Our company has been working on the design of a concrete pond for winery wastewater and the contractor proposed to replace the concrete liner with a geomembrane (canal 3) covered by shotcrete. Have you seen cases of this application being successful for wastewater holding? As an alternative we are considering applying the shotcrete over a clay liner. Are there any concerns or recommendations for this approach?

Shotcrete is a placement method for concrete. Shotcrete has been successfully used for over 70 years in thousands of industrial wastewater treatment/storage tanks, as well as replacement lining of sewers and manholes. Thus, exposure of the shotcreted pond to wastewater should be as good or likely even better than the original cast concrete liner. Long term durability of the shotcreted section will be dependent on the concrete mixture design. Many contractors use supplemental cementitious materials (SCMs) like silica fume or fly ash to improve the pumping or shooting characteristics of the mix. These SCMs also help to reduce permeability, increase strength, and thus make the concrete more durable. Fly ash also has the benefit of adding some sulfate resistance that would be beneficial in wastewater exposure conditions. Shotcrete is often shot on geomembranes or directly on the subgrade soils if they are stable enough to hold the impact, and weight of the shotcrete.