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City Of Dearborn Tackles Complicated Grout ‘Curtain’ Project
To achieve project objectives in the contact zone, grout had to penetrate the formation and withstand the harsh environment produced by hydrogen sulfide and remain intact for the duration of the construction project, said Timothy J. Myers, M.ASCE, Layne GeoConstruction district manager. For this reason, sleeve pipes were used to perform up to seven grouting passes of the contact zone with a non-evolutive, low-viscosity solution grout with adjustable set times to permeate and hydrofracture the various soil strata encountered. Two cement grouting passes were needed to strengthen the soil: one pass prior to the excavation (preconditioning pass) and one after the completion of rock grouting (stitch grouting pass).
This summary of the grouting project is based on a paper prepared by Myers, Naudts, Holly Rabine, senior staff engineer with NTH Consultants, and David Magill, chief executive officer of Avanti International, the project’s grout supplier.
A major factor considered when selecting the grout was its strength -- it had to be weak enough to facilitate multiple injection passes with adjustable set times in the contact zone, yet strong enough to hold back significant head pressure. More importantly, it had to be compatible with the caisson sinking technique. The grout needed to withstand the harsh environment caused by hydrogen sulfide in the groundwater and remain intact for a long period of time. The cost of grout also was a factor.
Acrylamide-based solution grout meets these requirements and was determined to be the most suitable grout to permeate the soils in the contact zone and to fill fine fissures not amenable to cement grout within the bedrock. The Dearborn acrylamide grouting projects were some of the largest ever undertaken in North America.
Cement-based grout was selected to permeate the larger features encountered in the bedrock. The goal of the grouting program was to create a grout curtain with an average residual permeability of two Lugeon, the unit of measurement most often used to quantify water permeability of bedrock. One Lugeon corresponds with a flow of approximately 0.00001 cm/s.