How must permissible explosives be stemmed to remain compliant?

Permissible explosives must be stemmed with incombustible material in order to ensure safety and compliance with mining regulations. This practice is crucial because incombustible materials help to contain the pressure generated by the explosive charge and prevent the propagation of flames. By using incombustible material for stemming, the risk of ignition from hot gases is significantly reduced, which is vital in maintaining safe working conditions in a mine.

Using any material for stemming, as suggested in one of the other choices, could introduce combustible elements that might ignite and create hazardous situations. Plastic piping, while an interesting concept, does not provide the necessary properties to contain and safely control explosive forces, making it unsuitable. Leaving explosives unstemmed would be highly dangerous, as this could result in uncontrolled blasts and flying debris, further complicating safety measures.

Thus, the requirement to use incombustible material for stemming is firmly rooted in safety standards and regulations designed to protect miners and maintain the integrity of mining operations.