The issues of estimation of danger of collapse of collapsing rocks at joint horizontally underground and open-pit development of apatite deposits sites are considered. It is shown, that at known parameters of self-collapse of covering rocks and borders of their collapse there are organizational and technological possibilities of prevention of danger of ejection of rocks in a quarry by a choice of an appropriate order and parameters of development of clearing works in underground conditions, and also a place and time of performance of protective measures.

The methodology and results of laboratory studies of the process of collapse of overlying rocks are outlined. The researches are executed in the Leningrad Mining Institute for conditions of development of apatite deposits of Khibiny, hanging side of which is represented by strong fractured rocks.