Geotechnics, Volume 2, Issue 1 (March 2022) – 12 articles

The textural characteristics of rocks influence their petrophysical and mechanical properties. Such parameters largely control rock mass stability. The ability to evaluate both immediate and long-term rock behaviors based on the interaction between various parameters of rock texture, petrophysical and mechanical properties is therefore crucial to many geoengineering facilities. However, due to the common lack of high-quality core samples for geomechanics and rock texture laboratory tests, single and multivariable regression analyses are conducted between mechanical properties and textural characteristics based on experimental test data. This study presents a review of how rock texture characteristics influence the geomechanical properties of a rock, and summarizes the regression equations between two aspects. More specifically, a review of the available literature on the effects of mineralogy, grain size, grain shape, packing density, foliation index, porosity, degree of weathering, and other rock physical characteristics on geomechanics is presented. Similarly, a review of the literature discussing the failure criteria of anisotropic rocks, both continuous and discontinuous, is also presented. These reviews are accompanied by a comparison of the fundamentals of these methods, describing their equations and discussing their advantages and disadvantages. This exercise has the objective of providing better guidelines on how to use these criteria, allowing for safer underground excavations via an improved understanding of how rock texture parameters affects the mechanical behavior of rocks. Full article

In this paper, we report on the results of an experimental study of the yielding and shear strength behavior of a normally consolidated silty clay soil from eastern Germany. The shear strength tests were performed on remolded forms of the soil using a triaxial cell which can regulate the temperature of the specimen. The experimental program comprised a series of pressure stepping compression consolidated drained (CD) triaxial tests at temperatures of 20, 40 and 60 °C. Overall, although the volume change behavior of the soil was found to exhibit some variations with changes in temperature, possibly due to changes in the double layer thickness of the clay fraction of the soil upon heating, the shear strength behavior (in terms of effective angle of internal friction) of the silty clay soil was found to exhibit minor changes with variations in temperature and can be assumed to be independent of temperature for the range considered in this study. Full article

Underground structures are being constructed at an increasing rate in seismic prone areas, to facilitate the expanding needs of societies. Considering the vital role of this infrastructure in densely populated urban areas and interurban transportation networks, as well as the significant losses associated with potential seismically induced damage, its assessment against seismic hazard is of great importance for stakeholders, operators, and governmental bodies. This paper presents a state-of-the-art review of current developments in the assessment of seismic vulnerability of tunnels and underground structures. Methods for the development of fragility functions for the assessment of bored tunnels in rock or alluvial, and cut and cover tunnels and subways in alluvial, against ground seismic shaking and earthquake-induced ground failures are presented. Emphasis is placed on the estimation of the capacity of the examined structures, the selection of appropriate intensity measures to express seismic intensity, the development of rational probabilistic seismic demand models and the estimation of epistemic and aleatoric uncertainties, related to the seismic fragility of underground structures. Through the discussion, acknowledged gaps in the relevant literature are highlighted. Full article

A new analytical solution for the thermal-hydraulic coupling process is derived with a 1-D steady state conductive heat flow in the body of hot rock with perpendicular water flow in the single fracture and transient heat transfer from rock to water. The heat produced from the hot rock via water flow in the idealized single fracture is demonstrated by arithmetic equations. The applicability of the analytical solution is verified by numerical calculations and is limited to conditions with fast water flow rates or high water flux and long fluid pathways. The lifetime of an EGS reservoir in these reference conditions is 23.2 years and is confined by the produced water temperature of 150 °C for commercial utilization. The heat recovery factor is 12.4%. With a power plant capacity of 5 Mw installed, the total area for extracting recoverable heat within the projected lifetime of a fracture surface of 1.58 × 10⁶ m² was determined. The total mass flow rate of water injected into the large fracture was 57 kg/s. The discussion shows the ability of the model to estimate heat production and reservoir scale. Full article

The load-transfer factor ($\zeta$) in the concentric cylinder approach is often used in analytical formulation in axially loaded piles. The factor is a constant value (in a given pile slenderness ratio and soil condition) devised under the elastic and ‘pre-failure’ perfect pile–soil bonding conditions (a non-slip analytical model). Given most numerical methods have already considered the pile–soil slipping in the ‘pre-failure’ stage, the limitations of non-slip analytical models have recently been discussed, and slipping analytical models have been recommended. Therefore, this research aims to investigate the load-transfer factor in slipping analytical models. This paper reviews that the factor in slipping analytical models is only constant in linear elastic and some Gibson soil conditions. Beyond these conditions, the factor varies as the pile-head load increases in some cohesionless soils. Adopting the existing constant factor in slipping analytical models will deviate the load–displacement results, as supported by numerical results. Therefore, a new equation is proposed to the load-transfer factor, and a new analytical method is proposed by varying the load-transfer factor during loading for improvement. Results presented in this paper demonstrate improved load–displacement results. Full article

Strong interactions between mechanical deformation and chemical reactions may play a critical role in the response of geomaterials or geological systems to evolving environmental circumstances that may occur in both natural and engineered processes. The present study focuses on mineral dissolution and precipitation at the intergranular contact whose consequences are often manifested at the macro-scale where the mechanical and transport properties of the geomaterial may be altered. Discrete element modeling is employed to explore two applications involving such mineral transformations. The first example is primarily focused on the chemo-mechanical coupling mechanisms of intergranular contact in the natural process of pressure solution and secondary compression. The effect of the mineral dissolution on the mechanical response at the grain contact is incorporated into the contact model. Discrete element simulations are performed to examine the overall mechanical response of particle assembles subject to mineral dissolution and the results demonstrate the important role of the kinetic rate characteristics of the dissolution process. The second part of the present study revolves around the effect of mineral precipitation in an engineered process known as microbially induced calcite precipitation for potential soil improvement. The kinetics of involved bio-chemical process is incorporated into on the contact model and the simulation results indicate considerable strengthening effect. Overall, the present study demonstrates the feasibility of discrete element approach as a numerical tool to model coupled chemo-mechanical phenomena across the scales. Full article

Evaluation of the sliding stability of concrete dams requires the use of numerical tools not only able to simulate the coupled hydromechanical behavior but also able to adequately represent the foundation discontinuities and the specific features of dam foundations. The formulation of a three-dimensional (3D) small displacement finite element model based on interface elements to simulate the discontinuities is presented. In this model, the hydraulic behavior is simulated assuming that the water flow occurs only along channels located at the edges of the triangular interface elements that simulate the discontinuities. The model is used to perform coupled hydromechanical analysis of a large arch-gravity dam and to assess safety against dam base sliding, assuming different constitutive models at the dam/foundation interface and two different approaches: (i) strength reduction method and (ii) amplification of the hydrostatic pressure, assuming an increase in the reservoir level. The present study shows that consistent results are obtained with the proposed numerical model and that stability analysis should preferably be carried out using the method of increasing the hydrostatic pressure and the corresponding uplift pressures, as this methodology leads to significantly lower safety factors. Full article

The use of optimization methods in geotechnics dates back to the 1950s. They were used in slope stability analysis (Bishop) and evolved to a wide range of applications in ground engineering. We present here a non-exhaustive review of recent publications that relate to the use of different optimization techniques in geotechnical engineering. Metaheuristic methods are present in almost all the problems in geotechnics that deal with optimization. In a number of cases, they are used as single techniques, in others in combination with other approaches, and in a number of situations as hybrids. Different results are discussed showing the advantages and issues of the techniques used. Computational time is one of the issues, as well as the assumptions those methods are based on. The article can be read as an update regarding the recent tendencies in the use of optimization techniques in geotechnics. Full article

The present paper provides a qualitative discussion of the evolution of contact traction fields beneath rigid shallow foundations resting on granular materials. A phenomenological similarity is recognized in the measured contact traction fields of rigid footings and at the bases of sandpiles. This observation leads to the hypothesis that the stress distributions are brought about by the same physical phenomena, namely the development of arching effects through force chains and mobilized intergranular friction. A set of semi-empirical equations are suggested for the normal and tangential components of this contact traction based on past experimental measurements and phenomenological assumptions of frictional behaviors at the foundation system scale. These equations are then applied to the prescribed boundary conditions for the analysis of the settlement, resistance, and stress fields in supporting granular materials beneath the footing. A parametric sensitivity study is performed on the proposed modelling method, highlighting solutions to the boundary-value problems in an isotropic, homogeneous elastic half-space. Full article

This work reviews the transient two-phase flow in porous media with engineering applications in Geotechnics. It initially overviews constitutive relationships, conventional theories, and experiments. Then, corresponding limitations are discussed according to conflicting observations and multiphase interfacial dynamics. Based on those findings, the dynamic nonequilibrium effects were so defined, which could also be abbreviated as dynamic/transient effects. Four advanced theories have already been developed to resolve these effects. This review collects them and discusses their pros and cons. In addition, this work further reviews the state-of-art in terms of experimental methods, influential factors in dynamic/transient effects, and modelling performance, as well as micromodel and numerical methods at pore-scale. Last, the corresponding geotechnical applications are reviewed, discussing their applicability in effective stress, shear strength, and deformation. Finally, the entire review is briefed to identify research gaps in Geotechnics. Full article

Offshore wind has become a major contributor to reducing global carbon emissions. This paper presents a probabilistic seismic hazard analysis for the Sofia Offshore Wind Farm, which is located about 200 km north-east of England in the southern North Sea and will be one of the largest offshore wind farms in the world once completed. The seismic source characterization is composed of two areal seismic source models and four seismic source models derived using smoothed gridded seismicity with earthquake catalogue data processed by different techniques. The ground motion characterization contains eight ground motion models selected based on comparisons with regional data. The main findings are (1) the variation in seismic hazard across the site is negligible; (2) the main source controlling the hazard is the source that includes the 1931 Dogger Bank earthquake; (3) earthquake scenarios controlling the hazard are M_w = 5.0–6.3 and R = 110–210 km; and (4) the peak ground accelerations on rock are lower than for previous regional studies. These results could help guide future seismic hazard assessments in the North Sea. Full article

Earthquake-induced liquefaction is one of the major causes of building damage as it decreases the strength and stiffness of soil. The liquefaction resistance of soils increases significantly as the degree of saturation decreases, making soil desaturation an effective measure for the mitigation of this phenomenon. This paper presents a comparative analysis of liquefaction resistance of an alluvial sand from Aveiro (Portugal) under fully and partially saturated conditions. For this purpose, an in situ characterisation based on CPTu and a laboratory series of cyclic triaxial tests were carried out. The cyclic triaxial tests were conducted under undrained conditions on remoulded specimens with different degrees of saturation, including the full saturation. On the other hand, the triaxial apparatus was instrumented with Hall-effect transducers to accurately measure the strains during all testing phases. In addition, it was equipped with piezoelectric transducers to measure seismic waves velocities, namely P-wave velocity, for evaluation of the saturation level of the specimen in parallel with the Skempton’s B parameter. Hence, relations between the B-value, and P-wave velocity and cyclic strength resistance are presented. The number of cycles to trigger liquefaction, considering the pore pressure build-up criterion, is presented for the different degrees of saturation. Results confirmed the increase in liquefaction resistance for lower degrees of saturation in this soil. Full article