Detection of Stratigraphic Interfaces and Thin Layering Using a Miniature Piezoprobe

Assessment of the depth location of stratigraphic interfaces and the depth and thickness of thin layers can be critical to the proper performance of geosystems. For example, stratigraphic interfaces can promote anisotropic soil strength response and potentially provide preferential slip planes that create slope instability. Similarly, the presence of thin, high permeability layers can alter groundwater flow regimes and rates of consolidation, which can hinder or accelerate methods of ground improvement. A miniature piezoprobe (MPP) with an apex filter location and a measurement frequency of 64 Hz was recently developed in an effort to detect and characterize these stratigraphic features. The piezoprobe's functionality is through the measured excess pore pressure profile, which reflects changes in the drainage conditions, and therefore soil conditions. This paper evaluates the piezoprobe's ability relative to the conventional piezocone penetrometer (CPTu) to characterize distinct interfaces and thin layers through the analysis of soundings performed at two highly characterized test sites. One test site consists of a near normally consolidated clay layer with distinct interfaces with the overlying backfill and interbedded layers of sand and silt below. The second site consists of varved clay, which is comprised of a repeating structure of thin pairs of silt and clay layers. Results demonstrate that the piezocone detects distinct interfaces at a higher resolution than the CPTu and that thin layers may go undetected by the CPTu.