The City of Carlsbad converted two old sewer lift stations to facilitate gravity conveyance to an expanded collection system to reduce operations and maintenance expenses associated with the old lift stations.
Horizontal Directional Drilling (HDD) was selected as the preferred method of construction for installing the new gravity flow pipeline stations because of cooperating soil conditions and minimizing impactd on environmentalyl sensitive habitat between the lift stations and the gravity collection system.
The project’s key challenges were:
- Sensitive Surroundings The project study area included nine vegetation communities; the two dominant communities being coastal sage scrub and southern mixed chaparral. Special-status wildlife and plant species that live within the project area include the coastal California gnatcatcher (Polioptila californica) and California adolphia (Adolphia californica).
- Tough Terrain The La Golondrina Lift Station abandonment pipeline alignment presented the unique challenge of threading the new pipeline through a ten-foot wide utility easement between two existing houses, down a steep slope with 98 vertical feet of fall, and negotiating a tight curvature to connect to the existing sewer manhole at the bottom of the hill; a total length of approximately 800 feet.
- Tight Tolerances The La Costa Meadows 3 Lift Station abandonment pipeline required tight controls on the vertical alignment, maintaining a constant slope of 2% for approximately 400 feet. As the only curves required for the alignment were vertical and limited to a 470 foot radius or greater, wire line tracking was required to ensure accuracy (wire line tracking records the location of the drill head to within 0.1 feet). The total length of the HDD installed eight-inch diameter pipeline was 585 feet.
HDD is a viable cost-effective method of pipeline installation in areas that restrict typical open-cut construction. For this project, the use of thinner walled FPVC (fusible polyvinyl chloride) pipe allowed for a smaller drilled hole, reducing the overall construction costs. The bend radius allowable with HDD is primarily determined by the drill string of the HDD equipment. But if highly accurate wire line tracking is required, the minimum curve radius is significantly larger than that of the HDD drill string and must be considered during the design phase.
The successful conversion of two pumped wastewater drainage basins to facilitate gravity conveyance to the existing collection system allows the City of Carlsbad to reduce operations and maintenance requirements and cost as well as inherent risks associated with lift stations. This dual-station project represents the second and third lift station abandoned within the past five years.