Lars Bo Ibsen

Calibration of Failure Criteria for Bucket Foundations on Drained Sand under General Loading

A new concept with respect to offshore wind turbines, the bucket foundation, is known in the oil and gas industry, although the load conditions for wind turbines are significantly different. An extensive number of loading tests with small-scale bucket foundations subjected to combined loading were carried out in the geotechnical laboratory at Aalborg University in Aalborg, Denmark. Tests were performed on buckets of various sizes, embedment ratios, and load paths on saturated dense Aalborg University Sand No. 1. The experimental results were used to evaluate the behavior of the bucket foundations in accordance with the macromodel approach. An expression to describe the combined capacity of bucket foundations was developed by calibrating the failure criteria for bucket foundations.

Adaptive plasticity model for bucket foundations

AbstractBased on experimental investigations, the literature proposes different methods for modeling the behavior and capacity of foundations subjected to combined loading. Generally, two methods are used to predict the behavior of foundations: traditional approaches and hardening plasticity solutions. The first method is only capable of determining the capacity of the foundations and not the prepeak behavior. Thus, a new strain-hardening criterion is developed by calibrating failure criteria by employing data from small-scale tests on bucket foundations subjected to static loads. The shape of the yield, potential, and failure surfaces are found to be dependent on the embedment ratio (i.e., ratio of skirt length to the diameter) and load path. For the models tested, associated flow is observed to be plausible in the radial planes, whereas nonassociated flow is observed in the planes along the V-axis.

Differential transform method for mathematical modeling of jamming transition problem in traffic congestion flow

In this paper we aim to find an analytical solution for jamming transition in traffic flow. Generally the Jamming Transition Problem (JTP) can be modeled via Lorentz system. So, in this way, the governing differential equation achieved is modeled in the form of a nonlinear damped oscillator. In current research the authors utilized the Differential Transformation Method (DTM) for solving the nonlinear problem and compared the analytical results with those ones obtained by the 4th order Runge-Kutta Method (RK4) as a numerical method. Further illustration embedded in this paper shows the ability of DTM in solving nonlinear problems when a so accurate solution is required.

THE DANISH RIGID BOUNDARY TRUE TRIAXIAL APPARATUS FOR SOIL TESTING

The intermediate principal stress plays an important role for the stress-strain behavior of soils. The effect of the intermediate stress cannot be examined in the conventional triaxial apparatus since axisymmetric stress states are prescribed in it. Thus, other testing devices capable of producing asymmetric stress states are necessary for more accurate determination of the stress-strain behavior. This paper describes the Danish rigid boundary true triaxial apparatus developed at Aalborg University and the techniques employed to prepare and test specimens of clay and sand for effects of the intermediate stress. Three series of true triaxial tests are presented together with various conventional triaxial tests to illustrate the capabilities of the apparatus. The first series of tests was carried out on cross-anisotropic specimens from a natural clay deposit. The other two series were carried out on air-pluviated (frozen and unfrozen) specimens of sand that also showed cross-anisotropic behavior.

Effect of Embedment on the Plastic Behavior of Bucket Foundations

AbstractA series of comprehensive research and development projects has validated the novel concept of the bucket foundation as an alternative to piled foundations, and bucket foundations are rapidly becoming a viable option for offshore wind farms and marginal fields.Within the last decade, experimental studies have indicated the possibility of defining foundation response using plasticity theory. Results of multiple loading tests addressing the effect of embedment on the strain-hardening behavior of shallow bucket foundations under combined loading are reported. The kinematic mechanisms accompanying pre-failure are presented. It is argued that the drained capacity of offshore bucket foundations and the ratio of plastic increments are largely influenced by embedment depth and the preload ratio V/Vpeak.

Vertical capacity of bucket foundations in undrained soil

AbstractOffshore wind turbine structures are traditionally founded on gravity concrete foundations or mono-piles. Bucket foundations were developed for the offshore oil and gas industry and are now being used in wind turbine construction. The loading in this application is characterized by a vertical load due to the slender construction combined with horizontal forces inducing a large overturning moment. Field tests on bucket foundations were performed to gain insight into the vertical load response of bucket foundations in clay soils. The field tests were accompanied by finite element numerical simulations in order to provide a better understanding of the parameters influencing bucket foundation behaviour.

The stable state in cyclic triaxial testing on sand

Abstract The paper will describe a number of new characteristic phenomena of sand under cyclic loading, which have been discovered by triaxial cell testing, at the Soil Mechanics Laboratory at Aalborg University using test specimens with equal height and diameter. These phenomena have been compared with results from tests executed on specimens with double height. It will be explained why the new phenomena ‘Stabilization’ and ‘Instant Stabilization’ are seldom observed when the test is executed on specimens with double height. The conclusion recommends that the study of pore pressure variation due to cyclic loading should only be performed with smooth pressure heads and specimens with equal height and diameter because the phenomena leading to stress hardening would otherwise not be observed.

Embedment Effects on Vertical Bearing Capacity of Offshore Bucket Foundations on Cohesionless Soil

AbstractThis paper presents the results from a series of physical modeling and three-dimensional finite-element (FE) analyses in which the authors examined the uniaxial vertical capacity of suction caissons for offshore wind turbines. The experiments were carried out in quartz sand and involved monotonic application of vertical load. It was found that the drained capacity of suction caissons is dependent on embedment ratio. In contrast, predictions from conventional semiempirical depth factors were found to somewhat underestimate when applied to rough foundations. On the basis of the tests and FE analyses, new expressions for the depth factor of shallow foundations were validated for embedment ratios (aspect ratios) up to unity, calibrating the fitting parameters by using data from a range of soil profiles.

A Similitude Theory for Bucket Foundations Under Monotonic Horizontal Load in Dense Sand

This paper aims at finding force-displacement relationships to be employed in the design of bucket foundations for offshore wind turbine. This is accomplished by combining small-scale tests and element tests within a theoretical framework. A similitude theory, regarding the lateral displacement of bucket foundations under horizontal load, is put forward. A constitutive law of the soil and a load-displacement relationship for the bucket foundation are theoretically obtained. Triaxial tests of sand, and small-scale tests of bucket foundation, are respectively employed to corroborate the theory. Attention is given to the different behaviour shown during the compressive and dilative phases of the soil. Some analogy between triaxial tests and tests of bucket foundation are pointed out. A theoretically derived power law is found capable to represent the dimensionless horizontal load-displacement curves of experimental results. In accordance with the theory, the exponent of the power law slightly varies between tests with considerably different features. The non-dimensional moment-rotation relationship is represented by a power law as well. The approach is considered valid for fatigue design. The study may be an interesting source for further researches on long-term cyclic horizontal loading.

Predicting subsurface soil layering and landslide risk with Artificial Neural Networks: a case study from Iran

Predicting subsurface soil layering and landslide risk with Artificial Neural Networks: a case study from Iran This paper is concerned principally with the application of Artificial Neural Networks (ANN) in geotechnical engineering. In particular the application of ANN is discussed in more detail for subsurface soil layering and landslide analysis. Two ANN models are trained to predict subsurface soil layering and landslide risk using data collected from a study area in northern Iran. Given the three-dimensional coordinates of soil layers present in thirty boreholes as training data, our first ANN successfully predicted the depth and type of subsurface soil layers at new locations in the region. The agreement between the ANN outputs and actual data is over 90 % for all test cases. The second ANN was designed to recognize the probability of landslide occurrence at 200 sampling points which were not used in training. The neural network outputs are very close (over 92 %) to risk values calculated by the finite element method or by Bishops method.