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IC10_I

Sediment pore pressure response from penetrating objects

State of the art
It has long been known that the strength of sediment is a function of the effective stress, i.e. cohesive forces as a function of mineralogy and grain size distribution, plus the pore fluid pressure counteracting the cohesion (Hubbert and Rubey, 1959). Namely the sediment water interface with high-porosity sediment (up to >80% water) is a regime where very subtle changes in pore pressure may cause remobilisation of particles, creeping or flow of sediment at very shallow slope angles (<1°). For coastal zones, such remobilisation may have strong repercussions, for instance if pore pressure changes owing to tidal fluctuations cause the precipitation or remobilisation of fluid mud layers in estuaries and waterways. Similarly, dredging exposes material to the seabed, which is overconsolidated whereas dumping releases amalgamated, often fluidised material to the seafloor in an underconsolidated state. One efficient way to physically characterise these deposits affected by human activity are cone penetration tests (CPT), during which the stiffness, cohesiveness of the sediment and the pore fluid pressure between the particles are measured in a time-efficient deployment with a shallow water free fall lance (Stegmann et al., 2006a,b; 2007). A similar, slightly smaller probe is the NIMROD, similar to the XBP (eXpendable Bottom Penetrometer; for details see Stoll and Akal, 1999).
No matter how careful such in situ tests are carried out, the data always contain an inherent artefact from the penetrating probe. This artefact may be particularly strong in fine-grained materials where the pore pressure peak from the impact has insufficient time to dissipate (Lunne et al., 1997). This problem has attracted some critical awareness, and laboratory measurements aimed at its quantification (e.g. Jeng and Cha, 2003; Song and Voyiadjis, 2005). It seems particularly vital to carry out in situ tests in well-characterised seafloor sediments and compare those data to numerical models for different sediment composition and texture. The effect of the impact velocity is of particular interest, because earlier workers have suggested that the enhanced impact velocity serves to accentuate differences in the sub-seafloor sedimentary structure (Stoll and Sun, 2005). Some workers have tried to quantify the effect of impact velocity by introducing empirical factors (e.g. Dayal and Allen 1975), however, this simplified approach is something we suggest to refine by the numerical approach suggested here. In a second step, a patent issued to Prof. R. Stoll (1997, US patent# 5,681,982) may be used to numerically map wider areas of the seafloor and relate geotechnical properties to them (see details in Mulukutla, 2008).
  1. How do grain size distribution and sediment strength affect the pore pressure signal of penetrating CPT or NIMROD probe?
  2. Is it possible to identify characteristic pore pressure signatures as a function of penetration rate? How do standard, quasi- static CPT tests (2 cm/s) compare to ‘free-fall’, dynamic impacts (50 cm/s and faster)?
  3. How well do in situ data, as to be obtained with the Bremen FF-CPT devices, the NIMRO penetrometer, or the GOST seafloor penetrometer, agree with predicted pore pressure trends from numerical models?
Project description
Many offshore civil engineering applications require knowledge of undrained shear strength of soft marine clays under high strain rate conditions encountered during dynamic penetration of free falling probes into marine sediments. Assessing the effect of strain rate on dynamic penetrometer results in soft clayey sediments becomes important. An approach that uses the velocity profile of the penetrometer to estimate the undrained shear strength profile affected by strain rate will be assessed (Elsworth and Lee, 2005). The FF-CPT device measures acceleration, cone resistance, sleeve friction and pore pressure during penetration. The proposed analysis would be applied to a data set from a series of velocity-controlled deployments in Tauranga Harbour (Bay of Plenty, New Zealand). Undrained shear strength from FF-CPT would be then compared with reference laboratory strength tests such as miniature vane shear tests on sediment cores. From an in situ test such as FF-CPT, an increase in undrained shear strength occurs with increasing impact velocities; hence, an appropriate rate parameter needs to be chosen accordingly based on the impact velocities. Applicability of various strain rate models and rate parameter to FF-CPT data would be also assessed in this work.
The shear strength profile in the upper 1 to 2 m of the seabed is critical for pipeline, flowline, and riser design, and yet is the most difficult to assess by means of in situ testing and soil sampling. All of the strain rate models compared reasonably well with the FF-CPT data. For decaying strain rate effects encountered during a FF-CPT deployment, the use of an inverse hyperbolic sine model could be advantageous. In our case the best fit of average values ranges from 0.05 to 0.1 for the range of impact velocities encountered during the deployments for clayey soft marine sediments.
For very rapid loading conditions encountered especially during a FF-CPT, we observe a pronounced increase in dynamic cone resistance due to negative pore water pressures. Also we see that particle crushing makes an important contribution for the increase in strength of sands at higher strain rates. Various empirical strain rate models are applied to the FF-CPT data in this work. From an in situ test such as FF-CPT we see that an appropriate rate parameter needs to be chosen accordingly based on the impact velocities, different soil types encountered the best fit from this study ranges from 0.2 -0.95 for the silty sands to fine and medium sands using the logarithmic function also using the inverse hyperbolic sine function we see the values of lambda ranges between 0.4 - 2.0.
In the future, FF-CPT experiments could be considered for use as one of the in situ techniques for characterizing the undrained shear strength of uppermost soft sediments after appropriate correction for strain rate effects, as it would be fast and economical

Members

Proponents:Prof. Dr. Achim KopfUniversity of Bremen
Prof. Dr. Tobias Mörz
:Dr. Vicki MoonUniversity of Waikato
Prof. Dr. Karin Bryan
PhD Candidate:Vigneshwaran RajasekaranUniversity of Bremen

Publications

N / A

Miscellaneous

Research stay at the University of Waikato, Hamilton: 08.04. - 02.08.2011