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Graphical abstract Highlights • A unit cell model for predicting the ETC of granular geomaterials was proposed. • The cylindrical unit cell contains an irregular shape solid particle, water and/or air. • This theoretical model was deduced using lumped parameter method. • Effects of component, porosity and saturation degree, etc. were evaluated. • The model has clear physical origin and good agreement with experimental data. Abstract A cylindrical unit cell model for predicting stagnant effective thermal conductivity of two-phase or three-phase low porosity granular geomaterials was proposed in this paper. In the present model, the cylindrical unit cell is made of a quasi-hemispherical/cylindrical solid particle, and the solid grain is surrounded by water or air or the mixtures of two phases. In addition to the thermal conductivity of each phase, the effects of porosity and saturation degree on the stagnant effective thermal conductivity of geomaterials were investigated by introducing the coefficients α and β that control the porosity and saturation degree, respectively. The stagnant effective thermal conductivity of cylindrical unit cell was obtained via lumped parameter method and a further spatial correction was then conducted. Comparisons with some empirical models and experimental data for three types of selected geomaterials indicate that this theoretical model successfully captures the stagnant effective thermal conductivity with variation of saturation degree and thus provides the stagnant effective thermal conductivity of granular geomaterials with acceptable accuracy in the porosity range [0, 0.33] at most degrees of saturation. [ABSTRACT FROM AUTHOR]

A fixed-parameter ETC predictive model was proposed using simple combinatory rules, based on which the tri-weighted means (arithmetic, geometric and harmonic) of tri-upper and lower ETC bounds (Series-Parallel, two forms of Maxwell-Eucken, and Co-Continuous-Effective Medium Theory) can be unified. Theoretical analyses firstly explain the relative success of the weighted arithmetic combination between series and parallel models in the literature so far. Experimental validation was then conducted and the results indicate that the presented combinatory models can be well fitted to most ETC data of internal porosity geomaterials by adjusted values of weighting parameter f. However, this type of combinatorial model would give more uncertainty when applied to external porosity geomaterials such as granular sands. Finally, some meaningful analyses, conclusions and research perspectives were obtained, drawn and opened for future investigations in this field. [ABSTRACT FROM AUTHOR]

[Display omitted] • The extended unit divided method can capture heat and moisture sources in tunnel. • Airflow enthalpy augment per one meter tunnel represents the heat hazard degree. • Phenomenon of return heat easily occurs in underground long development tunnel. • Production activities have an impact on the thermal inertia of airflow parameters. The unit divided method, which initially enables zones of methane emission, distribution and concentration in long-wall coal mining face to be manually identified, was analogously utilized in this study to capture distribution characteristics and variations of the airflow thermodynamic parameters in an underground long distance development tunnel. Field observations and thereby analyses preliminarily provide a definition of the airflow 'return or turn back heat' phenomenon that commonly occurred in deep underground long distance cul-de-sac development tunnels. Furthermore, the cooling performance in heading face was illustrated and some regression relations among cooling parameters associated with the heading face were developed simultaneously to maintain the climatic condition within the design limit. Finally, the influence of production activities on the thermal inertia of airflow in horizontal development tunnel was analyzed, which is similar to that near the mine vertical shaft. These findings may provide beneficial guidance for existing practical applications of deep mine heat hazard control. [ABSTRACT FROM AUTHOR]

This letter is a comment to the published paper entitled, "Soil thermal conductivity estimated using a semi-analytical approach" (by Rubin and Ho, Geothermics, 2021, 92: 102051). In the reviewed article, an enhanced semi-analytical model was proposed to evaluate the effective thermal conductivity of three-phase (solid-liquid-gas) soils by changing the assumption of 1D monodirectional to 2D quasi-uniaxial parallel heat fluxes in pore-scale soil heat conduction process. The significant innovation, some shortcomings and further research directions were discussed in this comment. In addition, some statements, conclusions and misprints occurred in the published version were also mentioned, indicating that more careful proof, necessary corrigendum and some supplementary materials are required to avoid confused presentation and improve the reliability. [ABSTRACT FROM AUTHOR]

[Display omitted] • Mine-oriented geothermal system within the Earth Critical Zone was proposed. • 44 examples of mine-oriented geothermal projects were presented and classified. • Spatio-temporal feasibility study on mine-oriented geothermal system is requisite. • Reservoir recharge and structural feature play crucial role in its sustainability. Based on 44 documented demonstration projects worldwide, a novel conceptual aspect of mine-oriented low-enthalpy geothermal exploitation classification was initially proposed. For this new classification, special attention was paid to a spatio-temporal perspective. From the first temporal perspective, i.e. life-cycle of a mine, the projects implemented in mine legacy phase are dominated (55%), followed by production stage (36%). From the second space point of view, the low-enthalpy geothermal heat, i.e. ~<2000 m depth within the Earth's Critical Zone (corresponding to the earth temperature < 85 °C), can be recoverd using different geothermal media such as solid-mine rock stratum (7%), liquid-mine water (79%) and gas-mine ventilation exhaust (14%) in open pits and/or underground mines. Next, several engineering issues were discussed, including configurations, parameters, hydro-thermo-geological conditions etc., to analyze their effects on the performance and sustainability of dominated mine water-based geothermal systems. Another key outcome of this review is to apply three methods: theoretical analysis, numerical simulation and experimental test, on mine water-based scenario to acquire the current status of the feasibility study. Moreover, an analytical conceptual model, mainly focusing on the structural characteristics of long-wall coal mining gobs, shafts and galleries, was developed to preliminarily evaluate the static and dynamic geothermal potential of flooded Jiahe colliery with the traditional volume method. It is hoped that the findings, viewpoints and suggestions obtained from this review can be served as references for real geothermal installations associated with mine sites in the future. [ABSTRACT FROM AUTHOR]

This comment concerns a rightful doubt involved in the above paper. In the reviewed article, a novel model without empirical constants was proposed to evaluate the effective thermal conductivity of three-phase soils, and such a model was claimed to be workable for soils with porosity ranging from 0.0349 to 0.4764 at all saturation degrees. However, sufficient reasons and pictorial explanations were given in this comment to prove its inapplicability for high degrees of saturation. Besides, the inapplicable range is constantly changing, depending on the porosity, i.e. the larger the porosity, the smaller the inapplicable saturation region. Similar issues were further discussed due to its great influence on the follow-up research in this field, i.e. whether can achieve a full coverage of the applicable saturation range [0,1] for a given porosity. [ABSTRACT FROM AUTHOR]

A good knowledge of the effective thermal conductivity (ETC) of geomaterials is required to achieve accurate study on heat transfer in a variety of earth-contact engineering applications. On account of composite-porous characteristic of geomaterials, the thermal conductivity of each component is the foundation while the porous structure is the key influence factor. Accordingly, a fractal capillary bundle model at pore scale is proposed in this paper to predict the ETC of saturated/unsaturated composite-porous geomaterials. The combination of fractal theory and capillary bundle model hypothesis is used to describe the random, disordered and extremely complicated microstructure of geomaterials. The analytical solution of the ETC is obtained by using the thermal-electrical analogy technique, which demonstrates that the proposed model is a function of variety of composition and microstructure parameters. Both the effects of these two types of parameters on the ETC of geomaterials are analyzed theoretically. Compared with other models, the present model represents specific physical meaning and shows acceptable accuracy and performance. The model predictions are further compared with the available experimental data for three types of concrete, and a good agreement between them can be found in most of saturation degrees (0.2 < S r < 0.8). [ABSTRACT FROM AUTHOR]

Cretaceous sandstones have weak cementation and high porosity while exhibit a high apparent brittleness. Compression and torsion (C-T) fractures are widely distributed in Cretaceous sandstones due to asymmetric tectonic convergence action. However, studies on C-T fracture formation and the mechanisms causing variability in Cretaceous sandstones containing no oil or gas are rare due to the challenges in sampling intact sandstone cores, despite their significance to mine shaft sinking. Therefore, this study used binder jetting-based 3D printing to prepare artificial Cretaceous sandstone and developed a real-time X-ray computed tomography- (CT-) aided torsion shear apparatus to test them. The test results showed that the 3D printed (3DP) sandstone had characteristic indexes that approached and even exceeded the lower limits of Cretaceous sandstone cores, thereby accurately representing the unavailable cores. Furthermore, the 3DP sandstones had anisotropic properties comparable to the sandstone cores. Under C-T action, the 3DP sandstone exhibited a pronounced strain gradient of 2.0 %/mm perpendicular to fracture inclination. The inclination angles of fractures formed under C-T action tended to increase as the cell pressure increased, and that approached the orientation angles of maximal principal stress. The maximal and minimal principal stresses exerted inclination-slip and width-stretching effects, respectively, on C-T fractures. But the effect of inclination-slip on the C-T fractures was stronger than that of width-stretching. This insight into C-T fracture formation will guide future studies on the fracture evolution and its disaster-dominating mechanisms arisen from disturbances by shaft sinking. [ABSTRACT FROM AUTHOR]

Fractures in soils subjected to freeze–thaw around underground facilities initiated from a connection of pores occupied by isolated ice and expansion of spaces occupied by layered ice in freezing, reserved in thawing, and varied due to a pore pressure. A digital image correlation (DIC) and X-ray CT–aided experiment was thus conducted to investigate the quantitative relationship between such behaviors and pore pressure in saturated soil specimen subjected to freezing to a stable thermal gradient and then thawing under uniform temperatures. The DIC–based data showed that an increase in heterogeneity level k H weakened the peak strength and enhanced the apparent brittleness of specimen. The maximum lateral strain and its occurrence height increased as k H increased with the height of lateral strain location zone reducing. The CT–based lateral strain approached the DIC–based data with an averaged deviation ≤ 0.4 % . It was found that pore localization zone existed within the lateral strain localization zone with the height of maximum pore ratio being approximately identical with that of the maximum lateral strain. The pre-existing fracture expanded and the isolated pores were connected gradually due to the strength attenuation near pore edge and the pore pressure; they together formed a fracture network. However, the influence of pore pressure to initiation of fracture was weakened as k H further increased. These observations were distinguished from the deformation and fracture behaviors occurring in frozen soil specimen and presented significances to better understand structure variation within soil layers subjected to artificial freezing. [ABSTRACT FROM AUTHOR]