Torsten Wichtmann

Prediction of Permanent Deformations in Pavements Using a High-Cycle Accumulation Model

The present paper discusses the application of a high-cycle accumulation (HCA) model originally developed for sand for the prediction of permanent deformations in an unbound granular material (UGM) used for base and subbase layers in pavements. Cyclic triaxial tests on precompacted samples of an UGM have been performed in order to validate and calibrate the model. The stress amplitude, the initial density, and the average stress were varied. The test results are compared to those of air-pluviated samples of sand (subgrade material). Some significant differences in the behavior of both materials under cyclic loading are outlined. It is demonstrated that the functions describing the intensity of accumulation can be maintained for an UGM with different material constants, but that the flow rule must be generalized in order to describe the anisotropy. Recalculations of the laboratory tests show a good prediction of the modified HCA model.

Comparison of Cyclic Triaxial Behavior of Unbound Granular Material under Constant and Variable Confining Pressure

Cyclic stresses due to passing wheels impose an accumulation of permanent strains in layers of unbound granular materials (UGMs) of flexible pavements. The hollow cylinder triaxial test would be the most appropriate test to simulate the in situ stress conditions but it is difficult to perform on UGMs due to their large maximum grain size. The simpler axisymmetric cyclic triaxial test does not consider the shear stress components. It can be performed with a constant confining pressure (CCP) or a variable confining pressure (VCP). CCP and VCP tests are commonly assumed to deliver similar residual and resilient strains as long as the average stress is the same. Thus, the simpler CCP test is mostly used in pavement engineering. However, this assumption is based on limited test data in the literature and may not be on the safe side. The present paper documents a comparative study of CCP and VCP tests on UGM. The study is mainly dedicated to the permanent deformations. The results show that only for some special stress paths do both types of tests deliver similar permanent axial or volumetric strains. For some other stress paths the CCP tests may underestimate the permanent axial strain in comparison to the corresponding VCP test.

Stiffness and Damping of Clean Quartz Sand with Various Grain-Size Distribution Curves

AbstractApproximately 240 resonant column (RC) tests have been performed on 26 clean quartz sands with piecewise linear, gap-graded, S-shaped, and other smoothly shaped grain-size distribution curves. For each material, the small-strain shear modulus, Gmax, the shear modulus degradation curves, G(γ)/Gmax, and the damping ratio curves, D(γ), were measured at different pressures and densities. The applicability of the extended empirical equations proposed by the authors in two previous publications, considering the influence of the uniformity coefficient, Cu=d60/d10, of the grain-size distribution curve, has been inspected for the various tested materials. These equations were developed originally based on RC test data for linear gradations. It is demonstrated that the extended empirical equations work well also for most of the more complicated grain-size distribution curves tested in the current study.

Towards the FE prediction of permanent deformations of offshore wind power plant foundations using a high-cycle accumulation model

The paper discusses a possible application of the authors’ high-cycle accumulation (HCA) model for the prediction of long-term deformations of offshore wind power plant (OWPP) foundations. The calibration of the HCA model parameters for a typical North Sea fine sand is presented. These parameters have been used for exemplary finite element calculations of a monopile foundation, with variation of soil density, average load and cyclic load amplitude.

Peak Stress Obliquity in Drained and Undrained Sands. Simulations with Neohypoplasticity

The difference between undrained and drained peak friction angle is considerable (up tp \(10^\circ \)), despite identical densities and pressures (at peak). This cannot be explained using the elastoplastic formalism. An attempt is made to describe this effect with neohypoplasticity. For this purpose two types of nonlinearity are used, the well-known term \(Y m_{ij} \Vert \dot{\epsilon }\Vert \) and the novel skew-symmetric correction tensor which is added to the elastic stiffness.

Behaviour of Granular Soils Under Environmentally Induced Cyclic Loads

Cyclic loading and its impacts are of practical relevance for many problems in geotechnical engineering. Some examples of non-endogenous nature are illustrated in Fig. 1. A cyclic loading may be caused by traffic (high-speed trains, magnetic levitation trains), industrial sources (crane rails, machine foundations), wind and waves (on-shore and off-shore wind power plants, coastal structures) or repeated filling and emptying processes (watergates, tanks and silos). Furthermore, construction processes (e.g. vibroinstallation of sheet piles) and mechanical compaction (e.g. vibratory compaction) impose cyclic loads into the soil. A cyclic loading of the soil may be also caused by endogenous sources. Earthquake events due to a slip between adjacent tectonic plates lead to a propagation of shear waves. The shear waves induce a cyclic shearing of the soil. The cyclic loading of the soil can lead to an accumulation of permanent deformations or to a possible liquefaction due to a build-up of excess pore water pressure.

COMPORTAMIENTO DE UN MATERIAL GRANULAR NO TRATADO EN ENSAYOS TRIAXIALES CÍCLICOS CON PRESIÓN DE CONFINAMIENTO CONSTANTE Y VARIABLE

In a pavement structure, passing wheel loads impose cyclic stresses consisting of vertical, horizontal and shear components. Studies of the behavior of unbound granular materials (UGM, used for base and sub-base layers) under cyclic loading are mostly performed using the axisymmetric triaxial test with constant confining pressure (CCP test) and a cyclic variation of the axial stress. However, in this type of test only the vertical component of the cyclic stress path is considered. The oscillation of the horizontal stress can be reproduced by an additional cyclic variation of the confining pressure (VCP test). CCP and VCP tests are sometimes assumed to deliver similar residual and resilient deformations as long as the average stress is the same in both types of tests. However, this assumption is based on limited test data in the literature. The present paper documents a more detailed comparative study with CCP and VCP tests. The results show that only for some special stress paths both types of test deliver similar permanent axial or volumetric deformations.

Caracterización cíclica multidimensional de suelos no cohesivos

Las cargas con ciclos de deformacion multidimensional pueden ser provocadas tanto por el trafico, como por cargas de viento y oleaje (en el caso de turbinas eolicas mar afuera por ejemplo), o por terremotos. El presente articulo se enfoca en la acumulacion de deformaciones permanentes producto de carga ciclica de muchos ciclos, esto se refiere a una gran cantidad de ciclos de amplitud de deformacion pequena a mediana. Se provee evidencia experimental de ciclos de deformacion complejos. Se presentan resultados de ensayos triaxiales ciclicos drenados con oscilacion simultanea de la tension axial y lateral. Se ensayan ciclos de deformacion mediante la superposicion de funciones armonicas con diferentes frecuencias y amplitudes. Para comparar, en un segundo ensayo se aplican las mismas oscilaciones pero separadas una a continuacion de la otra. Tambien se varia la secuencia de aplicacion de las oscilaciones.

Kumulatives und dynamisches Verhalten von Böden

Ein Uberblick uber einige der letzten Forschungsergebnisse an der Ruhr-Universitat Bochum auf dem Gebiet der zyklischen bzw. dynamischen Belastung von Boden wird prasentiert. Sowohl residuale als auch reversible Effekte in Boden unter zyklischer Belastung werden untersucht. Ein hochzyklisches Akkumulationsmodell fur nichtbindige Boden wurde entwickelt. Das Modell wird zusammen mit den zugrundeliegenden Experimenten dargestellt. Fur eine nichtlineare Beschreibung des reversiblen Verhaltens wurden Versuche zur dynamischen Steifigkeit und zur Dampfung von nichtbindigen, bindigen und organischen Boden durchgefuhrt, welche in diesem Beitrag prasentiert werden.

New findings regarding the behaviour of soils under cyclic loading

The results from three series of cyclic triaxial tests with strong implica- tions on the practical application of the high-cycle accumulation (HCA) model of Niemunis et al. (13) are discussed. Tests with packages of cycles successively app- lied at different average stresses show that the memory of a cyclic preloading that is built up during the cycles can be partly or fully erased by a subsequent mono- tonic loading. In the present case the variation of the average stress between the packages of cycles represents such monotonic loading. The second test series with multiple changes of the polarization between succeeding packages of cycles reveals that the influence of such changes, that is described by the factor fπ in the HCA model, is probably less pronounced than previously thought. A significant accelera- tion of strain accumulation was observed due to the first change of the polarization only, while the effect of the subsequent variations of the polarization were rather moderate. The third test series with a comparison of samples prepared by either air pluviation or moist tamping demonstrates the important influence of the initial fabric of the sand, i.e. the sample preparation method in the laboratory, on the cumulative strains and thus on various components of the HCA model.