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Large deformations of soft rocks often occur during the excavation and support of the Chuxiong section tunnel in the Yunnan Central Diversion Project. The physical characteristics and creep mechanical properties of the red-bed soft rock were analyzed by combining engineering experience, laboratory tests, and scanning electron microscopy. Furthermore, the creep parameters of the red-bed soft rock were obtained. The issues related to large deformations of deeply buried tunnels with soft and weak rock masses were investigated. Based on construction engineering practice and numerical simulations, characteristics of extrusion damage of deeply buried tunnels with soft and weak rock masses were analyzed. Countermeasures were implemented to solve the problems of large deformations, which can be used as references for the construction of similar soft rock tunnels in the future. [ABSTRACT FROM AUTHOR]

In constructing hydraulic tunnels, construction disturbances and complex geological conditions can induce variations in the surrounding rock parameters. To navigate the complex non-linear interplay between rock material parameters and tunnel displacement during construction, this study proposes a hybrid learning model. It employs particle swarm optimization (PSO) to refine the hyperparameters of the eXtreme Gradient Boosting (XGBoost) technique. Sensitivity analysis and inversion of rock parameters is performed by using orthogonal design and the Sobol method to analyze the sensitivity of environmental and rock material factors. The findings indicate that the tunnel depth, elastic modulus, and Poisson ratio are particularly sensitive parameters. Mechanical parameters of the rock mass, identified through sensitivity analysis, are the focal point of this research and are integrated into a three-dimensional computational model. The resulting tunnel displacement calculations serve as datasets for the inversion of the actual engineering project's surrounding rock mechanical parameters. These inverted parameters were fed into the FLAC3D software (version 7.0), yielding results that align closely with field measurements, which affirms the PSO-XGBoost model's validity and precision. The insights garnered from this research offer a substantial reference for determining rock mass parameters in tunnel engineering amidst complex conditions. [ABSTRACT FROM AUTHOR]

To enrich understanding of how fluid flows through the natural filled jointed rock, the permeability experiments of the natural filled jointed dacite rock under repeated loading and unloading conditions are carried out. Experimental results show that the response of permeability of filled jointed dacite rock to confining pressure under repeated loading and unloading conditions is considered as porous media. Permeability and gas pressure are positively correlated under repeated loading and unloading conditions. Relationship between permeability and gas pressure shows that the higher confining pressure, the smaller permeability increasing rate. The calculated equivalent crack apertures are very small at the order of magnitude about 10−8 m. The power law is found to well describe the relationship between equivalent crack aperture and effective stress of natural filled jointed dacite rock. With the parameters determined based on experiment results, an empirical expression is proposed. [ABSTRACT FROM AUTHOR]

During the reservoir filling of many high arch dams, it was observed that the valley slopes on both sides deformed towards the river. The measured value of valley narrowing lateral deformation (VNLD) reached 89.54 mm by October 2018. This rare phenomenon contradicts the traditional experience of dam engineering and lacks a convincing explanation. An arch dam project in China is selected as a case study to analyze the mechanism of the VNLD. Based on a comprehensive analysis of the geological background and the VNLD measurements, a numerical study is performed to identify the main factors that may contribute to the VNLD phenomenon. It is concluded that the time-dependent valley deformation is due to the rheological behaviour of rock mass with the change of hydrogeological conditions. The artesian water head in the deep-confined aquifer is increased and the mechanical parameters in fault and shear zones are weakened after reservoir filling, which is associated with the specific hydrogeological structures. The rise of artesian water head is the main induced factor that contributes to the uplift deformation, which is observed at the dam site, and can cause the VNLD in the horizontal direction. The simulated results considering these factors agreed well with the observed data. [ABSTRACT FROM AUTHOR]

Purpose: The purpose of this paper is to investigate the mechanical behavior and propagation of cracks of numerical granite samples through the Brazilian split test and to provide a reference for predicting the behavior of real granite samples. Design/methodology/approach: The numerical models of granite containing two fissures are established using the parallel bond model (PBM) and the smooth joint model (SJM) in PFC2D. The peak stresses, number of cracks and anisotropic ratios are obtained to study the influence of the mineral composition and the angle of inclination of rock bridge on the strength, failure mode and deformation characteristics. Findings: The numerical results obtained show that the mineral composition has a marginal influence on the peak stress. When the angle of inclination of rock bridge β increases, the peak stress drops to its minimum value at β = 90° and then gradually increases to a relatively low level. The behavior of cracks falls into three categories based on the distribution of cracks. By analyzing the stress–strain curve and the process of crack propagation for sample No. 4 with β = 60°, it is found that the process of failure can be divided into four stages and tensile cracks dominate. The anisotropic ratios of peak stress and a number of cracks obtained show that the peak stress is low anisotropic and the number of cracks is medium anisotropic. Originality/value: This paper presents a numerical simulation method to analyze mechanical behavior and propagation of cracks under different conditions. The proposed method and the results obtained are useful for predicting the behavior of real granite samples in laboratory and engineering projects. [ABSTRACT FROM AUTHOR]

The altered granite gneiss is a special rock with loosely-structured and well-developed pores and cavities. The microstructure and mineral composition of the rock were analysed by SEM-EDS and creep triaxial tests were carried out and the axial and lateral deformations over time were discussed. Both the axial and the lateral creep curves show the typical three distinct creep stages (decay creep, steady state creep, accelerated creep). At a low stress level, the axial creep strain is unnoticeable, and only when loaded at a high stress level will the creep strain increase at an increasing rate until the rock specimen fails. The lateral creep strain is smaller than the axial creep strain, but the creep rate (dε/dt) in the lateral direction is higher than that in the axial direction at all stress levels. The failure pattern, which is affected by many factors, shows a shear failure with rather broken and rough failure surfaces. [ABSTRACT FROM AUTHOR]

The permeability of the rock specimen during the process of deformation and failure is a dynamic evolution, and it relates with the growth and coalesces of micro-cracks within the rock. Although extensive researches have been conducted on the relationship between the stress-strain and the permeability, the deformation of the specimen should be taken into account for the evaluation of the permeability. In this study, experiments on the hydro-mechanical coupling of sandstone are carried out. Change of the permeability with the axial, lateral and volumetric strains is investigated. The experimental results show that the change of the permeability during the process of deformation and failure has apparent phases, and the evolution of the permeability is closely correlated with the strain. According to the experimental data, two kinds of empirical equations are fitted to better describe the relationship between the permeability and the strain. One is the polynomial equation used to simulate the phase of compression of rock specimens, the other is exponential equation used to simulate the rest of deformation phases. The obtained equations are just the empirical ones for the sandstone and are not for the universal application. [ABSTRACT FROM AUTHOR]