Technical Viability of Replacing a 54 -Inch Sanitary Sewer Force Main via a Horizontal Directional Drill (HDD): Fid-Bau Portal

Technical Viability of Replacing a 54 -Inch Sanitary Sewer Force Main via a Horizontal Directional Drill (HDD)

Fernandez-Cuervo, Victor / Vieira, Ricardo

The following two options were contemplated: Alternative 1 - Approximately 9,500-linear-foot, dual-barrel, 36-inch diameter HDD with steel casing. This alternative recommended the use of the HDD "intersect" construction method because of the long lengths involved. Alternative 2 - approximately 4,500-linear foot, HDD, dual-barrel piping. Evaluation Approach: We reviewed the current state of the Horizontal Directional Drill/Bore technology with respect to completing 30-inch or greater diameter bores for drill lengths greater than 4000 linear feet. We identified contractors and installations where "single pass" or "intersect" drilling for long-length crossings were successful. Additionally, we contacted local and national HDD Contractors with a successful record of performing this work and solicited feedback on their perceptions of the risks and/or limitations in undertaking a similar exercise. The availability of the drilling equipment was assessed and lead time associated with mobilizing Contractors to the site was obtained. This element of the evaluation was crucial in determining whether the work was both: (1) technically feasible; and (2) capable of being accomplished within a reasonable timeframe. We reviewed the technical merits of the alternatives by: performing a hydraulic analysis of the scheme; determining operational pressures and potential head losses; and, finally, assessing whether the pipe sizes proposed were satisfactory. We conducted a complete geometric analysis of the HDD alignment, validating the recommendations for the horizontal and vertical curves. This analysis identified launch/exit angles, ultimate elevation, setback distances, horizontal length, and final lay lengths of the pipe. We determined the design requirements for the HDD casing by establishing the pipe diameter, thickness, and material of construction following a detailed analysis of the allowable bending and hoop stresses. We incorporated geotechnical data in the analysis of loads and stress in the various pipe segments (along the HDD bore path), and consequently calculated the total "pulling force" required to successfully complete the installation. Following review of the geotechnical data, we made recommendations on the minimum vertical or horizontal clearance required between the two bores, and consequently recommended the width of the easement required to accommodate the pipes. We evaluated the feasibility of constructing both options using standard drilling equipment on the market today. We sized the equipment, determined availability, staging requirements, and drilling risks by considering the reliability/accuracy of the cutting tools and guidance system of the HDD equipment. We provided recommendations on the methodology ("single pass" or "intersect" HDD) by considering the minimum clearances (vertical and horizontal) required to successfully complete a dual bore.