What is the required air delivery to the intake of the pillar line if a bleeder return is closed due to roof falls during recovery operations?

The correct answer reflects the necessary airflow requirements to ensure adequate ventilation and safety in mining operations, particularly in situations where a bleeder return is compromised, such as during roof falls. Adequate air delivery is crucial to dilute and remove harmful gases, maintain safe oxygen levels, and promote the overall health of the mine environment.

In the event of a closure of a bleeder return, which typically helps to ventilate areas where there is a potential buildup of hazardous gases, increasing the airflow to the pillar line becomes critical. The specified 20,000 cubic feet of air per minute is based on the mine's design standards, regulations, and best practices that aim to establish a balance in ventilation to prevent dangerous conditions.

Maintaining this volume of intake air ensures that the operational area is sufficiently ventilated, thereby reducing risks associated with inadequate air supplies, such as explosions or toxic atmospheres. It highlights the importance of monitoring air quality and adjusting airflow as necessary during recovery operations, especially following incidents that compromise existing ventilation pathways.