Geomechanical properties of seafloor surface sediment layers affect the characterization, assessment, and management of submerged munitions sites in multiple aspects including: sinkage and burial of unexploded ordnances (UXO), exposure or capping of UXOs through sediment transport processes, and interpretation of remote sensing (e.g., acoustic and electromagnetic) surveying methods. Traditional methods of seafloor sampling or cone penetration testing are either time- and cost-intensive or do not provide the required sensitivity to sample sediments in the uppermost surface accurately. Free fall penetrometers have been introduced as a cost- and time-effective method to assess the geomechanical properties of seabed surface properties and have been specifically suggested for UXO characterization. The overarching goal of this project was the development and proof of concept of an improved framework for the deployment and data analysis of a portable free fall penetrometer (PFFP) in stratified sediments to assist with a cost-effective and rapid characterization, monitoring, and management of submerged munitions sites.
The following research questions were identified as essential to achieve this goal: 1) What are the typical differences in geomechanical properties (such as sediment strength, erodibility, and permeability) between surficial seafloor layers in sandy and muddy environments, respectively? 2) Can key geomechanical properties for UXO site characterization be directly inferred from portable free fall penetrometer results? 3) Which potential uncertainties of current UXO site characterization and monitoring methods resulting from surficial seafloor stratification can be addressed by an advanced use of PFFP? The research strategy included field surveys in areas of varying sediment types and environmental conditions, laboratory testing (sedimentology and geomechanics), data analysis and correlation, and the development and proof of concept of a novel investigation framework. Seven main field sites were tested during this experiment and were complemented by additional measurements obtained through collaborative efforts.
The key findings of this project include: 1) Current methods of PFFP data analysis were improved and validated. It was found that undrained shear strength can be estimated from PFFP for muddy seafloor sediments and that friction angles and relative density can be derived from PFFP for sandy seafloor sediments. 2) Significant variations in geomechanical properties within uppermost seabed surface layers were identified even without significant changes in sediment type. The results even suggested that uppermost seabed surface layers may exhibit more suspension-like behavior than soil behavior depending on the water content. 3) The variability in geomechanical properties is of relevance to UXO site assessment and monitoring. 4) A novel PFFP deployment and analysis strategy was formulated that enables a rapid and cost-effective characterization of the upper meter of the seabed surface.
The results provide a new insight into effects of geomechanical seabed soil layers on the interpretation of remotely sensed UXO site monitoring methods and UXO risk assessment. The derived relationships improve PFFP data analysis and interpretation for geomechanical characterization of subaquatic sites. The developed PFFP deployment and analysis framework can provide rapid, site-specific insights into spatial and sediment-depth dependent variations in geomechanical properties.