Holistic Simulation of Geotechnical Installation Processes

The knowledge of the initial soil state (stress and density distribution) in geotechnical model tests is indispensable, particularly with regard to FE back calculation of experimental results. Usually, so-called \(K_{0}\)-conditions are assumed, which for many cases do not describe the soil stress state before the experiment begins adequately. Using an exemplary test device we present and discuss different measurement techniques for the interpretation of soil deposition procedures and the evaluation of the initial state. By means of stress and bearing force measurements, the stress state is captured representatively. The soil deformations during the filling of the test device are evaluated with Digital Image Correlation (DIC) methods and the initial density distribution is examined by cone penetration tests (CPT). Afterwards, a simple FE simulation method is presented, which models the soil deposition procedure by a weight increase layer-by-layer. It is shown that the method is suitable to provide a realistic initial soil state. The methods presented can be easily transferred to other geotechnical test devices and can in many cases ensure a better comparability of tests with their simulations.

Benchmark tests for the numerical simulation of pile installation require clearly defined boundary value problems with corresponding experimental data. These experiments have to provide quantitative information on the soil deformations and stresses. Large scale model tests in dry, granular soil were carried out for this purpose. The interface testing device, that is used for the tests, allows the investigation of selected aspects of pile penetration. The normal and shear forces on the pile structure are measured. The displacements in the surrounding soil zone can be evaluated via Digital Image Correlation (DIC). The test results concentrate on the interface behavior between the soil and the pile and the evolution of stresses and deformations around the pile tip. For rough pile surfaces the occurrence of dilatancy effects in the pile-soil interface is shown. The localization of deformations in the post-peak phase is analyzed for monotonic and cyclic test paths. The influence of the pile driving mode on the evolution of stresses around the pile tip is demonstrated.

The experimental study compares soil displacement trajectories around model pile tips obtained from Digital Image Correlation (DIC) for different penetration modes. Monotonic and cyclic quasi-static penetration under plane strain-resembling conditions in dry sand and vibratory model pile penetration in saturated sand are investigated. The comparison results agree well although the penetration mode and the degree of saturation differ considerably. In the experiments, the soil below the pile tip is first pushed downwards as the pile approaches and is then moved more and more sidewards. A slight uplift of the grains is observed when the pile tip has passed. Subsequently, a clear trend of the soil adjacent to the pile shaft to move towards the pile is measured in the case of quasistatic cyclic and vibratory penetration. This trend is considered to be an indicator for \(``{\textit{friction~fatigue}}\)”, the degradation of shaft friction at a certain depth as the pile penetrates further. A discussion on the comparability with numerical results and on the influence of disturbing boundary effects concludes this contribution.

The main goal of the project is the realistic simulation of pile installation processes. By considering this processes prediction of soil behaviour using numerical simulation is used. The boundary conditions, here the external loads, are the contact forces between soil and structure. For correct prediction of the external loads a suitable contact and friction model is required. During the relative movement of a pile or generally a body with a rough surface within sand, a shear zone actually develops within the sand, directly to the contacting surfaces. Thus the interaction behaviour between sand and pile results in varying coefficient of friction, which is assumed as a quantity dependent on the stress state within sand body near to the contact surface. This assumption leads to an extension of the classical formulation of friction laws used within the contact mechanics framework related to the inelastic material behaviour. As constitutive law a hypoplastic material model is used, which represents volume changing effects of sand due to loading, which are specific for granular media. The discretisation of the contact constraints based on mortar method will be described. A robust hypoplastic model will be depicted. A proposed projection procedure for calculating the coefficient of friction exploiting the mentioned localisation of the contact surfaces and thus the analogy of simple shear and triaxial test behaviour of sand will be described. For the validation of the finite element model the results are compared with experimental data obtained within a specific large scale shear test.

The installation of vibro-injection piles into saturated sand has a significant impact on the surrounding soil and neighboring buildings. It is generally characterized by a multi-material flow with large material deformations, non-stationary and new material interfaces, and by the interaction of the grain skeleton and the pore water. Part 1 in this series of papers is concerned with the mathematical and physical modeling of the multi-material flow associated with vibro-injection pile installation. This model is the backbone of a new multi-material arbitrary Lagrangian-Eulerian (MMALE) numerical method presented in Part 2.

In Part 1 of this series of papers a macroscopic two-equation (two-field) reduced model for the mechanics of the multi-material flow associated with vibro-injection pile installation in saturated sand was derived. Here we employ this model to develop a so-called multi-material arbitrary Lagrangian-Eulerian (MMALE) method. MMALE avoids the disadvantages of the classical approaches in computational continuum mechanics concerning large deformations and evolving material interfaces. The numerical implementation of this method will be outlined, and then the experimental investigations will be presented that have been carried out in order to validate the computational model. Among these investigations, small-scale model tests in chambers with observing window have been designed step-by-step to reveal penetration and vibro-injection pile installation phenomena.

The paper presents results of the numerical modelling of the effective-stress evolution in saturated granular soil around the toe of a vertically vibrating pile. The problem is solved in a spherically symmetric formulation using two different types of constitutive models. An incremental hypoplasticity model is used to calculate the stress state after a limited number of cycle at the beginning of the vibration. Further changes in stresses for a large number of cycles are calculated with an explicit cyclic model. The influence of soil permeability and relative density is investigated. It is shown that the cyclic soil deformation results in the reduction of the effective stress around the pile in spite of the pore pressure dissipation in the case of high soil permeability.

A dynamic boundary value problem for a fluid-saturated solid can be represented as two coupled boundary value problems for one-phase media. This allows us to solve the problem with a commercial computer program without a built-in procedure for the solution of dynamic problems with non-zero permeability, provided that the user is able to establish the required coupling between the two problems. This approach has been implemented in the present paper with the computer program Abaqus/Standard using the dynamic analysis for one-phase media as a built-in procedure without the need to construct a user-defined finite element.

Behaviour of soils under small cycles is examined in the triaxial apparatus and the results are used for the calibration of several constitutive relations. The small strain relation is not exactly linear and stiffness \( E_{ijkl}\) in \(\dot{\sigma _{ij}}= E_{ijkl}\dot{\varepsilon _{kl}}\) is not constant. The popular hypoplastic (HP) model describes the small strain behaviour using the intergranular strain (Niemunis, Herle, Mech Cohesive-Frictional Mater 2(4):279–299 1997). However, this idea with an additional strain has several shortcomings. A better approach is the paraelastic (PE) model (Niemunis et al, Acta Geotech 6(2):67–80 2011; Prada Sarmiento, Paraelastic description of small-strain soil behaivour 2012). In this study the paraelasticity has been used already while evaluating of the raw data from triaxial test results. Similarly a simplified high cycle accumulation (HCA) formula (Niemunis et al, Comput Geotech 32(4):245–263 2005) and a simple assumption of stress dependence of \( E_{ijkl}\) have been used to purify the measured test data. A general curve-fitting strategy for testing of different constitutive models is developed. Some shortcomings of PE and HCA could be observed.

The spatial variability of high-cycle accumulated strain is larger than the variability obtained from monotonic loading. Gradients of strain cause that the fit of a given strain field \(\epsilon _{ij}^{\text {acc}}\) is difficult for the conventional elements (if no refinement is used). Usage of identical mesh for monotonic and cumulative deformations leads to numerical self-stresses in elements. More flexible elements are therefore examined. They can reduce the self-stresses considerably.

The stability of structures strongly relies upon the strength and stiffness of the foundation soil underneath. If fluid-saturated or nearly saturated soils are subjected to rapid cyclic loading conditions, for instance, during earthquakes, the intergranular frictional forces might be dramatically reduced. Subsequently, the load-bearing capacity decreases or even vanishes, if the soil grains loose contact to each other. This phenomena is often referred to as soil liquefaction. Drawing our attention to fluid-saturated granular materials with heterogeneous microstructures, the modelling is carried out within a continuum-mechanical framework by exploiting the macroscopic Theory of Porous Media (TPM) together with thermodynamically consistent constitutive equations. In this regard, the present contribution proceeds from a fully saturated soil, composed of an elasto-plastic solid skeleton and a materially incompressible pore fluid. The governing material parameters of the solid skeleton have been identified for the research-unit sand. The underlying equations are used to simulate soils under rapid cyclic loading conditions. In this regard, the semi-infinite domain is split into a near field, which usually the domain of interest, and a far field, which extents the simulated domain towards infinity. In order to avoid wave reflections at the near-field boundaries an energy-absorbing layer is introduced. Finally, several simulations are carried out. Firstly, a parametric study of the particular far-field treatment is performed and, secondly, soil liquefaction is simulated, where the underlying initial-boundary-value problem is inspired by practically relevant scenarios.

To look onto the stress-path-dependent strain behaviour of granular soils at low-cycle and monotonous loading processes as a basis for the development of new, improved or enhanced constitutive models, drained, stress-controlled triaxial-tests with a fine grained sand have been carried out. The focus was to investigate total as well as quasi-elastic strains for different stress-states by means of strain-response-envelopes. By subtracting the quasi-elastic strains from the total strains, a separate evaluation of plastic strains was also possible. For monotonous loading one separate soil-specimen has been used for each monotonous loading-direction. The shapes of the response-envelopes for initially isotropic stress-states were similar to ellipses, for initially anisotropic stress-states their shape was elongated and shifted away in the direction of the failure lines. For low-cycle loading, cycles of relatively small stress increments were applied in different directions in the stress-plane. It is found that quasi-elastic behaviour can already occur at a low number of cycles. The shapes of the obtained strain-response-envelopes were similar to symmetrical ellipses. It could be observed that the size of the ellipses decreases with increasing mean pressure p. The major axis of the ellipses rotates depending on the initial stress-state \(\eta ~=~q/p\), indicating a stress-induced anisotropy. Preloading seems to have little effect on the stiffness or the directions of the quasi-elastic strains. In the future it is intended to simulate the observed stress-strain behaviour by means of new, improved or enhanced constitutive models.