Salt storage caverns are constructed within deep salt bed deposits by circulating fluid to dissolve the salt deposit. The resulting cavern or reservoir is used to store hydrocarbons and liquefied petrochemicals within the salt deposit. A salt cavern is constructed by dissolving the salt deposits underground and extracting the resulting brine solution and placing it into a holding pond. Layfield was tasked with lining a below ground storage pond in Ontario, Canada to prevent leakage and potential contamination of underlying aquifers. The brine storage pond was lined with a double liner system that consists of a geosynthetic drainage composite encapsulated between two layers of geomembrane. The drainage composite monitors and captures any leakage through the primary or overlying geomembrane and the secondary or underlying geomembrane contains the leakage that occurs through the primary geomembrane.
There were a series of challenges during the design phase for this large brine storage pond including:
1. Selection of geosynthetics and method of fabrication, e.g., factory fabrication v. field fabrication
2. Chemical resistance of the geosynthetics to the brine
3. UV resistance of the primary geomembrane due to the long-term exposure and weathering in this exposed application.
4. Performance of the geonet within the drainage composite when subjected to high compressive stresses and the sediment in the brine solution.
5. Local weather conditions, which included higher than average rainfall, resulted in the site being completely saturated in the fall months, but the owner wanted the pond operational by early Spring of the following year.
Available data from the manufacturer shows that the transmissivity of the geonet is 20 times higher than that of a double-sided drainage composite, i.e., a geonet with two non-woven geotextiles. A drainage composite has at least one non-woven geotextile heat-bonded to the top or bottom of the geonet, which over time can intrude into the geonet causing a reduction in transmissivity. Initial designs included a double-sided drainage composite, i.e., two heat-bonded geotextiles, between the two 40 mil (1.0 mm) thick geomembranes. Layfield proposed just using a geonet to prevent geotextile intrusion and maintain the full transmissivity of the geonet over time. However, questions were raised about the possibility of a flexible 40 mil (1.0 mm) thick geomembrane deforming into the openings of the geonet and reducing its transmissivity or the geonet puncturing the geomembrane at the places where seams of the geonet overlap and create a bump. Layfield simulated this possible puncture situation in the laboratory (see Figure 1a) and the test results show that the geonet overlap did not puncture the geomembrane at the field stresses (see Figure 1b). Based on the laboratory testing, a geonet instead of a double-sided drainage geocomposite was encapsulated between the two layers of geomembrane to create a double-liner system with a leak detection zone for this brine storage pond.
Due to difficult site conditions, Layfield proposed using factory fabricated geomembrane panels in order to: minimize the amount of field welding, increase quality of the seams and final liner system, and meet the short project construction schedule (see Figure 3). Factory fabrication involves welding rolls of the geomembrane into large panels in a controlled environment, which increases seam quality and allows faster seaming. The seam quality is better because the welding is not affected by weather changes, dirt and other impurities in the seam area, and a constant temperature so the welding temperature does not have to be varied. This results in higher efficiency and quality of the seams in the factory than in the field (see Figure 4).