Geotechnical Investigations & FLAC‑Based Engineered Fill Cap Design

The City of Irwindale contains several former gravel pits that were excavated up to 150 feet deep and later backfilled with inert construction and demolition debris. These fills include oversized materials as large as 5 to 10 feet and were placed as uncontrolled fills in very thick lifts, creating significant challenges for future commercial and industrial development. Completely removing and replacing these deep inert debris fills was not feasible for the owners and developers.

Verdantas performed geotechnical investigations and FLAC modeling for three inert debris fill sites—Nu‑Way, Live Oak, and Arrow Pits—to support redevelopment planning. The work focused on characterizing 160‑ to 200‑foot‑deep inert debris fills and designing a compacted fill cap capable of controlling differential settlement to meet Los Angeles County Public Works requirements.

The investigations included 80‑foot‑deep test excavations, large‑scale in situ density testing using ring/frame methods, grain size testing, and hydrocollapse testing using specially fabricated laboratory equipment for oversized materials. Laboratory testing was performed on the finer fractions of the fill, and test pads were constructed to determine achievable compaction and density. The results were used to evaluate settlement potential under both static and seismic conditions.

FLAC modeling was performed to determine the cap thickness required to limit differential settlement to the County’s standard of 1 inch over 30 feet. Verdantas also supported the permitting process required for commercial development of these sites. The project ultimately enabled previously unsuitable inert debris fills to be considered for successful industrial redevelopment and resulted in a published technical paper presented at an international geotechnical engineering conference.