New Installation Runner Up

As cities grow in size, their infrastructure must also expand to accommodate the new additions — before the burgeoning population becomes too great of a burden on existing structures. This sentiment proved true for Bellevue, Wash., just east of Seattle, as its downtown area experienced growth at a rapid pace. In the past few years, the city has seen numerous construction projects take place, in hopes to make room for additional office, residential and retail space.

Although the growth was promising, the quick building and development created a need to upgrade the capacity of the existing King County Bellevue Pump Station due to the projected increased flows to the station. To address this need before it became a serious problem, the King County Department of Natural Resources and Parks — Wastewater Treatment Division sought out contractors and engineers to plot out a way to upgrade the facility without causing major disruption to the urban downtown area.

Upgrading the facility would involve the installation of a new 5,300-l ft, 24-in diameter sanitary sewer force main designed to handle the increase in peak flow capacity from 11.03 to 14.17 mgd. To keep the project economically feasible and from disturbing any structures above ground, horizontal directional drilling (HDD) was chosen as the method for the installation. To properly install the force main, the project called for two segments of the structure to be installed using 32-in. steel casing pipe and 28-in. HDPE carrier pipe.

Since the project required such long bores in tight city boundaries, adequate planning and design were critical. In order to handle a project with such specifications in a complex environment, Jacobs Engineering Group was chosen as the engineer and Michels Directional Crossings as the contractor through a public contracting process.

During the design phase of the project, it was decided that two conductor casings were specified for the bores — one for the west end of the first crossing and the other for the east end of the second crossing. Settlement monitoring points along the HDD alignment were also included in the construction contract and the intersect method was selected for both HDD crossings as part of Michels’ plan. A 1.2 million-lb rig was used on the pullback side and a 330,000-lb rig on pipe side.

The first segment was a 3,800-ft long crossing that was 200 ft deep. Drilling was done directly under several vital and busy structures of the city including multi-story condominiums, Bellevue High School, several private homes and a courthouse building.

“The rig side of the 3,800-ft bore consisted of a 200-ft by 24-ft area on a residential street with a large apartment complex situated approximately 75 ft in front of the 1.2 million lb rig,” explains Marty Noble, King County project representative. “This proximity necessitated the erection of a 20-ft high sound wall to help deaden the noise from the sometimes around the clock operations. A comprehensive community relations outreach plan by the County afforded the residents with real time information on what they could expect through all phases of the construction.”  

The second segment involved a 1,000-ft long crossing under Interstate 405. To complete the crossing correctly, crews installed the 270-ft long steel conductor sleeve by using a pneumatic hammer because of poor soils and a high groundwater table at the high end of the crossing. The project was successfully completed on March 15.

“The Bellevue Pump Station Force Main Project demonstrated the advantages of utilizing HDD in an urban environment involving conditions that may have never been encountered before,” says Noble. “The success of this project is positive proof to municipalities that given the right soil conditions, an effective design and a qualified HDD contractor, the shortest distance between two points is actually a straight line, regardless of what may lie between.”   

Pam Stask is an assistant editor of Trenchless Technology.

Project Owner:
King County Department of Natural Resources and Parks — Wastewater Treatment Division
Engineer: Jacobs Engineering Group (prime); Shannon & Wilson (geotechnical)
Contractor: Michels Directional Crossings

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