Dam Safety Inspection Report: Investigation of Sidewalk Washout at Barnum Park

Strategic Insights -

A section of sidewalk at Barnum Park, adjacent to the Municipal Dam in St Louis, was found to have settled near the east retaining wall, prompting an investigation by the Federal Energy Regulatory Commission (FERC) and the St Louis Municipal Electric Department. The inspection revealed that a localized damage to the sock-tile underdrain due to large pieces of concrete left in the trench as backfill had led to the washout. The area experienced two short-duration, high-intensity rainstorms the week prior to the discovery, which removed enough material to make the slab displacement noticeable.

Key Takeaways:

  • The investigation revealed that the washout was caused by localized damage to the sock-tile underdrain, resulting from large pieces of concrete left in the trench as backfill.
  • The area experienced two short-duration, high-intensity rainstorms the week prior to the discovery, which removed enough material to make the slab displacement noticeable.
  • The FERC engineer and the Director of Public Services, Keith W. Risdon, P.E., determined that the fine sandy backfill used in 2012 had been slowly washing out, over the years, into the damaged underdrain.
  • The corrective measures implemented include repairing the damaged pipe, backfilling with gravel, adding fabric cloth to minimize further sand movement, and monitoring the sidewalk for any further movement.
  • Additional measures are being considered to reduce the possibility of water draining to the sidewalk next to the wall, including re-grading, berms, edge drains, or field drains.
  • A local contractor has placed a new 6" concrete sidewalk slab over the damaged area.

Statistics:

  • The washout was approximately 7 feet deep and averaged about 8' x 18' at the top.
  • The area had experienced two short-duration, high-intensity rainstorms the week prior to the discovery.
  • The underdrain had been damaged due to large pieces of concrete left in the trench as backfill.
  • The repairs included repairing the damaged pipe, backfilling with gravel, adding fabric cloth to minimize further sand movement, and monitoring the sidewalk.
  • A new 6" concrete sidewalk slab was placed over the damaged area.

Sources:

  • Report submitted by Keith W. Risdon, P.E., Dam Safety Engineer, Director of Public Services, on August 8, 2025.
  • Document Accession #20250812-5019, Filed Date: 08/12/2025.
  • FERC Engineer Adam Christy's annual inspection report, July 8, 2025.
  • St Louis Municipal Electric Department's submission to the Federal Energy Regulatory Commission.