Dynamic simulation of a caving-induced airblast

The simulation of airblasts occurring during the uncontrolled collapse of rock in a large underground open void is of particular importance due to the severe consequences associated with such events. The main difficulty herein resides in simulating the associated compressible and transient flow regime. Three different phases or phenomena are involved:

• An initial compression in the open chamber,
• Airflow through the muck pile at the bottom of the void, if any, and
• Airflow through the escape tunnel(s) connected to the void.

A2GC has developed a Python-based tool for the simulation of transient compressible air flows through ducts. It can be coupled to other numerical models simulating the compression in the caving chamber and the airflow through the muck pile (if any) between the open void and the escape tunnel(s).

The simulation is based on the Method of Characteristics (MoC), adapted for a 1D case. It allows to represent the various pressure and velocity fronts progressing through the exhaust drift(s). The acceleration of the air while it is flowing through the drift(s) is also well reproduced.

The following images show a few results of an airblast simulation. In this simulation, the caving was modelled as a porous falling piston, the porosity allowing some of the compressed air to escape from the caving chamber to the void being created behind the falling rock. No muck pile is considered in this particular example, which explains the very high velocities obtained.

Velocity and pressure in the middle of 200m-long escape drift.

Air velocity in the drift when the falling rock reaches the bottom of the void.

The various rebounds of the velocity and pressure fronts are readily seen on these plots. Depending on the boundary conditions retained, this phenomenon can be more or less mitigated. The sudden change in the flow regime that occurs when the falling rock reaches the ground (at around 7s into the simulation) is also well represented.

Don’t hesitate to contact us for further information, or if you just want to explore this methodology to your own case studies. We would be more than happy to discuss this with you.