Soil Liquefaction Analysis in Des Moines – IBC & ASCE 7 Site-Specific Studies

Des Moines grew along the Des Moines and Raccoon river valleys, where broad floodplains and alluvial terraces shaped the city grid. Those same Quaternary sediments, up to 30 m thick in the downtown corridor, can be a concern when a project triggers a liquefaction evaluation under the International Building Code. A site-specific analysis following ASCE 7-22 and the NCEER/Youd-Idriss framework determines if loose saturated sands and silty sands below the water table will lose shear strength during a design seismic event. Our geotechnical team runs the boreholes, collects the SPT data, and delivers the factor of safety against liquefaction that structural engineers need for foundation design. For deeper profiling we often pair the field work with CPT testing where continuous tip resistance and pore pressure data refine the stratigraphy.

In the Des Moines River valley, a clean sand seam at 5 m depth can carry a factor of safety below 0.8 under a 2,475-year ground motion — identifying that seam early changes the foundation strategy.

Local considerations

Central Iowa weather swings from frozen ground in February to saturated floodplain conditions in June, and both extremes affect liquefaction vulnerability. High spring water tables along the Raccoon River push the phreatic surface to within a meter of grade, saturating Holocene sands that would otherwise be above the zone of concern. That seasonal rise brings more soil layers into the potentially liquefiable category. The analysis must use the highest credible groundwater elevation, not the depth measured on a dry autumn day, or the factor of safety will be unconservative. We also check for cyclic softening in fine-grained soils when the plasticity index runs low, which matters in the thin interbedded clays common in the Des Moines Lobe till.

Typical values

Applicable standards

ASCE 7-22, IBC 2021 Chapter 18, ASTM D1586-18, ASTM D2487, NCEER/Youd-Idriss 2001

Questions and answers