A system of filling mined cavities in which crushed rock is carried along a pipeline by compressed air and discharged at high velocity into the space to be packed, the intense projection ensuring a very high density of packed material. For stowing shallow workings--up to 200 yd (183 m) in depth--the stowing plant may be installed on the surface. The air pressure is about 60 psi (414 kPa). For deeper workings, the plant may 2393 be installed underground, and the crushed rock taken down from the surface. The stowing pipes are about 5 to 6 in (approx. 13 to 15 cm) in diameter. The system is often employed if important surface structures require protection. The material used is from old dirt heaps, screen dirt, and washery rejects. The material is crushed to -2-1/2 in (- 6.35 cm) and preferably without the -1/2-in (-1.27-cm) material.