MICROTUNNELING - (Portland, OR)



On a project in Portland, Oregon, James W. Fowler Company of Dallas, OR is using a Derrick 300 m3/hr slurry separation plant in conjunction with a Herrenknecht AVN 2000, 104” microtunneling machine. Representatives from Derrick Equipment along with Barry Sorteberg of Clean Slurry Technology- Derrick’s exclusive Western US underground construction agent- worked closely with JW Fowler to design the separation plant to keep up with the large microtunnel machine in any ground condition, and at maximum penetration and volumetric flow rates. The anticipated geology called for glacial til, sands and potentially some clay, so the separation plant had to be flexible to handle most every imaginable ground condition.

Primary solids removal is accomplished by two (2) FLC-2000 4-panel shakers with 7.3 G’s of consistent acceleration, outfitted with 500 um (35 mesh) polyurethane screens. As protection in case sticky clay is encountered on the project- which does not convey well on vibrating shakers- a Flo-Line Primer with 5 mesh polyurethane belt is mounted above the shakers. This machine utilizes a variable speed belt that is designed to convey out sticky, gumbo clay.

After the primary separation, a 100HP centrifugal pump is used to feed three (3) 15” sand separator cones with underflow regulators. Oversized material recovered from the sand separators is fed to another 4-panel shaker with Derrick’s patented Pyramid screens with API-100 mesh openings for further dewatering and discharge into the muck pit.

Because of the size TBM, and the amount of fine solids that could be generated, a Derrick DE-7200 high speed, decanting centrifuge with VFD control was selected to maximize fine solids removal. This unit offers up to 500 GPM and 15-20 t/hr of solids removal.

The centrifuge is ran in two modes: One mode running without any chemical enhancement so that the centrifuge removes fine solids 5-75um. When a semi-clear effluent is required, a polymer injection system is used to add a mixed polymer solution to the feed line of the centrifuge. This helps remove solids down to 0um, and insures particles in the 0 to 5 micron range do not build up and cause problems with the mud system.

The “big bowl” DE-7200 centrifuge proved to be a critical part of the separation process right from the get go. Because Fowler had constructed several ground stabilization panels using a cutter soil mixing rig to mix cement with the native soils along the tunnel path, as the TBM mined through the cement panels the viscosity and PH of the drilling mud went up considerably. However, the DE-7200 was able to keep up with the TBM advance rates and remove the fine cement particles, keeping viscosities manageable in the mud system. This allowed the shakers to work to their maximum effectiveness so that negligible amounts of fluid were lost to the muck pit, allowing for neat piles of solids to form under the shakers. Provisions have also been made for PH adjustment to help lower the cost and increase the effectiveness of polymer when tunneling in cement stabilized ground.

As with most tunnel projects, expect the unexpected as one never knows what Mother Nature is going to throw at you. By installing a separation plant with excess process capacity at each stage of separation, JW Fowler is fully prepared to tunnel at their desired production rates and not be held up by the separation plant.
dtl_product2.aspx?id=85