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CONTENTS
Volume 25, Number 6, June25 2021
 

Abstract
In tunnel engineering, the selection of tunnel support patterns should be estimated accurately to ensure stability of the tunnel, which may be caused by unexpected hazardous zones ahead of tunnel face. This study presents a method to estimate the selection of support patterns using artificial neural network (ANN) based on 318, 649 Measurement While Drilling (MWD) data. Controlled trials are conducted considering different input layer sizes and hidden layer sizes to obtain the optimal ANN model. Combinations of 6 feature parameters including penetration rate (PR), hammer pressure (HP), rotation pressure (RP), feed pressure (FP), hammer frequency (HF) and specific energy (SE) correspond to the different input layer sizes of the ANN. Average accuracy (A), average computing-time (T), sensitivity and stability are adopted as the performance index. The results show that a strong correlation exists between MWD data and support patterns. The combination of 6 feature parameters outperforms the subset of the entire feature parameters in terms of A, sensitivity and stability. The ANN model with the combination of PR, HP, RP, FP, HF and SE as the input feature parameters has the highest estimation stability. The ANN model with 6 feature parameters and one hidden layer with 30 nodes is proposed as optimal model considering all indices. The results confirm that it is feasible to estimate support patterns ahead of tunnel face using ANN based on MWD data.

Key Words
tunnel support pattern; feature selection; measurement while drilling data; artificial neural network

Address
Jiankang Liu: College of Energy and Mining Engineering, Shandong University of Science and Technology, Qingdao 266590, China

Yujing Jiang and Yuanchao Zhang: Graduate School of Engineering, Nagasaki University, 1-14 Bunkyo-machi, 852-8521 Nagasaki, Japan

Osamu Sakaguchi: Department of Civil Engineering, Konoike Construction Co., Ltd., 3-6-1, Kitakyuhoji-machi, Chuo-ku, 541-0057 Osaka, Japan

Abstract
The objective of this study is investigating the effect of loading rates on the interaction between rock bolts and rock bridges using experimental test and numerical simulation. A new test set up was developed experimentally for determination of tensile strength of bridge area. A concrete block with dimensions of 15 x 15 x 10 cm consisting non-persistent notch was prepared and subjected to tensile loading using special loading set up. The configuration of non-persistent joint was different in various samples. A 30-ton hydraulic load cell applied tensile loading to concrete complex with a high-pressure rate of 0.01 mm per second. Simultaneously with experimental test, numerical simulation was performed on the tensile behavior of non-persistent joint adjacent to rock bolt. Two sets of non-persistent joint were prepared. The first sets were similar to experimental one while, in the second sets, two edge joints with lengths of 1.5 cm, 3 cm and 4.5 cm were prepared. The angle of these joint related to horizontal axis were 0, 15, 30, 45, 60, 75, and 90. Also, the rock bolts adjacent to joints were simulated and were subjected to tensile loading with two high and low loading rates i.e. 0.01 mm/sec and 0.0001 mm/sec. The results showed that the crack propagation angle related to tensile load direction was decreased by decreasing the tensile loading rate. The tensile failure stress decreased by presence of pre-existing crack within the model. Tensile failure stress had minimum value whenever the angle of pre-existing crack was 0o. The numerical results were in a good accordance with experimental ones.

Key Words
tensile strength; rock bolt; tensile crack; PFC2D

Address
Vahab Sarfarazi: Department of Mining Engineering, Hamedan University of Technology, Hamedan, Iran

Kaveh Asgari: Department of Mining Engineering, Shahid Bahonar University of Kerman, Kerman, Iran

Mehdi Nasrollahi: Department of Civil Engineering, Azad University of Hamedan, Hamedan, Iran

Abstract
The behavior of soil directly affects not only its stability condition but also structural response of structural systems. High-plasticity clay soil (CH) is vulnerable to volumetric swelling leading to different settlements in structural systems. Hence, it becomes indispensable to propose practical solutions to reducing this effect. In the present study, structural response of R/C frame buildings, resting on high plasticity clayey soils strengthened through the coal fly ash column technique, to earthquake motion is investigated. For this aim, the swelling behavior of high plasticity clay soil (CH) is identified with in-situ experimental tests on the regions with high swelling potential in the city of Kirikkale, Turkey. In order to reduce the swelling potential of the investigated regions, the coal fly ash column technique was implemented to the reference soil specimen with high swelling percentage of 15.6%. Experimental results obtained from the strengthened soil specimens were compared to those from the reference specimen. This comparison revealed that the coal fly ash column approach has a considerable effect on improving the swelling behavior of the high plasticity clay soil. The decrease in the volumetric swelling value is also thought to directly improve the response of a building structure settled on high plasticity clay soil. The improvement in the seismic response of existing R/C structures located in the regions with high swelling potential was identified by adopting the increased allowable bearing pressure value of the improved soil in the analyses. Based on the comparative study, structural earthquake response of R/C frame systems was investigated on the basis of the engineering parameters, including the base-shear force, base overturning moment, base axial force and settlement of foundation. The percent changes in these values showed that the base axial force and settlement of foundation were improved with the help of this strengthening application.

Key Words
fly ash column; high plasticity clay soil; earthquake performance; reinforced concrete structures, earthquake-resistant structure, time-history analysis

Address
Baran Toprak and Ilker Kalkan:Department of Civil Engineering, Faculty of Engineering and Architecture, Kirikkale University, 71451 Kirikkale, Turkey

Selcuk Bas : Department of Civil Engineering, Faculty of Engineering, Architecture and Design, Bartin University, 74100 Bart

Abstract
Hazardous failure phenomena such as rock bursts and slabbing failure frequently occur in deep hardrock tunnels, thus understanding the failure phenomena and mechanisms of the stress regime on tunnels is extremely critical. In this study, the tunnel system in a rock mass was physically modelled as a number of scaled openings in rock specimens, and the mechanical behavior of specimens having one to four horseshoe-shaped openings under uniaxial compression were investigated systematically. During the tests, the digital image correlation (DIC) and acoustic emission (AE) techniques were jointly employed to monitor the fracture response of specimens. After which, the stress distributions in the specimens were numerically analyzed and the stress concentration factor on the periphery of the opening was calculated. The results show that the number of openings have a significant impact on the weakening effect of rock mechanical properties. The progressive cracking process of the specimens with openings evolves from first-tensile cracks through second-tensile cracks and spalling cracks to shear cracks, and the crack threshold stresses are measured. Two failure modes are formed: shear failure and shear-tensile failure. According to the stress distribution law around the opening, the crack initiation mechanism can be fully explained. This research provides an insight to failure mechanism of hardrock tunnel.

Key Words
hardrock tunnel; fracture behavior; failure mechanism; digital image correlation; acoustic emission; stress distribution

Address
Hao Wu, Dan Ma and A.J.S. Spearing: School of Mines, China University of Mining and Technology, Xuzhou 221116, China

Guoyan Zhao: School of Resources and Safety Engineering, Central South University, Changsha 410083, China

Abstract
Water inrush generally has a serious impact on karst shallow tunnel construction. Because of in situ fault fracture zone, high degree of weathering and poor quality of rock mass,. karst shallow tunnel would therefore face high risk of water inrush from surface during the disturbance of construction. In addition, the greater the surface water flow would contribute higher probability of water inrush under the same disaster-causing environment. However, existing research has paid less attention to the influence of surface water flow on faults or fissures water inrush. In this study, a risk assessment system of water inrush in karst shallow tunnel with stable surface water supply was firstly proposed on basis of Qinling Water Conveyance Tunnel and the Yuelongmen Tunnel in China. Each indicator was quantified and classified into four risk levels by the attribute mathematics theory and analytic hierarchy process, the degree of confidence criterion was then applied to identify the risk level of the water inrush. The evaluation results were finally verified by actual scenario on site to confirm the validity of this risk assessment system in karst shallow tunnel with stable surface water supply. Accordingly, the proposed method could be popularized and applied in future tunnel projects, because it could provide safe construction reference for karst shallow overburden tunnel with stable surface water supply.

Key Words
risk assessment; water inrush; karst shallow tunnel; stable surface water supply; attribute recognition model

Address
Zengguang Xu, Yaping Wang and Junrui Chai: State Key Laboratory of Eco-hydraulics in Northwest Arid Region of China, Xi'an University of Technology, Xi'an 710048, Shaanxi, China

Meiting Xian:1.) State Key Laboratory of Eco-hydraulics in Northwest Arid Region of China, Xi'an University of Technology, Xi'an 710048, Shaanxi, China
2.) Sichuan Water Resources and Hydroelectric Investigation & Design Institute Co. Ltd., Chengdu 610072, Sichuan, China

Xiaofeng Li and Jiaming Wang: Hanjiang-to-Weihe River Valley Water Diversion Project Construction Co., Ltd., Xi'an 710010, Shaanxi, China

Wei Zhou: Shaanxi Province Institute of Resources and Electric Power Investigation and Design, Xi'an 710001, Shaanxi, China


Abstract
To reveal the failure mechanism of roadway surrounding rock under the partial confining stress in deep mining, by means of theoretical analysis and numerical simulation, this paper studied the distribution laws of the principal stress field around the circular hole and compared the shapes of the plastic zone surrounding rock under the same conditions. The results show that: under hydrostatic stress (λ=1), the circumferential principal stress around the hole is the same everywhere, and the shape of plastic zone is circular; under low partial confining stress (1<λ<2), the rock element at the abscissa axis is most likely to be destroyed, while it is the least likely to be destroyed at the ordinate axis, resulting in the formation of an elliptical plastic zone; under high partial confining stress (λ≥2), the rock elements near the middle axis are easier to be destroyed, while the destructive force decreases gradually when it approaches the two axes, resulting in the formation of a butterfly plastic zone. The lateral stress coefficient is the main factor causing the butterfly failure of the roadway surrounding rock. And the depth is the main factor causing the large-scale failure of the roadway surrounding rock. Under the condition of deep and high partial confining stress, the roadway surrounding rock will appear large-scale and butterfly failure zone.

Key Words
partial confining stress; plastic zone shape; lateral stress coefficient; circular roadway; deep mining

Address
Xiaofei Guo, Chen Li and Tianhong Huo: School of Energy and Mining Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China

Abstract
This work presents seismic stability analysis of tunnel faces under three-dimensional (3D) conditions. To consider the temporal and spatial features of seismic force, the pseudo-dynamic approach was employed and incorporated into the 'horn-like' mechanism. According to the limit analysis method and the Hoek-Brown strength criterion, analytical solution of collapse pressure on tunnel faces was derived. The permanent displacement of tunnel face was then calculated by virtue of the Newmark method. The effects of the parameters of seismic force and Hoek-Brown strength criterion on collapse pressure and failure range of tunnel faces were analyzed. The relationship between the Hoek-Brown strength criterion parameters, the supporting force, and the yield acceleration was discussed. Moreover, the permanent displacements at the top, center, and bottom of tunnel faces under seismic effect were examined. This paper proposes a new idea for seismic stability analysis of tunnel faces.

Key Words
Hoek-Brown strength criterion; tunnel faces; supporting force; the Newmark method; the kinematic approach

Address
Biao Zhang: 1.) School of Civil Engineering, Hunan University of Science and Technology, Hunan 411201, China
2.) School of Civil Engineering, Central South University, Hunan 410075, China

Jin Jiang and Dao-bing Zhang:School of Resource and Environment and Safety Engineering, Hunan University of Science and Technology, Hunan, 411201, China

Ze Liu: School of Civil Engineering, Hunan University of Science and Technology, Hunan 411201, China

Abstract
The Chinese Loess Plateau is located in northwestern China. During investigations in loess fields, it was found that, under dry climatic conditions, cracks were prone to appear and propagate in the loess plateau. The presence of cracks and their effect on the engineering properties of loess greatly influence the stability of soil-based structures and can cause severe damages. The purpose of this research was to analyze the drying-induced cracking mechanisms in clayey loess, and to highlight the effect of soil mineralogy. Laboratory tests were performed to simulate the cracking process in loess. The analysis is based on the local two-dimensional strain and displacement fields derived from the Digital Image Correlation (DIC) method and the software VIC-2D. The determination of the mechanical strain tensors, i.e., the difference between the total strain and shrinkage strain tensors, and their visual representation allow a deeper understanding of cracking mechanisms. Based on the above methods, crack development and crack coalescence processes were observed and analyzed. Besides, other mechanisms were identified, such as junction and bifurcation of cracks.

Key Words
clayey loess; digital image correlation; crack development; mechanical strain tensors; coalescence; bifurcation; junction processes

Address
Xin Wei and Ling Xu: School of Human Settlement and Civil Engineering, Xi'an Jiaotong University, 28 Xianning Road, 700054 Xi'an, Shaanxi Province, People's Republic of China

Mahdia Hattab; Laboratory of Microstructures and Mechanics of Materials, University of Lorraine, CNRS UMR 7239, Arts et Métiers ParisTech, 57000 Metz, France


Said Taibi: Laboratory of Waves and Complex Media, University of Havre, CNRS UMR 6294, 25 rue Philippe Lebon, 76600 Le Havre, France

Katia V. Bicalho: Department of Civil Engineering, Federal University of Espirito Santo, Av. Fernando Ferrari, 514 - Goiabeiras, Vitoria - ES, Brasil, 29075-910


Jean-Marie Fleureau: Laboratory of Mechanics of Soils, Structures and Materials, Paris-Saclay University, Centrale Supélec CNRS UMR 8579, 3 rue Joliot Curie, 91190 Gif sur Yvette, France


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