A joint venture of contractors Kiewit Corporation of Nebraska and Bilfinger Berger of Germany (KBB) is working to complete the last drive of the sometimes arduous East Side Combined Sewer Overflow (ESCSO) project for the City of Portland Bureau of Environmental Services.
Overall programme
Once operational the new sewer will allow the full interception of most combined sewer overflows before reaching the Willamette River, and so improve the quality of the water flowing through the City to join the Columbia River and the Pacific Ocean.
The East Side CSO Project is the largest construction project in Portland’s 20-year program to control or eliminate combined sewer overflows (CSOs). The programme combines major projects like ESCSO with more community schemes. CSO incidents have, in the past, occurred every time it rains, but once the overall scheme is complete it is reckoned that incidents will be limited to only two or three each year.
Utilising a series of nine microtunnelled drives totalling 2400m in length and all completed last year, the flows of existing outfalls will be diverted into special structures within new shafts along the east bank of the river and thence into the 6.7m-diameter main tunnel. This will carry the combined sewage and stormwater to the Swan Island CSO Pumping Station to the north of the combined sewer area of the city, and subsequent treatment, instead of being dumped in the river. One of the project’s many major achievements has been North America’s longest mechanised pipe-jack drive at 931m (3055 FT). This was achieved with a Herrenknecht AVN D2000 slurry machine with a cutting diameter of 104.5 in (2.654m), which allowed for an 84in. (2.134m) diameter pipe to be installed.
In addition 4,000ft (1,200m) of diversion sewers to the main shafts were laid by cutand- fill. All collector sewer pipes range in diameter from 18 to 84in. (457-2134mm).
Main drives
Mirroring, but substantially longer and of larger diameter than the West Side CSO project, completed in 2006 by a joint venture of S A Healy and Impregilo (Italy), the main tunnel drive route of the ESCSO will run from the McLoughlin Shaft in the South to the Swan Island Pump Station in the North where it will merge with the West Side CSO project flow at the Confluent Shaft, built as part of the West Side project.
Kiewit combined the internal expertise of three of its own divisions or districts with the slurry-shield and micro tunnelling experience of Bilfinger Berger to create a team capable of tackling the sometimes tough ground conditions as well as the size of the project, valued at USD 433M.
The project is led by Kiewit director Tom Corry and field operations by project manager Niels Kofoed, who joined Kiewit after working with them on Storebelt.
The main sewer runs for a total length of 8.9km, linking seven shafts and, with the microtunnels, intercepting combined sewer overflows that have been polluting the Willamette River. The main tunnel depth ranges from 25 to 50m in a very mixed geology. The Troutdale Formation, comprising 85 per cent of the tunnel horizon, is understood to be a fluvioglacial deposit but, explains Niels Kofoed, indicative of catastrophic flooding, such as might be caused by the breaching of a natural dam, which caused everything from sand to boulders to be deposited along the valley haphazardly. The material is therefore very unpredictable as well as abrasive.
Shafts
Construction of the seven main shafts (several smaller and shallower ones were required for microtunnelling) was no simple matter either, due mainly to the mixed and sometimes hard ground but particularly due to the high water table next to the river. The basic construction consisted of slurry wall support with subsequent cementation, and tremie construction of concrete poured under water in the flooded shaft to ensure no collapse of the shaft due to groundwater pressure at depth. Once the slurry walls had been installed, excavation could be carried out within the shaft area. This was possible in the dry down to 15m. Then the shaft was allowed to flood to hold back groundwater pressure, and excavated to the required depth, using clamshell excavators in both dry and wet.
At construction depth a rebar reinforcement mat was lowered through the water to the bottom of the shaft. Divers pinned the reinforcement mat to the side of the excavation using dowels, after which the concrete could be tremied over the mat to create a reinforced slab that was designed to hold back the groundwater pressure and prevent a blow-out.
TBM choice
Although only the second time that a large diameter pressurised slurry TBM had beenemployed in North America, the Herrenknecht Mixshield TBM design was chosen on the basis of its ability to control groundwater pressure at the face under variable permeable ground conditions. The groundwater at tunnel horizon is normally 2.8-3.2bar pressure, with 3.5bar maximum.
The cutterhead had to be able to withstand the abrasive and sometimes hard, blocky ground although, due to the nature of the formation, no one could be certain of how much of each type of material could be expected.
TBM control is aided by the use of a VMT guidance systems that also, tunnel party chief Burnel Preston explains that the guidance system was subject to regular survey checks by his team, it greatly reduced the amount of surveying required.
The tunnel drive from Opera Shaft to the north took from June 2007 to October 2009. It is chiefly in sand and gravel, but the northern end of the drive encountered large boulders that slowed progress both during tunnelling and overall. However the arrival of the TBM at Port Center Shaft matched what was originally scheduled some four years earlier. Face cutters became worn and damaged towards the end of the last drive neccesitating major refurbishment.
Even thought tougher cutter shoes and other measures had been installed during the break, the TBM was deliberately controlled so as not to cause major problems such as broken cutters or slurry blockages. TBM operator, Tracy Cordova, explains, “we normally operate at up to 1400kNm torque, slowing down at 2000kNm to avoid spoil removal blockages. The normal cutting speed is 1.5-1.7 rev/min using eight motors on the cutterhead drive gear. The shield thrust is carefully controlled to avoid any more cutter damage in boulder ground. Overcut up to 17.5mm is possible, chiefly to negotiate curves. We guide the shield with the thrust rams according to monitoring from the VMT laser guidance.”
A crusher also assists blockage prevention over a grizzly in the return line of the pumped slurry system within the shield. This can apply a force of 300bar to reduce boulders to a small enough size to pass through the grizzly for pumping.
All tunnelling and microtunnelling was subject to instrumentation for ground movement or changes in groundwater elevations, with more detaled checks of mre sensitive and high risk structures near the TBM. Corry explains, “for some of the riskier structures (e.g. bridges), we were required to tunnel 24/7 and monitor every four hours. The ground/water devices included inclinomaters, piezometers, deep and shallow surface settlement markers and utility monitoring points. Besides this we monitored many of the building with a ‘Cyclopes’ (a total station and prisms set up to continuously monitor the building) for movement and cracks.”
Lining & spoil
Although both the East Side and West Side tunnels have become known as the ‘Big Pipe’ locally by the client and community, they are actually constructed of one-pass pre-cast bolted concrete segments, 5ft (1.52m) wide and 14in. (356mm), made in a plant operated by Kiewit on the west bank opposite the Opera Shaft. Each lining ring, of two types, has eight segments with 15 per cent of the reinforcement provided by the usual rebar cage, and the rest by steel fibre. All segments, fitted with Phoenix joint seal gaskets, have now been made with the remainder stacked awaiting installation.
Curves or correction of deviation in the tunnel construction are, as usual, made by the position of the lining ring. The VMT TBM control system records all ring builds and has all possible ring positions programmed into the system so they can be called up according the line correction, if any, required. Each is designated according to the key segment position chosen from 14 possibilities.
As it is a pressurised slurry shield TBM system, the spoil is pumped direct from the cutterhead, via the crusher, to the surface, using intermediate pumps to keep up delivery as necessary. After settlement and removal of fines in a purpose-made treatment plant, fluid is returned to the slurry circuit whilst removed solids are bunkered or carried by cantilever-structure belt conveyor to barges on the river. From there the spoil is carried 1.5 miles (2.4km) up river to Ross Island where it could be used to reclaim a lagoon located a lagoon.
Personality
At its launch, the TBM was named ‘Rosie’; as part of a community spirit and support programme typical of city-managed construction projects in North America. While naming a TBM is no longer unusual worldwide, to have a TBM transmitting messages on the microblogging site Twitter was probably a first.
When Rosie reached the Port Center Way Shaft on Swan Island on 30 October last year, after boring 4 miles (6.4km) from the Opera Shaft, it ‘said’, “I’ve been building this big pipe for more than two years now. It’ll be nice to come up for some fresh air next month and take a break.”
Rather than taking a ‘break’, ‘Rosie’ was destined for a trip up the river to the Opera Shaft site and a major facelift.
Big move
At 417tonne (460 ton), Rosie is quite a weighty lady, and needed a 700t crane to lift the main section form the 100ft (30.5m) deep shaft, followed by the 63.5-tonne (70-ton) tail shield, all under the careful supervision of specialist heavy-lift movers Emmert International. The sections were laid on self-propelled hydraulic drive platforms to carry them to barges on the Willamette River to be towed up the river to a position on the east bank adjacent to Opera Shaft. Here Emmert had prepared the ground and laid a steel ramp at a suitable grade so that the TBM could be transported up the bank to a position on the construction site for refurbishment.
“Emmert put a good plan together and performed the move of our TBM proferssionally and without any mishaps,” say Corry and Kofoed.
Major refurbishment, chiefly to the cutterhead, was necessary after the four miles of tunnelling, even though backloading cutter change interventions were carried out before entering each shaft in order to maintain gauge cutting before boring through the concrete of the shaft. The major refurbishment was carried out by the joint venture on site. Tunnelling superintendent Helmut Schulmeyer, who was in charge of the refurbishment, explained that the abrasive ground had worn many of the cutter shoes virtually flat. It was also found that worn out shoe seating, causing cutters to wobble under loading, had led to the loss of many cutters on the first drive, especially when big boulders had been encountered.
“The last 2,000ft (210m) were in the worst ground conditions and very difficult,” comments Kofoed, “but, considering the ground, we’ve been very pleased with the machine.”
The refurbishment work included grinding out the old shoes of each cutter to allow welding in of new seatings. Also 5000lb (2,268kg) of hard facing was applied to the cutterhead for better general durability.
The rate of tunnelling has been greatly dependent on the type of ground material, but was helped initially by the experienced crews that could be recruited from the West Side CSO Project as well as the even more experienced site management of Kiewit Construction and Bilfinger Berger. The rate of tunnelling on the first drive was an average of 73m a week, 20 per cent better than schduled. On the south drive, where the most favourable ground conditions are present, ‘Rosie’ is chewing up the ground at almost double the scheduled rate.
The final breakthrough to the McLoughlin Shaft is scheduled for later this year. As at the last report before going to Press, 1,419m had been completed leaving 1266m to reach McLoughlin. At the current rate of progress the south drive is expected to hole through in October and the deadline for substantial completion of construction by 15 September 2011 should be met slightly early.
Meanwhile the Opera Shaft site has to be vacated quickly as other infrastructure projects are waiting to claim the space on the banks of the Willamette River including a streetcar route extension and TriMet’s Light Rail Extension Project.
Installation on the rest of the scheme is already under way, but once all valves, controls and monitoring devices have been fitted and commissioned, the client will activate the East Side ‘Big Pipe’ towards the end of 2011.
The main East Side CSO project tunnel over the Willamette River to the completed West Side CSO project joining at Swan Island Project director of KBB, Tom Corry Looking up the Opera Shaft showing lining structure Tracy Cordova training up a TBM opeator in ‘Rosie’s’ control cabin, showing VMT guidance display at top KBB general superintendent of tunnelling, Helmet Schulmeyer and project manager Niels Kofoed at the Opera Shaft A special ramp was laid to ease ‘Rosie’s’ passage up the river bank to the Opera Shaft site [Photo: KBB]
