Wind blast is the sudden movement of air displaced by the massive
collapse of roof rock in underground openings and is experienced as a
high velocity flow of air through adjacent workings. The initial flow in
the outbye direction is often followed by 'suck back', that is a reverse
flow of air towards the newly created void. As part of the Wind Blast Project, undertaken by The University of
New South Wales, Department of Mining Engineering, a physical model
was constructed. One of the functions of the laboratory wind blast model
is to provide insight into the dynamics of the interaction between the
falling strata and the air being compressed below. During a collapse, some
air is expelled into the surrounding roadways while the remainder
penetrates towards the top of the roof elements. In this paper two aspects of the rock mass properties which influence
the phenomenon are considered: - Flow paths through the falling roof element, ie the permeability of
the roof element; - Bed separation prior to the fall which introduces a void above the
roof element. A series of tests was carried out utilising the physical model in order to
investigate the relationships between the flow of air through the simulated
roadways, the variations in gauge pressure above and below the falling
roof element, and the acceleration and terminal velocity of the latter, all
as a function of the flow paths through the roof element and of the extent
of the void above. The results of comprehensive tests using the wind blast physical model
suggested that the time history of outbye air flow in the simulated
roadway comprises two distinct phases. a) A primary phase, characterised by a high velocity flow of air which
exhibits a peak and corresponds to the period during which the roof
element is accelerating. The peak air velocity is related to the
acceleration of the roof element. b) A secondary phase, characterised by a residual air flow of a lesser
velocity than in the primary phase and corresponding to the period
during which the roof element is falling at its terminal velocity. The
velocity of the residual air flow is directly proportional to the
terminal velocity of the roof element. In a monolithic collapse of a roof element it was shown that the presence
of flow paths in the roof element increased the air velocity in the roadway
in both the primary and the secondary phases. The presence of an
air-filled void above the roof element, on the other hand, influenced the
air velocity only in the primary phase by increasing the peak air velocity.
The overall duration of the process was however decreased as a result.
'plug type' collapses, where roof failure extends to the ground surface,
potentially generate the highest peak air velocities likely to be
encountered during wind blasts.