The concept of a bio-based economy has been adopted by many advanced countries around the world, and thermochemical conversion of waste biomass is recognized as the most effective approach to achieve this objective. Recent studies indicate that hydrothermal carbonization (HTC) is a promising method for the conversion of waste biomass towards novel carbonaceous materials known as hydrochars. This cost-effective and eco-friendly process operates at moderate temperatures (180–280 °C) and uses water as a reaction medium. HTC has been successfully applied to a wide range of waste materials, including lignocellulose biomass, sewage sludge, algae, and municipal solid waste, generating desirable carbonaceous products. This review provides an overview of the key HTC process parameters, as well as the physical and chemical properties of the obtained hydrochar. It also explores potential applications of produced materials and highlights the modification and functionalization techniques that can transform these materials into game-changing solutions for a sustainable future.

Disposal of waste in municipal solid waste landfills is the only way of waste disposal in Serbia, with daily compaction and covering with soil material. Due to less space for the construction of new landfills, it is necessary to use their maximum capacity. Well-compacted municipal solid waste takes up less volume and enables safer storage, so it is useful to previously determine the compaction parameters in the laboratory: maximum dry unit weight (γdmax) and optimal water content (wopt). In the practice so far, the standard method has been used to obtain these parameters (Proctor compaction test) which is common in soil mechanics. However, although this methodology has been adopted, different treatments of municipal solid waste at the landfill (including pre-treatment) indicate the need to change this classical approach. Therefore, during the research, various innovative solutions are introduced, such as the change of the classic flat Proctor hammer, by adding spikes, whose function, in addition to compaction, is partial destruction and shredding of municipal waste. In this way, the operation of compactors (hedgehogs) in the field is simulated. The paper presents the behaviour of four artificially prepared samples of municipal solid waste of different composition. The samples were tested in the standard Proctor apparatus at the same compaction energy, using different hammers, a standard flat hammer, and an innovative hammer with spikes. After that, the geotechnical effects of compaction depending on the applied approach were analyzed.

Properties of fine-grained soils, such as the liquid and plastic limits, are widely used as indicators of geotechnical behaviour. Unlike the liquid limit that can be reliably determined by mechanical testing, the determination of the plastic limit is significantly more complex, and the standard thread-rolling test is increasingly criticised for its subjective nature. Therefore, there is a need for a standardised, mechanical, more reliable and reproducible method to determine the plastic limit. This study compares several alternative methods with the standard rolling test, including tests using 240 g and 400 g cones, the cone penetration index (beta), and an empirical method based on undrained shear strength data. The accuracy of each method is evaluated using MAE, RMSE, and MAPE. The results show that the  method provides the highest reliability, while the empirical approach shows moderate accuracy.

Empirical correlations commonly used in geotechnical practice can serve as a valuable tool for estimating the required parameters during the design phase and execution of geotechnical analyses. One of the increasingly important parameters in geotechnical analysis is the undrained shear strength of soil, whose use is also recommended by Eurocode 7 standards, especially when examining the properties of fine-grained soils. Variations in water content significantly influence the consistency state of the soil and have a substantial impact on its behaviour, particularly on shear strength. This study presents correlations between undrained shear strength (cu), determined using the fall cone method, and water content. The results showed a high level of correlation between undrained shear strength and water content, confirming a logarithmic relationship. Empirical equations were applied to enable more accurate estimation of undrained shear strength, with a high coefficient of determination (R²).

Typical empirical correlations in geotechnical practice can be useful for estimating the values of necessary parameters to be used in design and geotechnical calculations. For fine-grained soils, undrained shear strength is a key parameter recommended by Eurocode 7 and is strongly influenced by water content. This paper investigates the relationship between water content, fall cone penetration, and undrained shear strength of medium-plasticity silty clays based on an extensive laboratory dataset obtained over a wide range of moisture conditions, including values both below and above natural water content. The analysis is conducted on remoulded soil samples. Power-law relationships between water content and cone penetration were established, and the corresponding parameters were evaluated and compared with values reported in the literature. The undrained shear strength-water content relationship was further analyzed using normalized expressions based on the water content ratio, enabling direct comparison with existing empirical models. A within-sample regression approach was applied to account for repeated measurements on individual soil samples, revealing consistently high coefficients of determination and a narrow range of regression slopes across materials. The results demonstrate that systematic laboratory testing combined with appropriate data normalization and within-sample analysis provides physically meaningful and robust empirical relationships. A site-specific empirical model for estimating undrained shear strength is proposed, offering improved reliability compared to generalized correlations and supporting more accurate site-specific geotechnical assessments.