Jürgen Grabe

Stability of Submarine Foundation Pits Under Wave Loads

Shallow foundation structures offer ecological benefits compared to pile foundations as less noise is emitted at sea floor level during construction process. On the other hand, shallow offshore foundations can rarely be placed on top of the sea floor. Weak soils usually need to be excavated to place the foundation structure on more stable ground and thus, anthropogenic submarine pits result. Steep but stable slopes of the pit meet both economic and ecologic aims as they minimise material movement and sediment disturbance. According to Terzaghi [1] the angle β between slope and the horizontal of the ground surface of cohesionless soil is at most equal to the critical state friction angle φc. However, it can be observed that natural submarine slopes of sandy soils are always much more shallow. Artificial (temporary) slopes do not appear and behave as natural submarine slopes, since the latter are already shaped by perpetual loads of waves, tide and mass movements. Physical simulations of different scales were presented at the OMAE 2011 [2] to analyse the stability of artificial submarine slopes of sandy soil in the North Sea. The laboratory tests focused on gravitational forces and impacts from the excavation processes. This paper presents additional numerical simulations of wave-induced bottom pressure on the suggested submarine foundation pits. Furthermore, in-situ tests will be performed in 2012 and 2013. Both dredging process and resulted foundation pits will be considerably surveyed.Copyright

A New Simple Shear Apparatus and Testing Method for Unsaturated Sands

This paper deals with a new simple shear apparatus for the investigation of mechanical soil behavior of unsaturated coarse-grained soils under monotonous and cyclic loading conditions. Non-cohesive soils, such as sands, typically encounter low capillary effects, and little research has been dedicated to them so far. This holds true especially for effects of repeated loading. The presented test setup allows control of small matric suction up to 85 kPa within the soil specimens with the help of a vacuum control method. Although the pore water pressures are negative as occurring in nature, the pore air pressure is kept at an atmospheric level. Depending on the pore water drainage condition of the specimen boundaries, either drained constant suction tests (CS-tests) or undrained constant water content tests (CW-tests) can be performed. The focus of the research is placed on the interaction of loading and volume change with matric suction and degree of saturation, i.e., the hydraulic-mechanical coupling, during monotonous and cyclic shear. Central questions are the impact of matric suction and degree of saturation on the densification behavior as well as, vice versa, the effect of cyclic loading on a change of matric suction and degree of saturation. The new simple shear apparatus and testing method will be explained, their benefits and shortcomings will be discussed, and selected results of a series of tests will be presented.

Laterally Loaded Piles With Bulge

This paper contains several investigations on the behavior of bulged piles. Bulge means the application of vertical steel plates somewhere near the ground surface to improve the lateral bearing capacity. The results of small scale tests in sand are illustrated, which demonstrate the effectiveness of such bulge. Some theoretical investigations are presented trying to apply standard methods like the p-y-curve procedure to the design of bulged piles. An outline of possible calculation methods is given. Investigations on two different exemplary pile systems demonstrate the behavior of the bulged structure and give an idea of the advantages of this innovative system.

Spatial variation of soil stiffness: Spectral density approach

Abstract Measured data from an entire area using a dynamic compaction control device with vibrating rollers are reported and deviations of the measured values are interpreted. Entire area measurement is helpful in reducing geotechnical risk. Even in nominally homogeneous layers the soil stiffness varies from point to point. Using this device, it is possible to detect soft as well as hard spots. Spectral densities of the wave length of soil stiffness are calculated to investigate the spatial variation of soil stiffness. It was found that the amplitude of the waves of soil stiffness is proportional to the wave length. This phenomenon is called a 1/f spectrum or flickering noise: the stiffness variation is composed of long waves with high amplitudes and small waves with small amplitudes. A dominant wave length due to patterns could not be found. The 1/f spectrum has often been observed in natural processes. The phenomenon is illustrated in terms of soil mechanics using a simple generating process.

On the Set-Up of Piles

It is well known that the bearing capacity and stiffness of displacement piles depends on short term effects during and shortly after pile installation as well as long term effects, a phenomenon usually referred to as set-up. During pile installation the surrounding soil is influenced by the installation process. The soil state e.g. the stress state, pore pressure or void ratio are changing due to complex mechanical processes in the soil. The change of the soil state has a great influence on the pile capacity and the behavior of the pile under vertical and horizontal loading. During the installation process phenomena like a temporary and locally limited liquefaction of the soil can occur. After the pile installation the bearing capacity may increase significantly with time due to set-up effects. In cohesive soils the set-up is commonly explained by consolidation processes in which the dissipation of the excess pore pressures around the pile leads to an increase in effective radial stresses. Case histories show that set-up may also occur in sand over a period which exceeds by far the consolidation process. Therefore, other effects apart from the dissipation of excess pore pressures must contribute to this long-term set-up effect. An understanding and a correct estimation of these effects is of great importance for a economical design of pile-founded structures e.g. offshore wind turbines. For the investigation of these phenomena different methods like numerical simulations, model scale or full scale tests are applicable. In this paper two methods are used: The installation process is investigated with numerical simulations and the set-up effects are investigated by long term in-situ measurements.Copyright

Fortlaufend inverse Berechnung der Bodensteifigkeit aus dem Schwingungsverhalten einer fahrenden Vibrationswalze

ÜbersichtFür das Bewegungsverhalten einer Vibrationswalze auf dem Boden wird ein mechanisches Schwingungsmodell vorgestellt. Die Walzenbewegung wird durch eine Folge linearer Differentialgleichungen beschrieben. Es gelingt, die Differentialgleichungen in ein nichtlineares Gleichungssystem für die aus der gemessenen Walzenbewegung invers zu bestimmenden Bodenparameter zu überführen. Dadurch wird es möglich, fortlaufend aus der Walzenbewegung die Bodenparameter des Modells numerisch zu berechnen. An Meßdaten werden die Vorteile des Verfahrens gegenüber den bisher bekannten empirisch entwickelten Verfahren der flächendeckenden Verdichtungskontrolle aufgezeigt.SummaryA mechanical model is introduced for the dynamic behaviour of a vibratory roller on the soil. The movement of the drum is represented by a sequence of piecewise linear differential equations. These equations are transformed into a system of nonlinear equations for the unknown soil parameters. In this way, it is possible to numerically back calculate the soil parameters from the behaviour of the drum. The main advantages of this method compared to the known empirically found methods of an entire area compaction control are shown using measured data.

A 3D smoothed particle hydrodynamics model for erosional dam-break floods

ABSTRACT A mesh-less smoothed particle hydrodynamics (SPH) model for bed-load transport on erosional dam-break floods is presented. This mixture model describes both the liquid phase and the solid granular material. The model is validated on the results from several experiments on erosional dam breaks. A comparison between the present model and a 2-phase SPH model for geotechnical applications (Gadget Soil; TUHH) is performed. A demonstrative 3D erosional dam break on complex topography is investigated. The present 3D mixture model is characterised by: no tuning parameter for the mixture viscosity; consistency with the Kinetic Theory of Granular Flow; ability to reproduce the evolution of the free surface and the bed-load transport layer; applicability to practical problems in civil engineering. The numerical developments of this study are represented by a new SPH scheme for bed-load transport, which is implemented in the SPH code SPHERA v.8.0 (RSE SpA), distributed as FOSS on GitHub.

Numerical Simulation of Ship Collision With Gravity Base Foundations of Offshore Wind Turbines

For the installation of offshore foundations several countries (e.g. Germany) require a proof of averting environmental disasters in case of ship collision. The aim is to prevent possible discharge of supplies or even loss of the vessel. Especially for gravity base foundations this load case is problematic due to their larger stiffness and mass compared to monopiles, tripods or jacket foundations. The finite element method provides a powerful tool to predict the collision behaviour in a realistic way taking into account the complex interaction between vessel, foundation and soil. The collision between a fully loaded single hull tanker and a gravity base foundation is subject of numerical analysis. The calculated contact forces between vessel and foundation are compared to a simplified calculation approach. For evaluation of the foundation deformations and areas of failure of the vessel are investigated. The influence of the water depth, the diameter of the foundation and an embedment in the seabed are determined in a parametric study. It can be shown that the finite element method is a suitable approach for investigation of the collision behaviour of offshore structures. The design of gravity base foundations can be optimized with respect to ship collision in a fast and cost-effective manner using this method.Copyright

Determination of Intergranular Strain Parameters and Their Dependence on Properties of Grain Assemblies

Intergranular strain parameters of the extended Hypoplastic model are determined from laboratory experiments. Simple static triaxial setup coupled with controlled stress path test method is employed to determine the parameters. Parameters are determined for 4 different naturally existing sands acquired from field. The dependence of the intergranular strain parameters, on density and stress state of the sand is studied and recommendations are made for the selection of mean values in the relevant range of stresses and densities. The variation in the magnitude of intergranular strain parameters is studied in accordance with the varying grain assembly properties. The strain range within which the incremental stiffness remains constant after strain reversal is studied in conjunction with the grain properties and the validity of the assumption that the governing parameter is a material independent constant is commented upon.

Simulation of sand particle transport by coupled CFD-DEM:first investigations

The continuum based Euler-Euler approach represents the main field of application for the simulation of sediment transport processes. Herein, the decisive phases of free water and soil are modelled by interpenetrating continua. Although mixing of the phases is possible, the multi-component character of the soil phase, as a mixture of solid grains and pore water, is neglected. Hence, a coupling between the free water and the pore water remains unnoticed as well. However, this coupling represents an important factor for determining the current state of the soil boundary near the transition zone. Due to water level changes caused by ship induced bow and stern waves, excess pore water pressure can occur in the upper soil layers. As a result, fluidisation effects can be initiated, which reduce the erosion resistance. To consider these fluidisation effects, the soil has to be treated as a mixture of dispersed grain particles and pore water by a Lagrange-Euler approach. The coupling of the Discrete Element Method (DEM) and the Computational Fluid Dynamics (CFD) approach offers this possibility. Thereby, the DEM is used for modelling the dispersed particles of the Lagrangian regime, while the CFD method models the continuum Euler-phase of the water. This paper introduces the coupled CFD-DEM method for simulating sand particle transport at the boundary layer transition zone. The model investigations and first results of simulations regarding the initiation of motion are presented.