A two-year study was conducted by CONSOL in a northern West Virginia coal mine to evaluate pillar design and support requirements to improve roof control in right-hand longwall headgates subject to high in situ horizontal stresses.

Involved with the project and co-authors of a paper presented at the International Conference on Ground Control in Mining was Consol’s geomechanical engineer D Su, superintendent D Draskovich and geologist E Thomas.

The research team said horizontal stress measurements conducted within undisturbed areas of the Consol mine revealed a pre-existing regional horizontal stress field with a significant difference in magnitude between the east-west maximum and the north-south minimum stresses.

To measure in situ horizontal stresses the CSIRO Minifrac system was employed. Minifrac is a miniature fracturing system for underground application in small diameter boreholes.

Measures were taken at a nearly virgin area within the longwall block and six CSIRO hollow inclusion stress cells were installed into the headgate roof to evaluate the effect of headgate pillar width on horizontal stress concentration at the headgate T-junction.

In addition, stress meters and roof extensometers were used to monitor pillar pressure changes and roof deformation associated with each headgate pillar design.

“Three-dimensional stress cells installed to monitor horizontal stress concentration in the headgate roof during mining revealed that a narrow (60-foot center) headgate pillar reduces the east-west horizontal stress concentration in the roof ahead of the face such that the headgate is more stable and requires less support,” the paper said.

“This 60 foot x 140-foot pillar design, which has been employed in subsequent panels, still provides adequate stress attenuation to protect future tailgates under a maximum cover depth of 1000 feet,” it said.

Roof geology from underground roof investigations and underground bolt load and roof displacement measurements confirmed the effectiveness of 12-foot long cable bolts for controlling the headgate roof subject to high horizontal stresses.

“With the new headgate pillar design and the supplemental cable bolts, headgate roof conditions in the subsequent panels improved significantly. Longwall production delay due to adverse headgate roof condition was totally eliminated, and record coal production was achieved by the subject mine in the subsequent years.”