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A mudmat supporting a pipeline end termination (PLET) sits on top of a silty sand deposit that could potentially liquefy during earthquake shaking. A conventional code-based bearing capacity analysis of the mudmat was initially performed that accounted for the effect of excess pore pressures generated during earthquake shaking on the shear strength of the sand. The excess pore pressures used in the code-based approach were estimated from one-dimensional free-field site response analyses. The conventional bearing capacity analysis predicted the mudmat would be unable to withstand the seismic design loads. As a result, two-dimensional (2D) non-linear seismic site response analyses (SRA) in a finite element program were performed using an advanced constitutive model. Excess pore pressures generated during the design earthquake for the free field case (i.e. no structure), and for the near-field case (with mudmat) were estimated. Then, a bearing capacity analysis was performed in the same finite element program using the generated excess pore pressure stress field for the near-field case. The 2D SRA results showed that liquefaction (excess pore pressure ratio equal or greater than 0.9) would only occur near the soil surface due to the low effective stresses. In general, the maximum excess pore pressure ratios (ru) were between 0.3 and 0.5 for the free-field case. The addition of the mudmat decreased the ru due to the added weight of the mudmat, which increased the effective stresses. In addition, the horizontal loading on the mudmat induces initial static shear stresses in the soil, which further decreases ru for the modelled relative density of the silty sand. The bearing capacity analyses predicted that the mudmat can tolerate considerably larger loads compared to the design loads during and following the design earthquake event. The case history presented in this paper provides an advanced design methodology for evaluating mudmat bearing capacity subjected to earthquake shaking on potentially liquefiable soil. In addition, it shows the benefit of performing advanced analyses with site-specific data, which reduces uncertainty and can provide increased reliability and significant cost savings to the overall project.

This study focused on the potential liquefaction due to earthquake shaking of a gravity based structure (GBS) located off the northeast coast of Italy in the Adriatic Sea. The reinforced concrete GBS is 88 m wide by 180 m long and is underlain by a sand unit with variable thickness that could cause differential settlement and sliding. We first estimated settlements using cone penetration test (CPT) based liquefaction triggering methods and simplified settlement calculations with adjustments to account for the influence of the GBS. We then performed the same calculations but with the earthquake demand estimated from 1D site response analyses. Finally, we performed 2D finite element analyses of the soil-structure system using the advanced constitutive model PM4Sand to model the liquefiable sand layers. The results show that the total amount of displacement is small (1.5–7.5 cm), and no significant differential settlement or sliding is expected for the design earthquake scenarios. A practical observation from the study is that the more detailed and integrated analyses resulted in lower estimates of settlements and a more accurate representation of the soil-structure response.

41 pages, 23 figures, 3 tables, Submarine landslides occur across all continental margins worldwide. They are diverse in nature and can involve tremendous volumes and long run-out distances. This paper provides an overview of the geographical extent and mechanisms of dense flowing submarine landslides. To this end, the review describes occurrence of past landslides, preconditioning and triggers, geophysical and geomechanical characterization, landslide dynamics, tsunami-genesis, and hazard analysis. Given the huge complexity submarine landslides encompass, an emphasis is here given on quantitative and methodological aspects such as landslide dynamics modeling and tsunami generation

The dynamic behavior of a peat deposit in the north of the Netherlands is described. The organic content ranges from 70% to 95%, which is high compared to the organic content generally pres...

The International Building Code requires a site-specific investigation for NEHRP F sites. However, established procedures for performing site-specific investigations of NEHRP F sites are limited. This study developed a simplified model to estimate design spectra for non-liquefiable NEHRP F sites to provide guidance for these cases. The model is based on the results of 4695 one-dimensional effective stress nonlinear site response analyses. It is intended to be used jointly with site-specific response analyses. The model is dependent on the period, design ground motion intensity, and site properties, and it characterizes the uncertainty of the estimate. The sensitivity of the results to reasonable variations in input parameters is investigated. The model is validated against field observations and compared with 80% NEHRP E site design spectra. The simplified model can capture the trends and magnitude of acceleration response spectra calculated from the site response analyses for all ground motions tested. Design spectra may fall below the 80% NEHRP E site design spectra for moderate to large levels of shaking intensity due to soil nonlinearity.

In this study we investigate the effect of prior seismic shaking on the monotonic shear strength of saturated Ottawa Sand 20/30. We perform a series of stress-controlled undrained cyclic triaxial tests with different seismic intensities intentionally without causing failure, followed by drainage of the excess pore pressure and an undrained monotonic loading test to determine the undrained shear strength. The experimental data show that small to moderate seismic events that do not fail the specimen can significantly increase undrained shear strength without much change in relative density. One prior seismic event with peak ground acceleration ~ 1.3 m/s2 may increase the undrained shear strength of a specimen at ~ 10 m depth by around 30%. The results also show that as the intensity of the shaking increases, the increase in the monotonic shear strength increases. However, the strengthening effect does not increase with the number of seismic events although a small degree of global densification in the sample is observed. The results of this paper will help assess the change in static slope stability after a single or multiple small to moderate events occurred without causing initial instability.

This paper proposes a unified model to estimate the in-situ small strain shear modulus of clays, silts, sands, and gravels based on commonly available index properties of soils. We developed a model to predict the laboratory small strain shear modulus (Gmax,lab) using a mixed effects regression of a database that contains 1680 tests on 331 different soils. The proposed model includes the effect of void ratio, effective confining stress and overconsolidation ratio as well as plasticity index, fines content, and coefficient of uniformity. We compiled a second database to estimate the in-situ small strain shear modulus (Gmax,in-situ) from laboratory (Gmax,lab) measurements. This study validated and compared the resulting model with other existing models using a third database of measured Gmax,in-situ values. The residuals of the proposed model had a mean and median closer to zero and the smallest standard deviation of all the models considered. By including a statistical description of the residuals, this work allows uncertainty of the small strain shear modulus to be included in probabilistic studies.

This paper compares 5,715 equivalent linear analyses to examine the sensitivity of the results to the shear strain ratio ( SR). The results show that as SR increases, equivalent linear analyses predict larger values of maximum shear strain ( γ max), peak ground displacement, mean period ( T m), and spectral acceleration ( Sa) at long periods, smaller values of peak ground acceleration ( PGA), Arias intensity, significant duration, and Sa at short periods, and similar values of peak ground velocity and Sa at middle periods. SR has a non-negligible effect on predicted values of γ max, T m, and Sa at short and long periods. This study also compares measured values from 80 ground motions and six vertical arrays with the results of equivalent linear analyses conducted using two methods to calculate SR. The results show that using SR = 0.65 gives a 2–10% better fit than using SR = ( M − 1)/10, where M is the earthquake magnitude.

We compared the results of equivalent linear (ELA) and nonlinear site response analyses (NLA) and found that the differences between the values of the peak ground acceleration ( PGA), peak ground velocity ( PGV), Arias intensity ( I a), significant duration ( D5–75), and response spectrum for periods between 0.025 s and 2 s predicted by each method are non-negligible for maximum shear strain values predicted by ELA ( γ max, ELA) greater than 0.04% to 1.0%. As γ max, ELA increases, ELA in general predict smaller shear strain and D5–75 values, and larger PGA, PGV, I a, mean period, and response spectral values for periods less than 0.1 s and periods near the natural site period than NLA. To help researchers and practitioners decide when to use ELA and/or NLA, we developed a model to estimate γ max, ELA before conducting a site response analysis.

Highlights • The Danube deep-sea fan offers best conditions for hydrate production. • Gas production out of a hypothetical methane hydrate reservoir was simulated. • Hazard assessment to investigate the hazard of production-induced slope failures. • Factor of Safety against slope failure is not affected by the production process. • Mobilized mass could hit the production site if landslide were to happen. Methane production from gas hydrate reservoirs is only economically viable for hydrate reservoirs in permeable sediments. The most suitable known prospect in European waters is the paleo Danube deep-sea fan in the Bulgarian exclusive economic zone in the Black Sea where a gas hydrate reservoir is found 60 m below the seafloor in water depths of about 1500 m. To investigate the hazards associated with gas production-induced slope failures we carried out a slope stability analysis for this area. Screening of the area based on multibeam bathymetry data shows that the area is overall stable with some critical slopes at the inner levees of the paleo channels. Hydrate production using the depressurization method will increase the effective stresses in the reservoir beyond pre-consolidation stress, which results in sediment compaction and seafloor subsidence. The modeling results show that subsidence would locally be in the order of up to 0.4 m, but it remains confined to the immediate vicinity above the production site. Our simulations show that the Factor of Safety against slope failure (1.27) is not affected by the production process, and it is more likely that a landslide is triggered by an earthquake than by production itself. If a landslide were to happen, the mobilized sediments on the most likely failure plane could generate a landslide that would hit the production site with velocities of up to 10 m s-1. This case study shows that even in the case of production from very shallow gas hydrate reservoirs the threat of naturally occurring slope failures may be greater than that of hydrate production itself and has to be considered carefully in hazard assessments.

This paper presents a probabilistic seismic hazard analysis (PSHA) for a site offshore Maheshkhali Island, Bangladesh. The tectonic setting of the area is complex, and the PSHA includes active crustal faults, megathrust and intraplate subduction faults, and a background gridded seismicity areal source zone based on historical and recorded earthquakes. In addition, we model several rupture scenarios for the closest megathrust source. The peak ground acceleration values for return periods of 475 years and 2475 years are 0.33 g and 0.63 g, respectively. These values are higher than previous studies conducted in the area, however, past studies only included areal source zones based on recorded seismicity whereas this study takes advantage of recent research to include more detailed fault data. The deaggregation plots show that the main contributors to the hazard are a magnitude 6.5–7.1 earthquake 15–20 km from the site on the Maheshkhali fault and a magnitude 8.0–8.8 earthquake 120–250 km from the site on the Ramree megathrust. © 2018. This is the authors' accepted and refereed manuscript to the article. The final authenticated version is available online at: http://dx.doi.org/10.1061/9780784481462.015

Summary The Norwegian government will replace many ferry crossings along the coastal highway E39 with bridges or tunnels. Geophysical data from most fjords show ample evidence of repeated slope instabilities, which are a hazard and risk for the foundations of the infrastructure. The fjords are often several km wide, and hundreds of metres deep. In this study, we combined very-high-resolution swath bathymetry and sub-bottom profiling data as well as in situ geotechnical data to investigate the hazard potential of such landslides in Bjornafjorden. We used a 1D slope stability screening study to find suitable anchoring locations. Back-analysis of a well-constrained landslide using a geometry reconstructed from the interpretation of the geophysical data and validated by 2D finite-element slope stability modelling provides key input to constrain the rheological parameters. The results were then used to assess slope stability across the fjord basin, as well as landslide dynamic simulations in quasi-2D and quasi-3D, resulting in flow velocities above 10 m/s and run-out distances of several hundreds of meters. The work concludes with some recommendations for similar investigation and site characterization.

This article describes the development of a screening tool to estimate factors of safety and probabilities against slope failure for static and pseudo-static analyses, as well as seismically induced permanent displacements, for both onshore and offshore slopes. The screening tool combines geophysical and geotechnical data to rapidly assess and identify potentially unstable regions over large areas. The results allow a quick, objective, and logical rationale for selecting regions to perform more in-depth and detailed slope stability analyses, and/or areas to avoid when selecting locations or paths for future infrastructure development. We apply the screening tool to part of the continental slope offshore the Lofoten islands, Norway.

Ground‐motion selection for dynamic analysis of structures is often based on the conditional mean spectrum (CMS). The CMS is a spectrum that matches the uniform hazard spectrum (UHS) at a conditioning period but that also represents the response from a single earthquake scenario more realistically than the UHS. This paper discusses and offers solutions for several issues related to implementing CMS in practice. The issues covered are the selection of the conditioning period, the number of CMS necessary to cover the range of the UHS, how to broaden the CMS to match the UHS with fewer conditioning periods, how to compute a CMS for several ground‐motion prediction equations that is consistent with probabilistic seismic‐hazard analysis, the application of correlation coefficients to hard‐rock sites, and the robustness of correlation coefficients.

Offshore landslides pose a major threat to subsea facilities, and one of the main triggering factors of offshore landslides is strong earthquakes. The state of the practice models offshore slope stability during strong shaking using one horizontal component of a selected representative ground motion. However, slopes and earthquakes are three-dimensional phenomena. Therefore, to provide much needed guidance in this area, this study performed 27 three-dimensional seismic slope stability analyses in a finite element program to investigate the change in the predicted results when applying one, two, or three ground motion components.The results showed that applying two horizontal components instead of one may increase the predicted total displacements and shear strains on the slope by 25% to 50% and by 10% to 50%, respectively. In addition, it may also increase the total displacements at the seafloor on the shelf above the slope by 20% to 40%. This could have implications when deciding the safe stand-back line for offshore facilities. However, applying three ground motion components has a negligible effect on the results compared with applying only the two horizontal components. The ratio of the response spectra on the shelf, slope, and basin varied by ± 25%, however no clear trend was observed.

BACKGROUND The frequency of landslide disasters is increasing as a result of exploitation of natural resources, deforestation, and greater population vulnerability due to growing urbanization and uncontrolled land-use. Earthquakes are a major triggering factor of landslides, and earthquake-induced landslides pose a major threat to infrastructure and human life. This paper presents the effects of slope angle, soil sensitivity, ground motion orientation, and multidirectional shaking on the results of seismic slope stability analyses. RESULTS The results show that permanent shear stresses due to sloping ground, strain softening and sensitivity, ground motion orientation, and multidirectional shaking all have a large influence on the permanent displacements estimated from earthquake-induced landslide hazard analyses. Multidirectional shaking also predicts larger excess pore pressures in deep layers than unidirectional shaking. CONCLUSIONS It is important that site investigations provide adequate information to model the correct slope angle and soil sensitivity. The ground motion orientation should be considered and chosen based on the specific needs of a project. Analyses with only one ground motion component could give unconservative results.