Grain breakage under high stress conditions can significantly affect the hydro-mechanical behavior of granular materials by altering grain size distribution, pore structure, porosity, and permeability. This study investigates these coupled effects in two sands with contrasting mineralogical characteristics, namely Hostun silica sand and carbonate sand, subjected to high oedometric stresses up to 105 MPa. High-pressure oedometer tests were performed on dense specimens prepared at a relative density of approximately 0.9. Grain size distribution was determined by post-loading sieve analysis, while permeability was measured at different stress levels using a constant-head system connected to the oedometer cell. Pore-access size distribution was characterized using two complementary techniques depending on material response and stress level: the tensiometric method, based on drying-path water retention measurements and the Young–Laplace law, and mercury intrusion porosimetry for carbonate sand specimens that developed sufficient cohesion after high-stress loading. The results show that increasing stress induces progressive grain fragmentation, leading to an evolution of the grain size distribution toward an ultimate grading state, together with reductions in pore size and porosity. The crushing threshold was identified at approximately 12.5 MPa for Hostun sand and 3 MPa for carbonate sand, indicating the strong influence of mineralogical composition on breakage resistance. Permeability decreased markedly with stress, with a reduction of about 50 % for Hostun sand and more than 70 % for carbonate sand. This stronger reduction in carbonate sand is associated with more pronounced particle breakage, pore structure modification, fines production, and reduced pore connectivity. The experimental results were interpreted using the Continuum Breakage Mechanics framework, and a power-law relationship between permeability and porosity was calibrated for both sands. The analysis indicates that grain rearrangement and grain breakage jointly control porosity and permeability reduction, with the influence of grain breakage becoming increasingly significant at higher stress levels, particularly for carbonate sand.These findings provide an experimental and modeling basis for understanding the coupled effects of high stress, grain breakage, pore structure evolution, and permeability reduction in sands, with implications for geotechnical, geomechanical, and petroleum reservoir engineering applications..
In engineering projects and rock mechanics studies, it is important to determine the physical strength properties of rocks. In these studies, the anisotropy property, which represents the expression of the change in the properties of the rocks according to the directions, is one of the main parameters that directly affects the preparation of project planning. A non-destructive, fast, and economical determination of this parameter in situ can be very useful. In this study, the anisotropic features of different types of rocks (sedimentary, magmatic and metamorphic) were researched using the non-destructive tests (NDT), commonly used in recent years, of surface hardness (Schmidt hammer rebound values and Leeb hardness) and P-wave velocity (Vp). According to the findings obtained, limitations of the surface hardness test devices (small device probe area) were determined to cause deviations in the measurement values. As P-wave measurements were less affected by the surface features of the samples and provided more reliable information about the structural-textural features of the samples, they have better useability for determining the anisotropic features of rocks.
GNSS vertical time series exhibit nonlinear and non-stationary characteristics, making it difficult for traditional time series forecasting methods to achieve high-precision predictions. To address this challenge, this study proposes ahybrid prediction model-CEEMDAN-VMD-LSTM-which integrates Complete Ensemble Empirical Mode Decomposition with Adaptive Noise (CEEMDAN), Variational Mode Decomposition (VMD), and Long Short-Term Memory (LSTM) networks. The model first applies the CEEMDAN algorithm to decompose the original GNSS time series into a set of Intrinsic Mode Functions (IMFs), and computes the Permutation Entropy (PE) and Sample Entropy (SE) of each IMF. A Composite Entropy (CE) is constructed by taking the equal-weighted average of PE and SE. Based on the composite entropy values, the K-Nearest Neighbors (KNN) algorithm is used to classify the IMFs into high-frequency and low-frequency components. These components are then linearly aggregated into high-frequency and low-frequency sequences, respectively. The high-frequency sequence undergoes a second decomposition using the VMD algorithm, resulting in k IMFs and a residual sequence. The low-frequency sequence (from the first decomposition), the IMFs, and the residual sequence (from the second decomposition) are separately modeled and predicted using the LSTM network. The final forecast is reconstructed by aggregating the predictions of the subcomponents. Experimental results on GNSS vertical time series from ten stations show that the proposed CEEMDAN-VMD-LSTM model achieves Root Mean Square Error (RMSE) values ranging from 2.20 mm to 3.69 mm, Mean Absolute Error (MAE) values between 1.75 mm and 2.95 mm, and coefficients of determination (R2) ranging from 0.80 to 0.89. Compared with baseline models including LSTM, VMD-LSTM, CEEMDAN-VMD-RNN, and CEEMDAN-VMD-GRU, the proposed model demonstrates varying degrees of improvement, validating its effectiveness for GNSS vertical time series prediction.
In view of the severe thermal hazards induced by high temperatures in deep underground engineering, a thermo-mechanical coupling model based on PFC3D is adopted to investigate the microstructural evolution and mechanical characteristics of sandstone subjected to high-temperature thermal damage. The results show that: (1) The expansion of thermally induced cracks has a threshold effect and heterogeneous characteristic. The cracking degree increased sharply in the temperature range of 800 degrees C-1000 degrees C. Thermally induced cracks presented a central concentration phenomenon and gradually weakened from the core region to both ends of the rock specimen; (2) The incubation stage of thermal damage was mainly dominated by local shear slip. As the temperature increased, thermal deformation triggered the widespread initiation of tensile fractures, leading to a transition of the failure mode to tensile-shear joint yield; (3) The crack network has undergone an evolutionary process from locally preferred orientation to overall randomness distribution. The comprehensive uniformity index indicates that thermal stress will drive the failure behavior of rock, prompting a transition from anisotropy to uniform disintegration; (4) Based on the three-dimensional contact microstructure diagram, it was found that heating below 400 degrees C would promote the polarization of the force chain. Local strengthening would lead to a slight increase in uniaxial compressive strength, and the thermal damage variable value was relatively low. However, after exceeding 600 degrees C, the main skeleton collapsed, causing the thermal damage variable approaching 0.8.
The selection and design of foundation and superstructure systems depend heavily on determining the soil parameters and bearing capacity. Currently, bearing capacity is commonly estimated using analytical equations that involve numerous variable parameters. However, employing multiple formulations and variables in calculating bearing capacity can introduce inaccuracies and computational difficulties. This study aims to reduce potential inaccuracies and computational complexities in calculating bearing capacity and to accelerate the process using a Visual Basic application. This application simultaneously calculates and compares bearing capacities by Terzaghi, Meyerhof, Hansen, and Vesic methods. For this purpose, an example problem involving the calculation of bearing capacity was solved using both the conventional methods and a Visual Basic application, and the results were compared. This study demonstrated that bearing capacity results can be obtained more rapidly and consistently using the Visual Basic application than with conventional methods.
