Despite significant advances in laboratory testing in recent decades, geotechnical designs that incorporate data from in-situ testing remain predominant worldwide. One of the most commonly employed techniques for correlating soil mechanical properties is the standard penetration test. However, while this test provides valuable information for identifying soil strata and offering general descriptions of soil characteristics, its correlation with shear strength parameters has several limitations that are often overlooked. In this article, we aim to i) present a critical literature review concerning the applicability of correlations between the undrained shear strength of fine-grained soils and standard penetration test data; ii) estimate the uncertainties associated with the adoption of these empirical correlations, which are frequently disregarded in engineering practice; iii) present simulation results from typical slope stability analyses, taking into account the uncertainties associated with the estimation of the undrained shear strength. The findings of our study suggest that geotechnical engineers should exercise caution when using such simplified equations, as they often lead to underestimations or overestimations of the stability of geotechnical structures.

The properties of soils are highly complex, and therefore, the classification system should be based on multiple perspectives of soil properties to ensure effective classification in geotechnical engineering. The current study of research demonstrates a lack of correlation between classification systems based on soil plasticity and those based on in-situ mechanical properties of soils. A CPTu-based plasticity classification system is proposed using the soil behaviour type index (Ic), with its reliability and limitations discussed. The results indicate that (1) Ic has the capacity to predict the stratigraphic distribution from the in-situ mechanical properties of soils. It showed a significant linear correlation with wL, which soil classification zone was similar to that of clay factor (CF); (2) A CPTu-plasticity classification system is proposed to characterize both plasticity and in-situ mechanical properties of soils. This system allows for the initial classification of soils solely based on CPTu data. Furthermore, it has been established that Ic = 2.95 can delineate the boundary between high- and low-compressibility soils. (3) The error is only 25.2% relative to the Moreno-Maroto classification chart, and the system tends to classify soils of intermediate nature as clay or silt. The distance between the data points and both the C-line and the new C-line (Delta Ip, Delta IpIc) showed a significant positive correlation. Only one data point was misclassified, considering human error in measuring Ip. (4) The new classification chart has been found to be more applicable to offshore and marine soils.

Fine-grained soils containing diatom microfossils exhibit unique geotechnical behavior due to their biological origins, but their strength properties controlled jointly by diatom content (DC) and stress history remain to be revealed. In this study, reconstituted diatomaceous soil was prepared by mixing pure diatom and kaolin powders in different proportions. These mixtures were subjected to undrained consolidated triaxial shear tests performed using the Stress History and Normalized Soil Engineering Properties (SHANSEP) procedures, revealing how the DC and stress history affect the soil strength. Adding diatoms improved the mixture strength, and a critical DC of approximately 20% was determined, beyond which the normalized undrained strength of the soil was considerably higher than that of common clay without diatoms. Also, a DC higher than 20% associates with the dilatancy of the studied soil with high OCR. Improving the strength of diatomaceous soil by adding diatoms differs essentially from the case of common clay because the plasticity index of the former remains almost unchanged. New formulas incorporating DC and OCR are proposed for predicting the strength of diatomaceous soil, and data for several well-studied soils confirm their validity. This study improves the understanding of fine-grained soils with biological origins and provides important data regarding the mechanical behavior of diatomaceous soil.

To improve the reinforcement effect of MICP technology on fine-grained soil, and consider the fine particle size and activity characteristics of red mud, the experiment of red mud strengthening MICP solidified fine-grained soil was designed and carried out. Combined with mechanical test and microstructural analysis, the enhancing mechanism of red mud on microbial solidified fine-grained soil was comprehensively evaluated. The results show that: (1) Red mud can significantly improve the production of cement during microbial reinforcement of fine-grained soils; the optimal dosage of red mud is 20 %, which increases the strength by 34.6 % and the production of cement by 42.9 %, compared with conventional MICP. (2) After red mud was incorporated into the soil, the pore volume and pore diameter of the treated soil were significantly reduced, and the overall compactness was further improved. (3) The enhancement mechanism of microbial consolidation of fine-grained soils by red mud is mainly due to the presence of chemically active b-C2S and calcium oxide in red mud. These active calcium-based components undergo hydration and carbonation reactions under the action of microbial mineralization, generating calcium carbonate and hydrated calcium silicate, which improves the cement yield and enhances the intergranular bond strength, compactness and overall reinforcement effect of the treated soil. (c) 2025 Production and hosting by Elsevier B.V. on behalf of The Japanese Geotechnical Society. This is an open access article under the CC BY- NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

This paper presents a simple thermo-elasto-plastic constitutive model for saturated fine-grained soils, addressing thermal volume change, excess pore pressure, and shear strength. The model incorporates a novel temperature-dependent plastic modulus formulation that attributes the thermoplastic strain to an internal state variable representing the thermal stabilization of soils due to cyclic thermal loading. It can capture the accumulative volume expansion of highly overconsolidated (OC) soils, and the accumulative contraction of normally consolidated (NC) and slight OC soils after several heating-cooling cycles. A thermally induced pore pressure formula is derived with consideration of thermo-elastic expansion of pore water and soil particles, thermo-plasticity of soil skeleton as well as the elastic unloading due to the decrease of effective stress under undrained heating. The effect of temperature on the shear strength was emphasized. An insight into the evolution of shear strength with temperature is provided. The consolidated stress history and stress path play a vital role in the thermal effect on the shear strength. The proposed model comprises nine parameters, which can be easily calibrated by element tests (triaxial tests and oedometer tests). The adequacy of the proposed model has been verified with experimental results from fine-grained soils documented in the literature.

Diatomaceous soils, composed of diatom microfossils with biological origins, have geotechnical properties that are fundamentally different from those of conventional non-diatomaceous fine-grained soils. Despite their high fines content, diatomaceous soils typically exhibit remarkably high shear resistance, approaching that of sandy soils. However, the exact role that diatoms play in controlling the mechanical properties of fine-grained soils and the underlying mechanisms remain unclear. In light of this, the shear strength response of diatomaceous soils was systematically investigated using consolidated undrained triaxial compression tests on diatom-kaolin mixtures (DKMs) with various diatom contents and overconsolidation ratios. The micro- and nano-scale structures of the soil samples were characterized in detail using scanning electron microscope (SEM) and atomic force microscope (AFM) to interpret the abnormal shear strength parameters of diatomaceous soils. The results indicated that the presence of diatoms could contribute to significantly higher strength, e.g. the friction angle of DKMs was improved by 72.7% to 37 degrees and the value of undrained shear strength tripled with diatom content increasing from 20% to 100%. Such significant improvement in soil strength with diatom inclusion could be attribute to the hard siliceous skeleton of diatoms and the interlocking between particles with rough surfaces, which were quantitatively analyzed by the surface roughness parameters with AFM. Furthermore, a conceptual model established based on the macro-mechanical tests and microscopic observations portrays a microstructural evolution of soils with increasing diatoms. The microstructure of soils was gradually transformed from the matrix-type to the skeletal one, resulting in a continual augmentation in shear strength through mutual interactions between diatom microfossils. This paper provides new insights into the multi-scale structural properties of diatoms and significantly advances our understanding of the mechanical behavior of diatomaceous soils. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

Numerous geotechnical applications are significantly influenced by changes of moisture conditions, such as energy geostructures, nuclear waste disposal storage, embankments, landslides, and pavements. Additionally, the escalating impacts of climate change have started to amplify the influence of severe seasonal variations on the performance of foundations. These scenarios induce thermo-hydro-mechanical loads in the soil that can also vary in a cyclic manner. Robust constitutive numerical models are essential to analyze such behaviors. This article proposes an extended hypoplastic constitutive model capable of predicting the behavior of partially saturated fine-grained soils under monotonic and cyclic loading. The proposed model was developed through a hierarchical procedure that integrates existing features for accounting large strain behavior, asymptotic states, and small strain stiffness effects, and considers the dependency of strain accumulation rate on the number of cycles. To achieve this, the earlier formulation by Wong and Ma & scaron;& iacute;n (CG 61:355-369, 2014) was enhanced with the Improvement of the intergranular strain (ISI) concept proposed by Duque et al. (AG 15:3593-3604, 2020), extended with a new modification to predict the increase in soil stiffness with suction under cyclic loading. Furthermore, the water retention curve was modified with a new formulation proposed by Svoboda et al. (AG 18:3193-3211, 2023), which reproduces the nonlinear dependency of the degree of saturation on suction. The model's capabilities were examined using experimental results on a completely decomposed tuff subjected to monotonic and cyclic loading under different saturation ranges. The comparison between experimental measurements and numerical predictions suggests that the model reasonably captures the monotonic and cyclic behavior of fine-grained soil under partially saturated conditions. Some limitations of the extended model are as well remarked.

In slopes where high pore water pressure exists, deep counterfort drains (also called drainage trenches or trench drains) represent one of the most effective methods for improving stability or mitigating landslide risks. In the cases of deep or very deep slip surfaces, this method represents the only possible intervention. Trench drains can be realized by using panels or secant piles filled with coarse granular material or permeable concrete. If the trenches are adequately socket into the stable ground (for example sufficiently below the sliding surface of a landslide or below the critical slip surface of marginally stable slopes) and the filling material has sufficient shear strength and stiffness, like porous concrete, there is a further increase in shear strength due to the shear keys effect. The increase in shear strength is due both to the intrinsic resistance of the concrete on the sliding surface and the resistance at the concrete-soil interface (on the lateral surface of the trench). The latter can be very significant in relation to the thickness of the sliding mass, the socket depth, and the spacing between the trenches. The increase in shear strength linked to the shear keys effect depends on the state of the porous concrete-soil interface. For silty-clayey base soils, it is very significant and is of the same order of magnitude as the increase in shear resistance linked to the permanent reduction on the slip surface in pore water pressure (draining effect). This paper presents the results of an experimental investigation on the shear strength at the porous interface of concrete and fine-grained soils and demonstrates the high significance and effectiveness of the shear keys effect.

Due to the increasing need to find new alternative energy sources, more attention has been given to the development of energy geostructures, which not only serve as foundations, but also employ the geothermal properties of soils for heating and cooling structures, inducing mechanical and thermal loads. Additionally, the up -growing effects of climate change are influencing the performance of foundations due to the increase in temperature and seasonal variations. The previously mentioned examples correspond to scenarios where soils are subjected to thermo-hydro-mechanical loading, which can vary cyclically. To predict this behavior, in this article a coupled thermo-hydro-mechanical hypoplastic model for partially saturated fine-grained soils that accounts for both monotonic and cyclic loading is presented. The proposed constitutive model is capable of reproducing temperature and suction effects at large strains and asymptotic states. Additionally, coupled effects are predicted by incorporating a Water Retention Curve (WRC) that depends on temperature and void ratio. Small strain stiffness effects are captured based on the Improvement of the Intergranular Strain concept (ISI), modified to include the influence of temperature under cyclic loading, as well as a temperature dependent secant shear modulus formulation at very small strains. The capabilities of the constitutive model were evaluated through element tests simulations of monotonic and cyclic mechanical loading tests under temperature- and suction- controlled conditions, as well as heating/cooling experiments at constant stress. The proposed constitutive model shows accurate predictions when compare to experimental data. Nevertheless, some limitations have been encountered and further discussed.

Suction stress, the part of effective stress induced by soil-water interaction, is the source for the intrinsic cohesion of fine-grained soil slurries. Here, the previous unified effective stress equation is generalized to extend the suction stress variation from the liquid state to the oven-dry state, yielding an augmented closed-form equation. This equation includes a new term, named slurry adsorptive suction stress, to incorporate the adsorptive mechanism of soil slurries at the liquid state. This adsorption mechanism involves the interparticle van der Waals attraction, face-to-edge attraction, and electrical double-layer repulsion when soils are in the liquid state. The proposed equation is validated with a wide array of 12 fine-grained soils' shrinkage curves, modulus, and suction stress data measured by the drying cake test. It is demonstrated that the proposed equation can excellently capture the experimental data across all saturations. Furthermore, the practical implications of the proposed model are illustrated via its relevance to rheological properties of soil slurries and correlations with both liquid limit and plastic limit. (c) 2024 American Society of Civil Engineers.