High Ridge Storm Drainage Improvement Project

Geotechnical investigations revealed approximately 20 to 28 feet of alluvial soil. Even undercutting and replacing five feet of unsuitable soil would lead to long-term settlement of the proposed culvert. Though a more costly option, the most effective solution was a deep foundation design that would avoid excessive settlement.

Bounded by several acres of wetlands on both the upstream and downstream side, preservation of the wetlands was a key driver of the design and construction process. Placing an impermeable dike in the wetlands was impractical. The materials and equipment could damage the fragile ecosystem and a dike would not cut off groundwater flow beneath it. The solution was a cofferdam of sheet piles, with the depth of the sheet piles set to effectively cut off the flow of groundwater. This allowed the culvert work area to be dewatered during construction without draining the wetlands.

To preserve the wetlands, the culvert design also included an upstream weir wall between the culvert wingwalls. The wall height was set to maintain the same elevation of the stream bed to avoid inadvertently draining the wetlands.

The box culvert has a sill at each end to keep one foot of soil in the culvert as needed to promote aquatic life passage and habitat.

Another obstacle during construction was working around a six-inch steel gas main. The line could not be relocated or taken out of service, so pile spacing near the gas main was adjusted to provide more clearance and the foundation design was revised as needed. The project also involved the relocation of a 12-inch water main to allow the culvert to be installed.

Despite challenges around soil, wetlands, and other utilities, the project was completed ahead of schedule and under budget!