MD 355 Lightweight Fill

Bethesda, MD


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 sat across two 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 needed lightweight fill material for the majority of the backfill to reduce loads on the new pedestrian tunnel.

The extensive utilities and limited working space made other lightweight fill materials. Alternates, such as EPS blocks and lightweight aggregate, were impractical to place. The Clark Foundations project team reached out to CJGeo to come up with a pumpable lightweight fill solution.  



CJGeo proposed using 30lb/cuft CJFill-Ultra Lightweight 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. Low density grout also significantly reduces encased utility buoyancy during placement.
  • Strength: the 125psi at 28 days strength far exceeded the requirement, and allowed for nearly immediate final backfill & paving
  • Safety: time in the tunnel to set the grout placement pipes is minimal. No personnel are in tunnel during placement.
  • Constructibility: aside from being lightweight, the primary concern was ensuring that all utilities were fully encapsulated. 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.

7 day breaks on the cellular concrete exceeded the minimum strength for backfilling. This allowed the customer to stay on their targeted schedule.

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