Solving soil interaction problems through a
fundamental science-based inquiry approach in a
creative, collaborative, and interdisciplinary environment.

Projects

Full-Flow Penetrometers

Overview

Offshore GeohazardsThis research program was initially funded by the National Science Foundation CMS division and has since received continuing funding as part of "Developing International Protocols for Offshore Sediments and their Role in Geohazards: Characterization, Assessment, and Mitigation", an NSF Partnership for International Research and Teaching Grant (see www.offshoregeohazards.org).

The purpose of this research is to evaluate the ability of "full-flow" penetrometers to characterize the soil properties, including the undisturbed undrained and remolded undrained strengths, of soft soil deposits at a level of accuracy and reliability greater than that obtained by conventional methods such as the cone penetration (CPT) and vane shear (VST) tests. This is being accomplished through testing of T-bar and Ball full-flow probes (Figure 1) at highly characterized test sites around the world and comparing the measurements with results from conventional insitu tests and high-quality laboratory tests on undisturbed block samples.

Figure 1. T-bar and ball full-flow probes.

T-bar and ball full-flow probes

Background

Soft soil sediments present a unique challenge to the geotechnical community in both their characterization and use as an engineering material primarily due to high sensitivity, underconsolidation, and very low strength. Conventional drilling and in situ testing methods, including thin-walled piston tube sampling, vane shear testing (VST), and cone penetration testing (CPT), do not quantify soft sediments at a level of accuracy and reliability now required for many geotechnical projects. While performance modifications can be made, these in situ devices are restricted by inherent characteristics of their test procedure and/or geometric design that cannot be overcome through minor improvements. The insufficiency of these methods is compounded by steady increases in the quantity and complexity of on-shore, near-shore, and off-shore soft sediment applications such as tunnels, bridges, wind turbines, submarine slope stability, buried pipelines, and oil and gas platforms.

"Full-Flow" Penetration Probes

"Full-flow" penetrometers provide a measure of the pressure differential necessary to induce soil flow around a symmetric geometric probe. In principal, the penetrometers are akin to a viscosity measurement as they induce plastic flow of the soil around a geometric probe during penetration (Figure 2). This unique flow measurement is free from the correction factors that limit the accuracy of state-of-practice devices including high ambient pore pressure, over-burden pressure, degree of saturation of the porous element, and load cell resolution. Therefore, "full-flow" penetrometers have strong potential in the applications mentioned above where one or more of these factors limit conventional penetrometer performance. This unique capability has been achieved through the "forward development" of the penetrometers, where fundamental analysis of the flow mechanism and scaled laboratory tests were performed prior to extensive field testing.

Figure 2. Schematic of various in test method (a) continuous penetration at 2 cm/s, (b) cycling at 2 cm/s, (c) twitch test with consecutively halved penetration rate. [d]

Figure 2

Note: Arrow indicates penetration direction and thickness indicates penetration rate.

To date "full-flow" penetrometers have been evaluated though analytical, numerical, laboratory, and centrifuge investigations as well as limited field trials (Randolph 2004). From these analyses, "full-flow" penetrometers, through monotonic and cyclic penetration, demonstrate clear potential to obtain profiles of the undisturbed undrained shear strength (su) and the undrained remolded shear strength (su-remolded) respectively, which in turn provide an in situ estimate of sensitivity (St). Through performing "twitch" tests, a series of consecutive short penetrations where the penetration rate is sequentially halved, an in situ estimate of the coefficient of consolidation (cv) may be obtained.

Figure 3. Example sounding of combined monotonic and cyclic penetration with full-flow probe. [d]

Figure 3

Research Findings

We have completed field testing programs with the Ball and T-bar at highly characterized test sites including Amherst, MA (CVVC), Newbury, MA (Boston Blue Clay), Gloucester, Canada, Louiseville, Canada, Perth, Australia, and Onsoy, Norway. Monotonic, cyclic, and twitch tests have provided new insights into probe design, recommended procedures, and interpretation of data, including the following:

These, and additional findings, are detailed in related publications.

Recent Presentations

"The Role of Full-Flow Penetrometer Area Ratio on Penetration Resistance and Shear Strength Estimates" - Presented at Society for Underwater Technology, 6th International Offshore Site Investigation and Geotechnics Conference: Confronting New Challenges and Sharing Knowledge, 11–13 September 2007, London, UK. [Slides]

More Information? Jason T. DeJong and Nick Yafrate