The East Side Combined Sewer Overflow (CSO) Tunnel Project will convey captured flows from Southeast Portland to a pump station on Swan Island, travelling under a busy industrial area along the Willamette River. The main tunnel is being constructed at a depth ranging from 30 – 48 metres and, when finished, will be approximately 9,144 metres long with a finished inside diameter of 6.7 metres. The $US426 million East Side CSO Tunnel Project, constructed by the joint venture of KBB, is scheduled for completion in 2011. Jacobs Associates assists the owner, Portland’s Bureau of Environmental Services (BES), with construction management (CM) services, including contract administration and resident engineering. Craig Kolell, Associate at Jacobs Associates, provides CM services to BES for the microtunnel and pipeline work on the East Side project.

The main tunnel will function as a storage and conveyance conduit, intercepting a series of gravity conduits and drop structures that will collect flows from 13 of the system’s existing outfalls. Flows from several of the existing outfalls will be conveyed to the new tunnel via nine 2 metre diameter reinforced concrete pipe microtunnels. The nine microtunnel drives will total approximately 2,385 metres. One of the drives recently completed, the Outfall 46 drive, is a 938 metre long microtunnel, currently the longest microtunnel driven in the United States. The Outfall 46 alignment passes between an active railroad yard and a cement storage and shipping facility.

Driven by design

A slurry pressure microtunnel boring machine (MTBM) was required to prevent the soil from running or flowing uncontrollably into the machine face. Slurry microtunnelling minimises groundwater drawdown and provides positive face support, thereby minimising the effects of tunnelling on nearby structures and utilities. The MTBM is capable of supporting the soil exposed in parts of the face while cutting harder layers and concretions in other parts. It is equipped with a stone crusher and disc cutters to break up boulders with compressive strengths up to 379211 kPa (55,000 psi). A manlock was included to allow hyperbaric interventions for removal of obstructions and changes to cutterhead tools.

The original design for the Outfall 46 drive called for two microtunnels of 589 metres and 30 metres respectively, with an intermediate shaft. During construction, KBB proposed eliminating the intermediate shaft. It should be noted that the construction contract is cost reimbursable fixed-fee with labour, equipment, and materials paid by the owner as reimbursable costs. BES accepted the higher calculated risk of the long drive, which brought potential cost savings from the elimination of the intermediate shaft. The decision was based on the department's experience with microtunnelling on the West Side CSO Project in similar ground conditions and with a similar machine, along with the technical feasibility performed for this long drive by KBB. The advantage of the longer drive to KBB was the schedule savings from the elimination of the shaft, which was located in a difficult position with limited space. After the decision was made, design modifications were made to the MTBM system to support the longer alignment, including changes to the slurry system, intermediate jacking stations controls, and electrical equipment.

Geological conditions

The Outfall 46 drive was driven through two geologic units: artificial fill (Qaf) and sand/silt alluvium (Qal and Qff). These two units were defined in the geotechnical data report as follows: Qaf mostly consists of gravel, sand, sandy silt, and silt with organic debris. However, in this case, it will also contain building debris, abandoned steel rails and timber railroad ties, concrete, logs, and wood waste including sawdust, branches, wood chips and fragments. Fill composition will vary within the project area depending on previous site use.

Qal and Qff predominately comprise interbedded sandy silt and silty fine sand deposited as recent alluvium (Qal) and late Pleistocene fine-grained catastrophic flood deposits (Qff). They are typically non-plastic to low plasticity, but some zones of moderate to high plasticity elastic silt are found. Some gravel lenses are also found in this unit. The alluvium is typically stratified with alternating layers of fine sand, sandy silt, and clayey silt. The consistency of the alluvium is most often described as soft to medium stiff for fine-grained layers or loose to medium dense for coarse-grained layers. Organic material was encountered during drilling within the Sand/Silt Alluvium, especially in sloughs and gulches. Organic material commonly consists of organic silts, wood fragments, logs, and wood debris.

Machinery – coming up roses

An AVN 2000D machine built by Herrenknecht Tunnelling Systems was purchased for the project and named Rosie. The 2.6 metre diameter cutting wheel can rotate in both directions to compensate for any rolling of the machine caused by reaction to the torque of the cutting wheel. Situated in the shield directly behind the cutting wheel, the excavation chamber processes all excavated material, crushing it to a grain size suitable for transportation by slurry circuit. The conveyance of the excavated and crushed material is undertaken in a slurry suspension. A feed pump pushes this suspension through a feed line into the excavation chamber, where it mixes with the excavated material. Then the mixture gets pumped via slurry pumps to the separation plant at the ground surface. The separation plant disjoins the material and slurry suspension. The addition of bentonite allows the variation of the density and viscosity of the slurry suspension to suit geological conditions.

The jacking shaft was a 7 metre diameter, 16 metre deep secant pile shaft with a jet grout bottom plug. Due to space limitations at the bottom of the shaft, a platform was erected ten feet off the bottom for the slurry pump. A 1,100 tonne indexing jacking frame was used to handle the ten foot pipe sections.

The receiving shaft is located at the existing 2 metre outfall that will eventually divert flow into the microtunnel. The floor and walls of the structure were completed prior to the drive and the MTBM was removed from within the 4 metre wide by 5.5 metre long chamber in the structure. A 3.3 metre by 3.3 metre opening was left in the diversion structure wall to allow for the break in.

Jacking Stations

Because of the length of the drive, seven 1,100 tonne Intermediate Jacking Stations (IJSs) were used. The length of the drive also required some additional electrical equipment be set in the pipes behind the machine. Therefore, the first IJS was installed approximately 45 metres behind the machine. Subsequent IJSs were installed approximately every 128 metres. The jacking forces slowly increased from 150 to 250 tonnes to approximately 700 tonnes towards the end of the drive. 900 to 1,000 tonnes was sometimes required to start the pipe moving again after down time for adding pipe, surveying, or maintenance. On most pushes, only one or two IJSs were needed.

The long and the east of it

At the start of the drive the high silt content in the ground required some modifications to the separation plant equipment and handling procedures. Production rates were reduced when wood piles and miscellaneous pieces of metal – including large spikes, nails, and bolts – were encountered. The average daily production was approximately 16.5 metres per day, with production on the best day being 46 metres. No hyperbaric interventions were required and no cutterhead tools needed to be replaced.

The Outfall 46 drive is now the longest microtunnel ever driven in the United States. Like most rewarding tunnelling achievements, it was accomplished through hard work and long hours by a team of labourers, operators, engineers, and managers. Most importantly, it was completed safely. Three hundred and six pieces of undamaged pipe were installed with no injuries.