The Upper Northwest Interceptor Section 9 and Associated NEA Projects (UNWI 9) involved the installation of 32,660 ft of new 36- and 24-in. interceptor and trunk sewer for the Sacramento Regional County Sanitation District (SRCSD) and the Sacramento Area Sanitation District (SASD), in Sacramento County, Calif.
These segments provide additional system capacity to accommodate final in-fill growth and to alleviate SSO risks within the city of Citrus Heights and the Orangevale community of northeast Sacramento County. All but the downstream 5,000 ft of the project was installed by microtunneling. The remaining 27,600 ft of sanitary pipeline was installed over 40 consecutive drives between 41 vertical shafts. Drives lengths varied from 588 to 978 ft along the 36-in. alignment, and from 410 to 547 ft along the 24-in. alignment. Shaft depths ranged between 27 and 64 ft, with an average shaft depth of over 41 ft. The ground conditions consisted of dense to very dense clean and cohesive sands and stiff to hard clay and sandy clay. Extensive perched groundwater was present along greater than 90 percent of the alignment, requiring watertight shaft construction and tunneling methods.
Due to the highly developed residential and light-commercial setting, the pipeline alignment was constructed almost entirely within the traveled lanes of the city streets. Shaft locations and work areas were sited between extensive overhead and buried utilities, driveways and protected oak trees while maintaining two-way traffic. The resulting shaft location work areas were extremely tight. Many locations had a minimum width of 25 ft, inside of concrete traffic barriers, with as little as 7,000 sq ft of area at jacking shafts and only 4,000 sq ft at reception shafts.
The engineer’s estimate was $64.8 million. The low bid of $52 million was submitted by a team consisting of Steve P. Rados as the general contractor with Vadnais Corp. performing the microtunneling work, and Anderson Drilling completing all of the shafts using large-diameter auger drilling. Unit costs for the trenchless work were: $800 per ft for 36-in. microtunneling, $730 per ft for 24-in. microtunneling, and approximately $200,000 per shaft. The complete project team consisted of the two owner-agencies, SRCSD and SASD; the prime design consultant, HDR Inc., Folsom, Calif.; the trenchless design consultant, Bennett Trenchless Engineers, Folsom; the geotechnical consultant, Fugro Consultants, Roseville, Calif.; traffic consultant, Y&C Transportation Consultants, Sacramento, Calif.; environmental consultant, EDAW Inc., Sacramento; and public outreach consultant, Crocker & Crocker, Sacramento. Construction management was provided by the Sacramento office of MWH. The construction team was led by Steve P. Rados Inc. from its Sacramento facilities. The shaft construction subcontractor was Anderson Drilling from Lakeside, Calif.
In addition to the impressive achievement of successfully installing 27,500 consecutive ft of microtunneled sewer in one contract, the project also included many other impressive innovations and accomplishments. The layout of 41 jacking and receiving shafts proved to be an extremely complex puzzle: providing adequate work areas for shaft construction and trenchless pipe installation, minimizing disruption to traffic flow and maintaining access to both private and commercial driveways, avoiding dense overhead and underground utilities, and minimizing any trimming or removal of oak trees. Complicating the puzzle was the need to maintain reasonable drive lengths to ensure that the pipe could be installed accurately and without damage.
Significant time was spent by all members of the design team to analyze the proposed schedule and ensure that the work could be completed within the tight schedule. While the overall completion schedule was comfortable at just under 3.5 years to achieve the needed system relief, the UNWI 9 and NEA-2 portions of the work had to be online in only 530 calendar days. More than 14,000 ft of the tunneling, the 5,250 ft of open-cut pipeline, 36 manholes and various tie-ins, testing and other ancillary work were completed within the first 18 months.
The construction team performed exceptionally to overcome the numerous project challenges. The auger-drilled shafts constructed by Anderson Drilling solved many of the challenges presented on this project by providing a highly efficient, watertight shaft methodology with integral portal stabilization. Anderson Drilling developed an innovative system to allow its very large rig to be used for every shaft excavation, despite the small work areas. A sliding spoil box was set on rails such that it slid over the shaft when the auger was withdrawn from the hole. The spoils were dumped into the box and then slid back from over the excavation to allow drilling to continue. For the microtunneling work, Vadnais contributed key performance as well, including the use of three MTBMs simultaneously, gantry cranes over the shafts, and compact separation plants. The gantry crane setups saved significant space over a conventional rough-terrain or all-terrain cranes. This extra space allowed for more on-site pipe storage, which in turn reduced the need for one-way road closures for pipe delivery. The separation plants used on the project were very compact setups built vertically to reduce the required footprint.
Project Owner: Sacramento Regional County Sanitation District, Sacramento Area Sanitation District
Engineer: HDR Inc., Bennett Trenchless Engineers, Fugro Consultants, Y&C Transportation
Consultants: EDAW Inc.
Contractor: Steve P. Rados, Vadnais Corp., Anderson Drilling