Sedimentary facies characterization through CPTU profiles: An effective tool for subsurface investigation of modern alluvial and coastal plains

Cone penetration tests, a method that is typically used to determine the engineering properties of soils, can be used as an effective tool for refined subsurface stratigraphic investigations of alluvial and coastal plains, aside from the geographic location. High-resolution calibration of piezocone penetration tests (CPTU) with 20 sediment cores enabled the detailed characterization of alluvial, deltaic and coastal depositional systems of the Po Plain. Twelve cored facies associations, typical of alluvial and coastal plain environments, were characterized based on four distinct CPTU profiles: basic cone resistance (Qc), sleeve friction (Fs), water pore pressure (U) and friction ratio (FR). Sandy facies associations (fluvial/distributary channel, bay-head delta, transgressive barrier, delta-front/beach-ridge) typically have high (>4 MPa) Qc, low-to-negative U and low (<2%) FR. Muddy deposits (well-drained/poorly-drained floodplain, swamp, lagoon and prodelta) exhibit opposite trends. Heterolithic facies associations (crevasse-levee, offshore/delta-front transition) display characteristic seesaw profiles. Plotting of late Quaternary deposits onto the latest version of the cone penetration test typical soil behaviour chart (Robertson, 2010) enables the identification of distinctive facies associations reflecting distinctive grain size. CPTU interpretation leads to sedimentary facies recognition well beyond the simple lithological differentiation and, in particular, allows the refined characterization of clay-rich and silt-rich depositional units (swamp clays and peats, central-inner and outer lagoon, proximal/distal prodelta deposits) that exhibit only subtle differences in lithology. CPTU data can also serve for the accurate detection of key stratigraphic surfaces with potential engineering applications, such as the Pleistocene-Holocene boundary. This latter, a common feature of several alluvial and coastal plain successions, is commonly marked by an abrupt upward decrease of basic cone resistance and sleeve friction from Late Pleistocene, pedogenized, stiff strata to overlying Holocene, organic-rich, soft deposits. This study offers an updated CPTU-facies characterization method that could be suitable for subsurface investigations of modern alluvial and coastal plains worldwide.