Formatted Title
Drilling Methods Matter Even More in Sediment: How to Best Characterize Sediment the First Time
Background/Objectives
Characterizing the nature and extent of contaminated soil is challenging enough in dry conditions, but collecting sediment samples at various depths below the mudline and analyzing them to establish the types and concentrations of chemicals of concern (COCs) is even more challenging. The results of the COC data are used to establish the maximum depth of contamination which can then be tied to an elevation of contamination (EOC) at the site. The accuracy of the EOC has significant impacts on remedial design and implementation. While the EOC is relative stable in dry soils, sediments add another layer of variability that can have huge impacts on remediation depths (and therefore costs).
Conventional approaches to sediment sampling often include deploying typically land-based drilling equipment (e.g., hollow stem auger, mud rotary, sonic, or geoprobe) from a barge. Alternatively, implementing long-tube samplers such as vibracore and impact core devices is a widely used and relatively low-cost approach to sediment coring. Each of these approaches can introduce control when compared to vibracore, but also have their own inherent limitations.
Vibracoring can result in uncertainty of the actual depth profile of the sampled sediment which can be due to multiple conditions that have traditionally been difficult to identify: compaction of the sediment in the core tube; development of a plug at the cutting edge; encountering debris during the drive; sediment falling out of the bottom of the tube during extraction; or some combination of all. Vibracore Sediment Acquisition Monitoring (V-SAM) helps resolve these unknowns and increases coring accuracy.
Approach/Activities
Conventional sampling equipment and approach were identified and compared to modified vibracore sampling equipment that was instrumented with V-SAM to simultaneously measure the penetration of the core tube into the sediment and the acquisition of sediment within the core tube during driving.
Results/Lessons Learned
Using conventional drilling techniques can lead to differences in design depths and actual depth of contamination, leading to increased remediation costs. Observations obtained from past projects and multiple sediment cores advanced using V-SAM will be presented. The measured recovery data show how changes in in situ conditions through the drive length affect sediment recovery through expansion, compaction, plugging and bottom loss, and the application of this data in real time resulted in the ability to acquire sediment more effectively through various intervals. Live observations of the V-SAM data during coring led to vastly improved quality of sediment cores, acquisition of sediment intervals, and overall reliability of the data for dredging design.