Under IBC Chapter 18 and ASTM D5333-03, collapsible soils must be identified before any structural fill or foundation work begins. Orange California sits on the eastern edge of the Los Angeles Basin, where Pleistocene alluvial fans and San Gabriel River terraces create a high potential for metastable soil structures. When dry, these soils appear competent; once wetted under load, the intergranular bonds collapse, producing sudden settlement. We have tested dozens of sites along the 55 and 22 freeway corridors where collapse potential exceeded 5% under an overburden of only 200 psf. To avoid surprises, we always pair the collapse test with a consolidation test to distinguish hydrocompression from primary consolidation, and we use classification of soils to correlate collapse with low-plasticity silts and fine sands.
Collapse indices above 5% under 200 psf overburden are common in the alluvial fans east of the 55 freeway, requiring deep compaction or over-excavation.
