Norwegian Geotechnical Institute (NGI) Digital Archive
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Modelling 2018 Anak Krakatoa Flank Collapse and Tsunami: Effect of Landslide Failure Mechanism and Dynamics on Tsunami Generation
Get PDFThe 2018 Anak Krakatoa volcano flank collapse generated a tsunami that impacted the Sunda Strait coastlines. In the absence of a tsunami early warning system, it caused several hundred fatalities. There are ongoing discussions to understand how the failure mechanism of this event affected landslide dynamics and tsunami generation. In this paper, the sensitivity to different failure scenarios on the tsunami generation is investigated through numerical modelling. To this end, the rate of mass release, the landslide volume, the material yield strength, and orientation of the landslide failure plane are varied to shed light on the failure mechanism, landslide evolution, and tsunami generation. We model the landslide dynamics using the depth-averaged viscoplastic flow model BingClaw, coupled with depth-averaged long wave and shallow water type models to simulated tsunami propagation. We are able to match fairly well the observed tsunami surface elevation amplitudes and inundation heights in selected area with the numerical simulations. However, as observed by other authors, discrepancies in simulated and observed arrival times for some of the offshore gauges are found, which raises questions related to the accuracy of the available bathymetry. For this purpose, further sensitivity studies changing the bathymetric depth near the source area are carried out. With this alteration we are also able to match better the arrival times of the waves.publishedVersio
On the landslide tsunami uncertainty and hazard
Get PDFLandslides are the second most frequent tsunami source worldwide. However, their complex and diverse nature of origin combined with their infrequent event records make prognostic modelling challenging. In this paper, we present a probabilistic framework for analysing uncertainties emerging from the landslide source process. This probabilistic framework employs event trees and is used to conduct tsunami uncertainty analysis as well as probabilistic tsunami hazard analysis (PTHA). An example study is presented for the Lyngen fjord in Norway. This application uses a mix of empirical landslide data combined with expert judgement to come up with probability maps for tsunami inundation. Based on this study, it is concluded that the present landslide tsunami hazard analysis is largely driven by epistemic uncertainties. These epistemic uncertainties can be incorporated in the probabilistic framework. Conducting a literature analysis, we further show examples of how landslide and tsunami data can be used to better constrain landslide uncertainties, combined with statistical and numerical analysis methods. We discuss how these methods, combined with the probabilistic framework, can be used to improve landslide tsunami hazard analysis in the future.publishedVersio
Intact, Disturbed, and Reconstituted Undrained Shear Behavior of Low-Plasticity Natural Silt
Get PDFThis paper presents a laboratory investigation of undrained triaxial shear behavior of a natural low-plasticity silt from Halden, Norway in the intact, disturbed, and reconstituted states. Sherbrooke block sample and reconstituted specimens were subjected to simulated tube sampling in a triaxial stress path cell system prior to reconsolidation and undrained shear to assess the effects of disturbance on undrained shear behavior, undrained shear strength, and effective stress friction angle. Shear stress and pore-pressure development were evaluated relative to that measured for the undisturbed reference state, i.e., that measured on specimens from the intact block sample. Specimens trimmed from fixed-piston tube samples collected from the field site were tested for comparative purposes. Collectively, the results demonstrated that neither the volumetric method of evaluating sample quality for clays nor shear wave velocity tracked sample disturbance well for this low-plasticity silt. Relative to the reference intact block sample tests, simulated tube sampling resulted in an increasingly pronounced dilative-type behavior during postdisturbance undrained shear and a general increase in undrained shear strength. Specimens from the block sample that were subjected to simulated tube sample disturbance had similar stress–strain behavior as that from conventional anisotropically consolidated triaxial compression tests conducted on specimens from the tube samples, suggesting that significant alteration of the intact soil state occurred during tube sampling. Practical suggestions for selection of undrained shear strength for intact low-plasticity silts that exhibit dilative behavior such as the Halden silt are proposed.acceptedVersio
Comments on “On a Continuum Model for Avalanche Flow and Its Simplified Variants” by S. S. Grigorian and A. V. Ostroumov
Get PDFThis note first summarizes the history of the manuscript “On a Continuum Model for Avalanche Flow and Its Simplified Variants” by Grigorian and Ostroumov—published in this Special Issue—since the early 1990s and explains the guiding principles in editing it for publication. The changes are then detailed and some explanatory notes given for the benefit of readers who are not familiar with the early Russian work on snow avalanche dynamics. Finally, the editor’s personal views as to why he still considers this paper of relevance for avalanche dynamics research today are presented in brief essays on key aspects of the paper, namely the role of simple and complex models in avalanche research and mitigation work, the status and possible applications of Grigorian’s stress-limited friction law, and non-monotonicity of the dynamics of the Grigorian–Ostroumov model in the friction coefficient. A comparison of the erosion model proposed by those authors with two other models suggests to enhance it with an additional equation for the balance of tangential momentum across the shock front. A preliminary analysis indicates that continuous scouring entrainment is possible only in a restricted parameter range and that there is a second erosion regime with delayed entrainment.publishedVersio
Befaringsnotat Grandefonna, Geiranger
Get PDFGrandefonna i Geiranger gikk 17. januar 2019 etter en periode med stort snøfall (NV-situasjon) og stengte vegen. I løpet av 3-5 dager i midten av januar kom det omkring 70-100 cm snø i området. Det var ikke spesielt mye snø i terrenget fra før. Noen sier at det er 40 år siden skredet var så stort og at det ikke har gått skred i løpet lengst vest på 10 år. Vegen forble stengt helt til 23. januar.NVE (Norges vassdrags- og energidirektorat
Soft computing approach for prediction of surface settlement induced by earth pressure balance shield tunneling
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Effects of Drilling for Tieback Anchors on Surrounding Ground: Results from Field Tests
Get PDFA full-scale field test program was carried out to investigate the effects of drilling for tieback anchors on the surrounding ground. The test anchors were drilled from the ground surface through a soft clay deposit and into bedrock. Five different drilling methods were compared. All methods caused excess pore pressures in the surrounding clay, up to 70 kPa, extending several meters away from where drilling took place. This impact on pore pressures was for most drilling methods significantly larger than what has been observed for driven piles in clay. High penetration rate combined with water flushing during drilling through soft clay is the main reason for the effects on the pore pressure. Drilling with a down-the-hole hammer and air flushing through a layer of moraine and into bedrock in one of the test areas (Area B) caused significantly larger excess pore pressures and ground settlements than the other drilling methods. Approximately half of the maximum resulting settlements of 12 mm in Area B was most likely caused by reconsolidation of remolded clay around the casing tubes. Drilling with water-driven hammer in Area C had less effect on both pore pressures and ground settlements.publishedVersio
Validation of a ground-based telescope-assisted hyperspectral remote sensor for soil measurements
Get PDFThe utility of the ground-based soil observation laser absorption spectrometer (SOLAS) was demonstrated through spectral reflectance measurements of five soil types during laboratory- and field-based collection. The SOLAS telescope-assisted measurements were compared with proximal measurements. The spectra that were acquired at an intermediate range of 40 meters compared well with the spectra that were acquired proximally. Specimen type, range-dependent spatial resolution, and environmental conditions are discussed. The signal-to-noise ratio (SNR) was assessed and is presented as a function of wavelength for the spectral range of the receiver for each measurement condition. The proximal measurements performed outdoors under solar illumination had the greatest SNR, while the remote measurements performed indoors under artificial illumination had the lowest SNR. For the outdoor measurements, loss of signal was observed around the 1400- and 1900-nm bands due to long-path atmospheric water vapor absorption. The discussed future improvements to the SOLAS remote sensor will enable measurements of reflectance over longer ranges. Envisioned applications include remote characterization of surface materials for large construction projects (e.g., surface mines and tailings) for geohazard investigations, or for ground truthing of current and future multispectral and hyperspectral satellite data.publishedVersio
Effects of rotational submarine slump dynamics on tsunami-genesis – new insight from idealized models and the 1929 Grand Banks event
Get PDFSediment slumps are known to have generated important tsunamis such as the 1998 Papua New Guinea (PNG) and the 1929 Grand Banks events. Tsunami modellers commonly use solid blocks with short run-out distances to simulate these slumps. While such methods have the obvious advantage of being simple to use, they offer little or no insight into physical processes that drive the events. The importance of rotational slump motion to tsunamigenic potential is demonstrated in this study by employing a viscoplastic landslide model with Herschel-Bulkley rheology. A large number of simulations for different material properties and landslide configurations are carried out to link the slump's deformation, rheology, its translational and rotational kinematics, to its tsunami-genesis. The yield strength of the slump is shown to be the primary material property that determines the tsunami-genesis. This viscoplastic model is further employed to simulate the 1929 Grand Banks tsunami using updated geological source information. The results of this case study suggest that the viscoplastic model can be used to simulate complex slump induced tsunami. The simulations of the 1929 Grand Banks event also indicate that a pure slump mechanism is more tsunamigenic than a corresponding translational landslide mechanism.publishedVersio
Earthquake response of monopiles and caissons for Offshore Wind Turbines founded in liquefiable soil
Get PDFMonopile has been the most widespread foundation type for Offshore Wind Turbines (OWTs) in shallow waters. Caisson (skirted) foundations have also been evaluated in some projects as an economical alternative. While the main concern in design of offshore foundations has been the environmental loads, the recent growth in construction of OWTs in seismic regions with the possibility of soil liquefaction has necessitated evaluation of the impact of earthquake and liquefaction from strong shakings on these structures. Several studies have reported the consequences of soil liquefaction for buildings and onshore structures; However, the effects of liquefaction on offshore foundations have not been sufficiently studied. This paper investigates the use of advanced liquefaction modeling in assessment of the response of monopiles and caissons for offshore wind turbines. The software FLAC3D and the SANISAND constitutive model are used to conduct the nonlinear dynamic analyses for OWTs. Excess pore water pressure during earthquake shaking and earthquake-induced displacements are computed at various points in the soil medium around the considered monopile and caisson foundations. The analyses reveal that SANISAND model is capable of simulating the pore pressure generation in the free-field as observed in a recent centrifuge test. The numerical results also indicate that both monopile and caissons in liquefiable soil deposits experience considerable rotations under the combined action of wind loads and earthquake shaking when liquefaction occurs.publishedVersio