Clean Water Services (CWS) Bull Mountain Trunk Sewer Main - AKS Engineering & Forestry

Clean Water Services

CWS’s Bull Mountain Trunk Sewer Main spans the residential communities of Bull Mountain, South Cooper Mountain, and River Terrace, running adjacent to the major cities of Tigard, Beaverton, and King City. This sewer main is a crucial element to the highly populated areas of Washington County as they continue to experience residential growth. After performing the Upper Tualatin Interceptor Study and Master Plan Update (Study), CWS found that the Trunk Sewer Main would not be large enough to provide sewer capacity to the growing communities and a redesign was on the horizon. Our team collaborated with CWS and Emery & Sons Construction to develop a reliable plan of expansion and the design, construction, and finalization of Bull Mountain Trunk Sewer Main was completed in just over three years.

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Overall, the Bull Mountain Trunk Improvements project was an outstanding success story. A project of its size and complexity, along with its many unique challenges, adverse conditions, and scheduling conflicts, would easily have taken up to five years to complete. Instead, these challenges and conditions were dealt with seamlessly, and the Bull Mountain Trunk project was completed not only one month ahead of an ambitious two-year schedule, but $1 million under budget.

ARBORIST

One of the key concerns of this project was working closely to the Tualatin River. The entire lower corridor of the alignment, which ran along the north bank of the river, was narrow with many well-established trees, providing little space to replace the main without sacrificing many of these trees—and preserving as many trees as possible was a top priority.

CWS and our team of arborists joined forces to evaluate the trees in the area for removal and preservation. Together, we carefully analyzed the existing trees and determined the potential impact that construction would have on them, working closely with the engineering team to minimize the number of trees to be removed through designed solutions. To provide maximum protection of trees, it was also determined that construction limits would have to be minimized to the greatest extent practical.

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For instance, there was not enough space to install a new line parallel to the old one without significant impact, so the design called for a dig-and-replace approach. This approach meant crews carefully removed small sections of the old pipe one section at a time and replaced them with new pipe in place. Arborists were on-site during construction and, by the time the project was completed, only 13 trees required removal.

All 13 removed trees lived a second life and were left within the adjacent vegetated corridor where they provide wildlife habitat and improve its functions. In fact, during construction along the Tualatin River, any downed wood was left at the site to be used in restoration work to increase habitat. In addition to this, branches and other woody debris were chipped and used as a base for the haul road, which prevented the compaction of native soils and damage to tree roots.

CIVIL ENGINEERING

Our engineers’ first deliverable for the project was an alternatives analysis, where we discovered that a portion of the existing sewer main was constructed through a mobile home park and private residential properties, which limited CWS’s ability to maintain the line and made the existing location undesirable. The alternatives analysis also included engineer estimates for routing options, preliminary plans and profiles, and technical memoranda from each of the specialty consulting groups. The analysis reviewed routing options with a triple-bottom-line approach that considered social, economic, and environmental impacts in addition to constructability, operation, and maintenance, and combined public use opportunities. Five alternatives were conceptualized and evaluated.

Several key considerations led to the selected alignment:

Property acquisition was minimized by utilizing existing easements on the project’s lower portion.
SW Fischer Road, which was being improved by the City of King City during the alternatives analysis, was not impacted.
Collaboration with the City of King City on a planned street improvement project at SW 131st Avenue became possible.
Maintenance access was improved, and constructability challenges were addressed by eliminating the alignment segment routed through the mobile home park.
Construction permitting was simplified because it avoided crossing a Bonneville Power Administration (BPA) easement.
A new sewer main outside of the Urban Growth Boundary (UGB) was not proposed, alleviating potential public opposition and land-use permitting risks.

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After alignment selection, our team took over as project manager and facilitated project meetings, managed subconsultants, and identified and coordinated the submittal of all permits. Our team prepared construction drawings, an engineer’s estimate, and special provisions, and performed hydraulic calculations to size the new trunk main, which included flow/capacity and buoyancy calculations to evaluate potential pipe uplift.

While completing design, CWS discovered that stormwater infrastructure that paralleled the sanitary sewer alignment in SW 131st Avenue had corroded, leaving large holes along the bottom of the storm pipe. CWS quickly added its repair to the project scope so that the road would not be impacted a second time, creating additional inconvenience and cost to the community. Our team worked with Emery & Sons, the contractor for this project, and determined that a dual trench with a 48-inch storm main and a 21-inch sanitary sewer main would be the most cost-effective and least disruptive. Thanks to this collaboration, the project team added $1 million in storm improvements to the project scope without adjusting the original schedule.

LAND USE PLANNING

For this major sanitary sewer trunk line upsizing project, our planners led a team of consultants for the planning, permitting, and design phases. The improvements included approximately 9,800 linear feet of sewer main in King City/Washington County, an above-grade structural drainage crossing, significant natural resource permitting, and coordination with multiple jurisdictions, including Washington County, King City, and CWS. Acting as the project manager for the project, our team oversaw coordination with multiple jurisdictions and was responsible for facilitating all project meetings, managing the four subconsultants required to execute the design of the trunk main improvement, and identifying and coordinating all permit submittals. We represented CWS at a pre-application meeting and created Washington County Type II flood plain and drainage hazard area land use applications with Goal 5 natural resource overlay designations.

In the end, the project was delivered on time and under the consulting fee budget, and it won APWA’s 2019 Public Works Project of the Year.

NATURAL RESOURCES

The partial extension of the new sewer line through wetlands, waters, and vegetated corridor was of great concern. Our natural resource team conducted a site visit to delineate potentially jurisdictional wetlands and waters as well as required protective vegetated corridor buffers during the early design phases of this project to install approximately 9,800 linear feet of 12- to 42-inch sewer main in King City/Washington County. For this high-visibility project, we prepared an Oregon Department of State Lands (DSL) wetland delineation report, CWS Natural Resource Assessment (NRA), Washington County Community Development Code Section 422 land use submittal, and DSL/US Army Corps of Engineers (USACE) permitting, and conducted a robust alternatives analysis to avoid and minimize wetland impacts for expedited permitting while meeting CWS’s design goals.

This helped the team determine an alignment that avoided or minimized impacts to protected resources. Our team reviewed the results of these studies with CWS staff and evaluated affordable alternatives to avoid potential harm to these sensitive areas. The resulting design called for a dig-and-replace approach. This approach meant crews carefully removed small sections of the old pipe one section at a time and replaced them with new pipe in place. Two small wetlands were avoided entirely. In the end, the project only impacted one wetland.

LAND SURVERYING

Our surveyors completed a partial field topographic survey of existing conditions in the project area to supplement CWS’s topographic base map. They documented manufactured and natural features to be considered in project design, provided accurate and representative 1-foot ground contours of the project area on the base map, installed a network of survey control monumentation, and collected survey data.

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