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Druid Hill Park was inaugurated in 1860 and is listed on the National Register of Historic Places, totaling 745 acres (300 ha) in size. Druid Lake was constructed (1863–1871) to provide a 1-billion-gallon (440 million m3) capacity, 55-acre (22 ha) freshwater reservoir supplying the city of Baltimore, Md., and surrounding counties with drinking water. The lined temporary stone cofferdam was a critical component of a much larger, $164 million, five-year project to modify the existing Druid Hill Reservoir.
Due to new federal government drinking water regulations to protect drinking water from potential contaminants, the city of Baltimore decided to install two precast, prestressed water storage tanks up to 548 feet (167 m) in diameter underground in the western end of the lake. The total lake area will be reduced by 6.3 acres (2.5 ha) and will remain an ornamental lake in keeping with the existing park setting.
The temporary cofferdam was needed to hold back and maintain the exposed treated drinking water and create a barrier to allow dewatering of the western half of the reservoir in advance of constructing the underground storage tanks.
To aid in hydraulic retention a 1,200 ft (365 m) temporary lining was installed on one face of a stone cofferdam. The cofferdam varied in depth based on the existing reservoir bottom, ranging from 0 to more than 59 feet (0 to 18 m) deep. A 40-mil (1-mm) reinforced potable-water-grade geomembrane (8138 XR-3 PW)—manufactured by Seaman Corp.—was chosen. The liner was extremely puncture resistant to survive the installation process over angular riprap. Another factor in choosing this particular potable water geomembrane was its higher density and being heavier than water to allow for the sinking of the liner. A double-sided geocomposite was installed directly on the stone cofferdam prior to the installation of the liner. A piping system with hardware was attached to the leading edge of the geomembrane panels to assist with floating on top of the water.
The installed stone cofferdam varied in length and depth over the entire area, requiring that the pre-fabricated geomembrane panels vary in length. Each had to be measured and cut precisely to ensure the leading edge of the cofferdam would be covered after the liner system was sunk below the waterline. Another project challenge was keeping the liner afloat as it was incrementally being pulled out across the water and cofferdam, prior to being sunk into place. In case of heavy rains, multiple pumps were ready in order to keep water off the top of the liner as it was deployed from the embankment.
One of the final challenges to ensure the successful installation of the liner system was the sinking of the liner. The installation crew worked systematically from boats to drill holes into the schedule 40 PVC pipes located on the leading edge. Water filled the 15 cm (6 inch) pipes, creating ballast at the leading edge. The liner system sank to the bottom of the reservoir and cofferdam over the course of several hours. Then, divers were used to make sure the liner was in contact with the cofferdam below the waterline. The geosynthetic liner installation phase was completed in just under two weeks.
The geomembrane installer, Hallaton Environmental Linings, was awarded IFAI’s (Industrial Fabrics Association International) 2019 International Achievement Awards (IAA) in the Geosynthetic Miscellaneous Category for the Druid Hill Reservoir—Temporary Lined Stone Cofferdam project.
SIDEBAR: Project Highlights
Druid Hill Reservoir
Owner: City of Baltimore
Location: Baltimore, Md.
General Contractor: Oscar Renda Contracting
Engineers: Brierly Associates and Whitman Requardt & Associates
Geosynthetic Products: 8138 XR-3® PW
Geosynthetic Manufacturers: XR Geomembranes by Seaman Corporation
Installer: Hallaton Environmental Linings