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CONTENTS
Volume 15, Number 5, August10 2018
 

Abstract
A superposition-iteration (S-I) model is proposed to simulate the jet grouting pre-reinforcing impact for a shallow-buried tunnel. The common model is deduced by theoretical (force equilibrium) analysis and then transformed into the numerical formulation. After applying it to an actual engineering problem, the most obvious deficiency was found to be continuous error accumulation, even when the parameters change slightly. In order to address this problem, a superposition-iteration model is developed based on the basic assumption and superposition theory. First, the additional deflection between two successive excavation steps is determined. This is caused by the disappearance of the supporting force in the excavated zone and the soil pressure in the disturbed zone. Consequently, the final deflection can be obtained by repeatedly superposing the additional deflection to the initial deflection in the previous steps. The analytical solution is then determined with the boundary conditions. The superposition-iteration model is thus established. This model was then applied and found to be suitable for real-life engineering applications. During the calculation, the error induced by the ill-conditioned problem of the matrix is easily addressed. The precision of this model is greater compared to previous models. The sensitivity factors and their impact are determined through this superposition-iteration model.

Key Words
superposition-iteration; tunnel reinforcement; horizontal jet grouting; analytical solution

Address
Ning Zhang: Institute of Hydroelectric and Geotechnical Engineering, North China Electric Power University, Beijing, 102206, China

Zhongyin Li and Qingsong Ma: Beijing Sany Heavy Machinery Co., Ltd, Beijing 102226, China

Tianchi Ma, Xiaodong Niu, Xixi Liu and Tao Feng: 1.) Shandong Provincial Key Laboratory of Ocean Engineering, Qingdao 266100, China
2.) Shandong Provincial Key Laboratory of Marine Environment and Geological Engineering, Ocean University of China,
266100 Qingdao, China



Abstract
This paper presents a limit analysis of the series of construction stages of shallow tunneling method by investigating their respective safety factors and failure mechanisms. A case study for one particular cross-section of Beijing Subway Line 7 is undertaken, with a focus on the effects of multiple soil layers and construction sequencing of dual tunnels. Results show that using the step-excavation technique can render a higher safety factor for the excavation of a tunnel compared to the entire cross-section being excavated all at once. The failure mechanisms for each different construction stage are discussed and corresponding key locations are suggested to monitor the safety during tunneling. Simultaneous excavation of dual tunnels in the same cross-section should be expressly avoided considering their potential negative interactions. The normal and shear forces as well as bending moment of the primary lining and locking anchor pipe are found to reach their maximum value at Stage 6, before closure of the primary lining. Designing these struts should consider the effects of different construction stages of shallow tunneling method.

Key Words
limit analysis; strength reduction; shallow tunneling method; stability

Address
Shengbing Yu: 1.) School of Civil Engineering and Water Conservancy, Ningxia University, Yinchuan 750021, China
2.) Engineering Research Center of Efficient Use of Water Resources in Arid Modern Agriculture, Ministry of Education,
Yinchuan 750021, China
3.) Ningxia Irrigation and Water Resources Control Engineering Technology Research Center, Yinchuan 750021, China


Abstract
This paper focuses on the cracking and fragmentation process in rock materials containing a pair of non-parallel flaws, which are through the specimen thickness, under vertical compression. Several numerical experiments are conducted with varying flaw arrangements that affect the initiation and tensile wing cracks, shear crack growth, and crack coalescing behaviors. To obtain realistic numerical results, a parallelized peridynamics formulation coupled with a finite element method, which is able to capture arbitrarily occurring cracks, is employed. From previous studies, crack initiation and propagation of tensile wing cracks, horsetail cracks, and anti-wing cracks are well understood along with the coalescence between two parallel flaws. In this study, the coalescence behaviors, their fragmentation sequences, and the role of an x-shaped shear band in rock material containing two non-parallel flaws are discussed in detail on the basis of simulation results strongly correlated with previous experimental results. Firstly, crack initiation and propagation of tensile wing cracks and shear cracks between non-parallel flaws are investigated in time-history and then sequential coalescing behavior is analyzed. Secondly, under the effect of varying inclination angles of two non-parallel flaws and overlapping ratios between a pair of non-parallel flaws, the cracking patterns including crack coalescence, fragmentation, and x-shaped shear band are investigated. These numerical results, which are in good agreement with reported physical test results, are expected to provide insightful information of the fracture mechanism of rock with non-parallel flaws.

Key Words
non-parallel flaws; progressive failure; crack coalescence; fragmentation; x-shaped shear band

Address
Jooeun Lee and Jung-Wuk Hong: Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, Daejeon 34141, South Korea

Abstract
The main purpose of retaining wall methods for deep excavation is to keep the construction site safe from the earth pressure acting on the backfill during the construction period. Currently used retaining wall methods include the common strut method, anchor method, slurry wall method, and raker method. However, these methods have drawbacks such as reduced workspace and intrusion into private property, and thus, efforts are being made to improve them. The most advanced retaining wall method is the prestressed wale system, so far, in which a load corresponding to the earth pressure is applied to the wale by using the tension of a prestressed (PS) strand wire. This system affords advantages such as providing sufficient workspace by lengthening the strut interval and minimizing intrusion into private properties adjacent to the site. However, this system cannot control the tension of the PS strand wire, and thus, it cannot actively cope with changes in the earth pressure due to excavation. This study conducts a preliminary numerical analysis of the field applicability of the controllable prestressed wale system (CPWS) which can adjust the tension of the PS strand wire. For the analysis, back analysis was conducted through two-dimensional (2D) and three-dimensional (3D) numerical analyses based on the field measurement data of the typical strut method, and then, the field applicability of CPWS was examined by comparing the lateral deflection of the wall and adjacent ground surface settlements under the same conditions. In addition, the displacement and settlement of the wall were predicted through numerical analysis while the prestress force of CPWS was varied, and the structural stability was analysed through load tests on model specimens.

Key Words
retaining wall methods, controllable prestressed wale system (CPWS), finite element analysis, indoor model test

Address
Chang Il Lee, Eun Kyum Kim and Yong-Joo Lee: Department of Civil Engineering, Seoul National University of Science and Technology,232 Gongneung-ro, Nowon-gu, Seoul 01811, Republic of Korea

Jong Sik Park: Civil Business Team, TAEYOUNG E&C, 111 Yeouigongwon-ro, Yeongdeungpo-gu, Seoul 07241, Republic of Korea


Abstract
The study of behavior and values of deformations in the geological medium makes the scientific basis of the methodology of synthesis of true values of parameters of its physico-mechanical and density properties taking into account the influence of geodynamic impacts. The segments of continuous variation of homogeneous elastic uniform deformations are determined under overall compression of the medium. The limits of these segments are defined according to the criteria of instability (on geometric form changes and on \"internal\" instability). Analytical formulae are obtained to calculate current and limiting (critical) values of deformations within the framework of various variants of small and large initial deformations of the non-classically linearized approach of non-linear elastodynamics. The distribution of deformation becomes non-uniform in the medium while the limiting values of deformations are achieved. The proposed analytical formulae are applicable only within homogeneous distribution of deformations. Numerical experiments are carried out for various elastic potentials. It is found that various forms of instability can precede phase transitions and destruction. The influence of these deformation phenomena should be removed while the physico-mechanical and density parameters of the deformed media are determined. In particular, it is necessary to use the formulae proposed in this paper for this purpose.

Key Words
nonlinear elastodynamics; instability of the equilibrium state; elastic potentials; physico-mechanical and density parameters

Address
Hatam H. Guliyev and Gular H. Hasanova: Department of Tectonopysics and Geomechanics, Institute of Geology and Geophysics of Azerbaijan National Academy of Sciences (ANAS), Ave. H. Javid 119, Baku, AZ 1143 Azerbaijan

Rashid J. Javanshir: Azerbaijan National Academy of Sciences, Str. Istiglaliyyat, 30, Baku, AZ 1001, Azerbaijan

Abstract
The deformation and strength of brittle rocks are significantly influenced by the crack closure behavior. The relationship between the strength and deformation of rocks under uniaxial loading is the foundation for design and assessment of such scenarios. The concept of relative crack closure strain was proposed to describe the influence of the crack closure behavior on the deformation and strength of rocks. Considering the crack compaction effect, a new damage constitutive model was developed based on accumulated AE counts. First, a damage variable based on the accumulated AE counts was introduced, and the damage evolution equations for the four types of brittle rocks were then derived. Second, a compaction coefficient was proposed to describe the compaction degree and a correction factor was proposed to correct the error in the effective elastic modulus instead of the elastic modulus of the rock without new damage. Finally, the compaction coefficient and correction factor were used to modify the damage constitutive model obtained using the Lemaitre strain equivalence hypothesis. The fitted results of the models were then compared with the experimental data. The results showed that the uniaxial compressive strength and effective elastic modulus decrease with an increase in the relative crack closure strain. The values of the damage variables increase exponentially with strains. The modified damage constitutive equation can be used to more accurately describe the compressive deformation (particularly the compaction stage) of the four types of brittle rocks, with a coefficient of determination greater than 0.9.

Key Words
uniaxial loading; constitutive equation; damage; acoustic emission; compaction coefficient; correction factor

Address
Qingheng Gu, Qing Ma and Qiang Xu: School of Mining and Safety Engineering, Shandong University of Science and Technology, Qingdao, Shandong 266590, PR China

Jianguo Ning, Yunliang Tan and Xuesheng Liu: 1.) 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, PR China
2.) School of Mining and Safety Engineering, Shandong University of Science and Technology, Qingdao, Shandong 266590, PR China


Abstract
In this study, the relationships between hydraulic conductivity of GCLs and physico-chemical properties of bentonites were assessed. In addition to four factory manufactured GCLs, six artificially prepared GCLs (AP-GCLs) were tested. AP-GCLs were prepared in the laboratory without bonding or stitching. A total of 20 hydraulic conductivity tests were conducted using flexible wall permeameters ten of which were permeated with distilled deionized water (DIW) and the rest were permeated with tap water (TW). The hydraulic conductivity of GCLs and AP-GCLs were between 5.2 10-10 cm/s and 3.0 10-9 cm/s. The hydraulic conductivities of all GCLs to DIW were very similar to that of GCLs to TW. Then, simple regression analyses were conducted between hydraulic conductivity and physicochemical properties of bentonite. The best correlation coefficient was achieved when hydraulic conductivity was related with clay content (R=0.85). Liquid limit and plasticity index were other independent variables that have good correlation coefficients with hydraulic conductivity (R~0.80). The correlation coefficient with swell index is less than other parameters, but still fairly good (R~0.70). In contrast, hydraulic conductivity had poor correlation coefficients with specific surface area (SSA), smectite content and cation exchange capacity (CEC) (i.e., R < 0.5). Furthermore, some post-test properties of bentonite such as final height and final water content were correlated with the hydraulic conductivity as well. The hydraulic conductivity of GCLs had fairly good correlation coefficients with either final height or final water content. However, those of AP-GCLs had poor correlations with these variables on account of fiber free characteristics.

Key Words
bentonite, clay content, consistency limits, swell index, geosynthetic clay liners (GCLs), hydraulic conductivity, physicochemical properties

Address
A. Hakan Ören, Yeliz Yükselen Aksoy and Okan Önal: Department of Civil Engineering, Dokuz Eylül University, 35390, Buca-Izmir, Turkey

Havva Demirk

Abstract
This study aims to present accurate soil modeling and validation of a single roadside guardrail post as well as a single concrete pile installed near cut slopes or compacted sloping embankment. The conventional Winkler\'s elastic spring model and p-y curve approach for horizontal ground cannot directly be applied to sloping ground where ultimate soil resistance is significantly dependent on ground inclination. In this study, both grid-based 3-D FE model and particle-based SPH (smoothed particle hydrodynamics) model available in LS-DYNA have been adopted to predict the static behavior of a laterally loaded guardrail post. The SPH model has potential to eliminate any artificial soil stiffness due to the deterioration of the node-connected Lagrangian soil mesh. For this purpose, this study comprises two parts. Firstly, only 3-D FE modeling has been tested to show the numerical validity for a single concrete pile in sloping ground using Mohr-Coulomb material. However, this material option cannot be implemented for SPH elements. Nevertheless, Mohr-Coulomb model has been used since this material model requires six input soil data that can be obtained from the comparative papers in literatures. Secondly, this work is extended to compute the lateral resistance of a guardrail post located near the slope using the hybrid approach that combines Lagrange FE elements and SPH elements by the suitable node-merging option provided by LS-DYNA. For this analysis, the FHWA soil material developed for application to road-base soils has been used and also allows the application of SPH element.

Key Words
finite element method; nonlinearity; numerical analyses; soil modeling; soil-structure interaction

Address
Kwang S. Woo and Seung H. Yang: Department of Civil Engineering, Yeungnam University, 280 Daehak-Ro, Gyeongsan, Gyeongbuk 38541, Republic of Korea

Dong W. Lee: 2Soohyung Industry Development Co. Ltd. 47-16 Gongdan-ro 11-gil, Waegwan-eup, Chilgok-gun, Gyeongbuk 39910, Republic of Korea

Jae S. Ahn: School of General Education, Yeungnam University, 280 Daehak-Ro, Gyeongsan, Gyeongbuk 38541, Republic of Korea

Abstract
Geological dynamic hazards during deep coal mining are caused by the failure of a composite system consisting of the rock and coal layers, whereas the joint in coal affects the stability of the composite system. In this paper, the compression test simulations for the rock-coal combined body with single joint in coal were conducted using PFC2D software and especially the effects of joint length and joint angle on strength and failure characteristics in a rock-coal combined body were analyzed. The joint length and joint angle exhibit a deterioration effect on the strength and affect the failure modes. The deterioration effect of joint length of L on the strength can be neglected with a tiny variation at alpha of 0o or 90o between the loading direction and joint direction. While, the deterioration effect of L on strength are relatively large at alpha between 30o and 60o. And the peak stress and peak strain decrease with the increase of L. Additionally, the deterioration effect of alpha on the strength becomes larger with the increase of L. With the increase of alpha, the peak stress and peak strain first decrease and then increase, presenting \"V-shaped\" curves. And the peak stress and peak strain at alpha of 45o are the smallest. Moreover, the failure mainly occurs within the coal and no apparent failure is observed for rock. At alpha between 30o and 60o, the secondary shear cracks generated in or close to the joint tips, cause the structural instability failure of the combined body. Therefore, their failure models present as a shear failure along partial joint plane direction and partially cutting across the coal body or a shear failure along the joint plane direction. However, at alpha of 60o and L of 10 mm, the \"V-shaped\" shear cracks cutting across the coal body cause its final failure. While crack nucleations at alpha of 0o or 90o are randomly distributed in the coal, the failure mode shows a V-shaped shear failure cutting across the coal body.

Key Words
particle flow simulation; strength and failure characteristics; rock-coal combined body; single joint in coal; joint length and joint angle

Address
Da W. Yin and Shao J. Chen: 1.) College of Mining and Safety Engineering, Shandong University of Science and Technology, 579 Qianwanggang Road, Huangdao District, Qingdao, Shandong Province, 266590, China
2.) Key Laboratory of Safety and High-efficiency Coal Mining, Ministry of Education, Anhui University of Science and Technology, Huainan 232001, China

Bing Chen, Xing Q. Liu and Hong F. Ma: College of Mining and Safety Engineering, Shandong University of Science and Technology, 579 Qianwanggang Road, Huangdao District, Qingdao, Shandong Province, 266590, China

Abstract
Real-time characterization of the rock thermal deformation and fracture process provides guidance for detecting and evaluating thermal stability of rocks. In this paper, time -frequency characteristics of acoustic emission (AE) and electromagnetic radiation (EMR) signals were studied by conducting experiments during rock continuous heating. The coupling correlation between AE and EMR during rock thermal deformation and failure was analyzed, and the microcosmic mechanism of AE and EMR was theoretically analyzed. During rock continuous heating process, rocks simultaneously produce significant AE and EMR signals. These AE and EMR signals are, however, not completely synchronized, with the AE signals showing obvious fluctuation and the EMR signals increasing gradually. The sliding friction between the cracks is the main mechanism of EMR during the rock thermal deformation and fracture, and the AE is produced while the thermal cracks expanding. Both the EMR and AE monitoring methods can be applied to evaluate the thermal stability of rock in underground mines, although the mechanisms by which these signals generated are different.

Key Words
rock thermal treatment; AE; EMR; coupling correlation

Address
Biao Kong: Key Lab of Mine Disaster Prevention and Control, College of Mining ad Safety Engineering, Shandong University of Science and Technology, Qingdao, Shandong 266590, China

Enyuan Wang and Zenghua Li: 1.) Key Laboratory of Coal Methane and Fire Control, Ministry of Education, China University of Mining and Technology, Xuzhou 221116, China
2.) School of Safety Engineering, China University of Mining and Technology, Xuzhou, Jiangsu, 221116, China



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