Abstract: This invention relates to a method for bracing an underground cavity, wherein the cavity is lined with a tensionable layer, an expansible arch is then inserted against said layer, the arch is then tensioned and clamped in position to exert pressure in accordance with a specified formula.

Abstract: Welded grid structure for securing underground cavities such as mine roads, consisting of two sets or grid systems of parallel straight bars disposed in parallel planes and connected by welding to a third set of bars that extend perpendicular to the first two sets of bars. The bars of the third set are bent to enable them to intersect the planes of the first and second sets but not all intersection points are welded. The non-welded points are in accordance with a predetermined scheme as a result of which the grid system is rigid in the principal direction along the straight bars of the first two sets but is flexible along a direction perpendicular thereto. A process for making the grid structure is also disclosed.