Anybody remember the red stripe that used to encircle the fuselage of early military jets? That marked the plane of rotation of the turbine of those early engines. Too often the disc failed, or sometimes uncontained turbine blades instead; any shrapnel from such an event would be thrown out in approximately that plane of rotation. Hence it was
not a good idea to linger in that plane during runups.
The principle still applies, even though the statistics are much more in your favor today.
Nevertheless, pay a visit to a good engine test bench (or test cell on this side of the pond) and inquire how many cm of concrete surrounds the tested engine. At some cells, you may find evidence of disc fragments impacting the sidewalls at predictable locations.
