Holistic Simulation of Geotechnical Installation Processes

Designing and performing adapted model tests related to pile penetration is a major target of the central project of the research group GEOTECH. These tests shall allow to capture major aspects of pile penetration quantitatively and to obtain input data for numerical simulations. The tests are focused on the interaction of the pile and the soil in dry or saturated conditions. Guidelines are to keep the tests as simple as possible, realize boundary conditions that are convenient for numerical simulations, and to provide reliable information on the state of the soil at the beginning of and during the tests. Furthermore, implications induced by the measurements, e.g., lower stiffness of an instrumented pile or the use of glass walls enabling the application of digital image correlation have to be evaluated and considered in the numerical simulations as well. Examples demonstrate how the concepts have been implemented for the measurement of tip and friction force on model piles under monotonic, cyclic, and dynamic loading as well as for the evolution of pore water pressure. Based on selected results, size effects of the test devices and the role of the model material resp. its state are pointed out. The contribution includes a discussion on disturbing influences such as friction in the linear guiding system or between pile and glass wall.

A combined interpretation of force measurements together with the evaluation of dynamic motion around the pile based on digital image correlation (DIC) is performed to identify soil deformation during vibratory pile driving in model tests. The tests are executed under water-saturated 1g-conditions. We prove the occurrence of the so-called cavitational pile driving but without the experimental evidence of the forming of a cavity under the pile tip. Using the DIC results, first attempts are made to evaluate the volumetric cyclic deformation of soil around the pile tip during the vibro-penetration. The results show an alternation of contractancy and dilatancy in proximity of the pile tip with volumetric peak-to-peak strain amplitudes up to 2 %. They indicate drained or at least partially drained conditions. Based on the test results, existing phenomenological interpretations of soil deformation due to pile penetration are reviewed.

A numerical study conducted recently by the authors showed that the vibration of a pile in saturated granular soil leads to the formation of a zone with nearly zero effective stresses (liquefaction zone) around the pile toe. The dynamic problem was solved with the finite-element program Abaqus/Standard using a hypoplasticity model for soil with the assumption of zero soil permeability and without a mass force. A question which still remained open was the influence of the soil permeability and the gravity force on the solutions. In the present study, the problem is solved with nonzero permeability and gravity, and the solutions are compared with those obtained earlier. For this purpose, a user-defined element has been constructed in Abaqus to enable the dynamic analysis of a two-phase medium with nonzero permeability. The solutions show that high permeability and gravity do not prevent the formation of a liquefaction zone around the pile toe in spite of the fact that a build-up of the pore pressure is inhibited by the pore pressure dissipation.

Numerical modelling of the cyclic deformation of soil can be performed with the use of two types of constitutive models: incremental plasticity models and so-called explicit cyclic models. The present study deals with the application of a model of the second type—the high-cycle accumulation model for sand—to the analysis of vibration-induced stress changes in saturated soil. The application of the model requires the concurrent solution of two coupled boundary value problems: a dynamic problem for the determination of strain amplitude and a quasi-static problem for the calculation of stress evolution. The coupling of the two problems has been implemented in a two-dimensional axisymmetric formulation with the finite-element program Abaqus. The model is applied to the analysis of stress changes near a concrete wall caused by a vibrating pile. The numerical results show that a large-amplitude vibration, e.g. during the installation of vibratory driven piles, may substantially reduce both the effective and the total stresses in front of the wall. The effective stress may be reduced to zero resulting in soil liquefaction.

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.

The Intergranular Strain Anisotropy ISA framework is a novel approach to develop elastoplastic models wherein a yield surface is defined in terms of strain increments. For this purpose, the loading-unloading conditions are satisfied within the space of the intergranular strain, this latter being a state variable “following” the strain rate. With this, the model aims to improve the simulations under cyclic loading while keeping their good capabilities at monotonic loading. Within this article, a constitutive model for clays is developed under the ISA plasticity framework. The model adopts some parameters from the modified Cam-Clay model and others to describe the evolution of the integranular strain and its effect on the model response. Some illustrative simulations are provided to analyze the model performance under cyclic loading. The simulations show a qualitative behavior in agreement with some experiments. Possible improvements are discussed at the end of the article.

Cyclic soil behavior plays an important role in geotechnical engineering, both in the installation phase as over the life span of constructions. Relevant application examples which find increasing attention nowadays are the dimensioning of on- and offshore foundation systems, the analysis of soil behavior due to mechanized tunneling processes as well as analyses of loading histories related to deep excavation walls. Thus, in the present paper fundamentals of cyclic soil behavior under partially drained, oedometric conditions are analyzed. Excess pore water pressure evolution and accumulated deformations are studied by both numerical and experimental approach. For this purpose, a new oedometer device is introduced which allows to measure complete stress state under transient loading. Additionally, by numerical experiments using FEM the influence of soil stiffness and permeability on the evolution of excess pore water pressures and accumulated deformations is studied. By comparison of numerical and laboratory experiments the ability of classical isotropic hardening plasticity to model cyclic consolidation phenomena is validated.

It is known that some common constitutive models show deficits when predicting elastic and plastic deformations due to high- and low-cycle loading resulting for example from geotechnical installation processes. The object of part I of subproject 8 within the DFG research group FOR 1136 (GeoTech) is to show the performance of different constitutive models and to compare them to experimental laboratory test results and among each other. To look onto the incremental stress–strain behaviour of sand, series of drained, stress-controlled triaxial tests have been carried out to obtain strain response envelopes for monotonous loading. Here, a soil element is subjected to a constant stress increment in different directions and its strain responses are evaluated graphically. The presented laboratory tests were performed at different initial stress states. The accumulation of strains due to low cyclic loading (\(N\le 50\)) has also been examined for different loading directions and different sizes of stress amplitudes. All experiments have been recalculated numerically with different constitutive models, amongst them some common as well as advanced constitutive models, which have been developed recently and partly within the research group GeoTech.

In civil engineering, the installation of a reliable foundation is essential for the stability of the emerging structure. Thus, already during the foundation process, a comprehensive survey of the mutual interactions between the preliminary established construction pit and the surrounding soil is indispensable, especially when building in the existing context. Addressing the simulation of geotechnical foundation processes using vibro-injection piles, complex initial-boundary-value problems are necessary. In particular, the numerical model is composed of several mutual interacting parts, such as retaining walls, anchors and vibro-injection piles, all interacting with the surrounding soil. Additionally, a fine mesh is required in order to adequately resolve local effects such as shear bands. However, when such complex simulations are inevitable, explicit time-integration schemes are advantageous over implicit schemes. In this regard, the present contribution addresses the development and application of a numerical soil model based on the Theory of Porous Media, which is suitable for simulations exploiting the explicit time-integration schemes of Abaqus/Explicit. The underlying numerical soil model is investigated in terms of accuracy and parallel efficiency.

Multi-material flow describes a situation where several distinct materials separated by sharp material interfaces undergo large deformations. The research presented in this paper addresses a particular class of multi-material flow situations encountered in geomechanics and geotechnical engineering which is characterized by a complex coupled behavior of saturated granular material as well as by a hierarchy of distinct spatial scales. Examples include geotechnical installation processes, liquefaction-induced soil failure, and debris flow. The most attractive numerical approaches to solve such problems use variants of arbitrary Lagrangian–Eulerian descriptions allowing interfaces and free surfaces to flow through the computational mesh. Mesh elements cut by interfaces (multi-material elements) necessarily arise which contain a heterogeneous mixture of two or more materials. The heterogeneous mixture is represented as an effective single-phase material using mixture theory. The paper outlines the specific three-scale mixture theory developed by the authors and the MMALE numerical method to model and simulate geomechanical multi-material flow. In contrast to traditional flow models which consider the motion of multiple single-phase materials or single multi-phase mixture, the present research succeeds in incorporating both the coupled behavior of saturated granular material and its interaction with other (pure) materials.

In the course of research group FOR 1136 GEOTECH the task of subproject 8 (TP 8, part II) is to generate a holistic three-dimensional finite-element model to give improved predictions of construction-induced deformations of deep excavation walls. For this purpose, the results of the other subprojects dealing with contact elements, describing material behaviour and the modelling of soil liquefaction around a vibrating pile should be implemented. This contribution presents the framework of the 3D-model, in which the results of the other subprojects can be adopted at a later stage. The deep excavation at Potsdamer Platz in Berlin in the 1990s serves as reference for the model, since unexpected deformations of the diaphragm walls arised after pile driving. First numerical studies are presented to demonstrate the possibilities of the model.

The clay phase of many natural soils comprises a micro-structure of clay aggregates. These can be formed during sedimentation, due to van der Waals attraction between negatively charged particle surfaces in saltwater environments, or can occur in partially saturated soils where colloidal iron acts as a cementing agent. In order to understand the formation of clay aggregates and their role in affecting properties at the macroscale/continuum level, we have carried out multiscale analyses, initially considering the formation and properties of the aggregates. Nanoscale numerical simulations consider interactions between two clay platelets. The analyses focus on Wyoming montmorillonite (Na-smectite) and use the CLAYFF force field to describe pairwise interactions between ions within the clay and surrounding bulk water (i.e., Coulombic and van der Waals forces). The analyses establish the potential of mean force at different spacings between the layers for edge-to-edge and face-to-face interactions. The results are then used to calibrate the Gay-Berne (GB) potential that represents each platelet as a single-site ellipsoidal body. It is then possible to simulate the process of aggregation for an assembly of clay platelets in mesoscale simulations. These simulations find that aggregates of Na-smectite typically form in face-to-face stacks with 3–8 platelets. The particle assemblies become more ordered and exhibit more pronounced elastic anisotropy at higher confining pressures. The computed elastic stiffness properties are in good agreement with previously measured nanoindentation moduli over a wide range of clay-packing densities.