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CONTENTS
Volume 17, Number 1, January20 2019
 

Abstract
This paper proposes a new numerical approach to model geocell reinforced soils, where the geocell is described as membrane elements and the complex interaction between geocell and soil is realized by coupling their degrees of freedom. The effectiveness and robustness of this approach are demonstrated using two examples, i.e., a geocell-reinforced foundation and a large scale retaining wall project. The first example validates the approach against established solutions through a comprehensive parametrical study to understand the influence of geocell on the improvement of bearing capacity of foundations. The study results show that reducing the geocell pocket size has a strong effect on improving the bearing capacity. In addition, when the aspect ratio maintains the same value, the bearing capacity improvement with increasing geocell height is insignificant. Comparing with the field monitoring and measurement in the project, the second example investigates the application of the approach to practical engineering projects. This paper provides a practically feasible and efficient modelling approach, where no explicit interface or contact is required. This allows geocell reinforced soils in large scale project can be effectively modelled where the mechanism for complex geocell-soil interaction can be explicitly observed.

Key Words
geocell; three-dimensional analysis; numerical modelling; foundation; retaining wall

Address
Fei Song: Institute of Geotechnical Engineering, School of Highway Engineering, Chang

Abstract
The formulas offered so far on the settlement of raft footings provide only a rough estimate of the actual settlement. One of the best ways to make an accurate estimation is to conduct 3-dimensional finite element analyses. However, the required procedure for these analyses is comparatively cumbersome and expensive and needs a bit more expertise. In order to address this issue, in this study, a raft footing settlement formula was developed based on ninety finite element model configurations. The formula was derived using multi-parameter exponential regression analyses. The settlement formula incorporates the dimensions and the elastic modulus of a rectangular raft, vertical uniform pressure and soil moduli and Poisson\'s ratios up to 5 layers. In addition to this, an equation was offered for the estimation of average deflection of the raft. The proposed formula was checked against 3 well-documented case studies. The formula that is derived from 3D finite element analyses is useful in optimising the raft properties.

Key Words
footing; raft formula; DIANA; settlement; PLAXIS 3D

Address
Murat Hamderi: Faculty of Engineering, Turkish-German University, Istanbul, Turkey

Abstract
Recent research on the behavior of silty sand tends to advocate the use of equivalent skeleton void ratio to characterize the density state of this type of soil. This paper presents an investigation to explore the physical meaning of the equivalent skeleton void ratio by means of DEM simulations for assemblies of coarse and fine particles under biaxial shear. The simulations reveal that the distribution pattern of fine particles in the soil skeleton plays a crucial role in the overall macroscopic response: The contractive response observed at the macro scale is mainly caused by the movement of fine particles out of the force chains whereas the dilative response is mainly associated with the migration of fine particles into the force chains. In an assembly of coarse and fine particles, neither all of the fine particles nor all of the coarse ones participate in the force chains to carry the external loads, and therefore a more reasonable definition for equivalent skeleton void ratio is put forward in which a new parameter d is introduced to take into account the fraction of coarse particles absent from the force chains.

Key Words
silty sand; equivalent skeleton void ratio; fine particles; force chain; anisotropy; discrete element method

Address
Bei-Bing Dai: 1.) School of Civil Engineering, Sun Yat-sen University, Guangzhou, 510275, China
2.) Department of Geotechnical Engineering & Key Laboratory of Geotechnical and Underground Engineering of the Ministry of Education, Tongji University, Shanghai, 200092 China

Jun Yang: Department of Civil Engineering, The University of Hong Kong, Hong Kong, China

Xiao-Qiang Gu: Department of Geotechnical Engineering & Key Laboratory of Geotechnical and Underground Engineering of the Ministry of Education, Tongji University, Shanghai, 200092 China

Wei Zhang: College of Water Conservancy and Civil Engineering, South China Agricultural University, Guangzhou, 510642, China

Abstract
In this study the spatial variability of soils is substantiated physically and numerically by using random field theory. Heterogeneous samples are fabricated by combining nine homogeneous soil clusters that are assumed to be elements of an adopted random field. Homogeneous soils are prepared by mixing different percentages of kaolin and bentonite at water contents equivalent to their respective liquid limits. Comprehensive characteristic laboratory tests were carried out before embarking on direct shear experiments to deduce the basic correlations and properties of nine homogeneous soil clusters that serve to reconstitute the heterogeneous samples. The tests consist of Atterberg limits, and Oedometric and unconfined compression tests. The undrained shear strength of nine soil clusters were measured by the unconfined compression test data, and then correlations were made between the water content and the strength and stiffness of soil samples with different consistency limits. The direct shear strength of heterogeneous samples of different stochastic properties was then evaluated by physical and numerical modelling using FISH code programming in finite difference software of FLAC3D. The results of the experimental and stochastic numerical analyses were then compared. The deviation of numerical simulations from direct shear load-displacement profiles taken from different sources were discussed, potential sources of error was introduced and elaborated. This study was primarily to explain the mathematical and physical procedures of sample preparation in stochastic soil mechanics. It can be extended to different problems and applications in geotechnical engineering discipline to take in to account the variability of strength and deformation parameters.

Key Words
inherent variability; undrained cohesion; direct shear test; Monte Carlo simulation

Address
Reza Jamshidi Chenari, Malahat Ghoreishi and Ali Taleb: Faculty of Engineering, University of Guilan, Rasht, Guilan, Iran

Behzad Fatahi: School of Civil and Environmental Engineering, Faculty of Engineering and Information Technology, University of Technology Sydney (UTS), City Campus PO Box 123 Broadway NSW 2007, Australia

Abstract
Crossing (X-type) flaws are commonly encountered in rock mass. However, the crack coalescence and failure mechanisms of rock mass with X-type flaws remain unclear. In this study, we investigate the compressive failure process of rock-like specimens containing two X-type flaws aligned in the loading direction. For comparison purposes, compressive failure behavior of specimens containing two aligned single flaws is also studied. By examining the crack coalescence behavior, two characteristics for the aligned X-type flaws under uniaxial compression are revealed. The flaws tend to coalesce by cracks emanating from flaw tips along a potential path that is parallel to the maximum compressive stress direction. The flaws are more likely to coalesce along the coalescence path linked by flaw tips with greater maximum circumferential stress if there are several potential coalescence paths almost parallel to the maximum compressive stress direction. In addition, we find that some of the specimens containing two aligned X-type flaws exhibit higher strengths than that of the specimens containing two single parallel flaws. The two underlying reasons that may influence the strengths of specimens containing two aligned X-type flaws are the values of flaw tips maximum circumferential stresses and maximum shear stresses, as well as the shear crack propagation tendencies of some secondary flaws. The research reported here provides increased understanding of the fundamental nature of rock/rock-like material failure in uniaxial compression.

Key Words
aligned X-type flaws; rock-like material; crack coalescence; strength; uniaxial compression

Address
Bo Zhang, Jiyang Liu and Shuai Guo: School of Civil Engineering, Shandong University, Jinan, Shandong 250061, P.R. China

Shucai Li and Shugang Wang: Research Center of Geotechnical and Structural Engineering, Shandong University, Jinan, Shandong 250061, P.R. China

Xueying Yang: Shandong Urban Construction Vocational College, Jinan, Shandong 250014, P.R. China

Kaiwen Xia:Impact and Fracture Laboratory, Department of Civil Engineering and Lassonde Institute, University of Toronto, ON M5S 1A4, Canada

Abstract
Water-induced strength reduction is one of the most critical causes for rock deformation and failure. Understanding the effects of water on the strength, toughness and deformability of rocks are of a great importance in rock fracture mechanics and design of structures in rock. However, only a few studies have been conducted to understand the effects of water on fracture properties such as fracture toughness, crack propagation velocity, consumed energy, and microstructural damage. Thus, in this study, we focused on the understanding of how microscale damages induced by water saturation affect mesoscale mechanical and fracture properties compared with oven dried specimens along three notch orientations-divider, arrester, and short transverse. The mechanical properties of calcite-cemented sandstone were examined using standard uniaxial compressive strength (UCS) and Brazilian tensile strength (BTS) tests. In addition, fracture properties such as fracture toughness, consumed energy and crack propagation velocity were examined with cracked chevron notched Brazilian disk (CCNBD) tests. Digital Image Correlation (DIC), a non-contact optical measurement technique, was used for both strain and crack propagation velocity measurements along the bedding plane orientations. Finally, environmental scanning electron microscope (ESEM) was employed to investigate the microstructural damages produced in calcite-cemented sandstone specimens before and after CCNBD tests. As results, both mechanical and fracture properties reduced significantly when specimens were saturated. The effects of water on fracture properties (fracture toughness and consumed energy) were predominant in divider specimens when compared with arrester and short transverse specimens. Whereas crack propagation velocity was faster in short transverse and slower in arrester, and intermediate in divider specimens. Based on ESEM data, water in the calcite-cemented sandstone induced microstructural damages (microcracks and voids) and increased the strength disparity between cement/matrix and rock forming mineral grains, which in turn reduced the crack propagation resistance of the rock, leading to lower both consumed energy and fracture toughness (KIC).

Key Words
mode I fracture toughness; crack propagation velocity; consumed energy; water saturation; digital image correlation (DIC); microstructural damage

Address
Varun Maruvanchery: Underground Construction and Tunneling Engineering, Colorado School of Mines, Golden, Colorado, U.S.A.

Eunhye Kim: 1.) Underground Construction and Tunneling Engineering, Colorado School of Mines, Golden, Colorado, U.S.A.
2.) Department of Mining Engineering, Colorado School of Mines, Golden, Colorado, U.S.A.


Abstract
This work presents an experimental study of the hydromechanical behavior of a natural swelling soil taken from Boumagueur region east of Algeria. Several pathological cases due to the soil shrinkage / swelling phenomenon were detected in this area. In a first part, the hydric behavior on drying-wetting paths was made, using the osmotic technics and saturated salts solutions to control suction. In The second part, using a new osmotic oedometer, the coupled behavior as a function of applied stresses and suction was investigated. It was shown that soil compressibility parameters was influenced by suction variations that an increase in suction is followed by a decrease in the virgin compression slope. On the other hand, the unloading slope of the oedometric curves was not obviously affected by the imposed suction. The decrease in suction strongly influences the apparent preconsolidation pressure, ie during swelling of the samples after wetting.

Key Words
clay swelling soil; suction; shrinkage-swelling; drying-wetting; hydromechanical behavior; compressibility

Address
Mehdi Mebarki and Toufik Kareche: Departement of Civil Engineering, Faculty of Technology, University of Batna 2, Batna 5000, Algeria

Feth-Ellah Mounir Derfouf: 1.) EOLE, Department of Civil Engineering, University of Tlemcen, Tlemcen 13000, Algeria
2.) Department of Hydraulic and Civil Engineering, University of Saida, Algeria

Said Taibi: LOMC, UMR CNRS 6394, University of Le Havre Normandy

Nabil Abou-bekr: EOLE, Department of Civil Engineering, University of Tlemcen, Tlemcen 13000, Algeria

Abstract
Due to the importance of soil reinforcement using geotextiles in geotechnical engineering, study and investigation into long-term performance, design life and survivability of geotextiles, especially due to installation damage are necessary and will affect their economy. During installation, spreading and compaction of backfill materials, geotextiles may encounter severe stresses which can be higher than they will experience in-service. This paper aims to investigate the installation damage of geotextiles, in order to obtain a good approach to the estimation of the material\'s strength reduction factor. A series of full-scale tests were conducted to simulate the installation process. The study includes four deliberately poorly-graded backfill materials, two kinds of subgrades with different CBR values, three nonwoven needle-punched geotextiles of classes 1, 2 and 3 (according to AASHTO M288-08) and two different relative densities for the backfill materials. Also, to determine how well or how poorly the geotextiles tolerated the imposed construction stresses, grab tensile tests and visual inspections were carried out on geotextile specimens (before and after installation). Visual inspections of the geotextiles revealed sedimentation of fine-grained particles in all specimens and local stretching of geotextiles by larger soil particles which exerted some damage. A regression model is proposed to reliably predict the installation damage reduction factor. The results, obtained by grab tensile tests and via the proposed models, indicated that the strength reduction factor due to installation damage was reduced as the median grain size and relative density of the backfill decreases, stress transferred to the geotextiles\' level decreases and as the as-received grab tensile strength of geotextile and the subgrades\' CBR value increase.

Key Words
geotextiles; installation damage; grab tensile strength; retained tensile strength; strength reduction factor

Address
Ehsan Amjadi Sardehaei and Gholamhosein Tavakoli Mehrjardi: Department of Civil Engineering, Faculty of Engineering, Kharazmi University, Tehran, Iran

Andrew Dawson: Nottingham Transportation Engineering Centre, University of Nottingham, Nottingham, U.K.

Abstract
Based on the results of Han et al. (2016), in the failure zone ahead of the tunnel face it can be obviously identified that a shear failure band occurs in the lower part and a pressure arch happens at the upper part, which was often neglected in analyzing the face stability of shield tunnel. In order to better describe the collapse failure feature of the tunnel face, a new improved failure mechanism is proposed to evaluate the face stability of shield tunnel excavated in layered soils in the framework of limit analysis by using spatial discretization technique and linear interpolation method in this study. The developed failure mechanism is composed of two parts: i) the rotational failure mechanism denoting the shear failure band and ii) a uniformly distributed force denoting the pressure arch effect. Followed by the comparison between the results of critical face pressures provided by the developed model and those by the existing works, which indicates that the new developed failure mechanism provides comparatively reasonable results.

Key Words
face stability; shield tunnel; layered soils; limit analysis; pressure arch effect

Address
Guang-hui Chen, Jin-feng Zou and Ze-hang Qian: School of Civil Engineering, Central South University, No.22, Shaoshan South Road, Central South University Railway Campus, Changsha, Hunan Province, People

Abstract
The movement of soil along a slope during rainfall can cause serious economic damage and can jeopardize human life. Accordingly, predicting slope stability during rainfall is a major issue in geotechnical engineering. Due to rainwater penetrating the soil, the negative pore water pressure will decrease, in turn causing a loss of shear strength in the soil and ultimately slope failure. More seriously, many constructions such as houses and transmission towers built in/on slopes are at risk when the slopes fail. In this study, the numerical simulation using 2D finite difference program, which can solve a fully coupled hydromechanical problems, was used to evaluate the effects of soil properties, rainfall conditions, and the location of a foundation on the slope instability and slope failure mechanisms during rainfall. A slope with a transmission tower located in Namyangju, South Korea was analyzed in this study. The results showed that the correlation between permeability and rainfall intensity had an important role in changing the pore water pressure via controlling the infiltrated rainwater. The foundation of the transmission tower was stable during rainfall because the slope failure was estimated to occur at the toe of the slope, and did not go through the foundation.

Key Words
slope failure; safety factor; rainfall infiltration; pile displacement

Address
An T.P. Tran and Gye-Chun Cho: Department of Civil Engineering, Korea Advanced Institute for Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea

Ah-Ram Kim: Department of Infrastructure Safety Research, Korean Institute of Civil Engineering and Building Technology (KICT), 283 Goyangdae-ro, Ilsanseo-gu, Goyang 10223, Republic of Korea




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