Cyclic and Static Liquefaction Study for a New Earth Dam in Colorado

A combination of Standard Penetration Testing (SPT) sampling and laboratory testing, Cone Penetrometer Testing (CPT), and geophysics are typically used for sites with liquefaction issues. Geotechnical studies for water dams in Colorado do not typically include a CPT investigation or continuous SPT sampling. The dataset size from the SPT or CPT has a paramount role in estimating the extent and strength of liquefiable soils. A preliminary geotechnical site investigation and SPT-based liquefaction screening analysis to support a design of a new earth dam near the Denver area indicated the presence of continuous liquefiable soils in the foundation soils for a design earthquake with a return period of 2,475 years. The SPT results, however, were inadequate to provide a reliable estimate of liquefiable zone thicknesses and post-liquefaction residual shear strengths. The CPT investigation was subsequently performed to better evaluate the thickness and post-liquefaction strength of liquefiable soils. The thickness of liquefiable zones with a reasonable precision was obtained from the CPT-based liquefaction triggering analysis. Additionally, large CPT dataset supported a use of higher post-liquefaction residual shear strengths for liquefiable soils resulting in a satisfactory stability factor of safety for the post-earthquake scenario. This study highlights the importance of data collection methods and procedures with respect to the dataset size during the geotechnical site investigation planning in support of liquefaction studies among water dam practitioners in Colorado.