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CONTENTS
Volume 26, Number 2, July25 2021
 


Abstract
Soil stabilization is widely used to favourably amend the soil behaviour. The use of biopolymers to treat soil is not only an eco-friendly but is also a sustainable approach. Biopolymers, xanthan gum and guar gum are used to augment the strength of clayey sand. Xanthan gum is anionic while guar gum is non-ionic. Triaxial tests were conducted on treated soil samples to understand the effect of biopolymer treatment on clayey sand at different dosages and curing periods. Shear strength parameters –angle of internal friction and cohesion increases appreciably on treating soil with xanthan and guar gum for all dosages investigated, though angle of internal friction decreases with the curing period in case of xanthan gum treated soil. Xanthan gum performs better in enhancing the strength and deformation behaviour of the soil compared to guar gum. There is a substantial gain in early strength but as the curing period increases further, the rate of increase in strength is marginal. The deformation modulus at failure also increases with the biopolymer content. The reduction in post-peak strength of treated soil is sudden and drastic indicating brittle behavior. The energy absorption capacity of the biopolymer treated soil increases with increase in biopolymer content and curing period. The strength gain in soil can be ascribed to the formation of hydrogels that are cementitious in nature. Strength is also improved through the ionic / hydrogen bonds that are formed by biopolymer addition.

Key Words
cohesion; deformation modulus; energy absorption capacity; friction angle; guar gum; Xanthan gum

Address
S. Anandha Kumar and Evangelin Ramani Sujatha: Centre for Advanced Research on Environment, School of Civil Engineering, SASTRA Deemed University, Thanjavur 613401, Tamil Nadu, India


Abstract
Tailing dams are complex geotechnical systems comprising of an embankment and a basin containing the waste products from the mining processes. These structures are characterized by a wide surface exposed to the atmosphere whose interaction governs the position of the phreatic surface within the basin. A detailed knowledge of the hydro-mechanical properties of the tailings is fundamental to reliably assess the stability of the tailing dams. While most of the previous studies have dealt with the response of tailings in saturated conditions, this research provides an extension of the hydraulic behaviour in unsaturated and nearly saturated state of tailings collected after the failure of the Stava basins. The hydraulic behaviour in unsaturated conditions was investigated by means of tests where the suction was imposed and the water content was monitored (axis translation technique and vapour equilibrium technique), and tests where the water content was imposed and the suction was measured with psychrometer (dew point method). To account for the in-situ heterogeneity of tailings, the dependency of the water retention relationship on the grain size distribution, the preparation technique and on the initial density / void ratio was studied. Denser tailings showed a higher water retention behaviour than that given in looser specimens. Similarly, the increase of the fine content was demonstrated to improve the water retention capability. As for standard soils, also statically compacted Stava tailings reveal lower retention capability than the slurry samples, thus confirming the importance of the preparation method in determining the hydro-mechanical response of such soils.

Key Words
fine content; soil water retention curve; tailing dams; unsaturated soil; void ratio

Address
Gianluca Bella: 1.) Politecnico di Torino, C.so Duca degli Abruzzi 24, Torino, Italy
2.) Pini Swiss Engineers, Via Besso 7, Lugano, Switzerland


Abstract
The construction of stable slopes and vertical cuts is an important step in many geotechnical projects. Limit equilibrium methods (LEMs) are well-accepted procedures to compute factors of safety (FoS); however, they fail to provide any information about the distribution of the field variables within the soil mass because they do not include any stress-strain relationship in their formulation. On the other hand, the iterative finite element method (FEM/I) can estimate the field variables, but in the current study, we show that, for steep slopes and vertical cuts, it underestimates the FoS compared to the LEM. To overcome the obstacles that exist in this method, this study proposes a new approach to define the initiation of instability based on an abrupt change in the kinetic energy of the system. We also suggest a procedure to calculate the minimum FoS based on the explicit finite element method (FEM/E). Comparison of the results obtained from the proposed method, LEM, and FEM/I revealed that the FoS computed by the proposed method is in good agreement with the results of the LEM for a wide range of material parameters, geometries and external loading conditions, while no assumption regarding the critical slip surface needs to be made.

Key Words
explicit finite element method (FEM/E); factor of safety (FoS); kinetic energy; strength reduction method (SRM)

Address
Morteza Naeij and Danial Ghafarian: Department of Civil and Environmental Engineering, Amirkabir University of Technology (Tehran Polytechnic), Hafez Ave., Tehran, Iran

Hussein Ghasemi: School of Civil Engineering, College of Engineering, University of Tehran, Tehran, Iran

Yousef Javanmardi: Department of Mechanical Engineering, University College London, London, U.K.

Abstract
Direct shear test has been widely used to examine the shear strength of geomaterials because of the simplicity of the testing method and apparatus. Three factors significantly affect the accuracy of the experimental results of direct shear tests, namely (1) the type of direct shear apparatus, (2) the specimen size (scale effect), and (3) the opening size between shear boxes. This study focused on the Threshold Line (TL), which is obtained based on experimental tests, as a guideline for setting the opening size between the shear boxes. The validity of the TL was examined using distinct element method (DEM) 3D simulations from the following four perspectives: the first and second perspectives investigated the influence of the mean particle size and particle size distribution for mean particle sizes larger than 0.8 mm. In the third perspective, the scale effect of the specimens for fixed and varying D:H ratios of the shear box to reduce the shear box size was examined. Lastly, in the fourth perspective, the validity of using the TL to determine the appropriate opening size for the samples with a mean particle size smaller than 0.8 mm was also examined based on the Threshold Point (TP). For each case, the results of the TPs obtained from the DEM simulations agreed well with those of the TL. These findings suggest that the TL is valid and the TL relational equation can be used for setting the opening size between the shear boxes in the direct shear test regardless of saturated and unsaturated soils.

Key Words
DEM simulation; direct shear test; opening size; threshold line (TL); threshold point (TP)

Address
Byeong-Su Kim: Graduate School of Environmental and Life Science, Okayama University,3-1-1 Tsushima-naka, Kita-ku, Okayama City, Okayama 700-8530, Japan


Abstract
To protect adjacent buildings is a major concern for the construction of excavation in urban areas. In practice, the impacts on neighboring pile-supported buildings are normally minimized by limiting the deflection of earth retaining wall. Existing deflection criteria, however, are often empirical. In this work, we employ an analytical model to relate the wall deflection of braced excavation to the response of adjacent pile-supported buildings and thus forming a theoretical tool for determining the allowable deformation of excavation support structures based on the tolerance of buildings to distortion. The model combines closed-form excavation-induced free-field soil movements with elastic continuum solution that explicitly accounts for the interactions between raft, piles, and soils. Following validation against field model test and finite element simulation, the model is utilized to reveal the correspondence between the angular distortion of pile-supported buildings and the maximum retaining wall deflection under different combinations of excavation geometry, soil properties, and parameters of pile foundation potentially encountered in practice. A dimensionless factor composed of these influencing variables is proposed, and its correlation with the ratio of the building angular distortion over the maximum retaining wall deflection provides a rational way to determine the serviceability limit states of braced excavation.

Key Words
angular distortion; braced excavation; pile-supported building; serviceability limit state; wall deflection

Address
Linlong Mu, Maosong Huang and Zhenhao Shi: 1.) Department of Geotechnical Engineering, Tongji University, Shanghai 200092, China
2.) Key Laboratory of Geotechnical and Underground Engineering of Ministry of Education, Tongji University, Shanghai 200092, China

Gholam H. Roodi: Department of Civil, Architectural, and Environmental Engineering, University of Texas at Austin, Austin, TX 78712, U.S.A.

Abstract
Here, the results of a three-dimensional finite element study of the complex interaction of horizontal and moment loads (HM) on offshore monopiles as failure envelope, are reported. A new design criterion is described which is based on critical length, ultimate limit states, load characteristics and Eigen-frequency to ensure stable behavior of laterally loaded monopiles. Numerical analyses were performed to examine nonlinear interaction of a soil-pile system for 10,000 load cycles. The resulting framework can predict angular rotation due to cyclic loading. According to the loading level and duration of a load, elastic strains accumulate in the vicinity of a pile. Fairly intermediate two-way cyclic loading induced the largest rotations irrespective of the analysis performed (i.e., drained versus partially drained). Based on the regression coefficients of the non-dimensional frameworks used, accumulating rocking deformations of a pile at seabed level appear to be dependent on cyclic load ratio, drainage condition, and duration of loading. For safe design, sensitivity of the natural frequency of offshore wind turbine (OWT) at a monopile critical length as well as shorter lengths were also examined. The analytical model proposed here for determining the natural frequency of an OWT considers that soil-structure interaction (SSI) can be represented by monopile head springs characterized by lateral stiffness, KL, rotational stiffness, KR, cross-coupling stiffness, KLR, and parabolic soil stiffness variation with depth.

Key Words
accumulated strains; bearing strength envelopes; monopile; natural frequency

Address
Amin Barari and Lars Bo Ibsen: Department of the Built Environment, Aalborg University, Thomas Manns Vej 23, 9220 Aalborg

Abstract
The [BQ] method is a rock mass classification method to evaluate the quality of the rock mass and determine the construction parameters. This method is more empirical and cannot provide predictions for the deformation of tunnels after excavation. To predict the surrounding rock deformation of deep-buried tunnels by using the [BQ] method in China, first, data of 52 tunnels were collected and analyzed to determine the relationship between the grades of the surrounding rock, excavation method, burial depth, tunnel span, and surrounding rock deformation. Second, the equivalence of different surrounding rock grades to the range of geological strength index (GSI) scores were determined using methods, such as fitting GSI to another classification system RMR and RMR to BQ, and considering the correction factors of BQ values. This approach provides the basis for theoretical calculations based on the Hoek–Brown strength criterion. On the basis of the Hoek–Brown strength criterion, a theoretical approach to the deformation of surrounding rock under three failure models, namely, elastic–brittle–plastic, strain-softening, and elastic-perfectly-plastic, is presented when considering the installation time of primary support and the volumetric force of bolts. Finally, the theoretical approach is analyzed and compared with the measured data to verify its feasibility. Moreover, the effects of burial depth, grades of surrounding rock, support parameters, support time, and deformation allowance of the surrounding rock are analyzed. Analysis results can provide some guidance for the prediction of surrounding rock deformation of deep-buried tunnels in China.

Key Words
[BQ] method; deep-buried tunnels; deformation; failure model; Hoek-Brown strength criterion

Address
Jian Zhou and Xin A. Yang: The Key Laboratory of Road and Traffic Engineering, Ministry of Education, Tongji University, Shanghai 201804, China

Abstract
To maintain the stability of built structures, engineers employ various methods to increase ground strength. One such method is to exert mutual physical force upon a structure, thereby stabilizing it without external reinforcement. Typical examples include the stone mastic asphalt method and torsional structured stonework. By simulating a structural phenomenon, it is possible to increase the ground's strength simply by manipulating the distribution and spatial arrangement of soil particles; soil composed of two differently sized particles satisfying a specific ratio does not separate easily. The jamming of soil particles utilizes Plato's regular polyhedron model and assumes that soil particles are complete spheres. Larger soil particles are placed at each vertex of a regular polyhedron and smaller particles in the voids

Key Words
gap-graded soil; ground reinforcement; interlocking; particle; particle size distribution; replacement method

Address
Zhen-Hua Xin: Department of Infrastructure Safety Research, Korea Institute of Construction Technology, Goyang 10223, Republic of Korea

Jun-Ho Moon, Chan-Hee Kim and Young-Uk Kim : Department of Civil and Environmental Engineering, Myongji University, 116, Myongji-ro, Cheoin-gu, Yongin-si, Gyeonggi-do 17058, Republic of Korea

Kab-Boo Kim: Bosiddol Inc., Nonhyeon-ro 63, Gangnam-gu, Seoul, 06256, Republic of Korea


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