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CONTENTS
Volume 30, Number 5, September10 2022
 

Abstract
To obtain the dynamic mechanical properties, fracture modes, energy and brittleness characteristics of Furong Baijiao coal rock, the dynamic impact compression tests under 0, 4, 8 and 12 MPa confining pressure were carried out using the split Hopkinson pressure bar. The results show that failure mode of coal rock in uniaxial state is axial splitting failure, while it is mainly compression-shear failure with tensile failure in triaxial state. With strain rate and confining pressure increasing, compressive strength and peak strain increase, average fragmentation increases and fractal dimension decreases. Based on energy dissipation theory, the dissipated energy density of coal rock increases gradually with growing confining pressure, but it has little correlation with strain rate. Considering progressive destruction process of coal rock, damage variable was defined as the ratio of dissipated energy density to total absorbed energy density. The maximum damage rate was obtained by deriving damage variable to reflect its maximum failure severity, then a brittleness index BD was established based on the maximum damage rate. BD value declined gradually as confining pressure and strain rate increase, indicating the decrease of brittleness and destruction degree. When confining pressure rises to 12 MPa, brittleness index and average fragmentation gradually stabilize, which shows confining pressure growing cannot cause continuous damage. Finally, integrating dynamic deformation and destruction process of coal rock and according to its final failure characteristics under different confining pressures, BD value is used to classify the brittleness into four grades.

Key Words
average fragmentation; brittleness index; dissipated energy density; maximum damage rate; split Hopkinson pressure bar

Address
Xiaohui Liu: Key Laboratory of Fluid and Power Machinery, Ministry of Education, Xihua University, Chengdu, 610039, China; School of Energy and Power Engineering, Xihua University, Chengdu, 610039, China; Key Laboratory of Deep Earth Science and Engineering, Sichuan University, Ministry of Education, Chengdu, 610065, China
Yu Zheng: Southwest Municipal Engineering Design & Research Institute of China, Chengdu, 610081, China
Qijun Hao: Key Laboratory of Deep Earth Science and Engineering, Sichuan University, Ministry of Education, Chengdu, 610065, China
Rui Zhao: Key Laboratory of Fluid and Power Machinery, Ministry of Education, Xihua University, Chengdu, 610039, China; School of Energy and Power Engineering, Xihua University, Chengdu, 610039, China
Yang Xue: CHN Energy Dadu River Hydropower Development Co., Ltd, Chengdu 610041, China
Zhaopeng Zhang: Key Laboratory of Deep Earth Science and Engineering, Sichuan University, Ministry of Education, Chengdu, 610065, China

Abstract
TBM is widely used in the construction of various underground projects in the current world, and has the unique advantages that cannot be compared with traditional excavation methods. However, due to the high cost of TBM, the damage is even greater when geological disasters such as collapse occur during excavation. At present, there is still a shortage of research on various types of risk prediction of TBM tunnel, and accurate and reliable risk prediction model is an important theoretical basis for timely risk avoidance during construction. In this paper, a prediction model is proposed to evaluate the risk level of tunnel collapse by establishing a reasonable risk index system, using analytic hierarchy process to determine the index weight, and using the normal cloud model theory. At the same time, the traditional analytic hierarchy process is improved and optimized to ensure the objectivity of the weight values of the indicators in the prediction process, and the qualitative indicators are quantified so that they can directly participate in the process of risk prediction calculation. Through the practical engineering application, the feasibility and accuracy of the method are verified, and further optimization can be analyzed and discussed.

Key Words
analytic hierarchy process; collapse risk; normal cloud model; prediction model; TBM tunnel

Address
Peng Wang, Yiguo Xue, Maoxin Su, Daohong Qiu and Guangkun Li: Geotechnical and Structural Engineering Research Center, Shandong University, Ji'nan, Shandong, China

Abstract
Ground vibration generated repeatedly in blasting tunnel excavation sites is known to be one of the major hazards induced by blasting operations. Various studies have been conducted to minimize these hazards, both theoretical and empirical methods using electronic detonator, the deck charge method, the center-cut method among others Among these various existing methods for controlling the ground vibration, in this study, we investigated the cut method. In particular, we analyzed and compared the V-cut method, which is commonly used in tunnel blasting, to the double-drilled parallel method, which has recently been introduced in tunnel excavation site. To understand the rock fragmentation efficiency as well as the ground vibration controllability of the two methods, we performed in-situ field blasting tests with both cut methods at a tunnel excavation site. Additionally, numerical analysis by FLAC3D has been executed for a better understanding of fracture propagation pattern and ground vibration generation by each cut method. Ground vibration levels, by PPVs measured in field blasting tests and PPVs estimated in numerical simulations, showed a lower value in the double-drilled parallel compared with the V-cut method, although the exact values are quite different in field measurement and numerical estimation.

Key Words
blasting vibration; cut method; double-drilled parallel cut; finite difference method; V-cut

Address
Seung-Joong Lee: Smart Mining Team, Hanwha Corporation/Global, Pangyo-ro 305, Bundang-gu, Seongnam-si, Gyeonggi-do, Korea
Byung-Ryeol Kim: Energy Environment Team, Korea Institute of Limestone & Advanced Materials, Udeok-gil 18-1, Maepo-eup, Danyang-gun, Chungcheongbuk-do, Korea
Sung-Oong Choi: Department of Energy and Resources Engineering, Kangwon National University, Gangwondaehakgil 1, Chuncheon-si, Gangwon-do, Korea
Nam-Soo Kim: NSB Now ENC, Ttukseom-ro 19, Gwangjin-gu, Seoul-si, Korea

Abstract
Alluvial soil is challenging to work with due to its high compressibility. Thus, consolidation settlement of this type of soil should be accurately estimated. Accurate estimation of the consolidation settlement of alluvial soil requires accurate prediction of compressibility parameters. Geotechnical engineers usually use empirical correlations to estimate these compressibility parameters. However, no attempts have been made to develop correlations to estimate compressibility parameters of alluvial soil. Thus, this paper aims to develop new models to predict the compression and recompression indices (Cc and Cr) of alluvial soils. As part of the study, geotechnical laboratory tests have been conducted on large number of undisturbed samples of local alluvial soil. The obtained results from these tests in addition to available results from the literature from different parts in the world have been compiled to form the database of this study. This database is then employed to examine the accuracy of the available empirical correlations of the compressibility parameters and to develop the new models to estimate the compressibility parameters using the nonlinear regression analysis. The accuracy of the new models has been accessed using mean absolute error, root mean square error, mean, percentage of predictions with error range of +-20%, percentage of predictions with error range of +-30%, and coefficient of determination. It was found that the new models outperform the available correlations. Thus, these models can be used by geotechnical engineers with more confidence to predict Cc and Cr.

Key Words
alluvial soil, compression index, mean absolute error, nonlinear regression, recompression index, root mean square error

Address
Saif Alzabeebee: Department of Roads and Transport Engineering, University of Al-Qadisiyah, Al-Diwaniyah, Al-Qadisiyah, Iraq
Abbas Al-Taie: Department of Civil Engineering, Al-Nahrain University, Baghdad, Iraq

Abstract
Safe underground construction in a rock mass requires adequate ground investigation and effective determination of rock conditions. The estimation of rock mass behavior is difficult, because rock masses are innately anisotropic and heterogeneous at different scales and are affected by various environmental factors. Quantitative rock mass classification systems, such as the Q-system and rock mass rating, are widely used for characterization and engineering design. The measurement of rock classification parameters is subjective and can vary among observers, resulting in questionable accuracy. Geophysical investigation methods, such as seismic surveys, have also been used for ground characterization. Torsional shear wave propagation characteristics in cylindrical rods are equal to that in an infinite media. A probabilistic quantitative relationship between the Q-value and shear wave velocity is thus investigated considering long-wavelength wave propagation in equivalent continuum jointed rock masses. Individual Q-system parameters are correlated with stress-dependent shear wave velocities in jointed rocks using experimental and numerical methods. The relationship between the Q-value and the shear wave velocity is normalized using a defined reference condition. This relationship is further improved using probabilistic analysis to remove unrealistic data and to suggest a range of Q-values for a given wave velocity. The proposed probabilistic Q-value estimation is then compared with field measurements and cross-hole seismic test data to verify its applicability.

Key Words
rock mass classification; Q-system; jointed rock mass; shear wave velocity; equivalent continuum

Address
Ji-Won Kim: Disposal Performance Demonstration Research Division, Korea Atomic Energy Research Institute, Daejeon 34057, Korea
Song-Hun Chong: Department of Civil Engineering, Sunchon National University, 255 Jungang-ro, Sunchon, Jeollanam-do 57922, Korea
Gye-Chun Cho: Department of Civil and Environmental Engineering, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Korea

Abstract
One of the most important factors that should be considered for using any ground improvement technique is the stability of stabilized soil and the durability of the provided solution for getting the required engineering properties. Generally, most of the earth structures that are constructed on clayey soils are exposing movements due to the long periods of drying or wetting cycles. Over time, environmental changes may result in swells or settlements for these structures. In order to mitigate this problem, this research has been performed on mixtures of high plasticity clay with traditional additives such as lime, cement and non-traditional additives such as polypropylene fiber. The purpose of the research is to assess the most appropriate ground improvement technique by using commercially available additives for resisting the developed desiccation cracks during the drying process and resisting the volume changes that may result during wet/dry cycles as an attempt to simulate the changes of environmental conditions. The results show that the fiber-reinforced samples have the lowest volumetric deformation in comparision with cement and lime stabilized samples, and the optimum fiber content is identified as 0.38%. In addition, the desiccation cracks were not visible on the samples' surface for both unreinforced and chemically stabilized samples. Regarding cracks resistance resulting from the desiccation process, it is observed, that the resistance is connected with the fiber content and increases with the increase of the fiber inclusion, and the optimum content is between 1% and 1.5%.

Key Words
desiccation test; dry/wet cycle test; environmental changes; fiber-reinforcement; soil stabilization

Address
Talal Taleb and Yesim S. Unsever: Department of Civil Engineering, Bursa Uludağ University, 16059 Bursa, Türkiye

Abstract
Creep of soils has a significant impact on mechanical properties. The one-dimensional consolidation creep test, thermal analysis test, scanning electron microscope (SEM) test, and mercury compression test were performed on Guilin red clay to study the changes in bound water and microstructure during the creep process of Guilin red clay. According to the results of the tests, only free and weakly bound water is discharged during the creep of Guilin red clay. When the consolidation pressure p is in the 12.5-400.0 kPa range, it is primarily the discharge of free water; when the consolidation pressure p is in the 800.0-1600.0 kPa range, the weakly bound water is converted to free water and discharged. After consolidation creep, the microstructure of soil changes from granular overhead contact structure to flat sheet-like stacking structure, with a decrease in the number of large and medium pores, an increase in the number of small and micro pores, and a decrease in the fractal dimension of pores. The creep process of red clay is the discharge of weakly bound water as well as the compression of large pores into small pores and the transition of soil particles from loose to dense.

Key Words
consolidation creep; microstructure; pore water; red clay; thermal analysis tests

Address
Dajin Zhang: Key Laboratory of Geotechnics of Guangxi, Guilin University of Technology, 541004, China
Guiyuan Xiao: Key Laboratory of Geotechnics of Guangxi, Guilin University of Technology, 541004, China; School of Engineering, China University of Geosciences, Wuhan 430000, China
Le Yin: The Guangxi Zhuang Autonomous Region Company of China National Tobacco Corporation, China
Guangli Xu: School of Engineering, China University of Geosciences, Wuhan 430000, China
Jian Wang: Key Laboratory of Geotechnics of Guangxi, Guilin University of Technology, 541004, China

Abstract
This study evaluates the performance of gravel impact compaction piers system (GICPs) in strengthening retrofitting a very loose silty sand layer with a very high liquefaction risk with a thickness of 3.5 meters in a multilayer coastal soil located in Bushehr, Iran. The liquefiable sandy soil layer was located on clay layers with moderate to very stiff relative consistency. Implementation of gravel impact compaction piers is a new generation of aggregate piers. After technical and economic evaluation of the site plan, out of 3 experimental distances of 1.8, 2 and 2.2 meters between compaction piers, the distance of 2.2 meters was selected as a winning option and the northern ring of the site was implemented with 1250 gravel impact compaction piers. Based on the results of the standard penetration test in the matrix soil around the piers showed that the amount of (N1)60 in compacted soils was in the range of 20-27 and on average 14 times the amount of (1-3) in the initial soil. Also, the relative density of the initial soil was increased from 25% to 63% after soil improvement. Also the safety factor of the improved soil is 1.5-1.7 times the minimum required according to the two risk levels in the design.

Key Words
gravel impact compaction piers; liquefaction; multilayer soil; soil compaction; soil improvement

Address
Bahman Niroumand: Department of Civil Engineering, Faculty of Engineering, Persian Gulf University, Bushehr, Iran
Hamed Niroumand: Department of Civil Engineering, Faculty of Engineering, Buein Zahra Technical University, Qazvin, Iran

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