Most shotcrete slopes are placed without moisture barriers and are constructed to ensure that water pressure does not build up behind the slope and create hydrostatic pressure on the backside of the shotcrete. This is generally done with drainage material and weep holes or vents near the base of the shotcrete slope. Please bear in mind that shotcrete slope paving alone is not generally considered as a retaining wall. If shotcrete slope paving is to be used as a retaining structure, it is generally done in conjunction with soil nailing, tie backs, or some type of structural footing. If the shotcrete is intended to be used as a structural wall, a structural engineer must be consulted to be sure all structural issues are addressed.