Volume 49 Issue No. 1 March 2018 – SEAGS

Soil Reinforcement under Oblique Pull – An Updated Discretization

SEAGS Secretary — Wed, 21 Feb 2018 09:20:20 +0000

Geotechnical Engineering Journal of the SEAGS & AGSSEA ISSN 0046-5828

Vol. 49 No.1 March 2018

Soil Reinforcement under Oblique Pull – An Updated Discretization

S. Patra and J.T. Shahu

ABSTRACT: Reinforced soil structures are gaining popularity for a variety of reasons mainly because it is safe, economical, aesthetic and rapid in constructions. However, the actual behaviour of these structures at failure is still not properly understood. The present study attempts to evaluate the internal stability of these structures against pullout failure. Kinematics of failure suggests that the failure surface intersects the reinforcement obliquely causing an oblique pullout of the reinforcement. In this paper, an updated discretization technique is used to determine the pullout capacity of an inextensible reinforcement resting on a linear elastic Pasternak subgrade and subjected to an oblique end force. A parametric study is conducted and a new factor, length correction factor is introduced in the present analysis. The correction factors have a significant influence on the pullout response especially for high values of obliquity and end displacement. Present analysis thus gives a more realistic value of pullout capacity which is required for the internal stability analysis and design of reinforced soil structures. A case study is also presented to validate the proposed analysis. The maximum reinforcement tension is predicted for top few reinforcements using the proposed method and the AASHTO Simplified Method. The present analysis gives a better prediction of the mobilized reinforcement tension compared to the AASHTO method.

KEYWORDS: Numerical analysis, Discretization, Pullout capacity, Pasternak subgrade, Inextensible reinforcement, Rigid plastic interface.

DOI: 10.14456/seagj.2018.55

Seismic Analysis of Reinforced Soil Wall Considering Oblique Pull-out of Reinforcements: A Review

SEAGS Secretary — Wed, 21 Feb 2018 09:12:37 +0000

Geotechnical Engineering Journal of the SEAGS & AGSSEA ISSN 0046-5828

Vol. 49 No.1 March 2018

Seismic Analysis of Reinforced Soil Wall Considering Oblique Pull-out of Reinforcements: A Review

By Ritwik Nandi and Deepankar Choudhury

ABSTRACT: Several methods are available for stability analysis of reinforced soil structures. However, most of these methods mainly concentrated on the horizontal pull-out of the reinforcement in spite of the evidences available that show the failure surface of reinforced soil structure will always intersect reinforcement layers diagonally due to the failure kinematics. It will cause oblique/transverse deformation to reinforcements across the failure surface. In the present paper, state-of-the-art review of earthquake stability analysis of reinforced soil-wall by employing the oblique/transverse pull of reinforcements is discussed. Formulations that are developed in various studies to determine the mobilization of diagonal pullout resistance of reinforcements, the amount of drag force triggered in the reinforcement sheets due to instability in the structure and the factor of safety against pull-out are presented. A comparative study is also carried out between existing models and methods that are used in determining the seismic stability of reinforced soil structure subjected to diagonal pullout of soil reinforcements. The comparative study shows the effect of various models and methods on the factor of safety against reinforced soil-wall stability and the influence of different parameters i.e., horizontal seismic acceleration, internal friction angle of soil, interface friction angle of soil and reinforcement, relative subgrade stiffness factor etc. Depending on the model used in analyses, the computed factor of safety may vary significantly.

KEYWORDS: Reinforced soil-wall, Oblique pull, Horizontal slice method, Earthquake, Soil reinforcement.

DOI: 10.14456/seagj.2018.58

A Critical and Comparative Study on 2D and 3D Analyses of Raft and Piled Raft Foundations

itsupport installer — Sun, 18 Feb 2018 16:26:37 +0000

Geotechnical Engineering Journal of the SEAGS & AGSSEA ISSN 0046-5828

Vol. 49 No.1 March 2018

A Critical and Comparative Study on 2D and 3D Analyses of Raft and Piled Raft Foundations

By V. Balakumar, Min Huang, Erwin Oh and A. S. Balasubramaniam

ABSTRACT: The piled raft foundation has gained a very high level of acceptance as a foundation system whenever settlement alone governs the design. In the design of piled raft many of the traditional methods could not be applied due to the complex nature of interactions involved. Hence there is a need to use detailed three dimensional finite element analyses for the final design. But in the initial stages of design a simpler but effective analytical process need to be used to save the computational efforts. Since the primary requirement in the piled raft design is the design of optimum pile group to achieve the desired settlement reduction, through number of trials, the applicability of simpler two dimensional analyses are examined to save the computational efforts during the initial trials. It was found that simple two dimensional analyses provide results of acceptable accuracy for the design office requirements.

KEYWORDS: Plane strain, Axisymmetric, ANSYS, PLAXIS.

DOI: 10.14456/seagj.2018.65

Biogeotechnological Methods for Mitigation of Liquefaction

itsupport installer — Sun, 18 Feb 2018 16:23:13 +0000

Geotechnical Engineering Journal of the SEAGS & AGSSEA ISSN 0046-5828

Vol. 49 No.1 March 2018

Biogeotechnological Methods for Mitigation of Liquefaction

By S. Wu, B. Li, J. He and J. Chu

ABSTRACT: Liquefaction of granular soils during earthquake has long been identified as one of the major geohazards. Conventional soil improvement methods for mitigating liquefaction such as dynamic compaction or deep mixing are costly for large-scale applications. Recently some biological processes have shown significant influence on both the physical and chemical performance of geotechnical systems. Two types of biogeotechnological methods, biocementation and biogas desaturation, have been experimentally examined in this study. For the former, a microbial induced carbonate precipitation (MICP) process has turned one cubic meter of loose sand into sandstone-like material. The shear strength of the sand is greatly improved whereas the permeability is reduced at the same time. For the later, tiny inert gas bubbles are generated microbiologically within liquefaction prone ground to increase the resistance of sand to liquefaction. A series of shaking table model tests on biogas treated sand have demonstrated that this biogas desaturation method is effective for reducing pore pressure generation and shaking induced settlement during cyclic loading. When the degree of saturation of the soil is controlled to be around 90%, the generation of pore pressure in sand and the potential for liquefaction could be largely contained.

KEYWORDS: Liquefaction, Biocementation, Biogas, Strength, Pore pressure, Seismic response

DOI: 10.14456/seagj.2018.64

Stone Columns/Granular Piles for Improving Liquefiable Sites: Case studies

itsupport installer — Sun, 18 Feb 2018 16:19:32 +0000

Geotechnical Engineering Journal of the SEAGS & AGSSEA ISSN 0046-5828

Vol. 49 No.1 March 2018

Stone Columns/Granular Piles for Improving Liquefiable Sites: Case studies

By A. Murali Krishna, A. Madan Kumar, Utpal Kr. Baruah

ABSTRACT: Liquefaction is considered as a major hazard among different seismic risks. Ground improvement methods are commonly adopted to improve the liquefiable sites. The paper presents various aspects of liquefaction mitigation strategies to be implemented for liquefaction susceptible sites with focus on granular inclusions. A short discussion on liquefaction susceptible soils and its evaluation followed by outlines of the ground engineering applications is presented herein. Mechanisms that function at sites treated with stone columns/granular piles for liquefaction mitigation are discussed. Design aspects of granular piles for liquefaction mitigation are outlined. Few case studies, wherein stone columns have been adopted for improving the liquefiable sites, are presented. The paper concludes and highlights the effectiveness of granular inclusions in improving the liquefiable sites through various mechanisms.

KEYWORDS: Earthquakes, Liquefaction, Ground improvement, Stone columns, Granular piles

DOI: 10.14456/seagj.2018.63

Interference of Two Closely-Spaced Footings on Finite Sand Layer

itsupport installer — Sun, 18 Feb 2018 16:13:30 +0000

Geotechnical Engineering Journal of the SEAGS & AGSSEA ISSN 0046-5828

Vol. 49 No.1 March 2018

Interference of Two Closely-Spaced Footings on Finite Sand Layer

By Macharam Rohith, Sasanka Mouli, and Umashankar Balunaini

ABSTRACT: Bearing capacity of footing is influenced by the presence of adjacent footing. In this study, two closely-spaced strip, square, and circular footings are modelled in finite elements using commercially available software – PLAXIS 2D and 3D. Analysis is done considering both smooth and rough footing bases. The effect of spacing between the footings is examined for footings resting on both semi-infinite and finite sand layers. In addition, angle of shearing resistance of foundation soil is varied from 30° to 40° to investigate its effect on the bearing capacity. Bearing capacity of footings with rough base are found to attain a peak value at a particular spacing indicating the “blocking effect”. For square and circular footings, interference due to spacing is found to be insignificant compared to strip footing. Interference factors for rough footings are found to be higher than that for smooth footings.

KEYWORDS: Bearing capacity, Interference, Finite element method, Finite layer

DOI: 10.14456/seagj.2018.62

Influence of Shear Stiffness of Geocell Mattress on the Performance of Strip Footings: A Numerical Study

itsupport installer — Sun, 18 Feb 2018 16:10:30 +0000

Geotechnical Engineering Journal of the SEAGS & AGSSEA ISSN 0046-5828

Vol. 49 No.1 March 2018

Influence of Shear Stiffness of Geocell Mattress on the Performance of Strip Footings: A Numerical Study

By P. A. Faby Mole, S. Sireesh and M. R. Madhav

ABSTRACT: A modified Pasternak model was proposed to predict the behavior of a strip footing resting on a geocell reinforced granular layer overlying weak soil, especially considering the variation of shear stiffness of the geocell mattress. Both linear and nonlinear responses of the geocell reinforced beds were considered in the analysis. Results from the present model were validated with independent experimental load-deformation responses. The model parameters viz. inverse of normalized shear stiffness of the geocell and inverse of normalized ultimate bearing capacity of foundation soil were varied for the parametric study. It was found that the shear stiffness of the reinforced granular bed i.e. the product of shear modulus and the height of the geocell reinforced granular bed plays an important role in improving the performance of the foundation system. Design charts are presented in the form of improvement factors for the practical range of shear layer width, shear stiffness of the geocell reinforcement and ultimate bearing capacity of the soft soil.

KEYWORDS: Strip footing, Geocell reinforcement, Shear stiffness, Soft soil

DOI: 10.14456/seagj.2018.61

Encased Columnar Inclusions in Soft Grounds – A Review

itsupport installer — Sun, 18 Feb 2018 16:08:11 +0000

Geotechnical Engineering Journal of the SEAGS & AGSSEA ISSN 0046-5828

Vol. 49 No.1 March 2018

Encased Columnar Inclusions in Soft Grounds – A Review

By J. Jayapal and K.Rajagopal

ABSTRACT: Even before the evolution of soil mechanics, the research on mitigating the problems induced by soft soils has started. The granular column is one of the promising ground improvement technique widely accepted as a solution to soft soil problems all over the world. Recently the performance of it is improved by encasing with geosynthetic products like geogrid and geotextiles. This paper gives an insight into the technical aspects of encased granular columns by reviewing the advancements that have happened in the published literature. The focus of this paper is more on the problems associated with soft clay deposits, although granular columns can also be employed to mitigate liquefaction in saturated loose sand deposits. Discussions on the key technical aspects associated with encased granular columns and its applicability in the field are provided.

KEYWORDS: Ground improvement, Granular column, Soft soil, Geogrid, Encasement.

DOI: 10.14456/seagj.2018.60

Characterization of the Soil Samples from the Lonar Crater, India

itsupport installer — Sun, 18 Feb 2018 16:00:51 +0000

Geotechnical Engineering Journal of the SEAGS & AGSSEA ISSN 0046-5828

Vol. 49 No.1 March 2018

Characterization of the Soil Samples from the Lonar Crater, India

By Nevin Koshy, S. U. Sushalekshmi, Susmita Sharma, Jeevan Joseph, Vikas Sharma, D. N. Singh, Bhagwanjee Jha and M. Singh

ABSTRACT: The Lonar crater and its enclosed lake have been a universally recognized young and well preserved meteoritic formation in the state of Maharashtra, India. Previous studies on the uniqueness (salty and alkaline nature) of sediments (the crater soil) and the lake water, hint at its creation by meteor impact and post-impact induced hydrothermal interaction between the meteor and the then earth surface in the region. Also, the earlier reports confirm the sediments as basaltic rock, in nature. However, not many efforts have been made by the present generation of researchers for detailed chemical and mineralogical characterization of the sediments, which may reveal an analogue relationship between the crater sediments and a meteor (the lunar or the Martian soil) from the space. In this context, the present study attempts to understand the characteristics of the soil samples extracted from the crater region, with respect to their physical, chemical, mineralogical, electrical and magnetic properties. The findings also shed light into the response of the crater samples when subjected to different energy fields (viz., mechanical, chemical, electrical and X-rays). Based on a critical synthesis of the results, the characteristics (viz., alkalinity, saltiness, geological-structural properties, water-sediment interaction) of the sediments have been showcased and evaluated for their partial conformity with extraterrestrial objects (i.e., the meteors).

KEYWORDS: Lonar crater, Mineralogy, Morphology, Mechanical characterization, Chemical characterization, Electrical characterization

DOI: 10.14456/seagj.2018.59

Evaluation of Resilient Modulus of Geosynthetic Reinforced Layers Using Repeated Load Triaxial Tests

itsupport installer — Sun, 18 Feb 2018 15:44:47 +0000

Geotechnical Engineering Journal of the SEAGS & AGSSEA ISSN 0046-5828

Vol. 49 No.1 March 2018

Evaluation of Resilient Modulus of Geosynthetic Reinforced Layers Using Repeated Load Triaxial Tests

By Sudheer S. Prabhu, Lekshmi Suku and G. L. Sivakumar Babu

ABSTRACT: The stiffness and strength of the pavement layers are the major parameters that influence the design of highway pavements which in turn decides the thickness of various pavement layers. Studies have shown that the thickness of the base layer plays a crucial role in limiting the rutting of the in situ subgrade soil. Due to the lack of availability of aggregates, there is a dire need to minimize the thickness of the base. Geosynthetics in the form of geogrid and geocell have long been used for reinforcing unbound base/subbase layers in paved and unpaved roads and have been found to be effective in reducing the base thickness. A few laboratory studies have been conducted to evaluate the different aspects of geosynthetic reinforced base layers, and further studies are required to examine the behavior of these reinforced sections under elastic and plastic shake down range. The purpose of the current study is to evaluate and compare the resilient modulus of geogrid reinforced, geocell reinforced and the unreinforced granular base under repeated loading using the Repeated Load Triaxial tests. The response of aggregate under repeated loading expressed in terms of resilient modulus is a key parameter in the new Mechanistic Empirical Pavement Design Guide (MEPDG). The permanent strains of aggregates are also compared in the study to get an overall idea about the reinforcement effect in the granular base.

KEYWORDS: Repeated Load Triaxial Test, Resilient Modulus, Deformation, Geogrid, Geocell.

DOI: 10.14456/seagj.2018.57