Beginning in the later half of the nineteenth century, the City of Portland, Oregon, built a sewerage system that combined sanitary and storm flows and dumped them into the Willamette River and Columbia Slough through a series of outfalls.  An interceptor system was constructed in the late 1940s to collect those flows and transport them to the city's new sewage treatment plant at Columbia Boulevard.  Unfortunately, the interceptor system was not sized to handle peak storm events and combined sewage overflows (CSOs) continued to discharge into the city's waterways.  This situation worsened as Portland continued to grow, reaching the point where approximately 60 overflow events occurred each year, discharging an annual average of 6 billion gallons (23 billion liters) to those waterways.

In 1991, the City of Portland through its Bureau of Environmental Services (BES) embarked on a 20-year program to improve the quality of its waterways.  In a mandated agreement with the State of Oregon, the City began the study that has led to the $1.4 billion CSO Management Program undertaken to reduce CSOs by 94 percent.  PB was asked by the city to assist in this monumental effort as the design team lead for the West Side CSO Tunnel, Shafts, Pump Station, and Pipelines Project and the East Side CSO Tunnel Project (Figure 1).

The focus of this article is on two particular project features:      

West Side CSO Tunnel, Shafts, Pump Station and Pipelines Project

In June 2000, a PB-led team began working with BES to provide final design of the West Side CSO Project.  The 3.4-mile (5.5-km)-long, 14-foot (4.3-m)-diameter soft-ground tunnel was located along the west bank of the Willamette River under Tom McCall Waterfront Park and the central business district, crossing under the Willamette to the 160-foot (49-m)-deep 135-foot (41-m)-diameter Swan Island CSO Pump Station, a 220-mgd (1 bld) combined sewage pump facility.¹  The pump station will pump the flows by means of the Peninsular and Portsmouth Forcemains to the existing Columbia Boulevard Wastewater Treatment Plant.

Five shafts connecting the city's existing sewer system tothe new tunnel provide:

• A location to safely drop the collected CSO flows to the tunnel      
• Surge storage capacity
• Air venting of the tunnel      
• Equipment and personnel access.

Consolidation pipelines will bring the CSO flows from the existing outfalls to the major shaft structures, the longest of these being the 7,400-foot (2260-m) Southwest Parallel Interceptor (SWPI). 

East Side CSO Project

The design of the East Side CSO Tunnel began in February 2003, with construction scheduled for completion by the end of 2011.  It is 5.5 miles (9 km) long with an inside diameter of 22 feet (6.7 m), and has four times the storage capacity of the West Side CSO Tunnel.  The tunnel will slope at a constant down gradient of 0.0012 from south to north over its length.  Its grade was set so that the elevation of the crowns at the upper ends of both tunnels are similar to allow the two tunnels to operate either separately or together, depending on the location and size of the storm.  Six shafts connecting  the ground surface to the tunnel will transfer flows from existing outfalls to the tunnel.  As with the West Side CSO Project, consolidation pipelines will be constructed to bring CSO flows from the existing outfalls to the shafts.

In-depth Geotechnical Program Leads to Successful Tunneling Effort

Geotechnical Exploration.  As the West Side CSO tunnel design got underway, one of our main challenges was conducting an efficient geotechnical exploration program that would characterize the geologic conditions along the tunnel route and provide adequate information for selection of the optimal tunneling machine and appropriate shaft construction techniques.  For these purposes we undertook a detailed four-phase geotechnical investigation program.      

• Phase 1.  Conducted as part of preliminary design, this phase included 34 mud rotary borings.  These relatively inexpensive borings provided sufficient geotechnical information for refining the geologic profile and selecting the initial tunnel profile and alignment.      
• Phase 2.  This phase included twelve additional mud rotary borings, eight rotosonic boreholes and three large-diameter [3-foot (1-m)] boreholes taken to obtain a better characterization of the selected alignment with deeper information consistent with a revised lower tunnel alignment.  Although more costly than the mud rotary borings, the rotosonic boreholes provided a much greater wealth of information regarding the gravel alluvium and troutdale formations that comprised the majority of the proposed tunnel alignment.  The large-diameter boreholes provided information on the actual drillability of the geologic formations and allowed recovery of cobbles and boulders for physical property testing.
• Phase 3.  This phase concentrated on specific, critical project areas, including the river crossing, pump station site, shaft sites, and the SWPI pipeline.  It included both mud rotary and rotosonic boreholes.
• Phase 4.  Additional mud rotary holes were drilled to confirm geologic conditions at several new sites to which proposed facilities had been relocated during the latter stages of design.

Geophysical Exploration.  Additional geotechnical information was obtained using down-hole seismic surveys (tomography).  This survey technique is used to locate obstructions, such as former bridge foundations, by recording changes in seismic wave velocities using receiver strings of borehole hydrophones.  These surveys were performed at the locations of the Old Morrison Bridge and the Steel Bridge.  Obstructions were found in both instances, but they were well above the tunnel horizon.

Surface magnetometer surveys were conducted at the Liberty Ship Memorial site to locate old ship hulls in the subsurface.  Although results showed no significant magnetic anomalies above the tunnel alignment, some ship hulls were observed sticking up from the surface along a bike path in the memorial near the Willamette River.  The magnetometer survey detected major anomalies in the vicinity of the visible ship hulls that extended 100 feet (30 m) inland from the riverbank, but sufficiently away from the tunnel alignment.²

A seismic reflection survey was conducted along two potential river crossing alignments in an attempt to identify the elevation of the contact between the sand/silt alluvium and the gravel alluvium within the river channel.  Unfortunately, the seismic reflection data yielded no interpretable horizons.  The silt alluvium that underlies the channel has a high gas content due to the presence of abundant decomposed organic material.  The gas reflected and absorbed the seismic signal, thereby restricting penetration.  Multiple reflectors from the channel bottom further masked the gravel contact.

First Use in U.S. of Large-Diameter Slurry Face TBM. This staged approach and the use of unconventional geotechnical investigative techniques provided us with the information we needed about the geology of the project alignment, which ranged from densely packed gravels and boulder fields to loose sand-filled lake beds and stream channels, and the locations of potential obstructions to construction.  With this information gathered, the contractor was able to select the appropriate tunneling machine for the project-the first large-diameter slurry mix-shield tunnel boring machines (TBM) used in the U.S.

By using a TBM that applied a positive pressure to the exposed soil face, the ground could be better maintained in place as it was excavated than if other (that is, open-face) tunneling methods had been used.  The slurry face TBM maintained a positive pressure on the tunnel face by means of a film of bentonite that penetrated the pore spaces at the face.  The slurry was contained in a working chamber at the tunnel face within which the cutting wheel was enclosed.  The slurry was recirculated through the system and was the means for removal of the muck from the tunnel face.  With this method, it was also expected that the volume of ground loss at the tunnel heading would be limited, resulting in minimal surface effects, and that the groundwater table would not be lowered.

The final proof of the success of this endeavor was the completion of the tunneling effort several months ahead of schedule without significant problems encountered.  The measured ground settlement at the surface above the tunnel was less than that anticipated based on a predictive model. The success of the geotechnical program for the West Side CSO Project led to the implementation of a similar program for the East Side CSO Project.

Hydraulic Design

Also critical to the design of the tunnel system was an extensive hydraulic analysis to confirm the final diameter and grade of each tunnel.  BES maintains models of the contributing system basins for the entire City of Portland's CSO System.  These models were used by BES to simulate the hydraulic performance of the overall system and ensure compliance with the the Oregon Department of Environmental Quality's Amended Stipulated Final Order, which requires the city to control its 55 CSO outfalls by 2011 and meet specific milestones during this timeframe.  The results provided inputs to the design such that the tunnel profiles and layout of drop structures could be made more efficient.  The specific input parameters include:      

• Tunnel size and minimum length needed to provide required storage
• Inflows to the tunnel at the proposed outfall diversions      
• Identification of additional storm water sources that should be considered for capture. 

This last feature was quite unique in that this was the first time that storm sewers were captured in a CSO system.  This feature was important because stormwater is a polluter of receiving waters.

The captured design challenges were to:

• Confirm tunnel diameter and recommend adjustments to optimize layout      
• Determine tunnel vertical profile, connecting pipe geometry and configuration, hydraulic specifics of drop shafts, overflow consolidation and locations, low flow/flushing configuration, and general venting requirements
• Ensure hydraulics of the tunnel system were efficient      
• Establish a grade that would provide for self-flushing and prevent or minimize sedimentation      
• Establish a grade that would generate sub-critical flows under all flow conditions      
• Ensure sediment deposition and air intake and outlet were effectively managed.

The conclusions we made from the analysis included the following:

• The two tunnel system met the system-wide requirements set by the Amended Stipulated Final Order.
• The tunnel slopes would transport the required flows.      
• The diversions, consolidation conduits and drop shafts could convey the peak 25-year, 6-hour design storm.      
• There was no activation of the extreme event overflows for the water quality storm and the six summer storms.     
• Vent areas at the shafts were sized to meet required air venting rates. Odor control would be at the upper ends of the tunnels.      
• The hydraulic grade lines were close to the surface during a transient condition but did not cause surface flooding.

The profiles were selected to allow for self-cleansing velocities under as many flow conditions as is possible while maintaining sub-critical velocities.  The tunnels were sized by dynamic hydraulic analyses, considering rates of filling, dewatering, and storage.

Depending on the intensity of the storm, the tunnels may flow during the storm while being pumped out by Swan Island Pump Station.  In the event of a larger storm, the tunnels will store that flow to be pumped out when there is capacity at the plant.  The storage feature of the tunnels will allow for flexibility in pump out.  The tunnels can operate under open channel or surcharged conditions. 

Completion of West Side CSO Tunnel

At this time (September, 2006), the West Side CSO tunnel work has been completed (Figures 2 thru 5) except for system testing, sitework completion and various punch list items.  A celebration to commemorate completion of the project was held on September 14, 2006.  The first CSO flows to enter the tunnel occurred on that same day as a result of a rainstorm during the celebration. 

With construction of the West Side CSO tunnel and shaft system nearly complete, we have gained an in-depth understanding of the effectiveness of the tunnel and shaft designs in the local subsurface conditions, and of the capability of the construction equipment and contractors to install the system.  The implementation of the geotechnical program has contributed significantly to this success. 

It has been possible to design and construct these tunnels because of the relatively recent development of closed face TBMs.  One pass lining technology has also improved in structural and joint gasketting design such that a large diameter deep tunnel such as the now completed West Side CSO is in place and effectively watertight.

The new concept of shaft installation using slurry walls was used on this project design.  The carefully excavated, reinforced and concreted panels placed to form an effectively watertight circle allowed for the shafts to be installed to be connected to the driven tunnel in poor, water-bearing materials.

The depth of flow and storm variable data available to be input into the hydraulic design programs by BES allowed for the two deep tunnels to be sized and graded so that appropriate flow balance will occur between them, while the Swan Island Pump Station controls the filling and dewatering procedures.  The transient analyses showed that the hydraulic grade lines in the different tunnel diameters and grades work together to control the CSOs as required.