Improved Penetrometer Performance in Stratified Sediment for Cost-Effective Characterization, Monitoring and Management of Submerged Munitions Sites
Dr. Nina Stark | Virginia Tech
Sediment stratification at the uppermost surface of subaqueous sediments can vary in dimensions, physical characteristics and geomechanical behavior. This impacts munition deposition, burial depth and stability, as well as most surveying methods, and can lead to significant issues with munition detection, monitoring and munition site management. Portable free fall penetrometers (PFFPs) have been shown in the past to be a cost effective tool to characterize a munitions site. However, the calibration process previously used tends to blur interpretation of the data within the sediment seabed stratum. In this project, the research team will improve the use of the PFFPs by taking into account sediment strength, erodibility, permeability, and changes in the surface sediment layer as well as the seabed stratum, for rapid and cost effective geotechnical characterization of unexploded ordnance (UXO) sites as a complementary method to advance current UXO site characterization technologies. To achieve this overarching goal, the following research questions will be answered:
- What are the typical differences in geomechanical properties (such as sediment strength, erodibility, and permeability) between surficial seafloor surface layers?
- Can key geomechanical properties for UXO site characterization be directly inferred from portable free fall penetrometer results?
- Which potential uncertainties of current UXO site characterization methods resulting from surficial seafloor stratification can be addressed by an advanced use of portable free fall penetrometers?
The work will start with an initial field survey. The key aspect is that sites of different environmental conditions (sediment type, flow conditions, salinity, etc.) will be tested, and at each site a comprehensive set of PFFP data, and of sediment cores will be collected. Potential impacts of spatial seabed variability will be addressed by complementary acoustic and optic seabed imaging. The sediment samples will then undergo a detailed laboratory analysis, determining sediment properties (grain size, bulk density, permeability, organic content, X-ray imaging of layering, erodibility), as well as geotechnical properties (friction angles, cohesion, shear strength, bearing strength, void ratios). Differences in geomechanical properties of the surficial seafloor layers will be determined, enabling assessment and quantification of potential uncertainties in current survey and monitoring strategies by sediment layering. The PFFP in-situ measurements will be correlated to the geomechanical properties of the different sites and sediment layers, and relationships between the key geomechanical properties and the PFFP recordings will be established. At this point in the project, there will be a Go/ No-Go decision, depending on the completion of the previous research tasks. With the successfully established correlations, the development of a novel investigation framework and proof-of-concept will conclude the project. Here, the previous findings will be utilized to develop a novel UXO site investigation framework utilizing PFFPs. This novel framework will be demonstrated at a known UXO site.
One of the objectives of the Statement of Need (SON) MRSON-18-C1 is the improvement of the current knowledge of environmental conditions, specifically, sediment characteristics, of underwater sites that impact the performance of sensors and systems to detect and classify buried and proud munitions. This work will lead to the development and proof-of-concept of a novel investigation framework to conduct a rapid and cost effective site characterization of seafloor surface layering, providing information to help to assess potential uncertainties when using acoustic/visual seafloor inspection, and therefore, aid in assessment of risks of munition exposure or burial related to the sediment’s geomechanical behavior. The established correlations between the PFFP recordings and geomechanical properties such as erodibility and permeability will also be relevant for the investigation of sediment erosion and accumulation processes.