Analysis of potential cave-in from fault zones in hard rock subsea tunnels     

Prof. Dr. B. Nilsen 《Rock Mechanics and Rock Engineering》1994,27(2):63-75

Summary As a part of a research program on the rock engineering aspects of hard rock subsea tunnelling, analyses of potential cave-in from fault zones have been carried out at the Norwegian Institute of Technology. This is a topic of great importance for the planning of future subsea tunnels, and particularly for the selection of the minimum rock cover of such projects. The paper is divided into three main parts: a) review of cases of instability in Norwegian subsea tunnels, b) evaluation of theoretical maximum sliding, and c) discussion of cases of cave-in in tunnels under land. In theory, a cave-in during subsea tunnelling may propagate far higher than the normal minimum rock cover. Taking into consideration the comprehensive geo-investigations that are always carried out for subsea tunnel projects today, it would, however, be unrealistic to base the dimensioning of rock cover for future projects on worst-case scenarios. Consequently, the main result of this study is to emphasize the importance of comprehensive geo-investigations, detailed tunnel mapping, a high degree of readiness during tunnelling and a thorough quality control.  相似文献   

An attempt at estimating the effective fishing height of the bottom trawl using acoustic survey recordings     

Hjellvik  Vidar; Michalsen  Kathrine; Aglen  Asgeir; Nakken  Odd 《ICES Journal of Marine Science》2003,60(5):967-979

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Numerical Analysis of the 650,000 m<Superscript>2</Superscript> Åknes Rock Slope based on Measured Displacements and Geotechnical Data     

Vidar Kveldsvik  Herbert H. Einstein  Bjørn Nilsen  Lars Harald Blikra 《Rock Mechanics and Rock Engineering》2009,42(5):689-728

The unstable 650,000 m² Åknes rock slope (Western Norway) poses a hazard, as a sudden failure may cause a destructive tsunami in the fjord. In this study the slope was divided into blocks based on displacements measured at the slope surface. Discontinuous deformation analysis (DDA) showed that three or four blocks in the upper half may be considered as potential subareas that may fail catastrophically. The lower half may be divided into two or three blocks, but more limited data introduces more uncertainty into block definition. The Universal Distinct Element code (UDEC) was used for two-dimensional (2D) stability analyses. By varying fracture geometry, fracture friction, and groundwater conditions within reasonable limits based on site-specific data a number of possible models were compared. The conclusions show that models that were unstable to great depths were in closer agreement with shear strength parameters derived from an earlier study than models that were unstable to smaller depths. The length (depth) of the outcropping fracture, along which shear displacements are shown to occur, plays an important role. A (shallow) slide at 30 m, in which displacements have been documented by borehole measurements, will reduce the stability at greater depths. Increased groundwater pressure is demonstrated to be less critical for very deep slope instability. The results of the DDA and UDEC modelling will be useful for planning of future investigations, interpretation of the subsequent results, further development of the early warning system and in the tsunami modelling.  相似文献   

International viewpoint and news     

Dirk Notz  Peter Haugan  Karolina Widell  Frank Nilsen  Anne Marie de Grosbois 《Environmental Geology》2009,57(2):479-481

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Fjord–shelf exchanges controlled by ice and brine production: The interannual variation of Atlantic Water in Isfjorden,Svalbard     

F. Nilsen  F. Cottier  R. Skogseth  S. Mattsson 《Continental Shelf Research》2008

Exchanges between oceanic and coastal waters are fundamental in setting the hydrography of arctic shelves and fjords. In West Spitsbergen, Atlantic Water from the West Spitsbergen Current exchanges with the seasonally ice-covered waters of the coast and fjords causing a major annual shift in hydrographic conditions. The extent to which Atlantic Water dominates the fjord systems shows significant interannual variability. Hydrographic sections taken between 1999 and 2005 from Isfjorden and the adjacent shelf have been analyzed to identify the causes of the variability in Atlantic Water occupation of the fjord system. By treating the fjord system as a coastal polynya and running a polynya model to quantify the salt release each winter, we conclude that the critical parameter controlling fjord–shelf exchange is the density difference between the fjord water masses and the Atlantic Water. We provide a full dynamical mechanism for the interaction between water masses at the fjord entrance to rationalize the interannual variability.  相似文献   

Submarine-fan facies associations of the Upper Cretaceous and Paleocene Gottero Sandstone, Ligurian Apennines, Italy     

Tor H. Nilsen  Ernesto Abbate 《Geo-Marine Letters》1984,3(2-4):193-197

The Upper Cretaceous and Paleocene Gottero Sandstone was deposited as a small deep-sea fan on ophiolitic crust in a trench-slope basin. It was thrust northeastward as an allochthonous sheet in Early and Middle Cenozoic time. The Gottero, as thick as 1500 m, was probably derived from erosion of Hercynian granites and associated metamorphic rocks in northern Corsica. Outcrops of inner-fan channel, middle-fan channel and interchannel, outer-fan lobe, fan-fringe, and basin-plain facies associations indicate that the depositional model of Mutti and Ricci Lucchi for mixed-sediment deep-sea fans can be used. The original fan had a radius of 30 to 50 km. Margin setting represents fan and/or source area  相似文献   

Alternative approaches for analyses of a 100,000 m<Superscript>3</Superscript> rock slide based on Barton–Bandis shear strength criterion     

Vidar Kveldsvik  Bjørn Nilsen  Herbert H. Einstein  Farrokh Nadim 《Landslides》2008,5(2):161-176

A data set was derived for the Åknes rock slope, Norway, with the main focus on deriving input parameters for the Barton–Bandis shear strength criterion. Back-calculations of a 100,000 m³ rock slide were performed for evaluation of the data set. The limit equilibrium analysis showed that the joint roughness coefficient (JRC) has the greatest effect on the calculated safety factor of the slide. Probabilistic computations showed that the JRC stands out as the most important contributor to the total uncertainty over the whole set of variables and that the computed failure probability of the 1960 slide was very high, which may be interpreted that the input variables and the Barton–Bandis shear strength criterion are reasonable for the slide. JRC was measured on 0.25 m scale and on 1 m scale. The results from the two scales were different.  相似文献   

Combining the modified discrete element method with the virtual element method for fracturing of porous media     

Halvor Møll Nilsen  Idar Larsen  Xavier Raynaud 《Computational Geosciences》2017,21(5-6):1059-1073

Simulation of fracturing processes in porous rocks can be divided into two main branches: (i) modeling the rock as a continuum enhanced with special features to account for fractures or (ii) modeling the rock by a discrete (or discontinuous) approach that describes the material directly as a collection of separate blocks or particles, e.g., as in the discrete element method (DEM). In the modified discrete element (MDEM) method, the effective forces between virtual particles are modified so that they reproduce the discretization of a first-order finite element method (FEM) for linear elasticity. This provides an expression of the virtual forces in terms of general Hook’s macro-parameters. Previously, MDEM has been formulated through an analogy with linear elements for FEM. We show the connection between MDEM and the virtual element method (VEM), which is a generalization of FEM to polyhedral grids. Unlike standard FEM, which computes strain-states in a reference space, MDEM and VEM compute stress-states directly in real space. This connection leads us to a new derivation of the MDEM method. Moreover, it enables a direct coupling between (M)DEM and domains modeled by a grid made of polyhedral cells. Thus, this approach makes it possible to combine fine-scale (M)DEM behavior near the fracturing region with linear elasticity on complex reservoir grids in the far-field region without regridding. To demonstrate the simulation of hydraulic fracturing, the coupled (M)DEM-VEM method is implemented using the Matlab Reservoir Simulation Toolbox (MRST) and linked to an industry-standard reservoir simulator. Similar approaches have been presented previously using standard FEM, but due to the similarities in the approaches of VEM and MDEM, our work provides a more uniform approach and extends these previous works to general polyhedral grids for the non-fracturing domain.  相似文献   

Virtual element method for geomechanical simulations of reservoir models     

Odd Andersen  Halvor M. Nilsen  Xavier Raynaud 《Computational Geosciences》2017,21(5-6):877-893

In this paper, we study the use of virtual element method (VEM) for geomechanics. Our emphasis is on applications to reservoir simulations. The physical processes behind the formation of the reservoirs, such as sedimentation, erosion, and faulting, lead to complex geometrical structures. A minimal representation, with respect to the physical parameters of the system, then naturally leads to general polyhedral grids. Numerical methods which can directly handle this representation will be highly favorable, in particular in the setting of advanced work-flows. The virtual element method is a promising candidate to solve the linear elasticity equations on such models. In this paper, we investigate some of the limits of the VEM method when used on reservoir models. First, we demonstrate that care must be taken to make the method robust for highly elongated cells, which is common in these applications, and show the importance of calculating forces in terms of traction on the boundary of the elements for elongated distorted cells. Second, we study the effect of triangulations on the surfaces of curved faces, which also naturally occur in subsurface models. We also demonstrate how a more stable stabilization term for reservoir application can be derived.  相似文献   

Siliciclastic influx and burial of the Cenozoic carbonate system in the Gulf of Papua     

Evgueni N. Tcherepanov  André W. Droxler  Philippe Lapointe  Kenneth Mohn  Odd A. Larsen 《Marine and Petroleum Geology》2010

An extensive carbonate system in the Gulf of Papua (GoP), developed in the late Oligocene–middle Miocene, was buried by huge influx of siliciclastics originated from Papua New Guinea. Major episodes of siliciclastic influx in the carbonate system are related to tectonic activity in the fold and thrust belt during the Oligocene Peninsular Orogeny, late Miocene Central Range Orogeny, and late Pliocene renewed uplift and exhumation of peninsular region. Siliciclastics did not influence the carbonate deposition during the late Oligocene–middle Miocene, since they were accumulated in the Aure Trough, proximal foreland basin protecting the carbonate system. The most significant burial of the carbonate system started during the late Miocene–early Pliocene in the result of the Central Range Orogeny. However, the largest influx was related to the renewed uplift of the Papuan Peninsula during the early late Pliocene. The shelf edge prograded ∼150 km and formed more than 80% of the modern shelf. This high siliciclastic influx was also enhanced by the “mid” Pliocene global warmth period and intensified East Asian monsoons at 3.6–2.9 Ma. Although many publications exist on carbonate–siliciclastic mixing in different depositional environments, this study helps understand the carbonate–siliciclastic interactions in space and time, especially at basinal scale, and during different intervals of the carbonate system burial by siliciclastic sediments.  相似文献