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New sewer tunnel for Portland |
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The seaport Portland in the US state Oregon adorns itself with the sobriquet "City of roses". But during the past years, fumes with the smell of waste water have occasionally been lying above Portland, particularly after heavy rain falls. Now, new sewer tunnels shall solve that problem.
Continuous heavy rain falls had caused the flooding of the sewer system, and waste water not clarified reached the Willamette river. To come up against that problem, the municipality of Portland put out a tender for the sewer system amounting to a total of more than 1.2 billion $. A consortium consisting of an Italian and an American underground engineering company obtained the acceptance of bid for one of the largest contract sections of this project. For more than one year, multiple tunnel boring machines, made by the machine construction company Herrenknecht in Schwanau / Baden, have been driving new sewer tunnels under Portland within this phase of construction. In contrast to other cities in North America, rain water is not discharged separately from waste water. Instead of that, the sewage system is designed such that waste waters and rain water are flowing together to the sewage-treatment plant. Particularly after heavy rainfalls, waste water not clarified again and again reached the river. The environmental offices of Oregon obliged the city to make sure until the year 2010 that no more waste waters not clarified reach the river. In order to avoid the flooding of the sewage-treatment plant in the future, Portland decided in favor of new sewer tunnels with diameters of more than 5 m. In case of heavy rainfalls, these tunnels not only discharge rain water, at the same time they serve as large collecting and retaining basins. In technical respect, the project makes great demands on the tunnel constructors. On the one hand, they continuously range within aquiferous strata. On the other hand, geological conditions in Portland are extremely complex. The underground of the plain the Williamette river is flowing through consists of not consolidated sediments from the ice-age, that means sands, gravels, boulder, mixes of clay and lime as well as numerous huge erratic blocks. Moreover, the river had to be undercut on a distance of almost 1000 m, in which the thickness of the sediment stratum between riverbed and top of the tunnel does not exceed 9 m. Therefore, the consortium decided to order two tunnel boring machines, mix shield type, from Herrenknecht. With such machines, the unstable drift face, so miners call the rock being present straight in front of the cutting wheel, will be reinforced. The work at the contract section "West bank" started with the sinking of a 41 m deep shaft with a diameter of almost 20 m. Then the machine parts were brought through that shaft to the tunnel level and erected underground. The first machine headed the north tunnel of 1.2 km length which undercuts the river in a transverse angle. A second machine headed the southern sewer tunnel of 4.5 km length. The tunnel constructors also undercut six bridges. They all are high bridges so ocean-going vessels with their high superstructure are able to reach the arbors located upstream. When undercutting the constructions, the ground may not descend for more than one centimeter, otherwise the stability of the busy arterial roads would be put at risk. In addition to the large machines, another microtunneling boring machine, also made by Herrenknecht, is used in Portland. The term "micro" has very little relation to the diameter of the tunnels as they are between 1.84 m and 2.74 m.The term has to be understood historically. In the large tunnel boring machines, the operator is sitting in a control module at the face. In contrast, micro machines are controlled from a control stand above ground. All in all, the small machine will bore almost 3.5 km of tubes leading from the suburbs of the city to the great tunnel at the west bank of the river. Now, the next construction phase of the restoration project on the east side of the river starts. The municipality of Portland is hoping that in the year 2011 the project will be completely finished. Source: VDI news, H. Rademacher
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