Recent Developments of Soil Mechanics and Geotechnics in Theory and Practice

We present a coupled hydromechanical phase-field framework for capturing the formation and propagation of compaction bands in porous media under wet and dry conditions. Breakage mechanics is used to characterize the free energy function in the intact and damaged material, while the phase-field variable provides a measure of the degree of grain crushing. Permeability reduction in the zone of compaction localization is modeled using the Kozeny-Carman equation accounting for microstructural evolution. The effect of water-weakening on the rock’s plastic response is described by a modified Cam-Clay model that is enhanced to accommodate partial saturation. Numerical simulations on Savonnières limestone samples demonstrate the ability of the model to capture compaction band formation in dry and wet rock samples.

As a result of the early phase-out of coal mining, new geotechnical questions have to be answered in connection with the design of opencast mine lakes, in particular with regard to the excavated lake slopes.

Answering these questions requires extensive fundamental research. This is particularly true as the mine slopes are designed for an unlimited lifetime in an earthquake region, and especially as the Hambach “deep lake” will be the lake with the greatest water depth in Germany in the future.

As shown, a wide range of geotechnical questions (stability, earthquake impact, liquefaction resistance, wave impact and erosion, long-term material behaviour) need to be answered, and the results need to be scientifically validated, as subsequent changes to the design or reworking of the slopes are hardly possible in view of the dimensions of the future opencast mine lakes.

On the other hand, the early phase-out of coal requires timely findings for operational decisions. In order to take these operational requirements into account, safe geotechnical approaches were determined in an initial processing phase and corresponding pre-dimensioning was carried out. These have been and will continue to be successively validated and optimised. The work is carried out in cooperation with our external scientific partners and in close coordination with the internal specialist departments involved, in order to be able to evaluate and implement the results directly in terms of planning and operation.

Borehole integrity is a critical concern in drilling operations, as failure induced by stress redistribution poses a significant threat to both the borehole’s stability and economic viability. Partial failure takes place in the form of borehole breakouts, where a stable configuration is reached, though failure at the borehole wall takes place. Such breakouts, though endangering stability, can also be used to gain information on the primary stress state. Conventionally, isotropic simple models are used to assess both the likelihood of collapse and the primary stress state. In the present work, the influence of yield surface anisotropy on the shape and magnitude of borehole breakouts is investigated by means of finite element numerical calibrations. The influence of the post-yield behavior of the material is touched upon. To this end, a nonlocal model is used. The implications for the selection of a suitable boring fluid and the assessment of the in situ stress state are discussed.

The soil fabric can be manifold depending on the art of the deposition process creating the grain skeleton. It can be expected that a certain deposition method creates a specific type of the soil fabric. This deposition fabric is linked to a particular soil density. When expressed as a relative density with help of the limit densities from the standard index tests, a large range of the relative densities of different sands for one deposition method can be observed. The impact of this variation of the relative density at one particular deposition method on the build-up of the excess pore water pressure (PWP) should be clarified.

The newly developed PWP Tester enables a fast testing of the excess PWP build-up during undrained cyclic shearing in sandy soils. The tested specimens are created by water sedimentation and thus have a comparable fabric. Although the relative densities after the soil deposition  are significantly different for different soils, the rate of the PWP evolution during undrained shearing is well comparable. Consequently, the relative density should not be considered as a main factor controlling the excess PWP build-up.

The article summarizes the authors experience with different concepts for the analysis of cyclically loaded foundations based on cyclic laboratory test results. This is linked to the recent development of the offshore wind industry, especially related to the foundations of offshore wind turbine generators. Examples for different concepts are given and the fields of application are described.

The Site selection process for deep geological repositories for radioactive waste in Switzerland is defined by a respective sectoral plan. Siting regions are compared based on thirteen criteria specified by the sectoral plan. This paper presents the geotechnical risk analysis which is used for the assessment of constructional suitability of the three siting regions. The analysis considers project-specific geological and hydrogeological conditions and construction measures. The geotechnical risk analysis identifies and evaluates risks arising from deviations of ground conditions during the construction, operation, and closure of the repository (i.e. fault zones and adverse ground parameters). The analysis is based on selected underground structures which are representative for the entire repository. Mitigation measures are proposed for inacceptable initial risk values to minimize the residual risk to an acceptable level.

Particularly the risk analysis identifies the following main hazards: (i) inadmissible ground displacements and damage of the lining due to squeezing ground, (ii) tunnel face collapse on repository level and (iii) inadmissible water ingress during the construction of the access shafts. Special attention is given to hazards with a very low probability but a very high impact, which cannot be further reduced by reasonable measures. Respective risks are associated with the tunnel boring machine excavation of the drifts for high level waste (i.e. jamming of the shield, face collapse and damage of the lining).

Overall, the results of the geotechnical risk analysis confirm the technical feasibility and suitability of all three sites for the construction, operation, and closure of geological deep repositories.

This paper introduces a novel high-cycle accumulation model for sand, referred to as the NHCA model. While many empirical formulas for deformation accumulation rates (such as stress or density dependence) remain consistent with the HCA model proposed in 2005 (referred to as the OHCA), the NHCA model brings essential novelty through the definition of strain amplitude and the cycle counting method.

The definition of strain amplitude is rooted in kinematic hardening principles, commonly found in multi-surface plasticity models. Notably, for polychromatic cycles (comprising oscillations with different frequencies), the amplitude lacked clear definition within the OHCA framework. This paper specifically explores the ovality and polarization of strain loops, as well as stochastic and polychromatic multiaxial cycles, as key points of interest. Additionally, experimental results and model calibration procedures are presented to support the NHCA model.

An important engineering challenge of today is developing and harvesting alternative sources of energy and thus offshore wind farms have a great potential as an emerging technology. Unfortunately, the most common technique of wind turbine’s foundation establishment, pile driving, seems to influence marine mammals negatively. The noise generated from driving a foundation pile into the seabed affects the aquatic life several kilometers away, since lower frequencies with larger wavelengths (100-1kHz) do not dissipate in the sea water easily. To mitigate the ecological footprint, several techniques have been introduced, main of which is the placement of an air bubble curtain (ABC) around the pile to increase the compressibility, thereby reducing the velocity and increasing attenuation of acoustic waves. During the last decade, several numerical approaches for the prediction of offshore pile driving noise have been developed and applied for large-scale simulations. The majority of these methods utilize the Finite Element Method (FEM) for near field calculations and eigenfunction expansions for far field predictions. The purpose of this paper is to solve the challenging problem of pile driving, with and without ABC, by employing the Boundary Element Method (FEM). The problem is treated in frequency domain for specific frequencies between 100 Hz and 1 kHz and for noise predictions, with and without ABC, in a distance up to 1.1km from the pile. Also, a practical Iterative Effective Medium Approximation (IEMA) method is proposed for the evaluation of the frequency dependent phase velocity and attenuation that experiences a plane wave propagating in bubbly water.

Geotechnical boundary value problems involving soil-structure interaction and large deformations can be simulated well using the Coupled Eulerian-Lagrangian (CEL) method. That method has gained widespread acceptance as a numerical approach to analyze geotechnical processes with large soil deformations such as pile installations. There are two numerical meshes that overlap at least in some parts of the model, one Lagrangian and the other Eulerian. The soil is usually represented by the Eulerian part while the Lagrangian part is the structure. The material within the Eulerian region of the mesh overlap may be considered as ‘virtual’ or ‘ghost’ material, as there is only the material of the structure in reality and no additional material at the same place. It’s worth to investigate how much the existence of ‘ghost’ material in CEL simulations affects the numerical results. The investigation is done here by two different soil-structure interaction examples: (i) a laterally loaded pile in undrained soil, and (ii) the numerical simulation of single pile installation. Both examples are also modeled using the Arbitrary Lagrangian Eulerian (ALE) approach which does not require any ‘ghost’ material definition. A comparison of the modelling and results from these different methods – CEL and ALE – gives insights into pitfalls of the analyses. Further analysis challenges arise from the definition of void material on top of the free ground surface as well as the contact models in the numerical simulations, which will also be considered here.

Small strain shear modulus (G₀) or the associated shear wave velocity (V_s) is a basic soil property with clear physical meaning, and has a wide range of applications in geotechnical engineering. While it has been extensively studied in the past decades, some aspects of this property are not yet completely understood, due primarily to the particulate nature of soils. The characteristics of individual grains can significantly affect the packing pattern and the grain-to-grain interactions, and hence the overall mechanical properties. This paper presents selected results and findings yielded from a long-term research program which was aimed at understanding the roles played by grain attributes in the small strain stiffness of granular soils. Focus is here placed on grain shape, surface roughness, fines, and the fabric.