In cases where power-driven machinery is employed in the excavation of a trench, steep working faces may necessarily result from the nature of the machinery used. These steep faces shall be broken down to a stable slope as they occur, except at the working face. In such cases, however, every precaution shall be taken to prevent unprotected workmen from entering the zone of danger either on the surface above,the face or at the bottom of the trench near the face.