City Of Dearborn Tackles Complicated Grout ‘Curtain’ Project

By Jeff Griffin, Senior Editor | April 2010 Vol. 65 No. 4
Production oriented drill rigs during drilling of grout holes. (Source: Layne GeoConstruction)

Many United States municipalities and utility districts face federal Environmental Protection Agency (EPA) mandates to correct water quality problems caused by discharges from combined sanitary and storm sewer systems.

The EPA cites combined sewer overflows (CSOs) as a significant source of pollution of surface waters, and the U.S. Clean Water Act of 1972 calls for correction of water quality issues resulting from CSOs.

In Michigan, the city of Dearborn is addressing CSO issues by separating combined sanitary and storm sewers and in situations where that is impractical, constructing facilities to store and treat discharges to meet federal water quality standards.

One of the most ambitious elements in Dearborn’s program is construction of two massive concrete “treatment shafts” to capture and hold CSO overflows for direct treatment and discharge into the Lower Rouge River. Each shaft is approximately 120 feet in diameter and 150-feet deep.

Posen Construction Inc. is general contractor for one of the city of Dearborn projects (Contract 7) and Ric-Man Construction Inc. is the general contractor for the other (Contract 8).

Geotechnical investigations of sites where the two shafts were to be constructed revealed layers of highly permeable soil directly above bedrock (designated as the contact zones) and fractured limestone bedrock under artesian pressure capable of producing water inflows of thousands of gallons per minute. The large projected volumes of water and the required treatment of naturally-occurring hydrocarbons plus methane and hydrogen sulfide gases were considered too significant for dewatering to be feasible.

These conditions required special pretreatment measures to facilitate the sinking of the shafts and were themselves complex projects.

Grout curtain
The solution was to design and construct a circular grout curtain around the footprint of the shafts that extended through the contact zone and bedrock to significantly reduce water inflow during construction.

Layne GeoConstruction was selected as drilling and grouting contractor to perform pre-excavation grouting around the two shafts. NTH Consultants Ltd. was the design engineer and retained Alex Naudts, P.E., president of ECO Grouting Specialists Ltd., to produce the design for the grouting program.

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