Shotcrete is routinely used to seal shale after excavations. It is typically done as soon as possible after the excavation because the shale will deteriorate when exposed to the air. When shotcreting, it is considered good practice to wet the receiving surface prior to gunning to create a saturated surface-dry (SSD) condition so the substrate will not draw moisture from the newly placed shotcrete. A good SSD condition is where the surface is wet without any standing water on it. Gunning over wet shale should not be a problem unless the water seeping from the shale is moving. If that is the case, we would recommend installing weep holes with plastic pipe at the locations where the water is seeping from and using an accelerator to flash-set the material immediately around the weep-hole pipe. It is also a good idea to install weep holes at regular intervals along the excavation or exposed hillside. It is important to use a qualified shotcrete subcontractor for this or any high-quality shotcrete installation. A qualified shotcrete contractor will use ACI-certified nozzlemen and should provide you with a résumé of similar, successfully installed projects, along with the up-to-date contact information of representatives from the owners or engineers involved in those projects. The ASA Buyers Guide (www.shotcreteweb.wpengine.com/pages/products-services-information/buyers-guide/) is an excellent source of shotcrete contractors.