preferably an air knife as is well known in the art for delivering a high velocity jet of air this suction nozzle 11 has a circular cross section of about 25 cm diameter and in this case the periphery of the tip of the nozzle 13 follows an undulating path which is less likely to damage subterranean pipes which the free end of the nozzle 11 may encounter the nozzle 11 is provided with couplings or brackets 14 into which any number of extension pipes 15 may be inserted to extend the length of the nozzle 11 in this example the nozzle is made from aluminium which is strong and light the nozzle is provided with an on off switch in this case on a handle 16 used by the operator to direct the nozzle the on off switch immediately starts continues or stops suction through the nozzle 11 the switch is preferably arranged such that an operator must constantly apply pressure to it to continue the sucking operation when the operator stops applying pressure to the switch suction is then immediately stopped the ability to immediately disengage suction is particularly useful to enable blockages to be cleared from the end of the nozzle and to prevent injury in case the operator or his clothes are accidentally caught in the nozzle the nozzle is provided with a flexible hose 17 which may be made from heavy duty rubber to connect the nozzle to a boom 18 which may be hydraulically supported for easy operation and which is mounted on the vehicle 12 containing the air pump and filtering equipment fig2 diagramatically.