A recent trenchless utility installation beneath Holt Avenue encountered unexpected running ground within the Pomona alluvial fan deposits, forcing a costly redesign mid-project. This scenario plays out more often than contractors expect when tunneling through the soft, stratified sediments that define the eastern Los Angeles Basin. In Pomona, where the groundwater table can fluctuate significantly between the dry summer months and winter recharge, a standard desktop study is never enough. We approach each project by integrating field data from CPT testing to map continuous stratigraphic profiles, ensuring the geotechnical baseline report reflects actual conditions rather than interpolated assumptions. This level of scrutiny is critical when advancing through the young alluvium and older San Dimas Wash deposits that underlie much of the city.
Tunneling through Pomona's alluvium requires anticipating the transition from stiff clay to running sand within a single drive, a condition that standard SI classifications often miss.
