Characterization of Complex Soil Stratigraphies by VisCPT and Adaptive Remeshing

Regents Of The University Of Michigan - Ann Arbor, Ann Arbor MI

Investigators

Abstract

State-of-the-art geotechnical and geoenvironmental site characterization now ubiquitously employs direct-push technologies. The cone penetration test (CPT) and its variants, the piezo-cone (uCPT) and seismic-cone (SCPT), lead the direct-push technologies in providing both the most comprehensive assessment of site stratigraphy and determination of the engineering (mechanical and geohydrologic) soil properties. However, if the soil deposits are complex, the CPT often fails in providing accurate stratigraphic information and yields unreliable properties. These complexities may include unusual site stress history (geomorphology) and thin layering of strata. The present research effort utilizes two recently developed technologies to create procedures for comprehensively assessing site stratigraphy and for determining the soil properties of even thin soil layers such as sand seams and clay lenses. The first tool is "adaptive remeshing" in finite element modeling. Large penetration of soil by stiffer foreign objects such as a steel cone penetrometer creates large displacements and strains in the softer material. Therefore, a finite element mesh becomes very compressed and distorted leading to numerical instability. However, by adaptively remeshing the soil, penetrations on the order of meters can be simulated. The present research will therefore utilize this tool to study the response of soils to the CPT. Penetration across interfaces between dissimilar soils as well as through thin strata will be simulated to gain an understanding of the typical CPT signature in such conditions. Both normally consolidated and overconsolidated stress states will be explored. Extrapolation of the adaptive remeshing results to field conditions and specifically the determination of engineering properties of thin strata will require knowledge of the actual thicknesses and locations of these layers in-situ. For this, a recently developed "vision cone penetrometer" (VisCPT) will be utilized. Previous research has resulted in development of this augmentation to the conventional CPT, which allows for continuous image recording of the soil column. Even layers as thin as 0.5 cm are easily detected by the VisCPT thereby providing the missing information from the conventional CPT. Once a layer thickness is established, the soil properties can be predicted by the results of the finite element studies with adaptive remeshing. The broader impacts of the study include extensive interdisciplinary research, immediate technology transfer to industry and redesign of undergraduate laboratory experience to include VisCPT technology. Improvements to site characterization will lead to safer and more cost effective civil engineering construction. With more precise subsurface information, the construction industry would be able to reduce the typically high factors of safety that are presently needed to protect against unexpected conditions.