Stefan Kreiter

Influence of Shear Rate on Undrained Vane Shear Strength of Organic Harbor Mud

Dredging operations in European harbors for maintenance of navigable water depth produce vast amounts of harbor mud. Between 2005 and 2007, the second largest harbor construction project in Germany was designed as a pilot study to use dredged harbor mud as backfill material to avoid expensive disposal or ex situ treatment. During this project, a partial collapse of the backfill highlighted the need for an improved assessment of undrained shear strength of naturally occurring liquid harbor mud. Using vane shear testing, this study evaluates the effect of shear rate on the undrained shear strength of harbor mud. It is shown that measured values for both peak and residual shear strength are significantly influenced by shear rate effects. Furthermore, the influence of shear rate on the peak shear strength is found to be independent of water content while the influence of the shear rate on the residual shear strength strongly depends on water content. New shear rate dependent correction factors μ are proposed ...

Finneidfjord: a Field Laboratory for Integrated Submarine Slope Stability Assessments and Characterization of Landslide-Prone Sediments: A Review

Sorfjord outside the village of Finneidfjord has a history of landsliding throughout the Holocene. The 1996 landslide – the focus of this study – has many characteristics typical of submarine landslides (well-developed slip plane, outrunner blocks, peripheral thrusting and lateral spreading). Due to its sheltered and accessible location, Finneidfjord has become a natural laboratory for testing high-resolution and multidisciplinary techniques to improve our understanding of landslide development. This study integrates multiple sediment cores, swath-bathymetry surveys, single- and multi-channel 2D seismic data (Topas, boomer, sparker, airgun), very-high-resolution 3D chirp seismics, ocean-bottom seismometer as well as free fall and traditional cone penetration testing (CPTU). The cores have been subjected to both geological and geotechnical laboratory analyses. Of particular interest is the correlation of the regional slip plane as a high-amplitude package of reflections in the geophysical data with the results of the sediment and in situ measurements. Comparison of 3D traces with synthetic seismograms based on multi-sensor core logs show that the most prominent slip plane lies within a thin clay unit sandwiching a sand seam. The slip plane is difficult to identify from CPTU data alone. The top part of this composite unit has in places been eroded under the 1996 mass-transport deposit (MTD). This composite unit’s formation is associated with turbidite deposits from terrestrial quick clay landslides and possibly river floods in the catchment of the fjord. While the MTD is extensively deformed, different flow facies are identified within the landslide body revealing a complex, multi-phase failure. The seismic data were also used to infer physical properties (mean grain size, gas saturation from P-wave attenuation). Interestingly, shallow gas adjacent to the landslide appears not to have played a role in the landslide development. Fjordbed stability is strongly influenced by shallow subsurface structure, with geotechnical properties and lateral continuity of stratified beds acting as primary controls on slide plane depth and failure mechanisms. This study can well form a template for near-shore areas prone to landsliding. Currently, a long-term pore pressure monitoring programme is in progress, after the installation of several piezometers close to the depths of the slip plane close to the shoreline in September 2012.

Advanced dynamic soil testing - introducing the new MARUM dynamic triaxial testing device

Soil mechanical and submarine mass-movement initiation studies often use static and quasi-static approaches to determine the strength of soils against external mechanical stresses. However, many natural processes pose time variant stresses on soils, and hence exert key roles for submarine slope stability and submarine mass-movement initiation. Prominent examples are earthquake-, wind-, wave- and current-forces and alternating man-made loading on offshore constructions. Most soils show a weaker response to periodic loading — making dynamic and cyclic loading experiments mandatory for offshore natural hazard and risk assessment. Dynamic and cyclic triaxial testing are essential in liquefaction studies of granular soils and creep investigations of cohesive and granular sediments. So far, competing setups are used with mechanical spindles, pneumatic actuators or full hydraulic drives.

A new attraction-detachment model for explaining flow sliding in clay-rich tephras

Altered pyroclastic (tephra) deposits are highly susceptible to landsliding, leading to fatalities and property damage every year. Halloysite, a low-activity clay mineral, is commonly associated with landslide-prone layers within altered tephra successions, especially in deposits with high sensitivity, which describes the post-failure strength loss. However, the precise role of halloysite in the development of sensitivity, and thus in sudden and unpredictable landsliding, is unknown. Here we show that an abundance of mushroom cap–shaped (MCS) spheroidal halloysite governs the development of sensitivity, and hence proneness to landsliding, in altered rhyolitic tephras, North Island, New Zealand. We found that a highly sensitive layer, which was involved in a flow slide, has a remarkably high content of aggregated MCS spheroids with substantial openings on one side. We suggest that short-range electrostatic and van der Waals interactions enabled the MCS spheroids to form interconnected aggregates by attraction between the edges of numerous paired silanol and aluminol sheets that are exposed in the openings and the convex silanol faces on the exterior surfaces of adjacent MCS spheroids. If these weak attractions are overcome during slope failure, multiple, weakly attracted MCS spheroids can be separated from one another, and the prevailing repulsion between exterior MCS surfaces results in a low remolded shear strength, a high sensitivity, and a high propensity for flow sliding. The evidence indicates that the attraction-detachment model explains the high sensitivity and contributes to an improved understanding of the mechanisms of flow sliding in sensitive, altered tephras rich in spheroidal halloysite.

Utilizing cone penetration tests for landslide evaluation

Pore pressure and shear strength are two important parameters that control the stability of slopes. These parameters can be derived in-situ by cone penetration testing (CPT) with pore pressure measurements. This paper presents the results from three static, vibratory and dissipation CPT profiles deployed into a landslide headwall at Pyes Pa, Bay of Plenty, New Zealand. The landslide strata consist of volcanic ashes and ignimbrites. Studying the stability of slopes in this area using in-situ geotechnical testing is of societal-economic importance since several other landslides within comparable strata caused considerable property damage. Three CPT profiles were collected across the headwall of the slide scar with 2 m spacing in undisturbed sediments using static, vibratory and dissipation test modes. Static CPT results are used to evaluate soil grain size variations, geotechnical parameters of sediments such as shear resistance, probable slip surface and sensitivity of sediments. Liquefaction potential of sediments is assessed using vibratory CPT results. For dissipation tests, the cone remained stationary in the sediment for ∼60 min to monitor pore pressure dissipation at the depths of 6, 9 and 11 m. With the use of pore pressure dissipation data, values of soil horizontal permeability are calculated. The liquefaction probability from static CPT results is compared to liquefaction potential evaluation from vibratory CPT. Last but not least, an unstable soil layer is defined based on static CPT, vibratory CPT and dissipation results.

Subseafloor Investigation of Sediments at Southern Tauranga Harbour, New Zealand, before Capital Dredging

ABSTRACT Jorat, M.E.; Moon, V.G.; Hepp, D.A.; Kreiter, S.; de Lange, W.P.; Feldmann, S., and Mörz, T., 2017. Subseafloor investigation of sediments at southern Tauranga Harbour, New Zealand, before capital dredging. The Port of Tauranga plays a key role in New Zealands export–import industry, and capital dredging commenced in October 2015 to extend the shipping channels to accommodate larger container vessels. This study investigated two-dimensional and three-dimensional subsurface estuarine sediment stratigraphy to predict the sedimentological conditions encountered during dredging operations to ensure that appropriate dredging methodologies were used to minimise the generation of turbidity. Eight cone penetration tests (CPTs), 14 core descriptions, and a high-resolution seismic investigation in Stella Passage (the main shipping channel of Tauranga Harbour) provided the basis for this research. Six major units comprise the stratigraphy; in ascending order they are lower pumiceous sand and silt (UNIT6), quartz-feldspar sand and silt (UNIT5), middle pumiceous sand and silt (UNIT4), silt–sand–clay (UNIT3), upper pumiceous sand and silt (UNIT2), and Holocene marine (UNIT1) sediments forming the modern seafloor. Three paleovalleys filled with sediments from UNIT2 were identified extending west to east across the channel. The CPT profiles were correlated with the seismostratigraphic units and their corresponding soil behaviour type to characterise each units sediment stiffness. The UNIT6 is unlikely to be encountered in current and future capital dredging operations because it is the lowest observed unit. A special dredging methodology may be required for UNIT5 and UNIT4 as they have high CPT tip resistance and undrained shear strength. For UNIT3, UNIT2, and UNIT1, which have very low CPT tip resistance and undrained shear strength, trailing suction and cutter suction dredging is appropriate. The potential turbidity of the water column, however, could impose a significant threat to marine biota and has to be taken into account.

Static and Cyclic Shear Strength of Cohesive and Non-cohesive Sediments

Submarine slope failures are common along tectonically and seismically active margins and may have devastating impact on onshore and offshore infrastructure as well as coastal communities. Soils show a variable response to periodic loading compared to static loading – making static and cyclic loading experiments compulsory for submarine slope stability and mass-movement initiation studies. Results from (i) a generic study investigating the shear strength of water-saturated sediments upon drained static vs. undrained cyclic loading, and from (ii) a comparison to natural samples are presented. A direct shear apparatus and the MARUM Dynamic Triaxial Testing Device have been used to compare undrained, cyclic to drained, static shear strengths of reconstituted samples with different clay to quartz (sandy silt) ratios. With this experimental set-up we aim to identify the failure potential of cohesive to granular material under cyclic and static loading condition. Results indicate that the cyclic shear strengths of material mixtures with less than 20% clay mineral content are significantly lower than their static shear strengths. Mixtures with a clay mineral content exceeding 20% show converging cyclic, undrained and static, drained shear strengths. Ongoing studies build on the knowledge gained from the generic endmember tests and integrate natural samples from the Nankai Trough accretionary wedge (Japan).

First Results of the Geotechnical In Situ Investigation for Soil Characterisation Along the Upper Slope Off Vesterålen: Northern Norway

High-resolution geophysical data reveal the presence of several spatially-isolated, small-scale landslides along the gently dipping (~3–4°) upper slope off Vesteralen, Northern Norway. Dynamic slope stability analysis suggests that seismicity may be largely responsible for the occurrence of these slope failures. The landslides are clustered in two groups, with one group of parallel features with their headwalls in ~500 m water depths. The second group is found in ~800 m water depths.

In Situ Cyclic Softening of Marine Silts by Vibratory CPTU at Orkdalsfjord Test Site, Mid Norway

Earthquake induced cyclic loading has the potential to destabilize submarine slopes either by liquefaction in coarse-grained deposits or by cyclic softening in cohesive sediments. Vibratory cone penetration tests (VCPTU) represent a new approach for the evaluation of cyclic softening in fine grained sediments. In the past, VPCTU were utilized to evaluate liquefaction potential of sands, but cyclic softening of fine-grained marine sediments has not yet been tested with VCPTU in situ. At the study site in Orkdalsfjord, mid Norway marine clayey silt deposits are interbedded with coarse silt and clay layers. Static and vibratory CPTU were performed down to 19 m penetration depth using the Geotechnical Offshore Seabed Tool (GOST) and in addition, two gravity cores were taken for cyclic triaxial testing and geotechnical index tests. From static and vibratory CPTU a number of coarse silt layers with a distinct drop in cyclic cone resistance were identified. Compared to surrounding finer sediments the coarse silt layers exhibited a higher potential for cyclic softening. This assumption is supported by cyclic triaxial tests on very coarse and surrounding medium-coarse silts, respectively, revealing a strong loss of cyclic shear strength in a controlled and documented stress-strain regime. This study highlights the potential for VCPTU as a promising tool to qualitatively evaluate the vulnerability of marine silts to cyclic softening. In combination with advanced laboratory tests these results are envisioned to help better identifying submarine slopes subjected to failure during earthquakes.

A Small Volume Calibration Chamber for Cone Penetration Testing (CPT) on Submarine Soils

A new small volume CPT calibration chamber with dynamically controlled boundary conditions has been built to improve the correlation between in-situ data and soil parameters. The sample volume in the new CPT calibration chamber has a diameter of 30 cm and a height of 54.5 cm. Therefore, it is possible to use reconstituted samples of limited quantity, e.g. from boreholes. The chamber is able to simulate large overburden stresses and overconsolidation ratios (OCR) up to 5 MPa. Horizontal, vertical and pore pressures are independently applied via syringe pumps while recording volume changes. All pressures are dynamically controlled and allowing stress, strain and mixed stress-strain stress boundary conditions BC1–BC5 to be enforced. The sample deformation is measured by circumferential laser triangulation sensors. In a first series of tests using Cuxhaven Sand, a 12 mm cone and BC1 conditions, CPT tip resistances reach steady state about a length of at least 10 cm. The corrected tip resistances and the inferred relative densities for Cuxhaven Sand differ substantially from previously established correlations, confirming the need for more advanced correction factors and relationships between CPT data and in-situ soil properties.