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CONTENTS
Volume 29, Number 4, May25 2022
 


Abstract
The stability of the roof rock-coal pillar-floor rock composite structure is of great significance to coal mine safety production. The cracks existing in the composite structure seriously affect the stability of the roof rock-coal pillar-floor rock composite structure. The numerical simulation tests of rock-coal-rock composite structures with different crack characteristics were carried out to reveal the composite structures' mechanical properties and failure mechanisms. The test results show that the rock-coal-rock composite structure's peak stress and elastic modulus are directly proportional to the crack angle and inversely proportional to the crack length. The smaller the crack angle, the more branch cracks produced near the main control crack in the rock-coal-rock composite structure, and the larger the angle between the main control crack and the crack. The smaller the crack length, the larger the width of the crack zone. The impact energy index of the rock-coal-rock composite structure decreases first and then increases with the increase of crack length and increases with the increase of crack angle. The functional relationships between the different crack characteristics, peak stress, and impact energy index are determined based on the sensitivity analysis. The determination of the functional relationship can fully grasp the influence of the crack angle and the crack length on the peak stress and impact energy index of the coal-rock composite structure. The research results can provide a theoretical basis and guidance for preventing the instability and failure of the coal pillar-roof composite structure.

Key Words
crack characteristic; energy characteristic; mechanical property; rock-coal-rock composite structure; sensitivity analysis

Address
Tan Li: Institute of Mining and Coal, Inner Mongolia University of Science and Technology, Baotou 014010, China;
College of Energy and Mining Engineering, Shandong University of Science and Technology, Qingdao 266590, China
uangbo Chen: Institute of Mining and Coal, Inner Mongolia University of Science and Technology, Baotou 014010, China
Qinghai Li: College of Energy and Mining Engineering, Shandong University of Science and Technology, Qingdao 266590, China

Abstract
A newly proposed grouting simulation method, the sequential diffusion solidification method was introduced into the numerical simulation of combined bi-borehole grouting. The traditional, critical and difficult numerical problem for the temporal and spatial variation simulation of the slurry is solved. Thus, numerical simulation of grouting and blocking of flowing water in karst conduits is realized and the mechanism understanding of the combined bi-borehole technology is promoted. The sensitivity analysis of the influence factors of combined bi-borehole grouting was investigated. Through orthogonal experiment, the influences of proximal and distal slurry properties, the initial flow velocity of the conduit and the proximal and distal slurry injection rate on the blocking efficiency are compared. The velocity variation, pressure variation and slurry deposition phenomenon were monitored, and the flow field characteristics and slurry outflow behavior were analyzed. The interaction mechanism between the proximal and distal slurries in the combined bi-borehole grouting is revealed. The results show that, under the orthogonal experiment conditions, the slurry injection rate has the greatest impact on blocking. With a constant slurry injection rate, the blocking efficiency can be increased by more than 30% when using slurry with weak time-dependent viscosity behavior in the distal borehole and slurry with strong time-dependent viscosity behavior in the proximal borehole respectively. According to the results of numerical simulation, the grouting scheme of "intercept the flow from the proximal borehole by quick-setting slurry, and grout cement slurry from the distal borehole" is put forward and successfully applied to the water inflow treatment project of China Resources Cement (Pingnan) Limestone Mine.

Key Words
blocking mechanism; combined bi-borehole grouting; flowing water grouting; orthogonal experiment

Address
Dongdong Pan,Haiyan Li and Zhaofeng Li: Geotechnical and Structural Engineering Research Center, Shandong University,
Jinan, Shandong 250-061, China;
School of Civil Engineering, Shandong University, Jinan, Shandong 250-061, China
Yichi Zhang and Zhenhao Xu: Geotechnical and Structural Engineering Research Center, Shandong University,
Jinan, Shandong 250-061, China;
School of Qilu Transportation, Shandong University, Jinan, Shandong 250-061, China

Abstract
Embedded piles, which are typically used in Korea, are precast piles inserted into prebored ground with cement paste. Dynamic pile tests tend to underestimate the bearing capacity of embedded piles because of the undeveloped shaft resistance prior to the curing of the cement paste and the insufficient energy transferred after the curing. In this study, a resistance combination method using the base resistance before the cement paste is cured and the shaft resistance after the cement paste is cured is proposed to obtain a combined load–settlement curve from dynamic pile tests. Two pairs of embedded piles with diameters of 600 and 500 mm are installed. Each pair comprises one pile for the dynamic pile test and another pile for the static load test. The shape of the load–settlement curve obtained using the proposed method is similar to that obtained from the static load test. Thus, the resistances evaluated using the proposed method at selected settlements are similar to those obtained from the static load test. This study shows that the resistance combination method may be used effectively in dynamic pile tests to accurately evaluate the bearing capacity of embedded piles.

Key Words
base resistance; dynamic pile test; embedded pile; load–settlement curve; shaft resistance

Address
Mi Jeong Seo, Dongsoo Lee and Jong-Sub Lee: School of Civil, Environmental and Architectural Engineering, Korea University,
145, Anam-ro, Seongbuk-gu, Seoul, 02841, Republic of Korea
Jong-Bae Park: Land and Housing Institute, Korea Land & Housing Corporation,
99, Expo-ro 539beon-gil, Yuseong-gu, Daejeon, 34047, Republic of Korea

Abstract
In the present study, reference samples were prepared using ore preparation facility tailings taken from the copper mine (Kure, Kastamonu), Portland cement (PC) in certain proportions (3 wt%, 5 wt%, 7 wt%, 9wt% and 11 wt%), and water. Then natural zeolite taken from the Bigadic Region was mixed in certain proportions (10 wt%, 20 wt%, 30 wt% and 40 wt%) for each cement ratio, instead of the PC, to prepare zeolite-substituted CPB samples. Thus, the effect of using Zeolite instead of PC on CPB's strength was investigated. The obtained CPB samples were kept in the curing cabinet at a temperature of 25C and at least 80% humidity, and they were subjected to the Uniaxial Compressive Strength (UCS) test at the end of the curing periods of 3, 7, 14, 28, 56, and 90 days. Except for the 3 wt% cement ratio, zeolite substitution was observed to increase the compressive strength in all mixtures. Also, the liquefaction risk limit for paste backfill was achieved for all mixtures, and the desired strength limit value (0.7 MPa) was achieved for all mixtures with 28 days of curing time and 7 wt%, 9 wt%, 11 wt% cement ratios and 5% cement – 10% zeolite substituted mixture. Moreover, the limit value (4 MPa) required for use as roof support was obtained only for mixtures with 11% cement – 10% and 20% zeolite content. Generally, zeolite substitution seems to be more effective in early strength (up to 28th day). It has been determined that the long-term strength losses of zeolite-substituted paste backfill mixtures were caused by the reaction of sulfate and hydration products to form secondary gypsum, ettringite, and iron sulfate.

Key Words
cement; cemented paste backfill (CPB); tailings; uniaxial compressive strength (UCS); zeolite

Address
Hasan Eker: Eskipazar Vocational School, Property Protection and Safety Division, Occupational Health and Safety, Karabuk University, Karabuk, Turkey
Atac Bascetin: Department of Mining Engineering, Mining Faculty, Istanbul Technical University, Maslak, Istanbul, Turkey

Abstract
In this paper, unreinforced and geogrid-reinforced soil foundations were modeled by discrete element method and this performed under surface strip footing loads. The effects of horizontal position of geogrid, vertical position, thickness, number, confining pressure have been investigated on the footing settlement and propagation of tensile force along the geogrids. Also, interaction between rectangular tunnel and strip footing with and without presence of geogrid layer has been analyzed. Experimental results of the literature were used to validation of relationships between the numerically achieved footing pressure-settlement for foundations of reinforced and unreinforced soil. Models and micro input parameters which used in the numerical modelling of reinforced and unreinforced soil tunnel were similar to parameters which were used in soil foundations. Model dimension was 1000 mm* 600 mm. Normal and shear stiffness of soils were 5*105 and 2.5 *105 N/m, respectively. Normal and shear stiffness of geogrid were 1*109 and 1*109 N/m, respectively. Loading rate was 0.001 mm/sec. Micro input parameters used in numerical simulation gain by try and error. In addition of the quantitative tensile force propagation along the geogrids, the footing settlements were visualized. Due to collaboration of three layers of geogrid reinforcements the bearing capacity of the reinforced soil tunnel was greatly improved. In such practical reinforced soil formations, the qualitative displacement propagations of soil particles in the soil tunnel and the quantitative vertical displacement propagations along the soil layers/geogrids represented the geogrid reinforcing impacts too.

Key Words
geogrid; footing settlement; PFC 2D; strip footing

Address
Vahab Sarfarazi: Department of Mining Engineering, Hamedan University of Technology, Hamedan, Iran
Abdollah Tabaroei: Department of Civil Engineering, Eshragh Institute of Higher Education, Bojnourd, Iran
Kaveh Asgari: Department of Mining Engineering, Shahid Bahonar University of Kerman, Kerman, Iran

Abstract
The roadways surrounded by rock and coal will lose their stability or even collapse under rock burst. Rock burst mainly involves an evolution of dynamic loading which behaves quite differently from static or quasi-static loading. To compare the dynamic response of coal and rocks with different static strengths, three different rocks and bituminous coal were selected for testing at three different dynamic loadings. It's found that the dynamic compression strength of rocks and bituminous coal is much greater than the static compression strength. The dynamic compression strength and dynamic increase factor of the rocks both increase linearly with the increase of the strain rate, while those of the bituminous coal are irregular due to the characteristics of multi-fracture and heterogeneity. Moreover, the absorbed energy of the rocks and bituminous coal both increase linearly with an increase in the strain rate. And the ratio of absorbed energy to the total energy of bituminous coal is greater than that of rocks. With the increase of dynamic loading, the failure degree of the sample increases, with the increase of the static compressive strength, the damage degree also increases. The static compassion strength of the bituminous coal is lower than that of rocks, so the number of small-scale fragments was the largest after bituminous coal rupture.

Key Words
compression strength; dynamic loading; SHPB; strain rate; stress-strain

Address
Jingxuan Zhou: Faculty of Safety Engineering, China University of Mining and Technology, Xuzhou, Jiangsu, 221116, China;
Department of Structural Engineering, Tongji University, Shanghai, 200092, China
Chuanjie Zhu, Jie Ren, Ximiao Lu, Cong Ma and Ziye Li: Faculty of Safety Engineering, China University of Mining and Technology, Xuzhou, Jiangsu, 221116, China


Abstract
In NATM tunnels, water inrush and tunnel collapse are often encountered in silty-fine sand with abundant water during excavation. Because of the special engineering properties of this stratum, grouting effect is difficult to achieve as expected, and it is a major problem in the field of civil engineering. Taking Beijing Metro Line 10 as a case, we applied PFC3D to simulate the process of grouting in this stratum. By analyzing the law of grout diffusing and porosity change under different grouting pressures, the study found that grouting was a process of splitting, and grouting pressure played an important role. The numerical results were verified by theoretical calculation analysis, and the grouting parameters were determined under the various grouting pressures for practice. After the excavation of this tunnel, the concretions in silty-fine sand are similar to the results of PFC3D simulation, which indicates that the grouting mechanism is confirmed by field observation further.

Key Words
grounting pressure; NATM tunnel; silty-fine sand with abundant wate; splitting grounting

Address
Jun Liu, Liang Zhang, Tian You and Yuqian Wu: School of Civil and Transportation Engineering, Beijing University of Civil Engineering and Architecture, Beijing 100044, China
Hongsong Xue: BCEG Civil Engineering co.,LTD, Beijing 100044, China

Abstract
The effect of segmental joints is one of main importance for the segmental lining design when tunnels are excavated by a mechanized process. In this paper, segmental tunnel linings are analyzed by two numerical methods, namely the Beam-Spring Method (BSM) and the Solid-Interface Method (SIM). For this purpose, the Tehran Subway Line 6 Tunnel is considered to be the reference case. Comprehensive 2D numerical simulations are performed considering the soil's calibrated plastic hardening model (PH). Also, an advanced 3D numerical model was used to obtain the stress relaxation value. The SIM numerical model is conducted to calculate the average rotational stiffness of the longitudinal joints considering the joints bending moment distribution and joints openings. Then, based on the BSM, a sensitivity analysis was performed to investigate the influence of the ground rigidity, depth to diameter ratios, slippage between the segment and ground, segment thickness, number of segments and pattern of joints. The findings indicate that when the longitudinal joints are flexible, the soil-segment interaction effect is significant. The joint rotational stiffness effect becomes remarkable with increasing the segment thickness, segment number, and tunnel depth. The pattern of longitudinal joints, in addition to the joint stiffness ratio and number of segments, also depends on the placement of longitudinal joints of the key segment in the tunnel crown (similar to patterns B and B').

Key Words
beam-spring; joint position; rotational stiffness; segment thickness; soil-segment; solid-interface

Address
Alireza Rashiddel and Mohsen Hajihassani: Department of Mining, Faculty of Engineering, Urmia University, Urmia, Iran
Mehdi Kharghani: Department of Mining, Faculty of Engineering, Islamic Azad University, Science and Research Branch of Tehran, Iran
Hadi Valizadeh: Civil Engineering Department, Özyeğin University, 34794 Çekmeköy, Istanbul, Turkey
Reza Rahmannejad: Department of Mining, Faculty of Engineering, Shahid Bahonar University, Kerman, Iran
Daniel Dias: Department of Civil Engineering, Grenoble Alps University, Laboratory 3SR, Polytech Grenoble, France;
Geotechnical Expert, Antea Group, Antony, France



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