As part of the construction of a pedestrian underpass at the Medical Center WMATA station, an open cut tunnel was installed. The tunnel crossed under MD-355, but over the underlying Metro Red Line tunnel. To maintain vehicular traffic during construction, a temporary bridge was constructed spanning the H-pile & lagging open cut walls.
After the tunnel was completed, the 7′ of space between the top of the new pedestrian tunnel and the bottom of the temporary bridge had to be filled. Numerous utilities, including gas transmission mains, a power vault & duct bank, municipal water, gravity sewer, and numerous communication duct banks and lines all passed through the fill area. The tunnel designers had planned on lightweight fill material being used for the majority of the backfill in order to reduce loads on the new pedestrian tunnel.
The extensive utilities and limited working space made other lightweight fill materials such as EPS blocks and lightweight aggregate impractical to place. The Clark Foundations project team reached out to CJGeo to come up with a pumpable lightweight fill solution.
CJGeo proposed using cellular concrete for the lightweight backfill. Cellular concrete had a few distinct advantages:
- Unit weight: the 30lb/cuft density met the designer’s requirement for unit weight, and also reduced buoyancy of to-be-embedded utilities
- Strength: the 125psi at 28 days strength far exceeded the requirement, and allowed for nearly immediate final backfill & paving
- Safety: since cellular concrete is pumped into place, just a few hours of labor were required in the tunnel to set the grout placement pipes
- Constructibility: aside from being lightweight, the primary concern was ensuring that all utilities were fully encapsulated and that there were no voids left. Since the fill had to go to within 12″ of the bottom of the temporary bridge beams, this would have been exceptionally difficult with any type of non-pumped lightweight fill.
CJGeo mobilized a single cellular concrete crew to the site, capable of producing up to 150CY/hour of cellular concrete. Over the course of three days, CJGeo placed 990CY of 30lb/cuft non-pervious cellular concrete, using Aerlite preformed foam. The placement schedule was primarily driven by DOT-imposed maximum lift thicknesses. Placement into the tunnel was done via sacrificial tubes suspended from the bridge decking, in addition to hose discharge into the tunnel ends.
7 day breaks on the cellular concrete exceeded the minimum strength for backfilling. This allowed the customer to stay on their targeted schedule.