UndergroundQ We are considering a shotcrete application over a weathered rock outcrop (consisting of sandstone, siltstone, and clay stone) in our backyard to prevent further erosion and unstable conditions. Does the outcrop need to be prepared as described in your previous Shotcrete FAQs (loose material removed, saturated surface-dry [SSD]) if a mesh that is anchored to the outcrop will be used? Also, will the shotcrete need to have fibers in the mixture? Do we need joints?
It is always a good practice to scale off the loose material from the rock face, particularly when dealing with shale or clay stone, as they degrade when exposed to the air. In addition to scaling the rock face, it should be washed down with air and water prior to gunning. As for expansion joints, they are not normally used when gunning over natural rock. The shotcrete is typically gunned continuously across the hillside without any expansion joints, with a natural gun finish following the natural contours of the rock face. With an anchored mesh in place, the use of fibers is not necessary. In many applications, fibers can be used in place of or in addition to mesh.
