to-floor entry closures, manual and datalogger-monitored measurements of roof sag with extensometer, and manual measurements of roof sag using sonic probes. These data sets are described in detail in this report.

Methods were established to determine load transfer distance with various methods by detecting with borehole pressure cells, instrumented support Cans, closure measurements, and ground condition surveys the distance ahead of the face at which a mining-induced load is first sensed in the pillars and abutments. A knowledge of load transfer distance is important for calibrating numerical models for evaluating mine layout design to reduce risk of ground instabilities. Load transfer distance also forms a basis for empirical methods. Measuring first arrival of mining-induced abutment stress is necessary to determine load transfer distance.

In detecting first arrival of mining-induced stress, BPCs required the most installation effort, were most sensitive to abutment load, and yielded information about behavior of the pillars. Support Can loads, entry closures, and Can closures generally were not consistent enough to yield a reasonable degree of success in detecting first arrival of mining-induced abutment stress. Ground condition surveys were least sensitive to abutment stress but are the least costly option. Such surveys might be used alongside limited instrumentation to discern variations in the first arrival of abutment stress.