SMC recently completed a contract for Jones Lang LaSalle Americas, Inc. (JLL) to repair and rebuild an underground sand filter for one of their property management locations.  During  SMC’s annual confined space inspection of the underground sand filter, the inspector discovered the underground sandfilter wall between the sand filter chamber and outfall chamber had collapsed and approximately 8,000 gallons of sand and gravel had migrated into the downstream storm drain pipes.

Results of Collapsed Wall in Underground Sand Filter

SMC notified JLL of the failed wall via SMC’s inspection report and devised a plan for the work to be completed in three phases.  Phase 1 consisted of our in-house Underground Maintenance Department bulk heading the upstream flow splitter to divert water away from the underground sand filter to prevent additional material from being lost.  Additionally, SMC removed the 8,000 gallons of material that had migrated into the downstream storm drain pipes.

Phase 2 consisted of SMC’s engineers assessing why the wall had collapsed and the recommended repair method.  SMC’s engineers determined the wall had failed due to the reinforcement brackets being installed on the upstream side of the wall rather than on the downstream side of the wall during the original construction.  This subjected the brackets and bolts to tension forces rather than compression forces, which ultimately lead to the failure of the wall.  The repair methodology chosen by SMC’s engineers included designing a modified wall.  The existing wall would be jacked back into place, a new wall be cast behind the existing wall for structural support, and design of new brackets and bolts to be installed on the correct side of the wall.  These design plans were submitted and approved by Baltimore County.  

Construction Rebar

JLL took the approved design plans to bid for construction of the repair and rebuild of the sand filter, and received quotes from SMC and others.  SMC was awarded the contract for Phase 3 as the low bidder.  The Underground Maintenance Department then cleaned the area immediately upstream of the wall to allow the wall to be jacked back into place.  The wall was jacked into place and a new wall was cast behind the wall for structural support.  Once the wall was inspected and approved by SMC’s engineers, the Underground Maintenance Department rebuilt the sand filter in accordance with the original approved plan.  After approval by the engineers, the flow splitter bulkhead was removed to bring the underground sand filter back online. 

Construction Phase

This project involved three different departments of SMC: Inspection, Consulting, and Underground Maintenance.  Their effective collaboration was able to take this project from start to finish, in-house.  By having SMC conduct an annual, thorough underground inspection of the facility, SMC was able to minimize the financial cost of the repairs, and was able to identify the problem before the facility owner was notified by Baltimore County.  

SMC’s Completed Work of the Underground Sand Filter Rebuild and Repair