Geotechnics, Volume 1, Issue 1 (September 2021) – 11 articles

On behalf of the editorial board and MDPI Publishing, may we extend a very warm welcome to this first editorial of Geotechnics—a new and international, open access, scholarly journal aimed at showcasing and nurturing high-quality research and developmental activities in soil and rock engineering and geo-environmental engineering, worldwide [...] Full article

A new evaluation method for the dilatancy of fine-grained soils based on monotonic and cyclic undrained triaxial tests has been established using two elasticity approaches: isotropic and transverse isotropic hypoelasticity. The evaluation of two clays, Kaolin and Lower Rhine Clay, with the new method also shows that the dilatancy of fine-grained soils is dependent on the stress ratio, the void ratio, and the straining direction along with the intrinsic material parameters. Similar to sand, we can observe a Phase Transformation Line beyond which further shearing induces a volume increase. A generalization of the Taylor dilatancy rule from direct shear to multiaxial space is established, and an extension accounting for the behaviour of soft soils is proposed. We formulate a simple hypoplastic constitutive relation with a modified flow rule that reproduces the observed dilatant as well as contractant behaviour. Some simulations of monotonic as well as cyclic tests prove the accurate performance of the proposed dilatancy relation. Full article

Weak and weathered rocks are well known for their sensitivity to changes in moisture content. Degrading behavior is common in weak rocks with moisture-sensitive mineral components and present numerous stability problems. The slake durability is a measure of the resistance to weakening and disintegration of rock materials which quantitatively distinguishes durable from non-durable rock materials. Several rock material parameters interact on the process of disintegration when exposed to cyclic moisture changes, whereby the content of clay is believed to play a major role. This manuscript evaluates the overall material composition of flysch and serpentinite rocks cored from the wall of the shotcrete-lined headrace tunnel of a hydropower project, including minerals, structure, porosity, the presence of micro-discontinuities, and swelling potential, and links these properties to the slake durability. Further, the different methods used to assess compositional features affecting the durability of weak rocks are evaluated and discussed. The manuscript argues that the mineralogical composition and microstructures present in the intact rock and the content of moisture-sensitive constituents, as swelling clays, control the long-term durability of weak rock material. It is demonstrated that XRD assessments are not sufficient to detect the content of brucite and swelling components, and that methods as thin section and SEM analyses should be carried out in the assessment of weak and weathered rock mass. Full article

This study presents a literature review on the use of artificial neural networks in the prediction of geo-mechanical properties of stabilised clays. In this paper, the application of ANNs in a geotechnical analysis of clay stabilised with cement, lime, geopolymers and by-product cementitious materials has been evaluated. The chemical treatment of expansive clays will involve the development of optimum binder mix proportions or the improvement of a specific soil property using additives. These procedures often generate large data requiring regression analysis in order to correlate experimental data and model the performance of the soil in the field. These analyses involve large datasets and tedious mathematical procedures to correlate the variables and develop required models using traditional regression analysis. The findings from this study show that ANNs are becoming well known in dealing with the problem of mathematical modelling involving nonlinear functions due to their robust data analysis and correlation capabilities and have been successfully applied to the stabilisation of clays with high performance. The study also shows that the supervised ANN model is well adapted to dealing with stabilisation of clays with high performance as indicated by high R² and low MAE, RMSE and MSE values. The Levenberg–Marquardt algorithm is effective in shortening the convergence time during model training. Full article

The presence of complex discontinuity patterns, the inherent statistical nature of their geometrical parameters, the uncertainties involved in the estimation of the discontinuity geometrical and geo-mechanical properties and complex three dimensional (3-D) in-situ stress make the accurate prediction of rock mass strength a difficult task. It has been a great challenge for the rock mechanics and rock engineering professions to develop a rock mass strength criterion in three dimensions that incorporates the effect of the minor and intermediate principal stresses and captures the scale dependent and anisotropic properties resulting from the discontinuity geometry parameters, such as the number of discontinuity sets, 3-D discontinuity intensity, and the distributions of the discontinuity orientation and size. Rock mechanics and rock engineering researchers have dealt with this topic for more than 55 years. The paper provides a critical review of the current state of the art regarding 3-D jointed rock mass strength criteria. The shortcomings of several rock mass strength criteria are discussed. The historic development of rock mass strength criteria that incorporate the effect of the minor and intermediate principal stresses and capture the scale dependent and anisotropic properties is presented. The most advanced 3-D rock mass strength criteria currently available in the literature are presented, including suggested future improvements. Full article

The critical state theory is a robust conceptual framework for the characterisation of soil behaviour. In the laboratory, triaxial tests are used to assess the critical state locus. In the last decades, the equipment and testing procedures for soil characterisation, within the critical state framework, have advanced to obtain accurate and reliable results. This review paper summarises and describes a series of recommended laboratory procedures to assess the critical state locus in cohesionless soils. For this purpose, results obtained in the laboratory from different cohesionless soils and triaxial equipment configurations are compiled, analysed and discussed in detail. The procedures presented in this paper reinforce the use of triaxial cells with lubricated end platens and an embedded connection piston into the top-cap, together with the verification of the full saturation condition and the measurement end-of-test water content—preferable using the soil freezing technique. The experimental evidence and comparison between equipment configurations provide relevant insights about the laboratory procedures for obtaining a reliable characterisation of the critical state locus of cohesionless geomaterials. All the procedures recommended herein can be easily implemented in academic and commercial geotechnical laboratories. Full article

Critical responses are investigated for nonlinear base-isolated buildings considering soil–structure interaction under near-fault ground motions and long-duration ground motions. A double impulse and a multi impulse are employed to simulate the nonlinear critical responses of the models under such ground motions. The base-isolation story is assumed to consist of lead rubber bearings and to have a bilinear force–deformation relation. Two types of critical timings for a MDOF building model supported by a swaying-rocking spring-dashpot system are derived: (1) the timing that maximizes the total input energy to the whole system and (2) the timing that maximizes the instantaneous input energy to the base-isolated building excluding the swaying-rocking system. These two types of critical timings are compared through numerical examples. Finally, time-history response analyses were conducted under the critical double impulse, the corresponding one-cycle sine wave, and the critical multi impulse. The effect of the soil–structure interaction on the maximum responses of the nonlinear base-isolated building is clarified. Full article

Among the most commonly specified tests in the geotechnical engineering industry, the liquid limit and plastic limit tests are principally used for (i) deducing useful design parameter values from existing correlations with these consistency limits and (ii) for classifying fine-grained soils, typically employing the Casagrande-style plasticity chart. This updated state-of-the-art review paper gives a comprehensive presentation of salient latest research and understanding of soil consistency limits determinations/measurement, elaborating concisely on the many standardized and proposed experimental testing approaches, their various fundamental aspects and possibly pitfalls, as well as some very recent alternative proposals for consistency limits determinations. Specific attention is given to fall cone testing methods advocated (but totally unsuitable) for plastic limit determination; that is, the water content at the plastic–brittle transition point, as defined using the hand rolling of threads method. A framework (utilizing strength-based fall cone-derived parameters) appropriate for correlating shear strength variation with water content over the conventional plastic range is presented. This paper then describes two new fine-grained soil classification system advancements (charts) that do not rely on the thread-rolling plastic limit test, known to have high operator variability, and concludes by discussing alternative and emerging proposals for consistency limits determinations and fine-grained soil classification. Full article

Lightweight fill can be advantageous in embankment construction for the purposes of reducing the (i) bearing pressures on the underlying soil foundation, (ii) destabilizing moments for constructed earthen slopes, and (iii) earth pressures acting behind retaining walls. This paper investigates the merits/limitations of particulate expanded polystyrene (EPS) beads mixed with clayey sand (CS) soil as lightweight fill, considering both geotechnical and environmental perspectives. The bench-scale geotechnical testing programme included standard Proctor (SP) compaction, California bearing ratio (CBR), direct shear (sheardox), oedometer and permeability testing performed on two different gradation CS soils amended with 0.5, 1.5 and 3.0 wt.% EPS, investigating two nominal bead sizes equivalent to poorly-graded medium and coarse sands. Compared to the unamended soils, the compacted dry density substantially decreased with increasing EPS beads content, from 2.09 t/m³ (0 wt.% EPS) to as low as 0.33 t/m³ for 3 wt.% (73 v.%) of larger-sized EPS beads. However, from analyses of the test results for the investigated 50 to 400 kPa applied stress range, even 0.5 wt.% (21 v.%) EPS beads caused a substantial mechanical failure, with a drastic decay of the CBR and compressibility parameters for the studied CS soils. Given the more detrimental environmental cost of leaving myriads of separate EPS beads mixed forever among the soil, it is concluded that the approach of adding particulate EPS beads to soils for producing uncemented lightened fill should not be employed in geotechnical engineering practice. Full article

Back-to-back mechanically stabilized earth (MSE) walls can sustain significant loadings and deformations due to the interaction mechanisms which occur between the backfill material and reinforcement elements. These walls are commonly used in embankments approaching bridges, ramps, and railways. The performance of a reinforced wall depends on numerous factors, including those defining the soil, the reinforcement, and the soil/reinforcement interaction behavior. The focus of this study is to investigate the behavior of back-to-back mechanically stabilized earth walls considering synthetic and metallic strips. A two-dimensional finite difference numerical modeling is considered. The role of the soil friction angle, the distance of the reinforcement elements, the walls’ width to height ratio, and the quality of the soil material are investigated in a parametric study. Their effects on the critical failure surface, shear displacements, wall displacements, and tensile forces on the reinforcements are presented. The interaction between back-to-back reinforced walls strongly depends on the distance between walls and modifies the critical failure surface location. Full article

The increase in the number of discarded tires every year is becoming a major issue all over the world. Tires stockpiles and landfills have become a critical issue as they are considered a fertile environment for the breeding of rats and insects, a real fire hazard that may take up to months to extinguish and occupy a valuable, large area of land. One of the safest effective ways of recycling tires is that to use them as backfilling material, among different usages, in civil engineering projects due to their low unit weight and specific gravity. However, to use any material in the construction industry, several material properties must be evaluated, including the shear strength and stiffness parameters. Many factors control the measured parameters. One main factor that is known to have a significant effect is the particle size. This paper focuses on evaluating the effect of the particle size on the shear strength and stiffness parameters of six tire-derived aggregate (TDA) samples having particle sizes range between (9.5–101.6 mm) using a large-scale direct shear machine. The tests were conducted under three normal stresses: 50.1, 98.8 and 196.4 kPa using a constant shearing rate of 0.5 mm/min. The results of this study showed an increasing angle of internal friction as the maximum particle size increases. Moreover, the secant shear modulus also exhibited an increase by increasing the maximum particle size. Furthermore, equations to estimate the stress-strain curves of Type A-TDA for different confidence levels were developed, and their predictions were compared with experimental results to assess their suitability. Full article