**4. Caving behaviour**

The longwall started retreating on 12 June 2012 with a windblast management plan in place that required additional precautions to be used during mining until the caving commenced and the goaf developed. If the goaf behind the longwall face had not formed by the time the face had retreated to 25 m from the start position, additional work to induce caving was planned. However, the conglomerate caved, starting at the centre of the panel and progressing toward both gate roadways, after 24 m of retreat. This was a significant improvement over the estimated distance of more than 60 m for caving to start that was made based on modeling studies of the untreated conglomerate.

Beyond the startup area for a distance of 200 m, the conglomerate was preconditioned using boreholes located on approximate 80 m centres. The intensity of fractures placed in this main part of the longwall panel was approximately 25 percent of that applied along the startup section. A 100 m wide window was then left with no preconditioning to allow comparison of the fractured and unfractured conglomerate caving behavior. Mining under this section of conglomerate demonstrated that the preconditioning reduced the intensity of the periodic weighting events, but the events that still occurred were more random under the precondi‐ tioned roof. When mining under the conglomerate that was not preconditioned, weighting events could be anticipated to occur at regular intervals of about 15 m of longwall retreat. Therefore, adjustments to the daily longwall extraction plans were made so that any slowing or halting of mining was avoided when approaching an anticipated weighting event. Using this modified mining strategy, mining was continued without using preconditioning for the rest of Longwall 101.

Monitoring and Measuring Hydraulic Fracturing Growth During Preconditioning of a Roof Rock… http://dx.doi.org/10.5772/56325 911

**Figure 12.** Fracture orientation and spacing implied by intersection and temperature logging data collected in bore‐ hole 101AN during fracturing of borehole 101AM. The upper part of AM was fractured at 2.5 m vertical spacing which was reduced to 1.25 m spacing below 151 m depth.
