Helmut F. Schweiger

Numerical analysis of stone column supported foundations

Abstract In this paper, settlement and failure load of rafts resting on stone column reinforced soft clays are analyzed. The influence of the stone columns is assumed to be uniformly and homogeneously distributed throughout the reinforced region. It is also assumed that both columns and surrounding soil undergo the same total strains i.e. no slip occurs on the soil-column interface. A constitutive model is presented for an equivalent material. It combines different elasto-plastic laws, namely the Critical State model for clay and the Mohr-Coulomb criterion for gravel. Continuity of radial stresses is ensured by an additional pseudo-yield criterion. The model is incorporated in a finite element code and results for a circular footing are presented. The influence of dilatancy of the columns is highlighted together with the differences in the behaviour for columns situated at the centre or at the outer boundary of the footing. Flexible as well as rigid foundations are considered. It is emphasized that the finite element mesh is independent of the column spacing leading to considerable advantages in carrying out parametric studies.

Influence of Deep Excavations on Nearby Existing Tunnels

InShanghai,anumberofnewundergroundpassagesarebeingconstructed,oftenrequiringdeepexcavations.Whentheexcavations areinthevicinityof existingtunnels,it isnecessarytoassesstheinfluenceof theseexcavations onexistingtunnelssuchasdisplacementsofthe lining and additional loads on the lining. This paper presents a finite-element parametric study of tunnel behavior caused by nearby deep ex- cavation. This study investigates the effects of several parameters that may affect the tunnel response. These parameters are: Relative position of the tunnel with respect to the excavation, tunnel diameter, excavation dimensions, and tunnel protection measures. The results reveal that the influence of the excavation on the underlying tunnel is significant in a range of ∼53excavation width measured along the tunnel axis; unloading further than 10 m away from the axis of the tunnel has little effect on the tunnel structure. Based on the study in this paper a better understanding of the interaction between deep excavations and tunnels will be obtained. DOI: 10.1061/(ASCE)GM.1943- 5622.0000188.

Application of a Multilaminate Model to simulation of shear band formation in NATM-tunnelling

Abstract A constitutive model formulated within the framework of Multilaminate Models is applied to analyse the practical problem of a tunnel construction according to the principles of the NATM (New Austrian Tunnelling Method). It is demonstrated that the development of plastic shear strains leading to a failure mechanism that involves shear banding can be captured with this model. An enhanced strain softening formulation enables to model both, frictional and cohesive softening behaviour. Due to a simple regularisation technique mesh independent results are obtained with sufficient accuracy for practical purposes.

Numerical analysis of a floating stone column foundation using different constitutive models

This paper shows the results of a back analysis of a well instrumented field trial evaluating the performance of a floating stone column foundation for an embankment. The field test was carried out during the construction of a new football stadium in Klagenfurt, Austria. Different measuring devices such as exten- someters, piezometers, horizontal inclinometers and settlement gauges have been installed and readings were taken over a period of approximately 14 months. Thus the development of pore water pressures and settlements at different depths are available for various construction stages, including installation of columns, construction of the embankment and following consolidation. These provide the basis for back calculations presented in this paper. Advanced elastoplastic constitutive soil models have been used, some of them accounting for anisotropy or small strain stiffness effects. Reasonable agreement with in situ measurements could be achieved in general and it turned out that the anisotropic model gave the best fit to measurements.

On the use of drucker-prager failure criteria for earth pressure problems

Abstract Numerical methods are of considerable help in obtaining a better understanding of the behaviour of geomaterials and have become a standard tool for analysing complex problems in geotechnical engineering. Highly sophisticated constituttive models have been developed and are employed in the scientific world. However, practical engineers very often prefer relatively simple elastic-perfectly plastic material models. In this paper the response of frequently used failure criteria, namely different forms of the Drucker-Prager criterion, are investigated by solving a simple earth pressure problem. The study reveals that all Drucker- Prager models represent a poor approximation of the Mohr-Coulomb failure surface and may lead to higher computing costs. This fact is well acknowledged by the scientific community but is nevertheless often ignored in practice. The results presented here strongly suggest that for earth pressure problems a proper Mohr-Coulomb failure surface should be used when the application of more refined models is not possible or justified.

Application of the random set finite element method for analysing tunnel excavation

c effective cohesion e eccentricity ea imperfections E Young’s modulus E50 ref secant modulus at 50% of failure deviator stress in triaxial compression at reference cell pressure Eoed ref oedometer modulus at reference stress Eur ref unloading/reloading modulus at reference stress EA axial stiffness EI flexural rigidity ESC y stiffness of ‘young’ shotcrete ESC o stiffness of ‘old’ shotcrete fc uniaxial compressive strength of concrete FOS, Fs factor of safety kx horizontal permeability ky vertical permeability Ko coefficient of earth pressure at rest L length of potential failure surface m power index for stiffness dependency in HS model M bending moment Mpl plastic bending moment N axial force Npl plastic axial force Nzul admissible axial force p reference stress in HS model V volume x, y, z cartesian coordinates g total unit weight of soil gsat saturated unit weight of soil G variance reduction factor U spatial correlation length nur unloading Poisson’s ratio 8 effective friction angle sG standard deviation of spatial average SMStageTH prerelaxation factor for top heading excavation SMStageBI prerelaxation factor for bench and invert excavation c dilatancy angle

Influence of Anisotropic Small Strain Stiffness on the Deformation Behavior of Geotechnical Structures

AbstractConstitutive models based on the multilaminate framework naturally account for strain-induced anisotropy, but inherent anisotropy needs additional considerations. A recently introduced feature is the capability of the model to take into account anisotropic elasticity in the small strain range. The implications of taking into account anisotropy of small strain stiffness for the analysis of geotechnical problems is addressed in this paper by solving two relatively simple benchmark problems: a deep excavation and a strip footing. It follows from these studies that, if displacements and thus strains remain moderate (which is often the case for these types of problems), the influence of small strain stiffness anisotropy is more pronounced compared with problems where relatively large strains occur. However, the influence of stiffness anisotropy can be approximately accounted for by using the average of the axial stiffnesses.

Basic Concepts and Applications of Point Estimate Methods in Geotechnical Engineering

A procedure for combining a point estimate method (PEM) and deterministic finite element methods for geotechnical analysis is presented. A brief description of the theoretical background is given and an illustrative example analysing a sheet pile wall supporting an excavation is presented to demonstrate the general applicability of the proposed approach. Finally the application to numerical analyses of tunnel excavation is discussed in some detail. A NATM tunnel construction is considered and the results obtained from the suggested approach are compared to the ones obtained from conventional finite element analyses. The results clearly reveal the applicability of the proposed concept for solving practical problems and it is argued that the validity of finite element analyses is extended by introducing stochastic properties of input parameters.

Application of the random set finite element method (RS-FEM) in geotechnics

Dealing with uncertainty, caused e.g. by material parameters varying in a wide range or simply by a lack of knowledge, is one of the important issues in geotechnical analyses. The advantages of numerical modelling have been appreciated by practitioners, in particular when displacements and deformations of complex underground structures have to be predicted. Therefore, it seems to be logical to combine numerical modelling with concepts for the mathematical representation of uncertainties. Recent theoretical developments and advances made in computational modelling have established various methods which may serve as a basis for a more formal consideration of uncertainties as has been done so far. Random set theory offers one of these possibilities for the mathematical representation of uncertainties. It can be viewed as a generalisation of probability theory and interval analysis. After a brief introduction of the basics of the proposed approach an application to a boundary value problem is presented. The results show that the assessment of the probability of damage of a building, situated adjacent to the excavation, is in line with observed behaviour.

Design of deep excavations with FEM - influence of constitutive model and comparison of EC7 design approaches

Numerical analyses are performed on a routine basis in practical geotechnical engineering to assess the deformation behaviour of deep excavations under service load conditions, but it becomes increasingly common to use results from numerical analysis for ultimate limit state design (ULS). When doing so, compatibility of the design with relevant standards and codes of practice, valid in the respective country, has to be assured but there are no clear guidelines how this can be achieved. In this paper two aspects are addressed. First the influence of the constitutive model employed for modelling the mechanical behaviour of the soil on calculated structural forces of retaining walls is discussed and secondly the possibilities and limitations of introducing the partial factor concept as established in EC7 in combination with numerical analysis are highlighted.