Bridge Approach Lightweight Fill



A 30″ RCP culvert pipe was discovered under the footprint of a proposed bridge approach embankment during construction.  Due to the depth of existing fill over the pipe, and the proposed additional fill to be placed over the pipe, the pipe would be over loaded.  The pipe ran under a total of 11 traffic lanes and 6 shoulders of Interstate 395.  

The project’s geotechnical engineer determined lightweight fill needed to be installed over the pipe prior to construction of the MSE walls and fill material.  The contractor reached out to CJGeo for a solution.


  • The lightweight fill needed to be 15 pounds per cubic foot or lighter in order to protect the pipe.
  • The material had to have adequate bearing capacity to support the MSE wall footings, and the fill material.
  • Speed.  The project was stopped, and material had to be placed up to eight feet deep, so thin lifts weren’t feasible.
  • An area inside of a trench box also had to be filled, while removing the trench box (which supported the adjacent traffic lane).


bridge approach lightweight fill installation using geotechnical polyurethane by CJGEO

Spraying geotechnical polyurethane lightweight fill.

High density geotechnical polyurethane was chosen for the bridge approach lightweight fill material.  The chosen geotechnical polyurethane had a bearing capacity of 3700PSF, elastic modulus exceeding 600PSI, and a unit weight of 2PCF. All three of these characteristics exceeded the project requirements.  An added advantage of polyurethane grout is that it can be installed in very thick lifts.  Due to the effectively instant cure of the selected geotechnical polyurethane, the trench box area could be filled while slowly lifting it out of the excavation.

Cellular concrete was evaluated, however was ruled out.  Cellular concrete was ruled out due to not being able to achieve the 15PCF unit weight requirement, limits on lift thickness, and cure time.


CJGeo mobilized a polyurethane grouting truck to the site and placed 196 cubic yards of 2PCF geotechnical polyurethane.  Prior to the arrival of CJGeo, the customer excavated a trench over the pipe.  During placement of the polyurethane grout, the trench box was slowly lifted up.  The entire bridge approach lightweight fill installation was completed in two days, and construction of the MSE wall was able to resume immediately.