Abstract:Monopile foundations have been commonly used to support offshore wind turbine generators (WTGs), but this type of foundation encounters economic and technical limitations for larger WTGs in water depths exceeding 30m. Offshore wind farm projects are increasingly turning to alternative multipod foundations (for example tetrapod, jacket and tripods) supported on shallow foundations to reduce the environmental effects of piling noise. However the characteristics of these foundations under dynamic loading or long term cyclic wind turbine loading are not fully understood. This paper summarises the results from a series of small scaled tests (1:100, 1:150 and 1:200) of a complete NREL (National Renewable Energy Laboratory) wind turbine model on three types of foundations: monopiles, symmetric tetrapod and asymmetric tripod. The test bed used consists of either kaolin clay or sand and up to 1.4 million loading cycles were applied. The results showed that the multipod foundations (symmetric or asymmetric) exhibit two closely spaced natural frequencies corresponding to the rocking modes of vibration in two principle axes. Furthermore, the corresponding two spectral peaks change with repeated cycles of loading and they converge for symmetric tetrapods but not for asymmetric tripods. From the fatigue design point of view, the two spectral peaks for multipod foundations broaden the range of frequencies that can be excited by the broadband nature of the environmental loading (wind and wave) thereby impacting the extent of motions. Thus the system lifespan (number of cycles to failure) may effectively increase for symmetric foundations as the two peaks will tend to converge. However, for asymmetric foundations the system life may continue to be affected adversely as the two peaks will not converge. In this sense, designers should prefer symmetric foundations to asymmetric foundations.

The framework of anisotropic elasticity has been used to develop relationships between various anisotropic stiffness parameters found in the literature. It is shown that they are functions of the two Young's moduli and two Poisson's ratios that describe a cross-anisotropic soil, but not of the independent shear modulus. Multiple drained triaxial stress path excursions have been performed on 100 mm dia. samples of natural Gault Clay from Madingley in a stress path cell. Anisotropic parameters at small strains along different stress paths are reported and compared. It is shown that there are some advantages in performing tests at constant vertical and constant horizontal effective stress. Horizontally mounted bender elements on the same trixial samples have enabled the two anisotropic elastic shear moduli to be measured. Combining results from both sets of tests has enabled all five independent cross-anisotropic elastic parameters to be estimated. The Gault Clay at Madingley is highly anisotropic, although the degree of anisotropy depends on the way it is defined.

The inclusion of anisotropic stiffness parameters in sophisticated constitutive models necessitates their determination in the field and laboratory. Hitherto, anisotropy of small-strain stiffness of clays has occasionally been examined in the laboratory by measurements on specimens sampled at different orientations. The authors have developed a device to propagate and receive horizontal shear waves with both vertical and horizontal polarization through 100 mm triaxial samples. The device incorporates bender elements embedded in the pads of a horizontal belt. In tests on both reconstituted and natural samples of Gault Clay, each of the transmission velocities Vs(xh), Vs(hx) and Fs(hh) has been measured during stress path tests. The ratio G0(hh)/G0(vh) has been shown to be highly dependent on stress state. Laboratory data on natural samples show results broadly consistent with in situ results. Pour pouvoir inclure des paramètres de rigidité anisotrope dans des modèles constitutifs perfectionnés, il faut les déterminer sur le terrain et en laboratoire. Jusqu'à présent, on a parfois examiné l'anisotropie de la rigidité d'argiles aux petites contraintes en mesurant des échantillons prélevés à diverses orientations. Les auteurs ont mis au point un dispositif qui permet la propagation et la réception d'ondes de cisaillement horizontals avec polarisation verticale et horizontal dans des échantillons de compression triaxiale de 100mm. Le dispositif comprend des elements de flexion noyés dans les plaquettes d'une bande horizontal. Dans des essais de parcours de contrainte sur des échantillons reconstitués et naturels d'argile de Gault, on a mesuré les vitesses de transmission Vs(vh), Vs(hv) et Vs(hh). On a montré que le rapport G0(hh)/G0(vh) depend en grande partie de l'etat de contrainte. Les résultats obtenus en laboratoire sur des échantillons naturels concordent assez bien avec les résultats obtenus sur le terrain.

The one-dimensional consolidation behaviour of Bothkennar clay has been examined by testing high quality intact and reconstituted specimens recovered using the Laval sampler. The collaborative test programme consisted of incremental load, constant rate of strain, and restricted flow tests. During the test programme the variation of yield stress with depth, the effect of strain rate on the observed yield stress, the creep behaviour and the variability in soil compressibility were investigated. It was noted that the yield stress is influenced by the strain rate, with faster rates resulting in higher values of yield stress. When this was taken into account, it was found that specimens taken from a single depth showed reasonably consistent behaviour independent of the type of test. Creep occurred in the incremental load tests, with the maximum rate just after yield, indicating a clear linkage between creep and the structural breakdown at yield. The vertical permeability of the specimens was determined by flow pump tests and by observation of the differential pore pressure during constant rate of strain tests. Values were obtained for permeability at the in situ void ratio which were consistent between the two types of measurement, and were essentially constant over the full depth of the clay profile. Le comportement de l'argile de Bothkennar sous consolidation à une dimension a été étudié en faisant des essais sur des échantillons soit intacts de haute qualité soit reconstitués provenant de l'echantillonneur Laval. Le programme d'essais a comporté des essais de chargement par paliers, des essais à vitesse de déformation constante, et des essais à écoulements restreints. Durant les essais la variation de la limite d'élasticité avec la profondeur, l'effet de la vitesse de déformation sur la limite élastique observée, le comportement du fluage et la variabilité de la compressibilité du sol furent étudiés. Il a été noté que la limite élastique est influencée par la vitesse de déformation avec des vitesses plus grandes produisant des limites élastiques plus élevées. Quand ceci fut pris en compte on trouva que les échantillons à une même profondeur montrèrent un comportement compatible indépendant du type d'essai. Le fluage eut lieu dans les essais de chargement par paliers avec la vitesse maximale juste après la limite élastique, indiquant un lien clair entre fluage et la destruction de la structure au delà de la limite élastique. La perméabilité verticale des échantillons fut déterminée par des essais à la pompe à écoulement et par l'observation de la pression intersticielle différentielle pendant les essais à vitesse de déformation constante. Des valeurs de la perméabilité obtenues à l'indice des vides in-situ sont compatibles entre les deux types de mesures, et qui étaient pour ainsi dire constantes sur l'épaisseur totale de la couche d'argile.

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General rightsThis document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available: http://www.bristol.ac.uk/pure/about/ebr-terms 1 The prediction of the stress-strain response of granular soils under large numbers of repeated 19 loading cycles requires subtle changes to existing models, although the basic framework of 20 kinematic hardening/bounding surface elasto-plasticity can be retained. Extending an existing 21 model, an extra memory surface is introduced to track the stress history of the soil. The 22 memory surface can evolve in size and position according to three rules which can be linked 23 with physical principles of particle fabric and interaction. The memory surface changes in 24 size and position through the experienced plastic volumetric strains but it always encloses the 25 2 current stress state and the yield surface; these simple rules permit progressive stiffening of 26 the soil in cyclic loading, the accurate prediction of plastic strain rate accumulation during 27 cyclic loading, and the description of slightly stiffer stress-strain response upon subsequent 28 monotonic reloading. The implementation of the additional modelling features requires the 29 definition of only two new constitutive soil parameters. A parametric analysis is provided to 30 MEMORY SURFACE HARDENING MODEL FOR GRANULAR SOILS UNDER 1 REPEATED LOADING CONDITIONSshow model predictions for drained and undrained cyclic loading conditions. The model is 31 validated against available tests on Hostun Sand performed under drained triaxial cyclic 32 loading conditions with various confining pressures, densities, average stress ratios and cyclic 33 amplitudes. 34

Model tests have been performed to determine the mechanisms of response for beds of clay reinforced with stone columns subjected to surface footing loads. An exhumation technique has been used to discover the deformed shapes of the model stone columns and thence to deduce the way in which the columns have transferred load to the surrounding clay. Tests have explored the effect of varying the diameter, length and spacing of the model stone columns. These parameters control whether the columns act as somewhat rigid inclusions transferring load to their tips and eventually deforming either by bulging or by the formation of a failure plane, or whether they are able to compress axially or even, if sufficiently slender, to ‘bend’ and undergo significant lateral deformation. Miniature pressure transducers have been used to reveal the distribution of contact pressure between columns and clay at various stages during the loading of the footings. It appears that the columns at mid-radius of the footing are typically the most heavily loaded. Results from numerical analysis are used to provide qualitative support for some of the findings from the physical model tests.

For the assessment of the quality of laboratory samples, a number of methods are available, though not universally applicable to any soils. This paper examines the issue of sampling quality and its assessment using comparisons between shear wave velocity measurements in situ and in recovered samples as the base, which is very useful in naturally structured granular soils, like residual soils. For this purpose, cross-hole and down-hole tests were performed in thê eld and bender elements measurements were made on triaxial samples collected from two experimental sites on residual soil from Porto granite. Various sampling techniques and tools were used, including block sampling and diŠerent tube samplers. The analysis of the results has led to a new classiˆcation of sampling quality and sample condition based on the comparison of normalised shear wave velocities in theˆeld and in the laboratory.