Formatted Title

Why Vibracore Sediment Acquisition Monitoring is Being Considered at More Sites for Remedial Design: Ongoing Refinements to V-SAM Technology

Background/Objectives

Until recently, vertical control accuracy in dredging equipment exceeded the accuracy of vertical depth data from sediment vibracoring. Characterizing the nature and extent of contaminated sediment requires collecting sediment samples at various depths below the mudline and analyzing them to establish the types and concentrations of chemicals of concern (COCs). 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.

Recovered sediment often makes up only a fraction of the distance the tube was driven into the sediment, with recovery (length of sediment in core divided by distance the core was driven) often being in the range of 60 to 80 percent. The uncertainty inherent in conventional vibratory coring associated with in situ depths below mudline (bml) is well documented and can result in miss-classifying the EOC by as much as 1m or more, often increasing with core tube length.

Recovery less than 100% 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 vibracore sampling equipment was instrumented (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. For each core, the penetration and acquisition values were recorded and plotted on an acquisition curve at regular intervals in close coordination with the coring contractor. This curve was created in real time with the information intended for use during core processing to establish the in situ sampling depths. Throughout the implementation of V-SAM, it became evident that the information was valuable for use during the coring process as well, as the contractor could use the measured values to modify the coring methods and influence successful sediment acquisition. The system has been successfully implemented on over 140 cores to date.

Results/Lessons Learned

Observations obtained from three projects and more than 140 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 the coring contractors modifying their methods (e.g., by changing the speed at which the core is advanced and intensity of vibration) which improved the quality of the sediment cores, acquisition of sediment intervals, and overall reliability of the data for dredging design.