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City Of Dearborn Tackles Complicated Grout ‘Curtain’ Project
The grouting contractor designed a sophisticated acrylamide mixing and placement system to handle the large volume capacity of the project. The grouting plant, tank storage farm and grout mixing and pumping setup were designed to accommodate grouting in a wide range of weather conditions including subfreezing temperatures, rain and extreme heat. A state-of-the-art automated batching system, closed ventilation system and stringent personnel safety procedures met or exceeded industry standards for safety.
Grouting of the contact zone involved the installation of a double row grout curtain around the perimeter of the future shafts. There are two rows of grout holes that each contain 66 holes, 33 primary and 33 secondary, for a total of 132 grout holes. Grout holes were cased and drilled three feet into competent rock.
Overburden was drilled using rotary duplex with air and water flushed through 75 to 85 feet of silts and clays, 20 to 40 feet of glacial hardpan, and 10 to 20 feet of clay, silt, sand and gravel to the rock interface. An overburden casing outfitted as a sleeve pipe (OCSP) was installed into the bottom of the cased hole, and extended by riser pipe to the ground surface. All drilling and installation of OCSP was conducted prior to acrylamide grouting.
To eliminate artesian flow conditions during grouting operations, barrier bags inflated with cement-based grout were installed within the annulus around each OCSP in the hardpan zone.
To precondition the soils disturbed by drilling, a first grouting pass with cement-based suspension grout was performed in the contact zone. Several grout passes with acrylamide-based solution grout then were injected into the contact zone through the OCSPs. Grout was injected in a stage-up fashion with inflatable packers straddling one or more sleeves targeting each stratum in the contact zone.
Different grout formulations were selected to grout the contact zone; utilizing either 23 percent concentrated acrylamide or 11.5 percent concentrated acrylamide, depending on the number of injection passes performed in the area and soil conditions encountered.
Upon gelation, the acrylamide was flushed from each OCSP to facilitate the next grouting passes via the same sleeve. An engineering evaluation of the real-time grouting data was conducted to allow evaluation of the need for additional grouting passes in the particular strata.
After completing grouting of the contact zone, the second phase of pre-excavation grouting in bedrock was implemented via the same OCSPs.