Mörz, Tobias
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Mörz, Tobias
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Mörz, Tobias
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Moerz, Tobias
Tobias, Mörz
Mörz, T.
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tmoerz@marum.de
tmoerz@uni-bremen.de
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Item-typ:Veröffentlichung, Vibratory cone penetration test to investigate cyclic soil behavior in-situ(2020-03-11); ; ; Climate change increased the need of using renewable energy as replacement for fossil fuel. This led to a fast growth of the offshore wind energy sector, especially in Germany where offshore wind energy turbines are built in great numbers in the North Sea. Piles are usually used as foundations for these turbines. In recent years, new projects planned to install these piles with the vibratory pile driving technique which installs piles with axial vibrations. One of the main challenges with vibratory pile driving is the choice of the vibrator that should have a sufficient weight and energy to drive the pile to the designed depth. The selection of the vibrator usually depends on predictions from drivability analyses that use parameters obtained from the conventional in-situ soil investigation methods such as cone penetration test CPT as an input for the analysis. CPT is a cone that is pushed into the ground at constant speed while sensors in the cone measure the cone resistance, sleeve friction, and pore water pressure. The static soil behavior obtained from these conventional CPTs differs from the cyclic soil behavior exhibited during vibratory pile driving. In this research, vibratory cone penetration test VCPT is introduced to improve the geotechnical in-situ methods for evaluating the cyclic soil behavior during vibratory pile driving. The new device controls displacement amplitudes in real time and it penetrates the ground while inducing vertical cyclic strains. The cyclic motion of VCPT resembles the motion of the piles during vibratory driving, therefore it could be used to investigate the cyclic soil behavior and degradation during vibratory pile driving. Major obstacles with any in-situ soil investigation method are the inherent variations in soil properties which could affect the results obtained from in-situ tests. Performing a number of tests in small spacings increases the statistical significance and reduces the effect of soil variations on test results. Still, there is no consensus on how to define the minimum spacing between testing zones and no consensus on the effect of disturbance caused by in situ tests. This doctoral thesis has divided the in-situ soil investigations into three studies: In the first study, static cone penetration tests were used to develop a procedure to characterize the in-situ properties of soil and to investigate the minimum spacing between tests without the effect of soil disturbance. A systematic grid of 33 CPTs was performed in the field by sequentially and successively refining the grid spacing between CPTs, starting with a spacing of 119 cone diameters down to a grid spacing of 7 cone diameters. It was found that the cone resistance is affected by previous CPT measurements below a spacing threshold of 24 cone diameters in medium-dense sands. Silt and clay layers showed no reduction in the cone resistance for our minimum grid spacing of 7 cone diameters. The study also showed that the spacing between the tests and natural variations in soil properties are deciding factors for the number of CPTs needed in the field for a sufficient statistical significance. In the second study, VCPT was used to investigate the cyclic behavior of the soil. The device proved to generate constant amplitudes until the final depth of penetration. Two sand layers reacted to the applied cyclic loads by showing high reduction in the resistance. It was also found that even a distance of 50 cm does not guarantee a good correlation between CPT and VCPT. Therefore, two or more pairs of CPT and VCPT should be conducted in order to minimize misinterpretations of the data which are caused by small scale geological structures such as the presence of inclined layers, cross bedding, or other heterogeneities. In the third study, nine static CPTs and VCPTs were performed in a systematic grid. The vibratory CPTs were performed at a constant frequency of 20 Hz and at three different amplitudes of 3, 5, and 7 mm. It was found that the degradation of soil resistance to vibratory cone penetration increased with increasing cyclic displacement amplitudes. This degradation was not accompanied by any increase in pore water pressure. Cyclic cone resistance-displacement hysteresis loops indicated the formation of a cavity between the cone and soil during the upward movement of the cone. Furthermore, a distinct difference between the loading and the unloading stiffness during the vibratory penetration was observed. The results demonstrated that there is no unique relation between static and vibratory cone resistance, therefore, current practices that estimate the degradation in soil resistance due to vibratory pile driving from the static cone resistance are probably not sufficient. The results obtained from the three studies showed that the current civil engineering approaches that assess the cyclic soil behavior during vibratory pile driving from static cone resistance are not sufficient because there is no unique relation between static and vibratory cone resistance. VCPT could be used to assess the cyclic soil behavior, however, several tests should be performed in order to have statistical significance to consider the effect of the variation in soil properties.Dissertation495 437 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Principles of the axial pile setup(2022-04-29); ; ; Piles, as widely used foundations onshore and offshore for advanced structures or wind turbines are known for their frequently observed, long-lasting, capacity increases – known as the pile setup. Not seldom, this results in a doubling of the pile capacity within the first 100 days after their installation. Many sub-mechanisms are suspected to contribute to this increase, such as changes in the pile surrounding stress field, an aging of the soil, or modifications at pile shaft surface due to physiochemical processes. However, many of the existing studies focus too much on a single mechanism as a possible cause of the setup. As a result, attempts to transfer these findings to alternative sites and/or piles frequently did not produce satisfying results. The possibility, that this process might be multi-factorial is still often rejected or just ignored. As a result, there is still no conclusive theory to explain the setup in all its facets, even though it was first mentioned now 120 years ago. An improved understanding of the mechanisms and a consequent integration of the setup into daily engineering practice has the potential of considerable cost savings. In particular if smaller, thinner or shorter piles could be used to carry the same loads as before. When piles are used as foundations for wind turbines, eliminating such inefficiencies will be of utmost importance in a world of dwindling resources and for a humanity fighting the climate crisis. The present study aims at improving our understanding of this process in three, well defined sub-studies, which concern one constant test environment. An extensive, small-diameter field study (Chapter 4) reveals that the capacity of a pile and its setup is dependent on the respective installation method as well as on the corrosion vulnerability of the pile material. Piles installed by a rather soil disturbing installation method – i.e. by pile vibration instead of impact driving or pile jacking – potentially tend to a reduced initial capacity, but simultaneously, to a more pronounced setup. It is shown, that vibrated piles can reach capacities of identical impact driven piles after not more than 100 days of aging. Indications are provided, that for longer time ranges the vibrated piles might even outperform these impact driven piles. The second study (Chapter 5) evaluates three-years of capacity monitoring of six, offshore-size piles installed by pile vibration and by impact driving. It demonstrates that vibrated, large-diameter piles provide just a third of the capacity of impact driven equivalents even after an aging period of three years. The small-pile experiment hypothesis is consequently rejected, and a size dependence of the pile setup is assumed. A final laboratory and modeling study (Chapter 6) evaluates the impacts of physiochemical effects on the overall setup. Direct shear testing of naturally-aged surfaces and subsequent capacity modeling indicate that physiochemical effects and the formation of a pile adhering sand-crust have a high potential to increase pile capacity. With these three sub-studies, this doctoral thesis provides a significant contribution to the understanding of the setup as a multi-factorial and multidimensional process.Dissertation414 227 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Late Miocene-Pliocene glacial cyclicity in a deep-sea sediment drift on the Antarctic Peninsula continental margin: Sedimentary and diagenetic processes(2007-07-22); ; ; The presented sedimentological, geochemical and paleoceanographical work is intended to improve the understanding of the regional influence of West Antarctic Ice Sheet dynamics and changes in oceanographic conditions on (1) sedimentary processes and (2) the preservation of proxies in the sedimentary record. A crucial feature for interpreting ice sheet dynamics is the understanding of the glacial driven sedimentary transport system across the shelf to the slope and subsequently to deep-sea sediment bodies. Sediment physical and geochemical records, and X-ray images derived from ODP Site 1095 (West Antarctic Peninsula) were used to identify pattern in glacial-interglacial cyclicity and associated sedimentary and diagenetic processes of the late Miocene and Pliocene. A prominent feature is the cyclic loss of the magnetic susceptibility signal at the glacial-to-interglacial transitions, which is related to ice sheet collapses, meltwater formations, high fluxes of organic matter, and temporary suboxic to anoxic near surface sediment conditions at the end of the deglaciation phases. A quantification of the slope failure frequency reflects short and rapid but cyclic ice advances every ~375 yrs.Dissertation531 110 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Failure mechanisms and mobilization processes of coastal landslides in sensitive soils(2017-08-11); ; ; Landslides are widespread along coasts worldwide. Understanding initiation, type, and areas affected by such landslides, is thus one of the primary concerns for coastal communities and infrastructure projects, like harbor constructions and residential settlements. Important short-term, high-energy impacts that may trigger a landslide are earthquakes and heavy rainfall events. Weak zones within the depositional succession comprising the slope are another important factor contributing not only to landslide initiation, but also to post-failure landsliding. Assessing the landslide hazard in coastal regions therefore requires a good understanding of the complex interrelations between the various high-energy external impacts and the internal mechanical characteristics of the slope material. Some of the most damaging landslides in the past occurred in soil with high sensitivity, a measure of the post-failure strength loss in the failure zone during landsliding. Such soil exhibits very low shear strength after failure, predisposing highly mobile landslides with long runout distance and dimensions difficult to predict. High sensitivities have been described for post-glacial sediments in Norway, as well as for altered tephra in New Zealand. In both regions, it is of common interest to better understand the weakening and mobilization processes in sensitive slope forming soil subjected to cyclic loading such as earthquake shaking. Furthermore, the processes that lead to high sensitivities in altered tephra are still poorly understood. This doctoral thesis aims to broaden the understanding of failure mechanisms and mobilization processes in landslides at the interface between land and water. Two landslides were investigated, that occurred in sensitive soil and affected society, economy, and natural environment in coastal regions: (1) The coastal submarine Orkdalsfjord landslide in postglacial silt, Norway, and (2) the coastal subaerial Omokoroa flow slide in altered tephra, New Zealand. The vulnerability of the Orkdalsfjord landslide to cyclic loading was studied by using in situ vibratory cone penetration tests and laboratory cyclic triaxial testing. Very coarse silt layers, interbedded in the post-glacial silt unit overlying the failure surface, is more vulnerable to cyclic loading compared to surrounding finer silts. Accordingly, the very coarse silt layers may have contributed to the weakening and mobilization of the Orkdalsfjord landslide in case cyclic loading occurred during landsliding. The cyclic loading behavior of altered tephra from the Omokoroa flow slide was analyzed by monotonic and cyclic triaxial testing. The altered tephra experiences brittle failure and has higher friction coefficients than normally consolidated clay, being similar to granular soil. Comparing the cyclic shear strength of altered tephra with that of marine clays shows that for altered tephra the number of loading cycles to shear failure depends more strongly on the level of shear stress applied and that altered tephra is more resistant to small cyclic loading but fails within a narrower range of shear stresses. The development of high sensitivities in altered tephra was analyzed by scanning electron microscopy and laboratory vane shear measurements along a drill core comprising the intact tephra succession of the Omokoroa flow slide. The secondary clay mineral halloysite dominates the Pahoia Tephra, a sequence that was involved in the Omokoroa flow slide. The halloysite particle morphologies are highly variable with depth. While tubular morphologies are prevalent in the upper tephra successions, the lower Pahoia Tephra sequence is dominated by spheroidal halloysite. This change in halloysite morphology coincides with an increase in sensitivity with depth. Therefore, spheroidal halloysite is likely the key in the development of sensitivity in altered tephra from New Zealand and potentially elsewhere in regions of similar volcanic origin. In the failure surface of the Omokoroa flow slide, a new open-sided spheroidal halloysite particle shape in the form of a mushroom capsa is recognized for the first time that governs the development of high mobility in the failure surface during landsliding. Based on a new a attraction-detachmenta model, it is suggested that the rearrangement in the halloysite texture during the failure process reduces the attractions between the particles at nanoscale dimensions and thus predisposes flow sliding.Dissertation1025 190 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Design of a free-fall penetrometer for geotechnical characterisation of saturated sediments and its geological application(2007-10-26); ; ; Cone Penetration Testing (CPT) is a versatile, time efficient method to characterise sediment strength and pore pressure in offshore settings and on land. The majority of the penetrometers rely on heavy trucks or rigs to provide the necessary force to push the CPT probe into the ground. This laborious process usually deforms or otherwise affects the uppermost deposits, whose physical properties are in turn vital to understand processes related to scour, burial of mines, cable or pipeline laying, silting of water ways and harbours, or sediment transport and remobilisation. Owing to the shortcomings of heavy seagoing CPT gear, this thesis aimed to develop and use a cable-led marine penetrometer lance which profiles the uppermost sediments in a less destructive manner. The study summarises the development and deployment of a marine cone penetrometer system. It consists of an instrument for shallow-water application (200 m) and, based on the experience of the first, a second version operable down to 4000 m water depth. Design and construction of the instruments occurred at the Research Centre Ocean Margins, Bremen University (RCOM) in close and productive collaboration between Achim Kopf, Matthias Lange and myself during the first year. After an initial phase of researching for sensors and components, I contributed to the design. After construction, a total of 338 CPT experiments (both with the shallow-water [SW] and deep-water [DW] device) were carried out to date. From the wealth of deployments, 300 of them were performed by me while 204 raw data sets were processed by me over the course of this thesis (mostly 2nd and 3rd year). The CPT deployments were initially dedicated largely to instrument testing, and later focused on geological application. From 38 selected data sets, the strain-rate effects of the dynamic tests were assessed and, based on earlier empirical solutions from the CPT literature, corrected for. Overwhelmingly, the results with the RCOM lance agree well with those from the pushed tests, and further help to accentuate them. Within the spectrum of velocities tested , it is found that the faster the rate of initial penetration is chosen, the larger the discrepancy between the deviations caused by layering and variations in physical properties of the sediments. This observation may be vital when carrying out CPT experiments in geomaterials where lithological variability is small, because it helps identifying features otherwise undetectable by pushed profiling at the standard rate of 2 cm/s. Another five manuscripts summarise the geological application of the CPT devices. The tests were performed in geological environments as diverse as the Baltic Sea, Lake Lucerne, an active mud volcano in Azerbaijan, and the Cretan Sea in the Eastern Mediterranean. Regardless of the regional scenario, some overarching consistent results were obtained during the CPT deployments. As an outstanding finding, three types of characteristic pore pressure signals are recorded by CPT lance when deployed in "free-fall" mode on a cable. In granular, normally consolidated material, a pore pressure spike upon impact is usually followed by an exponential decay back to ambient values (if sufficient time is allowed for dissipation). Alternatively, a second pattern often met is a negative (i.e. sub-hydrostatic) pressure spike followed by an increase in pressure to ambient pore pressure values. The sub-hydrostatic signal is caused by displacement of pore fluid by the profiling CPT instrument, which results in flow away from the probe. This second pattern is restricted to coarse-grained deposits with high permeability. The third characteristic signal generally shows supra-hydrostatic pressures upon impact and during profiling, but then climbs to even higher pressures with time. Graphs like this are found in sediments of variable grain size distribution and are related to fluid overpressures. Interestingly, the third type is observed in clays as well as silt- or sand-bearing deposits, and appears no matter what the cause of the excess pore pressure is. In the various field studies, very similar pore pressure curves are seen although the reason for the overpressures were glacial loading (and potentially EQ tremor; Lake Lucerne), presence of microbial gas (Baltic Sea), hydrocarbon formation in folded and faulted shales of the Maykopian Formation (Azerbaijan, Greater Caucasus), or neo-tectonic movement and landsliding (Cretan Sea). In summary, this study has shown that velocity-controlled "free-fall" CPT lances are an efficient, user-friendly means to characterise geotechnically shallow sub-bottom deposits. They obtain reproducible results that can be linked with standard pushed tests, but have the added advantage of producing more pronounced excursions in cone resistance and sleeve friction as well as characteristic pore pressure responses indicative of geological conditions.Dissertation618 505 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Quaternary evolution of the drainage systems in the Southern and Central North Sea Basin deduced from high resolution reflection seismic and sedimentological data(2022-11-04); ; ; The North Sea shelf is one of the classic regions for studying Quaternary depositional and erosional sedimentary structures formed by discharging melt-water from retreating ice sheets and fluvial processes during periglacial shelf exposure. This provides an avenue to unravel the complex interplay between shelf morphology, changes in hydrodynamic regime, sediment influx and climate-driven eustatic changes. The ongoing political energy transition has prompted an intense geo-survey and sampling of the North Sea floor largely by industrial organizations, various Federal agencies such as the German Federal Maritime and Hydrographic Agency (BSH), German Federal Institute for Geosciences and Natural Resources (BGR) as well as some scientific institutions. The wealth of acquired data in the course of these various commercial, governmental and scientific expeditions has led to an improved understanding of the Quaternary geology of the North Sea sub-surface. Using a combination of tidally corrected acoustic profiles, shallower and deeper sediment cores and cone penetration test (CPT) data acquired from the German North Sea sector, this doctoral thesis aimed at contributing to the ongoing understanding of the morphology, the development of the drainage systems and the palaeo-landscape of the North Sea shelf since the Last Glacial Maximum. It also aimed at refining the stratigraphy and the geotechnical properties of the highly competent shallow sand units in the southern North Sea. The study unraveled the evolution, morphology and valley infill successions of the Elbe Palaeovalley (EPV). The EPV, located in the southeastern North Sea, occur as a shallow geomorphological trough spanning a length of about 210 km and width of about 40 km. The valley base is about 65 m below the present day sea-level. During the Marine Isotope Stage 2 sea level lowstand, this SE-NW oriented palaeo-drainage evolved as a braided fluvial system in a periglacial environment. Seismo-stratigraphic interpretation of the EPV infill successions revealed five major units. During deglacial sea-level rise, the EPV evolved into an estuary with tributaries, intertidal and subtidal flats. Towards the western flank of the EPV, the Palaeo-Ems, one of the known major tributaries that fed the southern head of the EPV was also identified and its overall course was seismo-stratigraphically described. The Palaeo-Ems river course was mapped as a buried, low gradient and meandering channel branching into two major pathways as it approaches a newly discovered delta flat at the western flank of the EPV. In its downstream direction, the Palaeo-Ems formed a unified depositional system with the early phase of the EPV. Based on available data, this study also shed light on the Palaeo Ems/EPV morpho-stratigraphic relationship for the first time. Ongoing sea-level rise since the early Holocene overwhelmed the adaptation capabilities of the joint system leading to the drowning of the whole drainage system. By focusing on the less well understood, regionally dominant sand units which were deposited after the retreat of the last glaciers, this study also refined the stratigraphic units and geotechnical parameters of the uppermost 50 m below the sea floor within the German North Sea sector. Two sandy units, the Aeolian Member and the Upper Fluvial Member, were identified as dominant deposits in the late- to post-Saalian geology within the study area. In addition, a detail seismic analysis revealed the occurrence of a Saalian Buried Valley Member believed to comprise fluvial deposits. Based on the integration of seismic facies analysis with core and CPT data, a detailed geotechnical parameter set for each identified stratigraphic unit within the study area was developed and correlated with those of the neighboring North Sea sectors. The findings from this study complements and details Coughlan et al. (2018) geotechnical and stratigraphic framework of the study area as well as the stratigraphic framework recently developed by the BSH (2021). These deductions offer a new insight about the soil competency of the North Sea sub-surface which is key for various offshore commercial and economic activities within the region including but not limited to wind farm developments. The findings from this study are thus crucial contributions in understanding the dewatering system and subsequently, the reconstruction of the palaeo-landscape development in the German North Sea sector since the Last Glacial Maximum. Lastly, the study contributes to an improved geotechnical understanding of the stratigraphy of the North Sea.Dissertation312 166 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Advanced CPTu and laboratory investigation of geotechnically critical on-shore and near-shore soft sediments in Germany and New Zealand(2014-07-09); ; ; The integrated Coastal Zone and Shelf Sea Research (INTERCOAST) organization, a cooperative German and New Zealand multidisciplinary research program, concentrates on scientific issues in social and natural science disciplines in the both countries. INTERCOAST 4 (IC4) Ph.D. program tackled geotechnically problematic soft soils such as peat, clay and clayey silts which have posed considerable challenges to geotechnical engineers in all parts of the world during design and construction process. In Germany, peat is one of the prevailing groups of soil which is present in both off-shore and on-shore areas and exhibit properties such as high compressibility and low shear strength; these properties may cause complications such as differential settlement or failure in structures built on such soils. Removal or stabilization are the most important methods used to overcome geotechnical problems related to peat soils engineering characteristics. In New Zealand, many off-shore and on-shore areas of the North Island are covered by volcanic ash, and weathering of this material has resulted in formation of clay minerals. Dredging of volcanic ash layers often causes major turbidity in the water column and poses risks to wildlife and humans. Due to very low effective shear strength and high sensitivity of volcanic ash, these sediments are not considered to be appropriate for off-shore construction and installation purposes. On-shore weathered volcanic ash having low permeability acts as a barrier to fluid flow, for example infiltration of rainfall, and prevents pore pressure from dissipating. This special characteristic may lead to failure of slopes with volcanic ash materials because increases in pore pressure lowers the effective normal (vertical) stress, and therefore shear strength. The aim of this dissertation is to utilize in-situ and laboratory measurements in order to (i) present soil mechanical intervention for stabilization of peat using cost-effective and environmentally-friendly stabilization method and focus on a comparison between mechanical characteristics of undisturbed and stabilized peat, (ii) investigate geological setting, lithology and depositional history of off-shore sub-seafloor volcanic soils and determine geotechnical properties of near-surface sediments and (iii) look into the role of volcanic soils in occurrence of on-shore landslides.Dissertation252 732 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Assessing prediction uncertainties in quantitative ground models for offshore wind farm areas(2026-02-19); 49 32 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Modellierung von Gashydraten und deren Wachstumsverhalten im Porenraum mariner Sedimente mit Hilfe der Distinkte Elemente Methode.(2007-07-24); ; ; The mechanical behavior of gas hydrate bearing sediment is a possible trigger for huge tsunami generating slides on continental margins, but little is known of gas hydrates mechanical behavior in sediment. Here a method is proposed to simulate the behavior of gas hydrate in sediment on the pore scale using the Distinct Element Method (DEM). The simulation is founded on the surface energy, which is the cause for the forces exerted by growing crystals. The simulation uses attractive particle interaction and random particle agitation to generate a surface tensed material. The method is calibrated and validated by a series of different experiments. Gas hydrate growth is simulated in an oedometer. The influence of the host sediment fabric on gas hydrate fabric is investigated in a series of element tests and compared to natural gas hydrate fabrics.Dissertation590 348 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Beneficial on-site reuse of dredged harbor mud: A geotechnical challenge in alternative sediment management(2008-12-12); ; ; Sediment accumulation in waterways, estuaries and harbors results in expenditures of more than 1 billion Euros annually in Europe alone. These costs are mainly due to dredging for maintenance of navigable water depth and subsequent relocation, transport, treatment and/or disposal of the dredged material. Of special concern is the treatment of sediments dredged from harbors. A pilot study to use dredged harbor mud as backfill material was conducted in the East Harbor of Bremerhaven, Germany between 2005 and 2007. During this project, a total of about 180,000 m3 of harbor mud was relocated by pumping behind a newly installed sheet piling, creating 14 acres of new harbor area. The subsequent occurrence of strong surface deformation of the backfill, indicating a partial collapse of the mud layer, highlights the need to better understand the geotechnical properties of harbor mud. The purpose of this work is to gain a better understanding of the usability of harbor mud as backfill material and therefore help improve the planning reliability in using dredged harbor sediments in future land reclamation projects.Dissertation486 228