Abstract
An unsaturated seasonal frozen soil slope of the Dandong-Altay highway is used as the research object. This paper is based on vapor-liquid migration and heat conduction multi-field coupling theory. The ice-water phase transition and vapor-liquid transition of water are also considered. The empirical relationship between unfrozen moisture content and ice volume fraction is introduced. A numerical model of the whole freeze-thaw process is established. The results show that: during the freezing period, the freezing depth increases faster before the temperature drops to the annual minimum temperature. Before entering the thawing period, the freezing depth is close to the maximum. After entering the thawing period, ice melting continuously absorbs heat from the frozen layer. Under the two-way action of external warming and geothermal heat, the deep freezing depth does not decrease for a long time. But it increases slowly and then decreases gradually. When the unsaturated soil slope is unfrozen, the engineering hazards of the slope caused by the migration of vapor are relatively small. After it enters the freezing state, the influence of vapor migration will increase significantly. The total volume moisture content of the frozen area is about 0.08 m3/m3 higher than without considering vapor migration.
Key Words
coupling water-vapor-heat migration; phase change; seasonal frozen soil region; unsaturated soil slope; VG model
Address
Yongxiang Zhan,Zheng Lu and Hailin Yao: State Key Laboratory of Geomechanics and Geotechnical Engineering Safety, Institute of Rock and Soil Mechanics,
Chinese Academy of Sciences, Wuhan 430071, China
Mingyang Zhao: State Key Laboratory of Geomechanics and Geotechnical Engineering Safety, Institute of Rock and Soil Mechanics,
Chinese Academy of Sciences, Wuhan 430071, China;
University of Chinese Academy of Sciences, Beijing 100049, China;
Hubei Communications Planning and Design Institute Co., Ltd., Wuhan 430051, China
Lang Qin: Sichuan Chengnan Expressway Co., Ltd., Chengdu 610051, China
Gang Liu: CCCC Second Highway Consultants Co., Ltd., Wuhan 430056, China
Abstract
The rapid expansion of underground transportation systems has given rise to a corresponding increase in the possibility of encountering soft strata, which usually causes substantial ground settlement during shield tunneling. Cement treatment is typically utilized to alleviate ground settlement induced by excavation. However, the implementation of an appropriate reinforcement approach and associated system parameters can be challenging due to the complexity of on-site geological conditions. For long-distance reinforcement projects, this study employs numerical simulations to evaluate the effectiveness of various reinforcement methods and associated system parameters on ground settlement reduction. The simulation results indicate that the triaxial mixing pile (TMP) outperforms the umbrella arch (UA) in settlement reduction and applicability, making it the preferred option for long-distance pre-reinforcement projects. Compared to the scenario without reinforcement, the ground settlement reduction using the UA is limited to 43.7%, while the TMP achieves a reduction of 67.5% with optimal parameters. (i.e. 0.5D (tunnel diameter) reinforcement width, 0.5D depth, 0.5 pile length ratio, 3.0D tunnel buried depth, 1Pref ~ 2Pref face pressure, and 22% cement incorporation ratios). These optimal TMP system parameters are also validated through on-site monitoring. Additionally, a new model in which a disturbance correction coefficient s(x) is introduced is proposed to predict ground settlement of reinforced strata. The findings presented in this study offer a detailed reference for similar tunnelling projects where ground settlement is a non-negligible concern.
Address
Yijie Jin, Ping Yang, Lin Li, Jiahui Wang, Yong Tao and Yanzi Wang: Department of Civil Engineering, Nanjing Forestry University, Nanjing, China
Zhiyu Zhang: CCCC Tunnel Engineering Company Limited, Nanjing, China
Abstract
At present, the widely used bishop stability analysis method does not consider the unsaturated effective stress, and the effective stress theory is unclear. To address these questions, this study initially explores a novel methodology for determining pore water pressure via the phreatic line. Subsequently, this study integrates the unified effective stress equation for both saturated and unsaturated soils, known for its clear physical significance, to propose an enhanced Bishop method(EBM) that accounts for effective stress in unsaturated soils. This EBM features a comprehensive theoretical framework, enabling direct stability analysis based on the phreatic line and demonstrating significant value for engineering applications. The reliability of the computational procedure of the EBM is validated through a saturated slope case study. Building upon this validation, the study further investigates the impact of incorporating unsaturated effective stress on slope stability. The results show that the deeper most dangerous sliding surface corresponds to an increase in the factor of safety(FOS) from 1.01% to 20.51% when considering the unsaturated effective stress. Overall, the integration of unsaturated effective stress exerts a positive influence on slope stability, underscoring the significance of this method for stability analysis and engineering design applications.
Address
Weiliang Ma: School of Mechanics and Aerospace Engineering, Dalian University of Technology, Dalian, Liaoning, China
Longtan Shao: State Key Laboratory of Structural Analysis, Optimization and CAE Software for Industrial Equipment,
Dalian University of Technology, Dalian Liaoning, China
Abstract
This study investigates the constant normal stiffness (CNS) boundary condition in deep rock engineering structures, which provides a more accurate representation of the stress environment of jointed rock masses than the constant normal load (CNL) boundary condition. Three CNS boundary conditions (0 GPa/m, 1.5 GPa/m, and 3.0 GPa/m) were designed to simulate the effects of confinement and various geological engineering conditions on different tunnel depths. Using direct shear tests on both anchored and unanchored joint samples under CNS conditions, this study incorporated the dilation curve of the joints into a model predicting joint shear strength. The model accounts for the effects of CNS boundary conditions, and combines the anchorage resistance model based on the theory of statically determinate beams. It also considers the relationship between axial and lateral displacements of anchors during shear deformation. Results demonstrate that both CNS boundary conditions and anchorage significantly influence shear mechanical properties. Anchor reinforcement exhibited a greater impact on peak shear stress than CNS boundary conditions, while both factors similarly affected peak normal displacement. The newly proposed model accurately predicts shear strength under different normal stiffness boundary conditions, aligning closely with experimental data. The study also analyzes the contribution of anchors to shear strength, revealing a 57.28% contribution under a stiffness condition of 0 GPa/m. With increasing normal stiffness, intrinsic shear resistance in jointed rock mass improves, while the relative contribution from anchors decreases.
Key Words
bolted jointed rock mass; constant normal stiffness; shear dilation curve; shear strength calculation model
Address
Yang Song, Jinghan Mao, Heping Wang and Bo Fan: School of Civil Engineering, Liaoning Technical University , Fuxin, Liaoning, 123000, China
Abstract
This paper presents a novel experimental investigation for improving the bearing pressure resistance of medium-dense sand deposits. Model footing tests were performed on similarly prepared sand beds to examine the effects of incorporating one, two and three separate geogrid-reinforced thin densified gravel layers at various depths within the beds. Additional load tests investigated incorporating only geogrid-reinforcement layers or densified gravel layers of different thicknesses placed separately within the sand beds for the purposes of establishing their resistance contributions. Furthermore, a dimensional analysis relating the model test results to the large-scale conditions is presented. Compared to the unreinforced medium-dense sand bed, superior bearing-pressure resistances (and reduced footing settlements) were achieved for including the geogrid-reinforced gravel layers. For example, at a settlement ratio of 2% (s/D = 2%), the inclusion of one, two, and three geogrid-reinforced thin densified SwG layers improved performance by 68%, 110%, and 124%, respectively, compared to the unreinforced sand bed. Increasing the number of these layers (from one to three) produced improved geomechanical performance, albeit more layers produced diminishing returns. The basic mechanisms responsible for the improvements in bearing pressure resistance are elucidated. This study demonstrates that when suitable engineering fill is unavailable, the concept of incorporating geogrid reinforced thin densified granular layers in the backfill could be of practical application for constructing reinforced footings, reinforced soil walls, etc.
Address
S.N. Moghaddas Tafreshi,H. Alizadeh Balf and H.R. Rezaeinejad: Department of Civil Engineering, K.N. Toosi University of Technology, Tehran, Iran
B.C. O'Kelly: Department of Civil, Structural and Environmental Engineering, Trinity College Dublin, Dublin, Ireland
A. Faramarzi: School of Engineering, University of Birmingham, Birmingham, UK
Abstract
The propagation attenuation law of elastic waves generated by rock mass fractures in strata is of great significance in explaining engineering problems, such as rock bursts. In this study, a wave velocity model was established based on the actual stratum conditions of the Jining No.2 coal mine in China. The propagation process of different types of elastic waves is simulated using the elastic wave equation and finite difference method. The attenuation coefficients of various elastic wave parameters propagating at different positions were calculated. Based on the calculated results, an attenuation equation for the elastic wave energy is proposed. The results show that the propagation of S-waves and P-waves in strata is similar to each other, and wave pattern transformation occurs at the interface of the strata. The magmatic rocks played a role in channeling and reflecting elastic wave propagation, leading to the occurrence of low-energy aftershocks in the coal seams. The peak displacement, peak stress, and energy density of the elastic wave that propagated to the coal seam after the siltstone layer fracture of the roof of the 3down coal seam decreased to 28%, 25%, and 9% of the source, respectively. The attenuation of elastic wave energy is highly correlated with the propagation distance, and the attenuation equation can be represented as an exponential function. These results are of great significance for revealing the propagation attenuation process of elastic waves in the stratum.
Key Words
elastic wave; energy attenuation; propagation characteristics; rock burst; roof fracture
Address
Peng-Fei Zhang, Tong-Bin Zhao, Xu-Fei Gong and Yan Tan: College of Energy and Mining Engineering, Shandong University of Science and Technology, Qingdao 266590, China;
State Key Laboratory of Mining Disaster Prevention and Control Co-founded by Shandong Province and the Ministry of Science and Technology, Shandong University of Science and Technology, Qingdao 266590, China;
Shandong Key Laboratory of Intelligent Prevention and Control of Dynamic Disaster in Deep Mines,
Shandong University of Science and Technology, Qingdao 266590, China
Chuan-Qing Guo: Energy Group Co., Ltd., Jining No.2 Coal Mine ,Jining 272012, China
