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CONTENTS
Volume 18, Number 3, June30 2019
 

Abstract
A numerical stepwise approach for cavity expansion problem in strain-softening rock or soil mass is investigated, which is compatible with Mohr–Coulomb and generalized Hoek-Brown failure criteria. Based on finite difference method, plastic region is divided into a finite number of concentric rings whose thicknesses are determined internally to satisfy the equilibrium and compatibility equations, the material parameters of the rock or soil mass are assumed to be the same in each ring. For the strain-softening behavior, the strength parameters are assumed to be a linear function of deviatoric plastic strain (yp*) for each ring. Increments of stress and strain for each ring are calculated with the finite difference method. Assumptions of large-strain for soil mass and small-strain for rock mass are adopted, respectively. A new numerical stepwise approach for limited pressure and plastic radius are obtained. Comparisons are conducted to validate the correctness of the proposed approach with Vesic\'s solution (1972). The results show that the perfectly elasto-plastic model may underestimate the displacement and stresses in cavity expansion than strain-softening coefficient considered. The results of limit expansion pressure based on the generalised H-B failure criterion are less than those obtained based on the M-C failure criterion.

Key Words
cavity expansion; strain-softening; numerical stepwise approach; undrained condition; rock or soil mass

Address
Jin-Feng Zou,Tao Yang and Wang Ling: School of Civil Engineering, Central South University, No.22, Shaoshan South Road,Central South University Railway Campus, Changsha, Hunan Province, People\'s Republic of China

Wujun Guo: Guizhou Zhongjiao Tonghuai Expressway Co., Ltd. Tongren, Guizhou 554300, People\'s Republic of China

Faling Huang: Guizhou Expressway Group Co., Ltd. T, No.266, Beijing Road, Guiyang, Guizhou Province, People\'s Republic of China

Abstract
This paper presents a novel methodology for face stability assessment of rock tunnels under water table by combining the kinematical approach of limit analysis and numerical simulation. The tunnels considered in this paper are excavated in fractured rock masses characterized by the Hoek-Brown failure criterion. In terms of natural rock deposition, a more convincing case of depth-dependent mi, GSI, D and sigmac is taken into account by proposing the horizontally layered discretization technique, which enables us to generate the failure surface of tunnel face point by point. The vertical distance between any two adjacent points is fixed, which is beneficial to deal with stability problems involving depth-dependent rock parameters. The pore water pressure is numerically computed by means of 3D steady-state flow analyses. Accordingly, the pore water pressure for each discretized point on the failure surface is obtained by interpolation. The parametric analysis is performed to show the influence of depth-dependent parameters of mi, GSI, D, sigmac and the variation of water table elevation on tunnel face stability. Finally, several design charts for an undisturbed tunnel are presented for quick calculations of critical support pressures against face failure.

Key Words
face stability; pore water pressure; numerical simulation; horizontally layered discretization technique; Hoek-Brown failure criterion

Address
T.Z. Li and X.L. Yang: School of Civil Engineering, Central South University, Changsha, 410075, China

Abstract
Soil strength and failure surface geometry directly influence magnitudes of passive earth thrust acting on geotechnical retaining structures. Accordingly, it is expected that as long as the shape of the failure surface geometry and strength parameters of the backfill are known, magnitudes of computed passive earth thrusts should be highly accurate. Building on this premise, this study adopts conventional method of slices for calculating passive earth thrust and combines it with equations for estimating failure surface geometries based on in-situ stress state and density. Accuracy of the proposed method is checked using the results obtained from small-scale physical retaining wall model tests. In these model tests, backfill was prepared using either air pluviation or compaction and different backfill relative densities were used in each test. When the calculated passive earth thrust magnitudes were compared with the measured values, it was noticed that the results were highly compatible for the tests with pluviated backfills. On the other hand, calculated thrust magnitudes significantly underestimated the measured thrust magnitudes for those tests with compacted backfills. Based on this observation, a new approach for the calculation of passive earth pressures is developed. The proposed approach calculates the magnitude and considers the influence of locked-in stresses that are the by-products of the backfill preparation method in the computation of lateral earth forces. Finally, recommendations are given for any geotechnical application involving the compaction of granular bodies that are equally applicable to physical modelling studies and field construction problems.

Key Words
model test; failure surface geometry; passive earth thrust; method of slices; locked-in stresses

Address
Ahmet Talha Gezgin and Ozer Cinicioglu: Department of Civil Engineering, Bogazici University, 34342 Bebek Istanbul, Turkey

Abstract
This paper presents a comparison between experimental measurements and numerical estimations of penetration length of a cement grout injected in discrete joints. In the experiment, a joint was generated by planar acryl plates with a certain separation distance (; aperture) and was designed in such a way to vary the separation distances. Since a cement grout was used, the grout viscosity can be varied by controlling water-cement (W/C) ratios. Throughout these experiments, the influence of joint aperture, cement grout viscosity, and injection rate on a penetration length in a discrete joint was investigated. During the experiments, we also measured the time-dependent variation of grout viscosity due to a hardening process. The time-dependent viscosity was included in our numerical simulations as a function of elapsed time to demonstrate its impact on the estimation of penetration length. In the numerical simulations, Bingham fluid model that has been known to be applicable to a viscous cement material, was employed. We showed that the estimations by the current numerical approach were well comparable to the experimental measurements only in limited conditions of lower injection rates and smaller joint apertures. The difference between two approaches resulted from the facts that material separation (; bleeding) of cement grout, which was noticeable in higher injection rate and there could be a significant surface friction between the grout and joint planes, which are not included in the numerical simulations. Our numerical simulation, meanwhile, could well demonstrate that penetration length can be significantly over-estimated without considering a time-dependency of viscosity in a cement grout.

Key Words
cement grout; penetration length; Bingham fluid; time-dependent viscosity

Address
Jong-Won Lee: 1.) Department of Energy & Mineral Resources Engineering, Sejong University, Seoul 05006 Korea
2.) Center for Deep Subsurface Research, Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon 34132 Korea

Hyung-Mok Kim: Department of Energy & Mineral Resources Engineering, Sejong University, Seoul 05006 Korea

Mahmoud Yazdani: Department of Civil and Environmental Engineering, Tarbiat Modares University, Tehran 14117-13116 Iran

Hangbok Lee and Eui-Seob Park: Center for Deep Subsurface Research, Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon 34132 Korea

Tae-Min Oh: Department of Civil and Environmetnal Engineering, Pusan National University, Busan 46241 Korea

Abstract
Tabriz is a large Iranian city and the capital of the East Azerbaijan province. The bed rock of this city is mainly consisted of marl layers. Marl layers have some outcrops in the northern and eastern parts of city that mainly belong to the Baghmisheh formation. Based on their colors, these marls are classified into three types: yellow, green, and gray marls. The city is developing toward its eastern side wherein various civil projects are under construction including tunnels, underground excavation, and high-rise building. In this regard, the swelling behavior assessment of these marls is of critical importance. Also, in lightweight structures with foundation pressure less than swelling pressure, several problems such as walls cracking and jamming of door and windows may occur. In the present study, physical properties and swelling behavior of Baghmisheh marls are investigated. According to the X-ray diffractometer (XRD) results, the marls are mainly composed of Illite, Kaolinite, Montmorillonite, and Chloride minerals. Type and content of clay minerals and initial void ratio have a decisive role in swelling behavior of these marls. The swelling potential of these marls was investigated using one-dimensional odometer apparatus under stress level up to 10 kPa. The results showed that yellow marls have high swelling potential and expansibility compared to the other marls. In addition, green and gray marls showed intermediate and low swelling potential and swelling pressure, respectively.

Key Words
Tabriz; swelling; swell pressure; Baghmisheh marls

Address
Ebrahim Asghari-Kaljahi, Ghodrat Barzegari and Shahrokh Jalali-Milani: Department of Earth Sciences, University of Tabriz, Iran

Abstract
In this study, an innovative anchoring approach has been developed dealing with all relevant aspects in consideration of previous works. An ultimate pulling force calculation of anchor is presented from a geotechnical point of view. The proposed umbrella anchor focuses not only on the friction resistance capacity, but also on the axial capacity of the composite end structure and the friction capacity occurring around the wedge. Even though the theoretical background is proposed, in-situ application requires high-level mechanical design. Hence, the required parts have been carefully improved and are composed of anchor body, anchor cap, connection brackets, cutter vanes, open-close ring, support elements and grouting system. Besides, stretcher element made of aramid fabric, interior grouting system, guide tube and cable-locking apparatus are the unique parts of this design. The production and placement steps of real sized anchors are explained in detail. Experimental results of 52 pullout tests on the weak dry soils and 12 in-situ tests inside natural soil indicate that the proposed approach is conservative and its peak pullout value is directly limited by a maximum strength of anchored soil layer if other failure possibilities are eliminated. Umbrella anchor is an alternative to conventional anchor applications used in all types of soils. It not only provides time and workmanship benefits, but also a high level of economic gain and safe design.

Key Words
umbrella anchor; anchorage; pullout strength; slope stability; field application

Address
Burak Evirgen, Ahmet Tuncan and Mustafa Tuncan: Department of Civil Engineering, Eskisehir Technical University, 26555, Eskisehir, Turkey

Abstract
In order to study the mechanical properties of rock salt, triaxial compression tests under different temperatures and confining pressure are carried out on rock salt specimens, the influence of temperature and confining pressure on the mechanical properties of rock salt was studied. The results show that the temperature has a deteriorative effect on the mechanical properties of rock salt. With the increase of temperature, the peak stress of rock salt decreases visibly; the plastic deformation characteristics become much obvious; the internal friction angle increases; while the cohesion strength decreases. With the increase of confining pressure, the peak stress and peak strain of rock salt will increase under the same temperature. Based on the test data, the Duncan-Chang constitutive model was modified, and the modified Duncan-Chang rock salt constitutive model considering the effect of temperature and confining pressure was established. The stress-strain curve calculated by the modified model was compared with the stress-strain curve obtained from the test. The close match between the test results and the model prediction suggests that the modified Duncan-Chang constitutive model is accurate in describing the behavior of rock slat under different confining pressure and temperature conditions.

Key Words
rock salt; triaxial compression test; temperature; confining pressure; mechanical properties; Duncan-Chang constitutive model

Address
Baoyun Zhao, Tianzhu Huang, Dongyan Liu, Yang Liu and Xiaoping Wang: 1.) School of Civil Engineering and Architecture, Chongqing University of Science and Technology, Chongqing, 401331, China
2.) Chongqing Key Laboratory of Energy Engineering Mechanics & Disaster Prevention and Mitigation, Chongqing, 401331, China

Shu Liu and Guibao Yu: Graduate Office, Chongqing University of Science and Technology, Chongqing, 401331, China


Abstract
This research investigated the nonlinear compressibility, permeability, the yielding due to structural degradation and their effects on consolidation behavior of structured soft soils. Based on oedometer and hydraulic conductivity test results of natural and reconstituted soft clays, linear log (1+e) ~ log sigma\'v and log (1+e) ~ log kv relationships were developed to capture the variations in compressibility and permeability, and the yield stress ratio (YSR) was introduced to characterize the soil structure of natural soft clay. Semi-analytical solutions for one-dimensional consolidation of soft clay under time-dependent loading incorporating the effects of soil nonlinearity and soil structure were proposed. The semi-analytical solutions were verified against field measurements of a well-documented test embankment and they can give better accuracy in prediction of excess pore pressure compared to the predictions using the existing analytical solutions. Additionally, parametric studies were conducted to analyze the effects of YSR, compression index (lambda_r and lambda_c), and permeability index (eta_k) on the consolidation behavior of structured soft clays. The magnitude of the difference between degree of consolidation based on excess pore pressure (Up) and that based on strain (Us) depends on YSR. The parameter lamda_c/eta_k plays a significant role in predicting consolidation behavior.

Key Words
nonlinear consolidation; soil structure; semi-analytical solutions; time-dependent loading

Address
Weizheng Liu and Zhiguo Shi: School of Civil Engineering, Central South University, Changsha, China

Junhui Zhang:National Engineering Laboratory of Highway Maintenance Technology, Changsha University of Science & Technology, Changsha, China

Dingwen Zhang: School of Transportation, Southeast University, Nanjing, China

Abstract
This paper presents an investigation on bearing capacity, load-settlement behavior and safety factor of rock-soil slopes reinforced using geogrid-box method (GBM). To this end, small-scale laboratory studies were carried out to study the load-settlement response of a circular footing resting on unreinforced and reinforced rock-soil slopes. Several parameters including unit weight of rock-soil materials (loose- and dense-packing modes), slope height, location of footing relative to the slope crest, and geogrid tensile strength were studied. A series of finite element analysis were conducted using ABAQUS software to predict the bearing capacity behavior of slopes. Limit equilibrium and finite element analysis were also performed using commercially available software SLIDE and ABAQUS, respectively to calculate the safety factor. It was found that stabilization of rock-soil slopes using GBM significantly improves the bearing capacity and settlement behavior of slopes. It was established that, the displacement contours in the dense-packing mode distribute in a broader and deeper area as compared with the loose-packing mode, which results in higher ultimate bearing load. Moreover, it was found that in the loose-packing mode an increase in the vertical pressure load is accompanied with an increase in the soil settlement, while in the dense-packing mode the load-settlement curves show a pronounced peak. Comparison of bearing capacity ratios for the dense- and loose-packing modes demonstrated that the maximum benefit of GBM is achieved for rock-soil slopes in loose-packing mode. It was also found that by increasing the slope height, both the initial stiffness and the bearing load decreases. The results indicated a significant increase in the ultimate bearing load as the distance of the footing to the slope crest increases. For all the cases, a good agreement between the laboratory and numerical results was observed.

Key Words
rock-soil slope; geogrid-box method; bearing capacity; safety factor; finite element analysis; limit equilibrium

Address
Gholam Moradi and Arvin Abdolmaleki: Faculty of Civil Engineering, University of Tabriz, Tabriz, Iran

Parham Soltani: Department of Textile Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran

Abstract
This paper presents an experimental investigation about visualization of bulging development of geosynthetic-encased stone column (GESC) based on the digital image correlation (DIC) technique and transparent soil. Visual model tests on GESC and ordinary stone column (OSC) were carried out. In order to delete the warping effect resulting from transparent soil and experiment setup, a modification for experiment results was performed. The bulging development process of the GESC and the displacement field of the surrounding soil were measured. By comparing with the existing experimental and theoretical results, it demonstrates that the model test system developed for studying the continuous bulging development of GESC is suitable. The current test results show that the bulging depth of GESC ranges from 1.05 to 1.40 times the diameter of GESC. The influence depth of GESC bulging on surrounding soil displacement is 0~3 the times diameter of GESC.

Key Words
geosynthetic-encased stone column; transparent soil; visualization; bulging development; displacement field

Address
Yang Zhou, Gangqiang Kong, Huaifeng Peng and Chunhong Li: Key Laboratory of Geomechanics and Embankment Engineering in Ministry of Education, Hohai University, No.1 Xikang Road, Nanjing, China

Hongyu Qin: Flinders University,1284 Clovelly Park South Australia 5042, Australia



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