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CONTENTS
Volume 22, Number 6, September25 2020
 

Abstract
The recent increase in the use of Expanded Polystyrene (EPS) geofoam in construction and geotechnical projects has driven researchers to investigate its behavior, more deeply. In this paper, a series of experimental tests to investigate the stress-strain behavior and the mechanical properties of EPS blocks, under monotonic axial loading are presented. Four different densities of cylindrically shaped EPS with different dimensions are used to investigate the effects of loading rate, height and diameter, as well as the influence of the density of EPS on the stress-strain response. The results show that increasing the height of the EPS samples leads to instability of the sample and consequent lower resistance to the applied pressure. Large EPS samples show higher Young\'s modulus and compressive resistance due to some boundary effects. An increase in the rate of loading can increase the elastic moduli and compressive resistance of the EPS geofoam samples, which also varies depending on the density of the samples. It was also determined that the elastic modulus of EPS increases with increasing EPS density. By implementing an efficient numerical procedure, the stress-strain response of EPS geofoam samples can be reproduced with great accuracy. The numerical analysis based on the proposed method can used to evaluate the effect of different factors on the behavior of EPS geofoam.

Key Words
expanded polystyrene (EPS); geofoam; sample size; sample slenderness; strain rate; monotonic loading

Address
Omid Khalaj, Miloslav Kepka, Tomas Kavalir, Michal Krizek and Hana Jirkova: Regional Technological Institute, Univerzitni 8, 30100, Plzen, Czech Republic

Seyed Mohammad Amin Ghotbi Siabil, Mehran Azizian and Seyed Naser Moghaddas Tafreshi: Department of Civil Engineering, K.N. Toosi University of Technology, Valiasr St., Mirdamad Cr., Tehran, Iran

Bohuslav Masek: COMTES FHT, Prumyslova 995, 334 41, Dobrany, Czech Republic


Abstract
This study proposes a new empirical model to effectively predict the excavation performance of a shield tunnel boring machine (TBM). The TBM performance is affected by the geological and geotechnical characteristics as well as the machine parameters of TBM. Field penetration index (FPI) is correlated with rock mass parameters to analyze the effective geotechnical parameters influencing the TBM performance. The result shows that RMR has a more dominant impact on the TBM performance than UCS and RQD. RMR also shows a significant relationship with the specific energy, which is defined as the energy required for excavating the unit volume of rock. Therefore, the specific energy can be used as an indicator of the mechanical efficiency of TBM. Based on these relationships with RMR, this study suggests an empirical performance prediction model to predict FPI, which can be derived from the correlation between the specific energy and RMR.

Key Words
TBM performance prediction; FPI; specific energy; RMR; regression analysis

Address
Kyoung-Yul Kim, Seon-Ah Jo and Hee-Hwan Ryu: Structural and Seismic Technology Group, Next Generation Transmission & Substation Laboratory, KEPCO Research Institute(KEPRI),
105 Munji-ro, Yuseong-gu, Daejeon 34056, Republic of Korea

Gye-Chun Cho: Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology(KAIST),291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea



Abstract
This paper presents the results of a three-dimensional numerical investigation into the effect of new tunnel construction on structural performance of existing tunnel lining. A three-dimensional finite difference model, capable of modelling the tunnel construction process, was adopted to perform a parametric study on the spatial variation of new tunnel location with respect to the existing tunnel with emphasis on the plan crossing angle of the new tunnel with respect to the existing tunnel and the vertical elevation of the new tunnel with respect to the existing one. The results of the analyses were arranged so that the effect of new tunnel construction on the lining member forces and stresses of the existing tunnel can be identified. The results indicate that when a new tunnel underpasses an existing tunnel, the new tunnel construction imposes greater impact on the existing tunnel lining when the two tunnels cross at an acute angle. Also shown are that the critical plan crossing angle of the new tunnel that would impose greater impact on the existing tunnel depends on the relative vertical location of the new tunnel with respect to the existing one, and that the overpassing new tunnel construction scenario is more critical than the underpassing scenario in view of the existing tunnel lining stability. Practical implications of the findings are discussed.

Key Words
conventional tunneling; crossing tunnel interaction; underpassing tunnel; overpassing tunnel; finite-difference analysis; lining member forces

Address
Chungsik Yoo: School of Civil, Architectural Engineering and Landscape Architecture, Sungkyunkwan University, 2066 Seboo-ro, Jangan-gu, Suwon, Kyounggi-do 16419, Republic of Korea

Shuaishuai Cui: School of Civil Engineering, Shandong University, No. 17923, Jingshi Road, Jinan, Shandong Province, China


Abstract
Multi-layered primary linings have been proved to be highly effective for tunneling in severe squeezing grounds. But there still has not existed well-established design method for it. Basically, there are two main critical problems in this method, including determinations of allowable deformation and distribution of support stiffness. In order to address such problems, an attempt to investigate the mechanical response of a circular tunnel with double primary linings is performed in this paper. Analytical solutions in closed form for stresses and displacements around tunnels are derived. In addition, the effectiveness and reliability of theoretical formulas provided are well validated by using the numerical method. Finally, based on the analytical solutions, a parametric investigation on the effects of allowable deformation and distribution of support stiffness on tunnel performance is conducted. Results show that the rock pressure and displacement are significantly affected by these two design parameters. It can be found that rock pressure decreases as either allowable deformation increases or stiffness of the first primary lining decreases, but rock displacement shows an opposite trend. This paper can provide a useful guidance for the design of multi-layered primary linings.

Key Words
squeezing tunnel; double primary linings; analytical solution; parametric investigation

Address
Kui Wu, Siyuan Hong and Su Qin: 1.) School of Civil Engineering, Xi\'an University of Architecture and Technology, Xi\'an 710055, China
2.) Shaanixi Ley Lab of Geotechnical and Underground Space Engineering (XAUAT), Xi\'an University of Architecture and Technology, Xi\'an 710055, China

Zhushan Shao: chool of Civil Engineering, Xi\'an University of Architecture and Technology, Xi\'an 710055, China


Abstract
A mathematical wetting model is usually used to predict the deformation of core wall rockfill dams induced by the wetting effect. In this paper, a series of wetting triaxial tests on a rockfill was conducted using a large-sized triaxial apparatus, and the wetting deformation behavior of the rockfill was studied. The wetting strains were found to be related to the confining pressure and shear stress levels, and two empirical equations, which are regarded as the proposed mathematical wetting model, were proposed to express these properties. The stress and deformation of a core wall rockfill dam was studied by using finite element analysis and the proposed wetting model. On the one hand, the simulations of the wetting model can estimate well the observed wetting strains of the upstream rockfill of the dam, which demonstrated that the proposed wetting model is applicable to express the wetting deformation behavior of the rockfill specimen. On the other hand, the simulated additional deformation of the dam induced by the wetting effect is thought to be reasonable according to practical engineering experience, which indicates the potential of the model in dam engineering.

Key Words
dams; wetting deformation; finite-element modeling; rockfills

Address
Wanli Guo and Yingli Wu: Geotechnical Engineering Department, Nanjing Hydraulic Research Institute, Nanjing 210024, China

Ge Chen: Key Laboratory of Ministry of Education for Geomechanics and Embankment Engineering, Hohai University, Nanjing, Jiangsu 210098, China

Junjie Wang: Key Laboratory for Hydraulic and Waterway Engineering of Ministry of Education, Chongqing Jiaotong University, Chongqing 400074, China

Abstract
Water inrush is a major hazard for mining and excavation in deep coal seams or rock masses. It can be attributed to the coalescence of rock fractures in rock mass due to the interaction of fractures, hydraulic flow and stress field. One of the key technical challenges is to understand the course and mechanism of fluid flows in rock joint networks and fracture propagation and hence to take measures to prevent the formation of water inrush channels caused by possible rock fracturing. Several case observations of fluid flowing in rock joint networks and coupled fracture propagation in underground coal roadways are shown in this paper. A number of numerical simulations were done using the recently developed flow coupling function in FRACOD which simulates explicitly the fracture initiation and propagation process. The study has demonstrated that the shortest path between the inlet and outlet in joint networks will become a larger fluid flow channel and those fractures nearest to the water source and the working faces become the main channel of water inrush. The fractures deeper into the rib are mostly caused by shearing, and slipping fractures coalesce with the joint, which connects the water source and eventually forming a water inrush channel.

Key Words
jointed rock; numerical simulation; fracture interaction; water inrush; FRACOD

Address
Shichuan Zhang, Xinguo Zhang, Yangyang Li, Wenbin Sun and Jinhai Zhao: State Key Laboratory of Mining Disaster Prevention and Control, Shandong University of Science and Technology, Qingdao, China

Baotang Shen: 1.) State Key Laboratory of Mining Disaster Prevention and Control, Shandong University of Science and Technology, Qingdao, China
2.) CSIRO Energy, Queensland Centre for Advanced Technologies, PO Box 883, Kenmore, Brisbane, QLD 4069, Australia

Abstract
With the increasing tension of current coal resources and the increasing depth of coal mining, the gob-side entry retaining technology has become a preferred coal mining method in underground coal mines. Among them, the technology of the gob-side entry retaining with the high-water filling material can not only improve the recovery rate of coal resources, but also reduce the amount of roadway excavation. In this paper, based on the characteristics of the high-water filling material, the technological process of gob-side entry retaining with the high-water filling material is introduced. The early and late stress states of the filling body formed by the high-water filling materials are analyzed and studied. Taking the 8th floor No.3 working face of Xin\'an coal mine as engineering background, the stress and displacement of surrounding rock of roadway with different filling body width are analyzed through the FLAC3D numerical simulation software. As the filling body width increases, the supporting ability of the filling body increases and the deformation of the surrounding rock decreases. According to the theoretical calculation and numerical simulation of the filling body width, the filling body width is finally determined to be 3.5m. Through the field observation, the deformation of the surrounding rock of the roadway is within the reasonable range. It is concluded that the gob-side entry retaining with the high-water filling material can control the deformation of the surrounding rock, which provides a reference for gob-side entry retaining technology with similar geological conditions.

Key Words
gob-side entry retaining; high-water filling material; filling body; numerical simulation; field observation

Address
Tan Li: 1.) Mining Research Institute, Inner Mongolia University of Science and Technology, Baotou, Inner Mongolia, 014010, China
2.) College of Mining and Safety Engineering, Shandong University of Science and Technology, Qingdao, Shandong, 266590, China

Guangbo Chen: Mining Research Institute, Inner Mongolia University of Science and Technology, Baotou, Inner Mongolia, 014010, China

Zhongcheng Qin: 1.) College of Mining and Safety Engineering, Shandong University of Science and Technology, Qingdao, Shandong, 266590, China
2.) National Demonstration Center for Experimental Mining Engineering Education, Shandong University of Science and Technology, Qingdao, Shandong, 266590, China

Qinghai Li, Bin Cao and Yongle Liu: College of Mining and Safety Engineering, Shandong University of Science and Technology, Qingdao, Shandong, 266590, China

Abstract
In this study, a simple numerical approach for a circular tunnel opening in strain-softening surrounding rock is proposed considering out-of-plane stress and seepage force based on Biot\'s effective stress principle. The plastic region of strain-softening surrounding rock was divided into a finite number of concentric rings, of which the thickness was determined by the internal equilibrium equation. The increments of stress and strain for each ring, starting from the elastic-plastic interface, were obtained by successively incorporating the effect of out-of-plane stress and Biot\'s effective stress principle. The initial value of the outmost ring was determined using equilibrium and compatibility equations. Based on the Mohr–Coulomb (M–C) and generalized Hoek–Brown (H–B) failure criteria, the stress-increment approach for solving stress, displacement, and plastic radius was improved by considering the effects of Biot\'s effective stress principle and the nonlinear degradation of strength and deformation parameters in plastic zone incorporating out-of-plane stress. The correctness of the proposed approach is validated by numerical simulation.

Key Words
strain-softening; out-of-plane stress; seepage force; new numerical procedure; surrounding rock; Biot

Address
Luo Wei: Department of Civil Engineering, East China Jiaotong University, No.808, Shuanggang East Street,
Nanchang, Jiangxi Province, People\'s Republic of China, 330013

Jin-feng Zou and Wei An: Department of Civil Engineering, Central South University, No.22, Shaoshan South Road,
Central South University Railway Campus, Changsha, Hunan Province, People\'s Republic of China, 410075

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