On the Use of CPT Correlations to Identify and Determine the Engineering Properties of Diatomaceous Soils

Diatoms are single-celled organisms that live in both marine and lacustrine environments worldwide. At the end of their lifecycle (typically several days), their silica frustules (analogous to exoskeletons) settle out of suspension and contribute to the sediment, creating diatomaceous soil deposits. The engineering behavior of diatomaceous soils differs from “textbook” soils of similar size and composition—individual particles are silt-to-clay sized, chemically inert, and topologically complex. Obtaining intact samples of diatomaceous soils is challenging because their texture can vary widely from mudstone-like, which can result in crushed sample tubes, to very soft, which can lead to poor sample recovery, except in the case of the most sophisticated samplers. Cone penetration testing (CPT) is a good alternative to intact sampling in all but the stiffest of profiles. CPTs can rapidly provide continuous information about soil behavior, but the existing database of CPT soundings in diatomaceous soils is limited. Furthermore, the applicability of existing correlations and semi-analytical relationships, such as those based on cavity expansion theory or critical state soil mechanics, is unknown. In this work, we isolate and analyze layers of diatomaceous soils from CPT soundings in diatomaceous deposits at four sites in Oregon. We identify the soil behavior index, Ic, and use published relationships to estimate unit weight, γ, stress history (i.e., effective yield stress, σ′y, and stress yield ratio, YSR), and effective stress friction angle, ϕ′. The resulting estimates are analyzed for internal consistency and compared to laboratory data (where available) to assess the efficacy of existing correlations for the determination of engineering properties from CPT data. The results of this investigation demonstrate that CPT-based relationships are an appropriate first step to estimate soil properties for initial design in these soils. Additionally, the investigation shows that CPT data from Oregon’s diatomaceous soils can be approximately categorized into two groups that show distinct soil behavior types and, in some cases, soil property interpretation.