This large CSO project in Alexandria, Virginia required pregrouting for hand mining. The hand mining was to install a new pipe below an existing influent tunnel, which is on piles. The hand mined tunnel was to connect two a secant pile shaft to a slurry wall pit, which are roughly 25′ apart.
The challenges on this project included:
- no drilling allowed without pre-probing
- 3′ thick secant pile walls
- unknown exact pile support layout of influent tunnel
- fully saturated soils
- potential debris in backfill under existing influent tunnel
CJGeo proposed performing the permeation grouting using acrylate. Acrylate is exceptionally low viscosity, so well suited to the organic, fine-grained soils. Previous permeation grouting on the project was done with acrylamide. However, acrylamide is neurotoxic and carcinogenic, so the owner was happy to learn about acrylate, which has very similar properties, but is not neurotoxic or carcinogenic.
To avoid drilling, CJGeo developed a pushed/driven probe grouting installation. This approach uses 1/2″ diameter steel tubing driven to depth, and then grouted through during extraction.
The 3′ thick secant pile walls didn’t pose too much of an issue, clearance holes were drilled with hand-held percussion drills.
Drilling wasn’t allowed. The owner and design team were concerned about potentially hitting and damaging the pile foundation. Because there was the potential for debris, there would be no way to distinguish debris from a pile until it was potentially too late. By using groutable probe rods, CJGeo avoided all of this risk.
The fully saturated soils had relatively low permeability, and the site was constrained on two sides by the existing SOE structures. Performing permeation grouting when below the water table you’re limits the placement rate by the ability to displace the existing groundwater. Because of the water-tight walls on two sides of this pregrouting for hand mining application, take was slower than originally anticipated.
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