Recent Advances in Seabed Liquefaction and Its Implications for Marine Structures
B. Mutlu Sumer
ABSTRACT: A review is presented of recent advances in seabed liquefaction and its implications for marine structures. The review is organized in seven sections: Residual liquefaction, including the sequence of liquefaction, mathematical modelling, centrifuge modelling and comparison with standard wave-flume results; Momentary liquefaction; Floatation of buried pipelines; Sinking of pipelines and marine objects; Liquefaction at gravity structures; Stability of rock berms in liquefied soils; and Impact of seismic-induced liquefaction.
KEYWORDS: Liquefaction, Waves, Marine structures, Seabed
Eulerian–Lagrangian Modeling of Current-Induced Coastal Sand Dune Migration
R. Sun, J. Wang, Y. Sakai and H. Xiao
ABSTRACT: In this work, an Eulerian–Lagrangian framework is developed for the modeling of current-induced sediment transport and sand dune migration. In this framework, the fluid flow is modeled by solving the Reynolds-averaged Navier–Stokes (RANS) equations, and a conservation equation is used to describe the morphological evolution of the sand bed, both of which are formulated in the Eulerian framework. Empirical models are used for the erosion, the dispersion, and the drag and lift forces exerted on the sediment particles. The trajectories of individual particles are tracked in the Lagrangian framework, which enables a high-fidelity representation of the particle motions and composition statistics, as well as direct representation of sediment deposition without the need of ad hoc models. This framework consists of four tightly coupled modules: (1) a fluid flow solver based on RANS equations, (2) a morphological evolution modeling equation, (3) a Lagrangian particle-tracking scheme for suspended sediments, and (4) a dynamic mesh motion solver which deforms the mesh to account for the effects of morphological evolution on the flow field. The developed framework is validated by using previous results in the literature and is used to simulate coastal sand dune formation migration. Favorable agreements with benchmark results are obtained, demonstrating potential of the developed Eulerian–Lagrangian modeling framework for sediment transport.
KEYWORDS: Sediment transport, Fluid–particle interaction, Computational fluid dynamics, Sand dune migration
Numerical Study of the Penetration Mechanism and Kinematic Behaviour of Drag Anchors Using a Coupled Eulerian-Lagrangian Approach
Haixiao Liu and Yanbing Zhao
ABSTRACT: The fundamental properties of drag anchors such as the movement direction of the fluke, the drag angle and drag force at the shackle and the anchor trajectory in soils are closely relevant to the penetration mechanism and kinematic behavior of the drag anchor during installation. In the present work, a large deformation finite element analysis using a coupled Eulerian-Lagrangian approach is performed to simulate the installation process of drag anchors with different fluke sections. The method for determining the reasonable mesh density and drag velocity is proposed based on the investigation on dependency of the numerical results on the mesh density and drag velocity. Through a systematic comparative study between numerical and theoretical analysis, clear knowledge of the movement direction, the drag angle and drag force at the shackle, the anchor trajectory, the effect of anchor geometry and the ultimate embedment depth of the anchor is obtained, which is beneficial to fully understanding the complex behavior of drag anchors in soils.
KEYWORDS: Drag anchor; Penetration mechanism; Kinematic behavior; Movement direction; Drag angle; Drag force; Trajectory; Coupled Eulerian-Lagrangian; Numerical
Cyclic Pore Pressure Generation in Silty Soils under the Action of Combined Waves and Current
Yi-Fa Wang, Fu-Ping Gao, and Wen-Gang Qi
ABSTRACT: Ocean waves and current always coexist in the offshore shallow water environments, which may bring the corresponding seabed responses more complex than those under pure waves. The responses of a silty soil under the action of combined waves and current were physically modeled with a specially designed water flume. A series of tests for the progressive waves with a following-current or with an opposing-current have been conducted, respectively. Both the water-surface elevation and the pore pressure, including the transient and residual components, at various depths in the silty soil were measured simultaneously. The effects of both wave loading history and superimposing current upon waves on the pore pressure responses are examined. It is indicated that the amplitude of the transient pore-pressure component is enlarged for the following-current case and reduced for the opposing-current case. The maximum amplitude of the residual pore-pressure component decreases gradually during the subsequent series of wave loading under the same wave conditions.
KEYWORDS: Pore pressure responses, Silty soil, Combined waves and current, Physical modeling
A Model for Predicting Pipeline Sinkage Induced by Tunnel Scour
Chengcai Luo, Hongwei An, Liang Cheng and David White
ABSTRACT: The current design practice for subsea pipeline on-bottom stability (e.g. DNV-RP-F109) does not account the effect of sediment transport around a pipeline. Both field survey and small scale model test results show that seabed scour has a significant effect on pipeline embedment and therefore stability. Physical model tests carried out in an innovative large experimental facility, named the O-tube, at the University of Western Australia, have shown that tunnel scour and the subsequent pipe sinkage into the scour hole tend to stabilize a pipeline which might otherwise become unstable on an assumed stationary seabed, under ramping-up flow conditions. A simple calculation model that incorporates the three-dimensional scour and pipe sinkage due to the soil bearing capacity failure at the supporting span shoulders is proposed. The model parameters were calibrated using the O-tube experimental results. The model serves as a key element of a new pipeline stability analysis method that takes into account seabed mobility.
KEYWORDS: Pipeline, On-bottom stability, Tunnel scour, Pipe sinkage
Predicting Spud Can Extraction Resistance in Soft Clay
Omid Kohan, Christophe Gaudin, Mark J. Cassidy, and Britta Bienen
ABSTRACT: Jack-ups are mobile offshore structures that are frequently relocated to new operation sites. To be relocated, the jack-up footings, known as spudcans need to be extracted from the seabed, using essentially the buoyancy of the hull as extraction force. This operation may be time consuming or even jeopardised if the spudcan extraction resistance is higher than the available extraction force. The maximum extraction (or breakout) resistance consists of suction at the spudcan base, weight of the soil above the spudcan, and soil shear resistance above the spudcan, with the contribution of the suction at the spudcan invert being the dominant component of the breakout resistance. This paper reviews an existing prediction method used to estimate spudcan extraction resistance and proposes an update of some of the input parameters based on insights obtained from a large database of experimental model data on two types of clays and for spudcan embedment up to three diameters.
KEYWORDS: Spudcan, Extraction resistance, Centrifuge modelling, Soft clay, Prediction method
A FE Procedure for Foundation Design of Offshore Structures – Applied to Study a Potential OWT Monopile Foundation in the Korean Western Sea
H.P. Jostad, G. Grimstad, K.H. Andersen, M. Saue, Y. Shin, and D. You
ABSTRACT: A finite element based calculation procedure that accounts for the effect of cyclic loading of soils under undrained conditions is presented. A material model called UDCAM that uses 3D strain contour diagrams from undrained cyclic and monotonic triaxial and DSS tests is used in the procedure. The model accounts for cyclic degradation by using the cyclic strain accumulation procedure developed at NGI in the seventies. The load history is idealized by a load composition containing load parcels with constant average and cyclic loads in each parcel. The applicability of the procedure is verified by back calculating a model test of a gravity base structure (GBS) in soft clay subjected to monotonic and cyclic loading. The procedure is then used to predict the behaviour of a monopile for a potential offshore wind turbine (OWT) in the Korean Western Sea. These results are compared with results obtained with traditional beam-spring analyses.
KEYWORDS: Offshore Engineering, Numerical modelling, Soil/structure interaction, Cyclic Loading, Clays, Monopiles, Wind Turbines
Compressibility as an Indicator of Liquefaction Potential
M. Murat Monkul, Poul V. Lade, Ehsan Etminan, Aykut Senol
ABSTRACT: It is difficult to impossible to obtain intact samples of loose, silty sand from coastal and offshore sandy soil deposits, which could potentially liquefy. To evaluate the liquefaction potential for such soils, a quantity that may be used as an indicator and can be measured in-situ is the volume compressibility. The more compressible the easier the soil will liquefy. Presented here is a study of three clean sands with the same geologic origin: Sile Sand 20/30, Sile Sand 50/55 and Sile Sand 80/100. Sile Sand 80/100 is also mixed with two types of non-plastic silts: TT Silt and IZ Silt. Three different fines contents of 5%, 15% and 25% are used for each of the two combinations of silty sands. Isotropic compression tests and undrained triaxial compression tests have been performed on these soils to determine their liquefaction potential and their compressibilities, and these are correlated with each other. Experiments have shown that volumetric compressibilities increase with increasing fines content for both silt types, which is similar to the observation of increasing liquefaction potential with fines content. Approximate boundaries for stable response, transition stage, and liquefaction region are determined. Accordingly, specimens with volumetric compressibility values smaller than 0.17 (1/MPa) were stable, while all specimens with volumetric compressibility values greater than 0.23 (1/MPa) liquefied. Further laboratory and in-situ tests on different sand and silt types are still needed to verify and tune those boundaries, which could potentially serve as indicators of liquefaction potential via in-situ compressibility tests.
KEYWORDS: Compressibility, Fines, Instability, Silty sand, Static liquefaction, Triaxial tests
Centrifuge Modelling of the Seismic Responses of a Gently Sloped Liquefiable Sand Deposit Confined within Parallel Walls
C.J. Lee, W.Y. Chung, and W.Y. Hung
ABSTRACT: A series of one-dimensional (1-D) centrifuge shaking table tests was performed to investigate the seismic responses of a 4° sloped liquefiable sand deposit confined within parallel walls, having various penetration depths and row distances, and with different fixed ends. The parallel walls relieved the build-up of excess pore water pressure in the deeper enclosed sand layer, but no obvious reductions were observed in the excess pore water pressure in the shallower sand layer during large earthquakes. The effective relief of the excess pore water pressure and decrease in the surface settlement within the walls would be expected to improve at deeper penetration depths and for higher wall bending stiffness values. Stiffer parallel walls with fixed ends can constrain the enclosed sands more effectively and prevent lateral displacement induced by lateral spreading occurred in gently sloped ground. The walls can also transmit larger accelerations into the enclosed soils. Protected structures would not, therefore, come in contact with the parallel walls, thereby avoiding experiencing larger accelerations.
KEYWORDS: Parallel walls, Liquefaction, Lateral spreading, Centrifuge modeling, Shaking table test
Eulerian Finite Element Analysis for Uplift Capacity of Circular Plate Anchors in Normally Consolidated Clay
Z. Chen, K. K. Tho, C. F. Leung and Y. K. Chow
ABSTRACT: Anchors are often used to provide uplift resistance for mooring of boats and floating decks as well as anchoring of pipelines offshore. Anchor is often idealized as a circular plate in the analysis of its uplift resistance. The uplift capacity of circular plate anchor in uniform soil is well documented in the literature. However, the pullout behavior of circular anchor in nonhomogeneous soil is less well studied and forms the motivation of this paper. In order to circumvent computational difficulties associated with severe mesh distortion during the pullout process, the Eulerian large strain, large deformation finite element approach is adopted in this study to investigate the pullout behavior of circular plate in normally consolidated clay. The applicability of the Eulerian numerical model is validated by comparing the numerical results with analytical solutions from lower bound limit analysis for a uniform soil as well as data for a centrifuge test conducted in normally consolidated Kaolin clay. Conventionally, it is generally accepted that the uplift behavior of a plate anchor in a normally consolidated soil can be inferred from that in a uniform soil by adopting the strength at the initial plate position as the reference strength. However, it is observed from the numerical results that the failure mechanisms corresponding to plate anchors in uniform clay and normally consolidated clay are different for the same set of reference undrained shear strength and geometric parameters. This implies that the conventional approach is not always applicable. A direct design method for obtaining the uplift capacity of a circular plate anchor embedded in a linearly increasing soil shear strength profile is then proposed.
KEYWORDS: Eulerian finite element, circular plate anchor, normally consolidated clay
Restoration Method of Artificial Tidal Flat by Use of Pressure Injection of Slurry Dredge Clay
Takahiro Kumagai, Takashi Tsuchida, Changjin Ko and Hiroaki Sugihara
ABSTRACT: The method of uplifting the ground surface by pressure injection of slurry dredge clay is proposed to restore the settled tidal flat without influencing the creatures living on and inside it. In order to establish the method, the study was carried out to examine the effectiveness of several technologies to uplift the ground smoothly avoiding the blowout of injected soil. As a result of laboratory experiment, it turns out that prior softening of original ground horizontally and placement of uplift restraint are effective for the purpose. The applicability of the proposed method was verified in situ contributing to the future practical application of technology by a field experiment.
KEYWORDS: Consolidation settlement, Dredged soil, Clam
Tsunami-Seabed-Structure Interaction from Geotechnical and Hydrodynamic Perspectives
S. Sassa
ABSTRACT: The paper reports some recent research advances on tsunami-seabed-structure interaction following the 2011 off the Pacific Coast of Tohoku Earthquake Tsunami, Japan. It presents a concise review of the latest research performed on the stability of breakwater foundation under tsunami by utilizing a geotechnical centrifuge and a large-scale hydro flume at Port and Airport Research Institute. I highlight here the role of tsunami-induced seepage in piping/boiling, erosion and bearing capacity decrease and failure of the rubble/seabed foundation. A comparison and discussion are made on the stability assessment for the design of tsunami-resistant structures on the basis of the results from both geo-centrifuge and large-scale hydrodynamic experiments.
KEYWORDS: Breakwaters, Geo-centrifuge, Large-scale hydro flume, Seepage, Tsunami
Feature Story on “Challenges in the Design of Tall Building Foundations”
Harry G. Poulos
ABSTRACT: This paper reviews some of the challenges that face designers of foundations for very tall buildings, primarily from a geotechnical viewpoint. Some characteristic features of such buildings will be reviewed and then the options for foundation systems will be discussed. A three-stage process of foundation design and verification will be described, and the importance of proper ground characterization and assessment of geotechnical parameters will be emphasized. The application of the foundation design principles to meet the challenges will be illustrated via three high-rise projects.
KEYWORDS: Case histories; design; foundations; piles; piled raft; settlement; tall buildings
