Recent and Current Research in Site Characterization

Seabed Sediment Sampling and Evaluation of Sample Quality

Don J. DeGroot in collaboration with Thomas C. Sheahan, NGI, and COFS

Offshore seabed sampling tools: Collection of high quality samples of offshore cohesive sediments is essential for accurate and reliable characterization of their mechanical properties. Without such data, any subsequent analytical and numerical analyses of offshore geohazards, such as submarine landslides, can be highly unreliable (i.e., they may misrepresent critical engineering properties). Significant research progress has been made on developing methods for improving sampling equipment for onshore practice. Numerical and experimental research (e.g., Hvorslev 1949; Baligh et al. 1987; Clayton et al. 1998) has highlighted the importance of sampler geometry (e.g., sharp cutting angle, small area ratio, and zero inside clearance ratio) and operation (e.g., fixed piston vs. free piston sampler) in order to obtain high quality "undisturbed" samples of cohesive soils deposits. And yet, many of these important aspects have not found their way into offshore practice. Research needs to be conducted on how to apply the results found for onshore sampling operations to the more challenging offshore environment (e.g., deep water, very soft sediments, vessel-based sampling operation, etc.). The US research team will partner with ICG/NGI to foster their seminal research on onshore and offshore sampling equipment and operations. NGI is currently leading a research effort on the design of a new continuous seabed sampler that incorporates many of the research findings noted above (Lunne and Long 2005). Dr. DeGroot will team with NGI on this effort. Specifically, UMass Amherst will provide the laboratory and human resources (research students) to conduct extensive advanced laboratory tests (triaxial, CRS consolidation, direct simple shear) on samples collected with the new sampler in order to evaluate its performance and, as appropriate, develop design improvements.

Development of portable nondestructive sample quality measurement equipment and test procedures for offshore drilling operations: Evaluation of sample quality is considered essential for cohesive sediments collected for laboratory measurement of engineering properties (Hight and Leroueil 2003, Ladd and DeGroot 2003). NGI pioneered the use of laboratory reconsolidation volumetric strain as an indicator of sample quality (Andresen and Kolstad 1979; Lunne et al. 1997). However, in spite of its great value (it is significant to note that it is rarely used in US practice, which is a technology transfer problem), this method is an a posteriori measure, i.e., one does not know a sample’s quality until a laboratory specimen has been trimmed and set up. Current NSF sponsored research being conducted at UMass Amherst and NU is focused on development of a nondestructive measure of sample quality using shear wave velocity. Field testing at onshore research sites by the UMass Amherst/NU team using bender elements shows that shear wave velocity is a viable parameter for nondestructive evaluation of sample quality immediately after sampling (e.g., Landon et al. 2004). Research needs to be conducted on how to implement such a tool in the more challenging offshore drilling environment. Both NGI and COFS have proven expertise in the use of bender elements (e.g., Dyvik and Madshus 1985; Ismail et al. 2003). Together the US Team, NGI and COFS will work towards developing a robust tool that can nondestructively measure shear wave velocity of offshore sediment samples immediately after sampling, i.e., on the drilling vessel. This will allow for "real-time" decisions to be made on sampling operations while the vessel is offshore and hence offers potentially significant cost savings. It also affords a systematic procedure for screening samples prior to setting up costly and often time consuming advanced laboratory tests for measurement of design parameters.

In situ measurement of sediment pore pressure

Don J. DeGroot in collaboration with Jason T. DeJong, NGI, and COFS

Determining the stability of soft sediments requires knowledge of the in situ pore pressure state. In offshore soft sediments, where strengths are typically very low and the deposits are either still consolidating (due to high sedimentation rates) or normally consolidated, accurate measurement of the pore pressure profile is critical. Excess in situ pore pressures are believed to have been a major contributing factor to the Storegga slide (Solheim et al. 2005) and many other offshore regions of the world have excess pore water pressure caused by rapid sedimentation rates, mud volcano activity, and other mechanisms (e.g., Gulf of Mexico, Caspian Sea, Offshore West Africa). While measurement of the water pressure at the seabed floor (top of sediment) is relatively straightforward, determination of the pore pressure within soft sediments is much more complex. The complexity in measuring the in situ pore pressure arises from the current state-of-the-art: measurement requires installation of a sensor or probe that in turn displaces soil and creates excess pore pressures. Immediately after installation, the probe measures the excess pore pressure during installation, and it is only after prolonged durations (often hours and even days; which is costly because it ties up the drilling vessel) that the excess pore pressure dissipates and hydrostatic pore pressures can be measured. Efforts to reduce the time required for dissipation have primarily focused on development of a miniature tapered piezoprobe since the degree of excess pore pressure is proportional to the square of the probe diameter (Houlsby and Teh 1988). Constrained by practical implementation issues, the miniature piezoprobe begins to taper up to a larger diameter about 100 to 150 mm behind the tip (Whittle et. al. 2001). During dissipation the excess pore pressure generated by the miniature tip dissipates relatively quickly. However, before it reaches the in situ equilibrium pore pressure, the excess pore pressure from the upper tapered section propagates forward and produces an increase in the excess pore pressure at the tip. This results in the equilibrium pore pressure at the tip not being measured until the excess pore pressure from the tapered section is also dissipated, effectively negating the tapered section’s benefit. To overcome this issue Whittle et. al. (2001), using numerical modeling, proposed the use of a dual element piezoprobe and a more complex analysis where the correlations between two dissipation curves are analyzed. This enables a rigorous prediction of the equilibrium pore pressure but not a direct measurement. Determination of the equilibrium pore pressure from piezoprobe dissipation is essentially a time-dependent process during which an instantaneous pore pressure differential must diffuse and return to equilibrium. To date, all piezoprobe methods have required dissipation excess pore pressure via flow of water away from the probe into the surrounding soil. The dissipation time has been accelerated by using a smaller probe since a smaller probe produces a smaller differential pressure. This research proposes a novel approach that enables accelerated dissipation of excess pore pressure via the flow of water into the piezoprobe. Prior to penetration, an estimate of the in situ equilibrium pore pressure will be made. After piezoprobe penetration to the target depth, the pore pressure will be measured continuously. For a specified time (to be determined by this research) water flow in/out of the probe will be regulated so that the measured pore pressure remains at the estimated in situ equilibrium conditions. After the specified time, water flow in/out of the probe will cease and the pore pressure will come to equilibrium. This approach will rapidly reduce the initial pore pressure differential since water flow into the probe will occur at the location of highest excess pore pressure. Once most of the excess pore pressure has been relieved, the time required for equilibrium conditions to be restored will be minimal and less than the time required for the pore pressure front generated by the tapered section to reach the measurement location. The novel device and technique proposed above requires significant design, analysis, modeling, and testing components. The external dimensions of the miniature piezoprobe will be maintained as closely as possible to those by previous researchers, enabling use of prior results and analysis (e.g. Whittle et al. 2001). The control of water flow in and out of the piezoprobe (fluid chamber in which the pore pressure sensor is located) will be performed using a stack of piezoceramic disks. The piezoceramic disk stack expands with positive voltage and contracts with negative voltage. During penetration the piezoceramic will be in the expanded state with high positive voltage. By varying the voltage the volume of the piezoceramic stack will be varied, which will be equal to the volume of water flow into the probe. In addition to the design details, analytical and numerical analyses will be performed to determine the time duration during which the flow should be regulated, the time required to reach equilibrium conditions, whether this time is less than that at which the tapered pore pressure front reaches the measurement, and how these parameters vary with soil properties.

Implementation of Full Flow Penetrometers in OffShore Practice

Don J. DeGroot in collaboration with Jason T. DeJong, NGI, and COFS

In-situ Determination of Peak and Remoulded Strength Using Full-Flow Penetrometers. Accurate and precise measurement of the peak and remoulded strength is critical for the assessment of geohazard stability and for the design and performance of all structures founded in soft sediments. Unlike onshore characterization, offshore sediment characterization is complicated by significantly higher cost per hourof investigation and execution and resolution limitations of all current state-of-practice and state-of-the-art in situ devices (DeJong et al. 2004). In recent years, full-flow penetrometers have shown to have the potential to measure undrained and remolded strength directly, quickly, and accurately, thereby solving amajor technical obstacle in geohazard characterization. Translation of full-flow penetrometers from potential to realization for characterization of offshore sediments is not trivial. However, progress has been made internationally (COFS and NGI) and at UMass Amherst, and the collaborative scope of work contained in this proposal has the potential to result in internationally standardized probe designs, testing methodology, and data analysis and interpretation. The full-flow research scope for this project will focus on (1) practical issues regarding development of international specifications that would enable consistent and reliable implementation by engineers at any potential geohazard site in the world and (2) phenomenological issues regarding testing specifications and data analysis and interpretation. Practical issues to be addressed include design of a mandrel with a temperature compensated, moment insensitive load cell and a pore pressure module, probe compatibility with offshore drilling pipe, procedures required for remoulded strength determination, penetration rate, and framework for initial analysis factors and site-specific factor calibration. Much of the practical issues are integrated with soil behavior phenomena and properties including: strain-rate effects (viscosity and partial consolidation), sensitivity, stratigraphic and stress anisotropy, strain-softening rate, and soil type. These series of issues will be investigated in an integrated collaborative research program that will use well characterized international test sites in the US and Canada as well as sites managed by NGI (Onsøy, Norway) and COFS-UWA (Burswood, Australia). With baseline, full-flow investigations (by UMass Amherst) and extensive laboratory investigations already performed at these sites, the background research is completed, and the potential and ability to research the above issues is established.

Vertical Variability of Hydraulic Conductivity of an Unconfined Aquifer Measured Using a Pneumatic Multilevel Slug Testing Packer System
Aaron Judge, advised by Don J. DeGroot

The Hydrogeologic Characterization of a Public Drinking Water Aquifer Site in Dedham, Massachusetts
Daniel .F. LaMesa, advised by Don J. DeGroot

Evaluation of an Automated Early Warning System for Unstable Soil Slopes
Jeff Lloyd, advised by Don J. DeGroot

High Quality Deep Water Geotechnical Sampling and Shear Wave Velocity
Cody .D. Jones, advised by Don J. DeGroot

For more information about past research projects click here