Reservoir meets water quality & design standards of Oregon city

Hillsboro Water Department, located approximately 15 miles west of Portland in the city of Hillsboro, Ore., serves nearly 80,000 customers. The city’s service area includes customers residing in the city’s municipal territory, more than 600 rural connections in Washington County and wholesale service to three water utilities.

Following a lengthy site selection process, the city began construction of the 10 million gal Will Crandall Reservoir and Pump Station in March 2012. The Crandall Reservoir, now completed, ensured Hillsboro met its required in-town potable water storage of three average days’ emergency demand. The new reservoir was Hillsboro’s third in-town reservoir and has increased the city’s total capacity to approximately 30.6 million gal. 

                                                                                                            

Challenges Addressed with Unique Project Delivery     

 The Will Crandall Project included many challenges from which the city benefitted by working with an experienced design team. The city of Hillsboro selected CH2M Hill to help locate and purchase a site for the facility, as well as provide engineering design and construction services.

During the development of the project, CH2M Hill and the city opted for an alternative project construction method known as the construction management/general contractor (CM/GC) approach for project delivery. Using the CM/GC approach for project delivery was beneficial for a large, complex project such as the Will Crandall Reservoir and Pump Station through engaging the construction contractor early, during the design phase.

The city solicited proposals for CM/GC services and ultimately selected Ward-Henshaw Construction. The CM/GC approach brought Ward-Henshaw Construction into the project early in the design phase, which allowed them to assist in the development of the project. Ward-Henshaw Construction assisted in the scheduling and phasing of construction, analyzed constructability issues, provided cost estimates to the team, and presented value-engineering options.

As the reservoir was classified by the building code as an essential facility, the ability of the reservoir and pump station to withstand a seismic event was of utmost importance. Geotechnical studies of the project site indicated that a probable earthquake could cause liquefaction of the soils at the project site. Therefore, strengthening of the ground under the project was the first challenge to overcome. Together, the city, CH2M Hill, and Ward-Henshaw Construction determined that the soil improvement option with the lowest impact to the community was cement deep soil mixing (CDSM).

Raito Inc. was selected as the subcontractor to perform the deep soil mixing by use of an auger to inject and mix cement into the existing soils in a grid underlying the reservoir, pipelines and pump station. This process increased stability and bearing capacity of the soils beneath the reservoir and reduced the potential of subsidence due to liquefaction.

To best address the concern for reservoir seismic performance, a prestressed concrete reservoir was selected and constructed by Ward-Henshaw Construction. Concrete offered a combination of the highest quality, most economical and lowest maintenance solution for the reservoir.

The reservoir included specialized seismic connections at the wall base and wall top, allowing for a free connection and ductility of the structure during a seismic event. Additionally, the reservoir walls were compressed both vertically and circumferentially to keep the concrete in its most efficient state which will give the structure strength and durability over its service life.

DN Tanks performed the specialized prestressing by use of a strandwrapping machine that tensioned galvanized 7-wire strand to 14,950 lb. Through their wrapping process, the prestressing was continuously recorded and electronically monitored, which ensured that a tight tolerance of the applied force was maintained. The prestressed reservoir ensured an economical project and a structure with the highest level of strength and durability to undergo horizontal and vertical ground accelerations due to a seismic event.

Energy Savings & Water Quality    

Though the primary function of the reservoir was to provide a potable water supply to the city of Hillsboro’s customers, the city also participated in the local power utility’s Dispatchable Standby Generation (DSG) Program with Portland General Electric (PGE). The pump station adjoining the reservoir required standby power in the event of power loss to the site. As the standby power system was for emergency purposes and rarely used, placing this asset into the DSG program and allowing PGE to put power back into the utility’s grid during peak demand periods seemed logical and feasible. As part of the DSG Program, PGE paid for generator fuel, all maintenance, monthly online testing, specialized equipment, and provided the city with an annual stipend for regulatory costs.

The 3,500-sq-ft pump station also housed a water-driven turbine-generator and a secondary pilot-operated, pressure-reducing flow control valve (FCV) used to fill the reservoir. The water main servicing the reservoir operated at 130 psi, which must be reduced before the water enters the reservoir. While filling the reservoir, excess pressure was converted into electricity through the hydro-turbine system and the power generated was credited to the city’s electrical bill in a net-metering arrangement. The FCV provided a backup means to fill the tank if necessary. The reservoir project has also been designed for future onsite solar power generation, reserving the area atop the reservoir, as well as a portion of the ground area, for additional power generation.

The city also reduced energy consumption by using a unique operational scheme. As opposed to their current operations, where the reservoirs fill during a three-day period and then are immediately pumped into the distribution system, the city of Hillsboro anticipated storing the water in the reservoir for up to 30 days between fill and draw cycles. Water quality was maintained with a pumped mixing system and re-chlorination. The planned energy savings associated with reduced pumping qualified the city for Energy Trust of Oregon grant money, which helped fund the mixing and re-chlorination systems.

Storing the water for these extended times meant that water quality degradation was a concern. As a result, the city utilized 12 unique sampling points, four locations at three different elevations within the reservoir. The sampling points allowed the city to closely and continuously monitor the water quality in the reservoir. All sampling was performed in a consolidated sampling location within the pump house.

Public Outreach & Sensitivity

With the challenges of the project, public outreach and communication was critical. One of the benefits of the CM/GC approach was that the contractor was given the opportunity to assist in developing a community disruption mitigation plan, which was vital to maintaining good community relations.

Several additional steps were also taken to minimize the impact to the community. CDSM was selected as the foundation system, as this type of installation was quieter than hammered piles and less costly than drilled piers. Noise and visual impact was reduced by berms that were built from the native soil, which in turn eliminated the need for trucks to haul the material to another location. The city also hosted site visits for local groups, neighbors, and schools, in an effort to educate the community on the project and how the facility was essential to the community.

A City Proud of its Reservoir

The two-year Will Crandall Reservoir and Pump Station project was completed in one and one-half years, in fall 2013. With the combined efforts of the design and construction team the city selected, the Will Crandall Reservoir and Pump Station Project will meet current water quality and design standards and will continue to offer safe and reliable potable water to Hillsboro’s water customers. On Oct. 1, 2013, the Will Crandall Reservoir was dedicated to Will Crandall, the former utilities commissioner who oversaw many water infrastructure improvements during his 29 years of service. In 2014, the project was selected as the winner of the Excellence in Engineering award, large project category, presented by the Pacific Northwest Section of the American Water Works Assn.

This article was written by Chris Young, Pacific NW Manager for DN Tanks.  It was featured in the publication, "Water and Wastes Digest," which has a circulation of over 60,000 people.