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CONTENTS
Volume 11, Number 3, September 2016
 

Abstract
A total stress-based bounding surface model is developed to predict the undrained behaviour of saturated soft clays under cyclic loads based on the anisotropic hardening modulus field and bounding-surface theories. A new hardening rule is developed based on a new interpolation function of the hardening modulus that has simple mathematic expression and fewer model parameters. The evolution of hardening modulus field is described in the deviatoric stress space. It is assumed that the stress reverse points are the mapping centre points and the mapping centre moves with the variation of loading and unloading paths to describe the cyclic stress-strain hysteresis curve. In addition, by introducing a model parameter that reflects the accumulation rate and level of shear strain to the interpolation function, the cyclic shakedown and failure behaviour of soil elements with different combinations of initial and cyclic stresses can be captured. The methods to determine the model parameters using cyclic triaxial compression tests are also studied. Finally, the cyclic triaxial extension and torsional shear tests are performed. By comparing the predictions with the test results, the model can be used to describe undrained cyclic stress-strain responses of elements with different stress states for the tested clays.

Key Words
soft clay; constitutive model; cyclic loading; hardening modulus field; bounding surface

Address
State Key Laboratory of Hydraulic Engineering Simulation and Safety of Tianjin University, Tianjin University, Tianjin 300072, P.R. China.


Abstract
In order to investigate the influence of the interfacial angel on failure characteristics and mechanism of combined coal-rock mass, 35 uniaxial/biaxial compressive simulation tests with 5 different interfacial angels of combined coal-rock samples were conducted by PFC2D software. The following conclusions are drawn: (1) The compressive strength and cohesion decrease with the increase of interfacial angle, which is defined as the angle between structure plane and the exterior normal of maximum principal plane, while the changes of elastic modulus and internal friction angle are not obvious; (2) The impact energy index KE decreases with the increase of interfacial angle, and the slip failure of the interface can be predicted based on whether the number of acoustic emission (AE) hits has multiple peaks or not; (3) There are four typical failure patterns for combined coal-rock samples including I (V-shaped shear failure of coal), II (single-fracture shear failure of coal), III (shear failure of rock and coal), and IV (slip rupture of interface); and (4) A positive correlation between interfacial angle and interface effect is shown obviously, and the interfacial angle can be divided into weak-influencing scope (0-15°), moderate-influencing scope (15-45°), and strong-influencing scope (> 45°), respectively. However, the confining pressure has a certain constraint effect on the interface effect.

Key Words
particle flow; combined coal-rock; interfacial angle; failure pattern; mechanical properties

Address
(1) Tong-Bin Zhao, Guang-Ming Zhao:
Key Laboratory of Safety and High-efficiency Coal Mining, Ministry of Education (Anhui University of Science and Technology), Huainan, Anhui, 232001, P.R. China;
(2) Tong-Bin Zhao, Wei-Yao Guo:
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, Shandong 266590, P.R. China;
(3) Cai-Ping Lu:
School of Mining Engineering, Key Laboratory of Deep Coal Resource Mining (Ministry of Education), China University of Mining and Technology, Xuzhou, Jiangsu 221116, P.R. China.

Abstract
The soil-concrete interface shear strength, although has been extensively studied, is still difficult to predict as a result of the dependence on many factors such as normal stresses, surface roughness, particle sizes, moisture contents, dilation angles of soils, etc. In this study, a well-known rigorous statistical learning approach, namely the least squares support vector machine (LS-SVM) realized in a ubiquitous spreadsheet platform is firstly used in estimating the soil-structure interface shear strength. Instead of studying the complicated mechanism, LSSVM enables to explore the possible link between the fundamental factors and the interface shear strengths, via a sophisticated statistic approach. As a preliminary investigation, the authors study the expansive soils that are found extensively in most countries. To reduce the complexity, three major influential factors, e.g., initial moisture contents, initial dry densities and normal stresses of soils are taken into account in developing the LS-SVM models for the soilconcrete interface shear strengths. The predicted results by LS-SVM show reasonably good agreement with experimental data from direct shear tests.

Key Words
soil-concrete interface shear strength; modified direct shear test; LS-SVM; statistical prediction

Address
(1) Chunshun Zhang, Sheng-Qi Yang:
State Key Lab for Geomechanics and Deep Underground Engineering, China University of Mining and Technology, Xuzhou, P.R. China;
(2) Jian Ji:
Key Lab of Ministry of Education for Geomechanics and Embankment Engineering, Hohai University, Nanjing, P.R. China;
(3) Chunshun Zhang, Jian Ji, Jayantha Kodikara:
Department of Civil Engineering, Monash University, Australia;
(4) Yilin Gui:
School of Civil and Environmental Engineering, Nanyang Technological University, Singapore;
(5) Lei He:
School of Civil Engineering, Southeast University, Nanjing, P.R. China.

Abstract
As one natural material, the physical and mechanical properties of rock will be affected very largely by chemical erosion environment. Under chemical environment, the strength of rock will be reduced. Considering the effect of the chemical erosion, fracture factor of rock is reduced. The damage variable is applied to express the change of fracture stress. Therefore, the fracture criterion of rock under chemical environment is constructed. By one experiment of rock fracture under chemical erosion environment, the proposed fracture criterion is verified. The results show that, the fracture path by theory is agree with the testing one well.

Key Words
chemical erosion; environment effect; fracture; rock; crack

Address
Key Laboratory of Ministry of Education for Geomechanics and Embankment Engineering, College of Civil and Transportation Engineering, Hohai University, Nanjing 210098, P.R. China.


Abstract
Bearing capacity of open-ended piles depends largely on inner frictional resistance, which is influenced by the degree of soil plugging. While a fully-plugged open-ended pile produces a bearing capacity similar to a closed-ended pile, fully coring (or unplugged) pile produces a much smaller bearing capacity. In general, open-ended piles are driven under partially-plugged mode. The formation of soil plug may depend on many factors, including wall thickness at the pile tip (or inner pile diameter), sleeve height of the thickened wall at the pile tip and relative density. In this paper, we studied the effects of wall thickness at the pile base and sleeve height of the thickened wall at the pile tip on bearing capacity using laboratory model tests. The tests were conducted on a medium dense sandy ground. The model piles with different tip thicknesses and sleeve heights of thickened wall at the pile tip were tested. The results were also discussed using the incremental filling ratio and plug length ratio, which are generally used to describe the degree of soil plugging. The results showed that the bearing capacity increases with tip thickness. The bearing capacity of piles of smaller sleeve length (e.g., ≥ 1D; D is pile outer diameter) was found to be dependent on the sleeve length, while it is independent on the sleeve length of greater than a 1D length. We also found that the soil plug height is dependent on wall thickness at the pile base. The results on the incremental filling ratio revealed that the thinner walled piles produce higher degree of soil plugging at greater penetration depths. The results also revealed that the soil plug height is dependent on sleeve length of up to 2D length and independent beyond a 2D length. The piles of a smaller sleeve length (e.g., ≥ 1D) produce higher degree of soil plugging at shallow penetration depths while the piles of a larger sleeve length (e.g., ≦ 2D) produce higher degree of soil plugging at greater penetration depths.

Key Words
bearing capacity; inner friction; sleeve height; soil plug; tip thickness

Address
Department of Civil Engineering, Tokyo University of Science, 2641, Yamazaki, Noda, Chiba 278-8510, Japan.


Abstract
The stability of deep coal roadways with large sections and thick top coal is a typical challenge in many coal mines in China. The innovative Universal Discrete Element Code (UDEC) trigon block is adopted to create a numerical model based on a case study at the Dongtan coal mine in China to better understand the failure mechanism and stability control mechanism of this kind of roadway. The failure process of an unsupported roadway is simulated, and the results suggest that the deformation of the roof is more serious than that of the sides and floor, especially in the center of the roof. The radial stress that is released is more intense than the tangential stress, while a large zone of relaxation appears around the roadway. The failure process begins from partial failure at roadway corners, and then propagates deeper into the roof and sides, finally resulting in large deformation in the roadway. A combined support system is proposed to support roadways based on an analysis of the simulation results. The numerical simulation and field monitoring suggest that the availability of this support method is feasible both in theory and practice, which can provide helpful references for research on the failure mechanisms and scientific support designing of engineering in deep coal mines.

Key Words
numerical modeling; deep coal roadway with large section and thick top coal; failure mechanism; crack propagation; combined support system

Address
(1) Miao Chen, Sheng-Qi Yang, Yuan-Chao Zhang:
State Key Laboratory for Geomechanics and Deep Underground Engineering, School of Mechanics and Civil Engineering, China University of Mining and Technology, Xuzhou 221116, P.R. China;
(2) Miao Chen, Chuan-Wei Zang:
College of Mining and Safety Engineering, Shandong University of Science and Technology, Qingdao 266510, P.R. China.

Abstract
Explosions inside transportation tunnels might result in failure of tunnel structures. This study investigated the failure mechanisms of circular cast-iron tunnels in saturated soil subjected to medium internal blast loading. This issue is crucial to tunnel safety as many transportation tunnels run through saturated soils. At the same time blast loading on saturated soils may induce residual excess pore pressure, which may result in soil liquefaction. A series of numerical simulations were carried out using Finite Element program LS-DYNA. The effect of soil liquefaction was simulated by the Federal Highway soil model. It was found that the failure modes of tunnel lining were differed with different levels of blast loading. The damage and failure of the tunnel lining was progressive in nature and they occurred mainly during lining vibration when the main event of blast loading was over. Soil liquefaction may lead to more severe failure of tunnel lining. Soil deformation and soil liquefaction were determined by the coupling effects of lining damage, lining vibration, and blast loading. The damage of tunnel lining was a result of internal blast loading as well as dynamic interaction between tunnel lining and saturated soil, and stress concentration induced by a ventilation shaft connected to the tunnel might result in more severe lining damage.

Key Words
blast loading; cast-iron lining; failure modes; soil-lining interaction; soil liquefaction

Address
(1) Yuzhen Han:
Beijing Urban Construction Design & Development Group Co., Limited, No. 5, Fuchengmen Beidajie, Xicheng District, Beijing 100037, China;
(2) Huabei Liu:
School of Civil Engineering and Mechanics, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, Hubei 430074, China.

Abstract
This paper presented solutions of displacement and stress for a circular opening which is reinforced with grouted rock bolt. It satisfies the Mohr-Coulomb (M-C) or generalized Hoek-Brown (H-B) failure criterion, and exhibits elastic-brittle-plastic or strain-softening behavior. The numerical stepwise produce for strain-softening rock mass reinforced with grouted rock bolt was developed with non-associative flow rules and two segments piecewise linear functions related to a principle strain-dependent plastic parameter, to model the transition from peak to residual strength. Three models of the interaction mechanism between grouted rock bolt and surrounding rock proposed by Fahimifar and Soroush (2005) were adopted. Based on the axial symmetrical plane strain assumption, the theoretical solution of the displacement and stress were proposed for a circular tunnel excavated in elastic-brittle-plastic and strain-softening rock mass compatible with M-C or generalized H-B failure criterion, which is reinforced with grouted rock bolt. It showed that Fahimifar and Soroush's (2005) solution is a special case of the proposed solution for n = 0.5. Further, the proposed method is validated through example comparison calculated by MATLAB programming. Meanwhile, some particular examples for M-C or generalized H-B failure criterion have been conducted, and parametric studies were carried out to highlight the influence of different parameters (e.g., the very good, average and very poor rock mass). The results showed that, stress field in plastic region of surrounding rock with considering the supporting effectiveness of the grouted rock bolt is more than that without considering the effectiveness of the grouted rock bolt, and the convergence and plastic radius are reduced.

Key Words
strain-softening; support effectiveness; numerical stepwise produce; circular opening; grouted rock bolt

Address
School of Civil Engineering, Central South University, No.22, Shaoshan South Road, Central South University Railway Campus, Changsha, Hunan Province, People's Republic of China.



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