This paper discusses the formation of arching phenomenon as a stable arch in granular materials. A series of laboratory tests are performed to investigate the formation in granular materials. For this aim, a new trapdoor apparatus is designed to find the dimensions of arches formed over the door in cohesionless aggregates. This setup has two new important applications. In order to investigate the maximum width of the opening generated exactly on the verge of failure, the door can be open to an arbitrary size. On the other hand, the box containing granular materials (or base angle) is able to set on angles of zero to 90 degrees with respect to horizontal. Therefore, it is possible to understand the effect of different gravity accelerations on arching. It is observed that for all tested granular materials, increasing the door size and decreasing the base angle, both causes to increase the width and height of the arch. Moreover, the shape of all arches is governed by a parabola. Furthermore, the maximum door width is approximately five to 8.6 times the particle size, regarding to the internal friction angle and base angle of materials.

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