Christos Vrettos

The Boussinesq problem for soils with bounded non‐homogeneity

The response of a compressible continuously non-homogeneous elastic soil to a static vertical point load on its surface is analytically investigated by using classical integral transform techniques and the extended power series method for obtaining the solution in the transform domain. The non-homogeneity is described by means of a depth-function which is non-zero at the surface and bounded at infinity and is capable in modelling both increasing and decreasing soil stiffness with depth. The influence of non-homogeneity on the displacements and stresses at the surface and in the interior is examined over a wide range on the governing parameters.

Vertical and rocking impedances for rigid rectangular foundations on soils with bounded non-homogeneity

The vertical and rocking response of rigid rectangular foundations resting on a linear-elastic, compressible, non-homogeneous half-space soil model is studied. The non-homogeneity is described by a continuous yet bounded increase of shear modulus with depth. The mixed boundary value problem is solved by means of the semi-analytical method of the subdivision of the foundation/soil contact area whereby the influence functions for the sub-regions are determined by integration of the corresponding surface-to-surface Greens functions for the particular soil model. Impedance functions are given for representative values of the non-homogeneity parameters, the Poissons ratio and the foundation geometry over a wide range of frequencies. Significant features associated with the soil non-homogeneity are pointed out.

An Initial Study on the Viscous Behaviour of a Partially Saturated Kaolinite Clay Based on Triaxial Tests

Creep plays an important role in the estimation of long-term behaviour of cohesive soils. It is, among other parameters, related to the viscosity of the soil, hence rate-dependent. Apart from the degree of saturation and the initial void ratio, the stress rates as well as isotropic / deviatoric stress states determine the behaviour. A partially saturated kaolinite is selected as the test material. The creep tests are performed in a triaxial apparatus with tensiometer-measured suction. To combine the viscous and the elasto-plastic behaviour, a new triaxial apparatus performing deviatoric triaxial tests has recently been developed, which is described in detail in this paper. Selected results of the triaxial and creep tests are presented. The creep tests show good agreement with results from the literature and may be directly used to formulate an elasto-visco-plastic constitutive law for this type of soil.

Simple inversion procedure for shallow seismic refraction in continuously nonhomogeneous soils

Seismic refraction analysis is presented for uniformly deposited shallow soil strata with wave velocity increasing continuously with depth due to differential compaction effects. Closed form solutions of the surface-to-surface travel time as well as for the depth to maximum penetration are derived for realistic velocity-depth functions. An inversion procedure based on simple formulas is presented and its applicability is demonstrated and discussed. The method takes into account the information along the entire refraction line in one step, in contrast to the discrete ray-tracing technique and is, therefore, less sensitive to the natural scatter of the data.

Penetration Tests in a Mold on Regolith Quasi-Analogues at Different Relative Densities

Know-how from terrestrial geotechnical engineering may be applied for a variety of space problems such as the prediction of the soil mechanical response of extra-terrestrial soils, the exploration of near-surface planetary soil, and the movement of vehicles. Penetration testing constitutes a well-established method for the back-calculation of the soil properties. In an initial study, indentation of a rod into granular material placed within a mold has been investigated for variable density states. Regolith simulants, natural soils, and glass have been considered. The test results are presented and key features are elucidated showing that the sole use of the grain size distribution curve as a prediction tool within each subgroup is not appropriate. The effects of the rigid base and the rigid walls are clearly revealed in the response.

Assessment of Swelling Deformation of Unsaturated Kaolinite Clay

The swelling behaviour of kaolinite clay is systematically investigated by means of two types of oedometer devices suitably modified to measure the variation of swelling pressure in dependence of the vertical and volumetric strain. A wide range of values for initial void ratio as well as initial and final degree of saturation has been covered. Based on the experimental results approximate equations for the swelling deformation are derived that are implemented in a FEM-Code. The application to a typical situation in practice is demonstrated by the numerical solution of the respective boundary value problem.

Ein Näherungsverfahren zur Berechnung von Pfahl-Plattengründungen mit unregelmäßiger Geometrie

Zunehmend werden Hochhausgrundungen in tragfahigen Boden als Pfahl-Plattengrundungen ausgefuhrt. Bei dem Entwurf werden die Position und die Geometrie der Pfahle optimiert, um die differentiellen Setzungen und somit auch die Beanspruchung der Platte zu minimieren. In der Praxis erfolgt die Berechnung dieses komplizierten Boden-Bauwerk-Interaktion-Problems mittels eines pseudo-gekoppelten Verfahrens; als Eingangsparameter dienen die Federkonstanten der einzelnen Pfahle und die mittlere Bettungsziffer der Platte. Die vorliegende Arbeit beschreibt ein Naherungsverfahren zur Bestimmung dieser Parameter fur Pfahl-Plattengrundungen mit unregelmasiger Geometrie. Es basiert auf dem elastischen Verfahren nach Randolph et al. mit Modifikationen zur Berucksichtigung des variablen Abstands und der Lange der einzelnen Pfahle sowie des unterschiedlichen Tragverhaltens von Rand- und Zentrumspfahlen. Durch Ersetzen des statischen Wertes des Bodenmoduls durch sein dynamisches Gegenstuck konnen die statischen Ersatzfederkonstanten der Pfahle als erste Approximation bei der Berechnung der dynamischen Antwort von Bauwerken benutzt werden.

Materialverhalten von gefrorenem Sand aus Triaxialversuchen an kubischen Proben

Die kunstliche Bodenvereisung findet vermehrt bei anspruchsvollen Aufgaben des Grund- und Tunnelbaus Einsatz. Bei der Dimensionierung dieser Masnahmen sind die Kenntnis des komplexen Materialverhaltens sowie die wirklichkeitsgetreue kontinuumsmechanische Modellierung erforderlich. Ein bestehender Ansatz fur ein elasto-viskoplastisches Stoffmodell wird zugrunde gelegt und anhand von Versuchsergebnissen an kubischen Frostproben weiter entwickelt. Neben der Darstellung von reprasentativen Versuchsergebnissen, werden die wesentlichen Merkmale der elasto-plastischen und der viskosen Komponenten des Stoffmodells in Abhangigkeit der Zustandsgrosen Spannungen, Deformationen, Temperatur, Sattigungsgrad, Porenzahl, Lode-Winkel und Zeit herausgestellt.

Influence of State Variables on the Shear Behaviour of an Unsaturated Clay

Suction is one important parameter affecting the shear behaviour of partially saturated soils. State variables such as void ratio and degree of saturation control the development of suction. A series of deviatoric triaxial tests on kaolinite clay with direct measurement of suction are reported. An appropriate elasto-plastic constitutive for this type of soil is presented and its parameters are determined from the test results.