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CONTENTS
Volume 20, Number 1, January10 2020
 

Abstract
In structures excavated in rock mass, load progressively increases to a level and remains constant during the construction. Rocks display different elastic properties such as Ei and v under different loading conditions and this requires to use the true values of elastic properties for the design of safe structures in rock. Also, rocks will undergo horizontal and vertical deformations depending on the amount of load applied. However, under constant loads, values of Ei and v will vary in time and induce variations in the behavior of the rock mass. In some empirical equations in which deformation modulus of the rock mass is taken into consideration, elastic parameters of intact rock become functions in the equation. Hence, the use of time dependent elastic properties determined under constant loading will yield more reliable results than when only constant elastic properties are used. As well known, rock material will play an important role in the deformation mechanism since the discontinuities will be closed due to the load. In this study, Ei and v values of intact rocks were investigated under different constant loads for certain rocks with high deformation capabilities. The results indicated significant time dependent variations in elastic properties under constant loading conditions. Ei value obtained from deformability test was found to be higher than the Ei value obtained from the constant loading test. This implies that when static values of elastic properties are used, the material is defined as more elastic than the rock material itself. In fact, Ei and v values embedded in empirical equations are not static. Hence, this workattempts to emerge a new understanding in designing of safer structures in rock mass by numerical methods. The use of time-dependent values of Ei and v under different constant loads will yield more accurate results in numerical modeling analysis.

Key Words
modulus of elasticity; Poisson\'s ratio; time-dependency; rock mass deformation; intact rock deformation

Address
C.O. Aksoy and H.E. Yaman: Department of Mining Engineering, Dokuz Eylul University, Izmir, Turkey

G.G. Uyar Aksoy: Department of Mining Engineering, Hacettepe University, Ankara, Turkey

A. Guney: Department of Mining Engineering, Mugla Sitki Kocman University, Mugla, Turkey

V. Ozacar: Torbali Vocational School, Dokuz Eylul University, İzmir, Turkey

Abstract
Three-point bending specimens have been used to investigate the mixed mode fracture of green sandstone. Dimensionless stress intensity factors and T-stresses were calculated first by using the finite element method for various crack lengths, crack angles and span to length ratios. It is shown that three-point bending specimens can provide the whole range of mode mixities from pure mode I to pure mode II, provided suitable values are chosen for the crack angle and span to length ratio. The fracture test results were also used to compare with predictions of different criteria. These comparisons show that modified criteria including the influence of the T-stress agree better with experiment than the conventional criteria but that no one criterion matches perfectly the test results.

Key Words
sandstone; fracture; T-stress; three-point bending; different criteria

Address
Yifan Li: 1.) Department of Mechanical Engineering, University of Bristol, Queens Building, University Walk, Bristol BS8 1TR, U.K.
2.) College of Architecture and Environment, Sichuan University, Chengdu 610065, China

Shiming Dong: College of Architecture and Environment, Sichuan University, Chengdu 610065, China

Martyn J. Pavier:Department of Mechanical Engineering, University of Bristol, Queens Building, University Walk, Bristol BS8 1TR, U.K.


Abstract
The spatial variability of geotechnical properties can lead to the uncertainty of settlement for frozen soil foundation around the oil pipeline, and it can affect the stability of permafrost foundation. In this paper, the elastic modulus, cohesion, angle of internal friction and poisson ratio are taken as four independent random fields. A stochastic analysis model for the uncertain settlement characteristic of frozen soil foundation around an oil pipeline is presented. The accuracy of the stochastic analysis model is verified by measured data. Considering the different combinations for the coefficient of variation and scale of fluctuation, the influences of spatial variability of geotechnical properties on uncertain settlement are estimated. The results show that the stochastic effects between elastic modulus, cohesion, angle of internal friction and poisson ratio are obviously different. The deformation parameters have a greater influence on stochastic settlement than the strength parameters. The overall variability of settlement reduces with the increase of horizontal scale of fluctuation and vertical scale of fluctuation. These results can improve our understanding of the influences of spatial variability of geotechnical properties on uncertain settlement and provide a theoretical basis for the reliability analysis of pipeline engineering in permafrost regions.

Key Words
foundations; spatial variability; geotechnical properties; settlement; permafrost regions

Address
Tao Wang, Guoqing Zhou, Jianzhou Wang and Di Wang: State Key Laboratory for Geomechanics and Deep Underground Engineering, School of Mechanics and Civil Engineering,
China University of Mining and Technology, Xuzhou, Jiangsu, 221116, China


Abstract
It is extremely important to obtain rock strength parameters for geological engineering. In this paper, the evolution of sandstone cohesion and internal friction angle with plastic shear strain was obtained by simulating the cyclic loading and unloading tests under different confining pressures using Particle Flow Code software. By which and combined with the micro-crack propagation process, the mesoscopic mechanism of parameter evolution was studied. The results show that with the increase of plastic shear strain, the sandstone cohesion decreases first and then tends to be stable, while the internal friction angle increases first, then decreases, and finally maintains unchanged. The evolution of sandstone shear strength parameters is closely related to the whole process of crack formation, propagation and coalescence. When the internal micro-cracks are less and distributed randomly and dispersedly, and the rock shear strength parameters (cohesion, internal friction angle) are considered to have not been fully mobilized. As the directional development of the internal micro-fractures as well as the gradual formation of macroscopic shear plane, the rock cohesion reduces continuously and the internal friction angle is in the rise stage. As the formation of the macroscopic shear plane, both the rock cohesion and internal friction angle continuously decrease to a certain residual level.

Key Words
strength parameter evolution; plastic shear strain; mesoscopic mechanism; numerical study;sandstone

Address
Hao Shi: 1.) State Key Laboratory for Geomechanics & Deep Underground Engineering, School of Mechanics & Civil Engineering, China University of Mining and Technology, Xuzhou, 221116, People\'s Republic of China
2.) GeoEnergy Research Centre, University of Nottingham, Nottingham NG7 2RD, U.K

Houquan Zhang and Lei Song: State Key Laboratory for Geomechanics & Deep Underground Engineering, School of Mechanics & Civil Engineering, China University of Mining and Technology, Xuzhou, 221116, People\'s Republic of China

Abstract
The limit analysis and response surfaces method were combined to investigate the reliability of pressurized tunnel faces subjected to seismic force. The quasi-static method was utilized to introduce seismic force into the tunnel face. A 3D horn failure mechanism of pressurized tunnel faces subjected to seismic force was constructed. The collapse pressure of pressurized tunnel faces was solved by the kinematical approach. The limit state equation of pressurized tunnel faces was obtained according to the collapse pressure and support pressure. And then a reliability model of pressurized tunnel faces was established. The feasibility and superiority of the response surfaces method was verified by comparing with the Monte Carlo method. The influence of the mean of soil parameters and support pressure, variation coefficients, distribution type and correlation of c-phi on the reliability of pressurized tunnel faces was discussed. The reasonable safety factor and support pressure required by pressurized tunnel faces to satisfy 3 safety levels were presented. In addition, the effects of horizontal seismic force, vertical seismic force and correlation of kh-kv on the reliability of pressurized tunnel faces were also performed. The method of this work can give a new idea for anti-seismic design of pressurized tunnel faces.

Key Words
3D failure mechanism; pseudo-static method; response surfaces method; reliability; correlation

Address
Biao Zhang: 1.) School of Civil Engineering, Hunan University of Science and Technology, Hunan 411201, China
2.) Hunan Province Key Laboratory of Geotechnical Engineering Stability Control and Health Monitoring,
Hunan University of Science and Technology, Hunan 411201, China
3.) School of Civil Engineering, Central South University, Hunan 410075, China

Zongyu Ma and Wenqing Peng: School of Resource and Environment and Safety Engineering, Hunan University of Science and Technology, Hunan, 411201, China

Xuan Wang and Jiasheng Zhang:School of Civil Engineering, Central South University, Hunan 410075, China

Abstract
The damage or failure of coal rock is accompanied by energy accumulation, dissipation and release. It is crucial to study the energy evolution characteristics of coal rock for rock mechanics and mining engineering applications. In this paper, coal specimens sourced from the Xinhe mine located in the Jining mining area of China were initially subjected to uniaxial compression, and the micro-parameters of the two-dimensional particle flow code (PFC2D) model were calibrated according to the experimental test results. Then, the PFC2D model was used to subject the specimens to substantial uniaxial compression, and the energy evolution laws of coal specimens with various schemes were presented. Finally, the elastic energy storage ratio m was investigated for coal rock, which described the energy conversion in coal specimens with various arrangements of preformed holes. The arrangement of the preformed holes significantly influenced the characteristics of the crack initiation stress and energy in the prepeak stage, whereas the characteristics of the cumulative crack number, failure pattern and elastic strain energy during the loading process were similar. Additionally, the arrangement of the preformed holes altered the proportion of elastic strain energy U_e in the total energy in the prepeak stage, and the probability of rock bursts can be qualitatively predicted.

Key Words
coal specimen; preformed hole; particle flow code (PFC); energy accumulation; energy dissipation; eastic energy storage ratio

Address
Na Wu and Zhengzhao Liang:State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology, Dalian, 116024, China

Jingren Zhou: State Key Laboratory of Hydraulics and Mountain River Engineering, Sichuan University, No. 24 South Section 1, Yihuan Road, Chengdu 610065, China

Lizhou Zhang: Chongqing Survey Institute, Chongqing, 401121, China

Abstract
With the increasing number of underground projects, the problem of rock-water coupling catastrophe has increasingly become the focus of safety. Grouting reinforcement is gradually applied in subway, tunnel, bridge reinforcement, coal mine floor and other construction projects. At present, cement-based grouting materials are easy to shrink and have low strength after solidification. In order to overcome the special problems of high water pressure and high in-situ stress in deep part and improve the reinforcement effect. In view of the mining conditions of deep surrounding rock, a new type of cement-based reinforcement material was developed. We analyses the principle and main indexes of floor strengthening, and tests and optimizes the indexes and proportions of the two materials through laboratory tests. Then, observes and compares the microstructures of the optimized floor strengthening materials with those of the traditional strengthening materials through scanning electron microscopy. The test results show that 42.5 Portland cement-based grouting reinforcement material has the advantages of slight expansion, anti-dry-shrinkage, high compressive strength and high density when the water-cement ratio is 0.4, the content of bentonite is 4%, and the content of Nano Silica is 2.5%. The reinforcement effect is better than other traditional grouting reinforcement materials.

Key Words
floor reinforcement with grunting; nano silica; scanning electron microscope

Address
Fei Zhou and Lingjun Kong: College of Mining and Safety Engineering, Shandong University of Science and Technology, Shandong 266590, China

Wenbin Sun: Shandong Key Laboratory of Wisdom Mine Information Technology,Shandong University of Science and Technology, Qingdao 266590, China

Xueyu Geng: The University of Warwick Coventry, CV4 7AL, U.K.

Abstract
This study aims to simulate the stabilization process of fibrous peat samples using end-bearing Cement Deep Mixing (CDM) columns by three area improvement ratios of 13.1% (TS-2), 19.6% (TS-3) and 26.2% (TS-3). It also focuses on the determination of approximate stress distribution between CDM columns and untreated fibrous peat soil. First, fibrous peat samples were mechanically stabilized using CDM columns of different area improvement ratio. Further, the ultimate bearing capacity of a rectangular foundation rested on the stabilized peat was calculated in stress-controlled condition. Then, this process was simulated via a FEM-based model using Plaxis 3-D foundation and the numerical modelling results were compared with experimental findings. In the numerical modelling stage, the behaviour of fibrous peat was simulated based on hardening soil (HS) model and Mohr-Coulomb (MC) model, while embedded pile element was utilized for CDM columns. The results indicated that in case of untreated peat HS model could predict the behaviour of fibrous peat better than MC model. The comparison between experimental and numerical investigations showed that the stress distribution between soil (S) and CDM columns (C) were 81%C-19%S (TS-2), 83%C-17%S (TS-3) and 89%C-11%S (TS-4), respectively. This implies that when the area improvement ratio is increased, the share of the CDM columns from final load was increased. Finally, the calculated bearing capacity factors were compared with results on the account of empirical design methods.

Key Words
stress distribution; soil cement columns; peat soil; rectangular foundation; numerical modelling

Address
Ali Dehghanbanadaki: Department of Civil Engineering, Damavand Branch, Islamic Azad University, Damavand, Iran

Shervin Motamedi: School of Energy, Construction and Environment, Sir John Laing Building, Coventry University, Coventry, CV1 2LT, Coventry, U.K.

Kamarudin Ahmad: School of Civil Engineering, University Teknologi Malaysia, 81310 UTM Skudai, Malaysia


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