Akshay Kumar Jha, M.R. Madhav and G.V.N. Reddy

ABSTRACT: Steepening of slopes for construction of rail/road embankments or for widening for other civil engineering structures is a necessity for development. Use of geosynthetics for steep slope construction or repair of failed slopes considering all aspects of design and environment could be a viable alternative to these problems. Literature survey indicates that efforts are being made for optimization of length of reinforcement for overall economy. The present paper details an analysis to optimize the length of geosynthetic reinforcement from the face or near end of the slope with respect to its location to obtain the desired minimum factor of safety. Unreinforced and reinforced slopes are analyzed using Morgenstern-Price method to obtain critical factors of safety. The effect of providing geosynthetic reinforcement layer in shifting the critical slip circle has been identified and quantified. Consequently relatively smaller magnitude of force gets mobilized in the reinforcement.

KEYWORDS: Reinforcement, Optimization of length, Critical slip circle, Reinforced slope, Geosynthetics

Fatigue Performance of Geosynthetic Reinforced Two-Layered Asphalt
Concrete Beams

V. Vinay Kumar and S. Sireesh

ABSTRACT: One of the most common rehabilitation techniques adopted for distressed pavements is hot mix asphalt (HMA) overlay. It is often practiced to include geosynthetic interlayers before placing an HMA overlay. The interlayers in HMA overlay not only improves the performance life of the pavement structure by increasing the stiffness, but also, reduces the maintenance cost and the cost of construction by reducing the thickness of HMA overlay.

In the current study, the performance of geosynthetic reinforced two layered asphalt beams is evaluated in two stages. During the first stage, the fatigue performance of the two layered asphalt beams is evaluated using a flexural fatigue test (four point bending). During the second stage, the fracture energy required for crack propagation in the beams during fatigue loading and the corresponding tensile stiffness of two layered asphalt beams with and without geosynthetic interlayers are determined using Fenix test. Three types of geosynthetics, namely biaxial polyester grids, woven geo-jute mat and biaxial polypropylene grids are used in the study. The results from fatigue and Fenix tests indicated that the fatigue life and the tensile stiffness of the geosynthetic reinforced asphalt beams have drastically increased against the control specimens. A 30 times increase in fatigue life is noticed in polyester grid reinforced asphalt beams against unreinforced beams at 10 mm vertical deformation, which is attributed to the increase in tensile stiffness of the specimens from 7.3 kN/mm to 17.6 kN/mm. A linear regression equation is proposed to correlate the normalized complex modulus and tensile stiffness index to estimate the complex modulus of the geosynthetic reinforced asphalt beams.

KEYWORDS: Fenix test, Flexural fatigue test, Geosynthetics, HMA overlay

Deformation Response of Geocells in Pavements under Moving Loads

Priti Maheshwari and G. L. Sivakumar Babu

ABSTRACT: Geocells are extensively used in pavements as one of the ground improvement techniques. Pavements are subjected to various types of loading pattern and its deformation under these loads plays an important role in its analysis and design. In the present work, a deformation model of geocell has been proposed in which geocell has been idealized as an infinite beam subjected to a concentrated load moving with constant speed. The foundation soil has been modeled as Winkler springs. Influence of magnitude and speed of applied load, flexural rigidity of geocell, modulus of subgrade reaction of foundation soil, mass of beam, viscous damping and interfacial resistance between geocell reinforcement and the neighboring soil on response of geocell has been studied. Non-dimensional charts have been developed for normalized deflection and the bending moment in geocell reinforcement. These charts will be useful while analyzing and designing the pavements under moving loads. A numerical example has also been presented for the better understanding of results from the proposed model.

KEYWORDS: Geocells, Deformation, Pavements, Infinite beams, Viscous damping, Ground improvement

Effect of Multilayered Geosynthetic Reinforcements on the Response of Foundations Resting on Stone Column-Improved Soft Soil

K. Deb

ABSTRACT: The present paper pertains to the development of a mechanical model based on soil-structure interaction to study the effect of multilayered geosynthetic reinforcements on the behaviour of footings resting on stone column-improved soft soil. The footing is idealized as a beam. The soft soil and granular layer are idealized as nonlinear spring-dashpot and Pasternak shear layer, respectively. The geosynthetic reinforcements are modelled by elastic membranes. The stone columns are idealized by nonlinear springs. The governing differential equations are solved by finite difference method and results are presented in non-dimensional term. It is observed that multilayered-reinforced system is not effective for settlement reduction, but it is effective for bending moment and shear force reduction. However, for higher modular ratio (>40), the multilayered-reinforced system is not useful for maximum bending moment reduction. As the modular ratio increases positive bending moment at the centre of the beam decreases and the positive bending moment of the beam above middle of the stone column becomes negative. The negative bending moment of the beam above middle of the stone column increases as the modular ratio increases. The maximum shear force is observed for s/bw ratio 3 and 5 corresponding to the modular ratio 10 and 100, respectively.

KEYWORDS: Beams, Granular layer, Multilayered geosynthetic reinforcements, Stone column, Soft soil, Soil-structure interaction

A Critical Review of the Performance of Geosynthetic-Reinforced Railroad Ballast

Syed Khaja Karimullah Hussaini, Buddhima Indraratna and J. S. Vinod

ABSTRACT: In the recent times, railway organizations across the world have resorted to the use of geosynthetics as a low-cost solution to stabilize ballast. In this view, extensive studies have been conducted worldwide to assess the performance of geosynthetic-reinforced ballast under various loading conditions. This paper evaluates the various benefits the rail industry could attain because of the geosynthetic reinforcement. A review of literature reveals that geogrid arrests the lateral spreading of ballast, reduces the extent of permanent vertical settlement and minimizes the particle breakage. The geogrid was also found to reduce the extent of volumetric compressions in ballast. The overall performance improvement due to geogrid was observed to be a function of the interface efficiency factor (α). Moreover, studies also established the additional role of geogrids in reducing the differential track settlements and diminishing the stresses at the subgrade level. The geosynthetics were found to be more beneficial in case of tracks resting on soft subgrades. Furthermore, the benefits of geosynthetics in stabilizing ballast were found to be significantly higher when placed within the ballast. The optimum placement location of geosynthetics has been reported by several researchers to be about 200-250 mm below the sleeper soffit for a conventional ballast depth of 300-350 mm. A number of field investigations and track rehabilitation schemes also confirmed the role of geosynthetics/geogrids in stabilizing the tracks thereby helping in removing the stringent speed restrictions that were imposed earlier, and enhancing the time interval between maintenance operations.

KEYWORDS: Cyclic loading, Geosynthetics, Vertical settlement, Lateral spreading, Ballast breakage, Differential settlement

Performance of Geosynthetic Reinforced Model Pavements under Repetitive Loading

K. H. Mamatha, S. V. Dinesh and B. C. Swamy

ABSTRACT: In this paper, the effectiveness of geosynthetic reinforcement materials such as geogrids and geocells in improving the pavement performance is investigated by carrying out a series of repeated load tests on unreinforced, geogrid and geocell reinforced model pavement sections. The effect of properties of geogrids and geocells on the improved performance is also studied. The provision of geogrid/geocell at the interface of subgrade and sub-base course is found to reduce the plastic settlement significantly with geocells being very effective when compared with geogrids. The reduced plastic settlement results in reduced rutting at the surface leading to increased service life of the pavements and also increased ride comfort to the road users. The geocells reinforcement results in higher TBR values when compared with that of geogrid.

KEYWORDS: Pavement, Geogrid, Geocell, Granular layer, Service life, Rutting

Lateral Response Analysis of GRS Bridge Abutments under Passive Push

M. Ramalakshmi and G. R. Dodagoudar

ABSTRACT: The objective of this study is to analyse the response of Geosynthetic Reinforced Soil (GRS) bridge abutments under lateral push towards the backfill. Hypoplastic constitutive model is adopted as the user defined material model in the subroutine, VUMAT, to represent the soil behaviour in finite element (FE) analysis. The unreinforced abutment and GRS abutments of eighteen different configurations are modelled using FE approach and analysed for static passive push. The passive force-displacement curves are obtained to study the lateral response of the GRS abutments. The inclusion of geogrid reduced the passive pressures behind the abutment wall. The GRS abutments with lesser geogrid spacing and longer geogrid reduced the passive pressures significantly compared to the other cases studied.

KEYWORDS: GRS bridge abutment, Lateral response, Hypoplastic soil model, Static push, Passive earth pressure

Numerical Analysis of Machine Foundation Resting on the Geocell Reinforced
Soil Beds

H. Venkateswarlu and A. Hegde

ABSTRACT: The foundation beds are often subjected to dynamic loads due to many circumstances, such as earthquakes, traffic loads, and the machine vibrations in the case of the machine foundations. Excessive vibrations caused by the dynamic sources can lead to the structural damage of the foundation soil. Over the years, geosynthetics have been effectively used in reducing the settlement of the foundations under static loads. However, the performance of geosynthetics is not fully analyzed under the dynamic loads. In the present study, the numerical analyses have been carried out to understand the performance of the machine foundations resting on the geocell reinforced beds. The analyses were carried out by using finite element software PLAXIS 2D. The hypothetical case of the circular machine foundation of 1 m diameter resting on the saturated silty sand was analyzed. Mohr-Coulomb failure criteria was used to simulate the behavior of the soil. Initially, the numerical model was validated with the existing results reported in the literature. The validated numerical model was further used to investigate the performance of the machine foundations. Three different cases, namely, unreinforced, geogrid reinforced and geocell reinforced were considered. The response of all the cases was studied by varying the frequency of dynamic excitation and maintaining the constant force amplitude. The depth of the placement of the geocell and geogrid was also varied. At the optimum location of geocell, 61% reduction in the displacement amplitude was observed as compared to unreinforced foundation bed. Similarly, as compared to geogrid, more than 50% reduction in the displacement was observed in the presence of geocell. In addition, 40% reduction in peak particle velocity was observed in the presence of geocell at the center of the footing. The resonant frequency was found to vary with the reinforcement system. Furthermore, 163% increase in the damping ratio of the soil was observed in the presence of geocell. In this way, the study highlights the possible new applications of geocell in supporting the machine foundations.

KEYWORDS: Dynamic response, Machine Foundation, Geocell, Amplitude, PLAXIS 2D

Finite Element Modeling of Embankment Resting on Soft Ground Stabilized with Prefabricated Vertical Drains

B. Giridhar Rajesh, S. K. Chukka, and A. Dey

ABSTRACT: This paper presents the numerical modelling of embankment resting on soft soil improved by the use of prefabricated vertical drains (PVDs). The study has been validated with the field measurements of settlements and excess pore pressures for a trial embankment at the Krishnapatnam Ultra Mega Power Project (KUMPP) in Nellore, Andhra Pradesh, India. The paper elaborately highlights the intricate effect of various parameters such as the drain spacing, reduction of permeability due to smear, and the efficiency of floating drains. Two-dimensional finite element modelling was carried out using PLAXIS 2D. In the analysis, classical axisymmetric solution for consolidation by vertical drains has been converted into an equivalent two-dimensional plane strain analysis. The comparatives reflect the agreements and differences between the field measurements and the results obtained from the numerical model. Based on the results, the state of smear prevailing in the field has been identified. The numerical study suggests that the optimal length of the partially penetrating drains (75-80% of the full penetration) would be efficient in aiding sufficient vertical consolidation of the soft soil site, thus making its usage more economical.

KEYWORDS: Preloadng embankment, Prefabricated vertical drains (PVDs), Smear effect, Floating PVDs, Soil constitutive models

Effect of Geobags on Water Flow through Capillary Barrier System

H. Rahardjo, N. Gofar, F. Harnas, A. Satyanaga

ABSTRACT: Capillary barrier is a two-layer cover system consisting of fine over coarse materials designed to protect slope from rainfall-induced failure. Previous studies have shown that the capillary barrier system (CBS) is effective for protection of gentle slopes, but the application of CBS on steep slopes requires further study. The fine materials are wrapped with geobags before laying them on top of the coarse materials. In this case, the bags serve as the separator between the fine and coarse materials. This paper highlights the effect of geobags on the effectiveness of CBS consisting of fine sand (Sand) as the fine material and reclaimed asphalt pavement (RAP) as the coarse material. Soil column tests were performed for two configurations (1) Sand overlying RAP (no-geo) and (2) Sand overlying RAP with geobags inserted at the interface (geo). The soil column was instrumented with tensiometer-transducer system, moisture sensors and electronic balance to measure pore-water pressures (PWP), volumetric water content (VWC) and outflow, respectively. Numerical simulations were carried out to support the findings from the soil column tests. Results of the soil column tests and numerical analyses on both configurations showed that the presence of geobags at the interface of Sand and RAP does not affect the effectiveness of CBS as slope protection from rainfall infiltration.

KEYWORDS: Geobags, Infiltration, Capillary barrier system, Soil column test, Numerical model

Performance of Ballasted Track under Impact Loading and Applications of Recycled Rubber Inclusion

Sanjay Nimbalkar, Sujit Kumar Dash, and Buddhima Indraratna

ABSTRACT: In this paper a review of the sources of impact loads and their effect on the performance of ballasted track is presented. The typical characteristics and implications of impact loading on track deterioration, particularly ballast degradation, are discussed. None of the procedures so far developed to design rail track incorporate the impact that dynamic loading has on the breakage of ballast and therefore it can be said to be incomplete. An intensive study on the impact of induced ballast breakage is needed in order to understand this phenomenon and then use the knowledge gained to further advance the design methodology. A stiff track structure can create severe dynamic loading under operating conditions which causes large scale component failure and increases maintenance requirements. Installing resilient mats such as rubber pads (ballast mat, soffit pad) in rail tracks can attenuate the dynamic force and improve overall performance. The efficacy of ballast mats to reduce structural noise and ground vibration has been studied extensively, but a few recent studies has reported how ballast mats and soffit pads reduce ballast degradation, thus obviating the necessity of a comprehensive study in this direction.

KEYWORDS: Impact loads, Recycled materials, Degradation, Rubber mats

Probabilistic Stability Analyses of Reinforced Slope Subjected to Strip Loading

Koushik Halder and Debarghya Chakraborty

ABSTRACT: The aim of the present study is to investigate the effect of uncertainty associated with soil friction angle (ϕ) and soil unit weight (γ) on the stability of both unreinforced and reinforced cohesionless soil slopes subjected to strip loading. The magnitude of CoV of ϕ and γ are varied to account uncertainties. The location of the footing on the top of the slope is also changed. Stability of both unreinforced and reinforced slopes is presented in terms of factor of safety (FoS). Deterministic FoS values are computed first by using a two-dimensional finite difference software FLAC. To perform probabilistic analyses, FLAC is combined with Monte Carlo simulations. The outcomes of the probabilistic analyses are presented in terms of probability of failure (pF) and reliability index (β). The value of β obtained from the present study is compared with the guidelines provided by USACE. It is found out that with the increase in the value of CoV, pF increases and β decreases. As expected, the failure probability of slope is found to be maximum, when footing is placed on the edge of the unreinforced slope. With the inclusion of a single layer of geotextile in the slope for the same footing position, pF reduces drastically, and β increases significantly. As footing position shifts from the slope edge, pF increases for a particular CoV value of ϕ and γ. The effect of uncertainty related to ϕ is found to be more prominent with compared to the uncertainty related to γ. The influence of cross-correlation between ϕ and γ is also studied. It is found that there is no significant change in the value of pF with the change in the value of cross correlation coefficient. Though the present study is related to a simple slope stability problem, but using the same methodology, probabilistic analyses of complex slopes can also be performed.

KEYWORDS: Probabilistic analyses; Reinforced slope ; Strip footing; FLAC; MCS

Advancing the Use of Geosynthetic Clay Liners as Barriers

J. Scalia IV, C.A. Bareither and C.D. Shackelford

ABSTRACT: Geosynthetic clay liners (GCLs) are effective barrier materials for liner and cover systems in waste containment applications. Exposure to non-standard chemical solutions can alter the chemical and mechanical properties of both the bentonite and geotextiles comprising a GCL. Considerable advances in laboratory testing and analysis of GCLs have occurred recently in regard to hydraulic conductivity, the existence and persistence of membrane behavior, and long-term shear strength of GCLs evaluated under stress-controlled conditions. The objective of this paper is to present a synopsis of advances in research related to GCLs that is focused on enhancing knowledge of GCLs used as hydraulic and chemical contaminant barriers.

KEYWORDS: Bentonite, Chemico-osmosis, Containment barrier, Enhanced bentonite, Geosynthetic clay liners, Hydraulic conductivity, Membrane behavior, Polymerized bentonite, Shear strength

Field and Laboratory Tests on the Bearing Behaviour of Unpaved Roads Reinforced by Different Geosynthetics

G. Bräu and S. Vogt

ABSTRACT: Field experiences have shown that the use of geosynthetics improves the trafficability of unpaved roads on soft subsoil. Specifically, the thickness of the base course and therefore the amount of high quality geomaterials e.g. crushed gravel can be reduced. Until now, the design is mainly based on empirical approaches based on results from experiments obtained in field tests. The thickness of the base course is increased until an adequate bearing capacity of the unpaved road is reached. There are extensive studies throughout the literature that confirm the mechanism of the bearing capacity improvement, but mostly cover only individual effects such as the influence of the bearing layer thickness at constant subsoil strength. Therefore, they cannot be extended to a general theory and design approach that can account for all of the important variables. To investigate the effectiveness of different geosynthetics in unpaved roads a series of loading tests on geotextile reinforced, unpaved roads were carried out both in the laboratory and in the field. Beside the bearing strength and stiffness of the soft subsoil, the base course thickness as well as the type, and hence the strength of the geosynthetics were varied in the tests. This paper presents a brief summary of the experimental results that may be used to evaluate models to predict the bearing capacity of unpaved roads.

KEYWORDS: Geosynthetics, Soil Reinforcement, Experiments, Unpaved Road, Soft Soil, Base Course, Cyclic Loading

Performance of Nonwoven Geotextiles as Separators for Pavement Applications

Maria P.S. Susunaga, Ennio M. Palmeira & Gregório L.S. Araújo

ABSTRACT: Geosynthetics can be used in several applications in geotechnical and geoenvironmental engineering, being geotextiles the most traditional and versatile type of geosynthetic. One of the applications of geotextiles is in separation between good and poor quality soils. This situation may occur in geotechnical structures such as roads and railways constructed on soft saturated subgrades. The presence of a geotextile separator avoids or minimize the contamination of the good quality base or ballast material with fines from the subgrade, increasing the life of the road and reducing maintenance costs. Despite its importance, very few studies on the behaviour of geotextiles in separation can be found in the literature compared to other applications of these materials. This paper investigates the performance of nonwoven geotextiles in separation. Laboratory tests on geotextiles with masses per unit area ranging from 200 g/m2 to 600 g/m2 were executed using an apparatus capable of applying repetitive loading to simulate traffic conditions. Measurements of surface displacements and pore pressures in the subgrade soil and the evaluation of geotextile mechanical damages at the end of the tests were carried out. The results obtained showed that the three geotextiles tested were effective separators, avoiding contamination of the base soil and accelerating the dissipation of excess pore pressures in the subgrade soil. However, significant mechanical damage was observed in the lighter geotextile used.

Geosynthetics Application in Indonesia – A Case Histories

Tjie-Liong GOUW

ABSTRACT: The first application of geosynthetics technology was back in 1983, where a high strength geotextile of 200 kN/m was laid to help stabilize the highway built on swampy land toward Soekarno Hatta airport, the gateway to Indonesia. Since then, geosynthetics have been gaining popularity in solving challenging ground conditions for civil engineering development, e.g. stabilization of road development over peat deposits, accelerating consolidation of soft clay, stabilization of foundation over expansive clays, slope stabilization over clay shales formation, retaining walls, ponds lining, breakwater, shore protection and river bank stabilization, etc. This paper presents the author experiences in applying geosynthetics technology in building geotechnical construction over difficult ground condition such as peat, soft clay, expansive soils, and clay shales. It also presents the application of geosynthetics tubes (geotubes) to build containment dykes over soft marine clays.

KEYWORDS: Geosynthetic Reinforcement, Vacuum, MSE wall, Geotubes, Peat, Soft Clay, Expansive Soils, Clay Shales, Tanah Merah

By Harry G. Poulos

ABSTRACT: The concept of modulus of subgrade reaction has been employed within the engineering world for almost 150 years. It has been especially embraced by structural engineers who have found it convenient to represent the behaviour of the ground supporting their structures by elastic springs. Despite the best efforts of the geotechnical profession to dissuade our structural colleagues from using this flawed concept in foundation design, requests to provide a modulus of subgrade reaction continue almost unabated. Given this situation, a suitable response is to provide such values via a rational process of evaluation, rather than by empirical correlations which have little   theoretical basis and which may not be applicable to the foundation being considered.

This paper sets out an approach to the estimation of values of modulus of subgrade reaction for various types of foundation. The key points made are that the modulus of subgrade reaction (k) is not a fundamental soil property, but varies with the foundation type, foundation dimension, and type of loading. k can be related to the Young’s modulus of the supporting soil and to the foundation dimensions, but for pile groups, account must be taken of the reduction in k because of group effects arising from pile-soil-pile interaction. It is also emphasized that careful distinction must be made between the modulus of subgrade reaction, k, and the spring stiffness K.

KEYWORDS: Foundation, Lateral loading, Modulus of subgrade reaction, Piles, Pile group, Settlement.

Effectiveness of Stone Column Reinforcement for Stabilizing Soft Ground with Reference to Transport Infrastructure

By S. Basack, B. Indraratna and C. Rujikiatkamjorn

ABSTRACT: The use of stone columns for soft soil stabilization has numerous advantages compared to other methods. There are many factors controlling performance of stone columns including column geometry and particle morphology. The reinforced soft ground supporting transport infrastructure like the railways and highways is subjected to cyclic loading, usually initiating a partially drained condition. The study reveals that the stone columns are more effective in mitigating the built up of cyclic excess pore water pressure compared to conventional vertical drains. This paper presents a brief overview on the rigorous theoretical and experimental studies carried out by the Authors to investigate the effectiveness of stone column reinforcement for stabilizing soft ground with particular reference to transport infrastructure.

KEYWORDS: Arching, Clogging, Cyclic stress, Ground settlement, Radial consolidation, Stress concentration

Pile Design and Group Behaviour; A Case Study of Large Tank Foundations in Soft Soil Conditions

By W.F. Van Impe, P.O. Van Impe and A. Manzotti

ABSTRACT: The paper presents the case study on the construction of three 48m diameter oil tanks in Ostend (Belgium), each founded on a group of 422 displacement cast in-situ screw piles. The three tanks are close enough to each other to induce interaction. Monitoring of the tanks’ movements has been performed during the hydro-testing of the steel tanks and during the subsequent working stage of the tanks. The bearing layer of the pile group is a 5m thick stiff sand layer at a depth of about 20m, overlain by a very heterogeneous soft clayey/silty fill containing sand pockets, and underlain by a very thick slightly over-consolidated clay. Some short and long term settlement prediction of the tanks have been done, assuming soil parameters derived from the CPT data on site, and compared to the measured settlements. The initially derived soil parameters are then re-evaluated in order to predict the long term settlement for the full life span of the construction.

KEYWORDS: Pile group, Screw pile, Group effect, Load-settlement behavior, Oil tanks, Pile loading testing, Residual stress

Granular Columns for Geotechnical Applications

By V. Sivakumar

ABSTRACT: Soft clay deposits are globally widespread and often coincide with strategic transport links and growing urban developments. These soft deposits are often waterlogged and are composed of clay with varying degrees of silt, sand and organic matter. These soils have low undrained shear strength and high compressibility, contributing to construction problems in relation to stability and settlement. Granular columns, also referred to as flexible piles, are one of the techniques widely considered in the industry for improving soft deposits for low-moderate structural loading. The purpose of this article is to highlight some of the key investigations carried out in the topic of granular columns at Queen’s University Belfast, the UK.

The investigations focused on several aspects: (a) the interaction between columns and surrounding clay (b) containment of columns in geo-grid for enhanced strength performance (c) settlement performance under single or multiple column configuration (d) stress distribution under the footing and along the column (e) assessment of consolidation and creep settlement under constant loading and (f) granular columns for anchoring purposes and therefore stabilization of slopes. Overall observations are: settlement improvement factors were moderate under isolated loading, but granular columns are very effective in providing pull-out capacity in the form of anchors.

KEYWORDS: Organic soils, Creep, Settlement, Consolidation, Anchors

Ground Engineering using Prefabricated Vertical Drains: A Review

By V. A. Sakleshpur, M. Prezzi, and R. Salgado

ABSTRACT: Improvement of soft ground by preloading with prefabricated vertical drains (PVDs) is a common practice in the field of ground engineering. PVDs accelerate the consolidation process of soft soils by providing a shorter drainage path for the pore water and thereby increase the strength and stiffness of soft soils over time. This paper presents a review of recent analytical, laboratory, numerical and field studies performed using preloading with PVDs for improvement of soft ground. The focus of the paper is on conventional PVDs without the use of vacuum, thermal and electro-osmosis techniques. Summary tables, which provide quick and easy access to the latest information from various research efforts, have been prepared and discussed. The review is complemented by two case histories that highlight the performance of PVDs in the field.

KEYWORDS: Ground improvement, Prefabricated vertical drain (PVD), Inner smear zone, Transition zone, Well resistance, Case history

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.

Effect of Facing Slope on the Seismic Response of Geocell Walls

By Madhavi Latha and Manju G. S.

ABSTRACT: This paper presents the effect of slope angle of facing on the seismic response of retaining walls with geocell facing. Keeping the dimensions and configuration of geocell layer same, shaking table model tests were carried out with vertical and battered walls retaining sand backfill. In case of battered walls, geocell layers were laid with an offset, resulting in an overall slope of the wall. Vertical walls were constructed with geocell layers stacked vertically above each other. Gravel was used as infill material in geocells. Models were subjected to different levels of ground motion conditions by controlling the acceleration and frequency of shaking. Acceleration amplitudes of 0.2g and 0.3g with frequencies ranging between 1 Hz and 7 Hz were used in the model tests. Response of models was monitored with cyclic shaking at intended acceleration and frequency by measuring the face deformations and acceleration amplifications along the height of the retaining wall, Results from model tests showed that battered walls perform better than the vertical walls since the measured deformations and acceleration amplifications were comparatively low in battered walls. The improved performance of battered walls is due to the increased stiffness and increase in dynamic impedance caused due to shifting of moment of inertia of pressure distribution at the back of the wall in case of walls battered towards the backfill.

KEYWORDS: Geocells, Retaining walls, Seismic performance, RMSA amplification factor, Wall slope, Shaking table model studies

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.

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.

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

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.

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

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

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

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.

Vol. 49 No. 4 December 2018

Geosynthetics Application in Indonesia – A Case Histories

Tjie-Liong GOUW

ABSTRACT: The first application of geosynthetics technology was back in 1983, where a high strength geotextile of 200 kN/m was laid to help stabilize the highway built on swampy land toward Soekarno Hatta airport, the gateway to Indonesia. Since then, geosynthetics have been gaining popularity in solving challenging ground conditions for civil engineering development, e.g. stabilization of road development over peat deposits, accelerating consolidation of soft clay, stabilization of foundation over expansive clays, slope stabilization over clay shales formation, retaining walls, ponds lining, breakwater, shore protection and river bank stabilization, etc. This paper presents the author experiences in applying geosynthetics technology in building geotechnical construction over difficult ground condition such as peat, soft clay, expansive soils, and clay shales. It also presents the application of geosynthetics tubes (geotubes) to build containment dykes over soft marine clays.

KEYWORDS: Geosynthetic Reinforcement, Vacuum, MSE wall, Geotubes, Peat, Soft Clay, Expansive Soils, Clay Shales, Tanah Merah

DOI: 10.14456/seagj.2018.16

Vol. 49 No. 4 December 2018

Performance of Nonwoven Geotextiles as Separators for Pavement Applications

Maria P.S. Susunaga, Ennio M. Palmeira & Gregório L.S. Araújo

ABSTRACT: Geosynthetics can be used in several applications in geotechnical and geoenvironmental engineering, being geotextiles the most traditional and versatile type of geosynthetic. One of the applications of geotextiles is in separation between good and poor quality soils. This situation may occur in geotechnical structures such as roads and railways constructed on soft saturated subgrades. The presence of a geotextile separator avoids or minimize the contamination of the good quality base or ballast material with fines from the subgrade, increasing the life of the road and reducing maintenance costs. Despite its importance, very few studies on the behaviour of geotextiles in separation can be found in the literature compared to other applications of these materials. This paper investigates the performance of nonwoven geotextiles in separation. Laboratory tests on geotextiles with masses per unit area ranging from 200 g/m2 to 600 g/m2 were executed using an apparatus capable of applying repetitive loading to simulate traffic conditions. Measurements of surface displacements and pore pressures in the subgrade soil and the evaluation of geotextile mechanical damages at the end of the tests were carried out. The results obtained showed that the three geotextiles tested were effective separators, avoiding contamination of the base soil and accelerating the dissipation of excess pore pressures in the subgrade soil. However, significant mechanical damage was observed in the lighter geotextile used.

KEYWORDS: Performance, Nonwoven Geotextiles, Separators, Pavement Applications

DOI: 10.14456/seagj.2018.15

Vol. 49 No. 4 December 2018

Field and Laboratory Tests on the Bearing Behaviour of Unpaved Roads Reinforced by Different Geosynthetics

G. Bräu and S. Vogt

ABSTRACT: Field experiences have shown that the use of geosynthetics improves the trafficability of unpaved roads on soft subsoil. Specifically, the thickness of the base course and therefore the amount of high quality geomaterials e.g. crushed gravel can be reduced. Until now, the design is mainly based on empirical approaches based on results from experiments obtained in field tests. The thickness of the base course is increased until an adequate bearing capacity of the unpaved road is reached. There are extensive studies throughout the literature that confirm the mechanism of the bearing capacity improvement, but mostly cover only individual effects such as the influence of the bearing layer thickness at constant subsoil strength. Therefore, they cannot be extended to a general theory and design approach that can account for all of the important variables. To investigate the effectiveness of different geosynthetics in unpaved roads a series of loading tests on geotextile reinforced, unpaved roads were carried out both in the laboratory and in the field. Beside the bearing strength and stiffness of the soft subsoil, the base course thickness as well as the type, and hence the strength of the geosynthetics were varied in the tests. This paper presents a brief summary of the experimental results that may be used to evaluate models to predict the bearing capacity of unpaved roads.

KEYWORDS: Geosynthetics, Soil Reinforcement, Experiments, Unpaved Road, Soft Soil, Base Course, Cyclic Loading

DOI: 10.14456/seagj.2018.14

Vol. 49 No. 4 December 2018

Advancing the Use of Geosynthetic Clay Liners as Barriers

J. Scalia IV, C.A. Bareither and C.D. Shackelford

ABSTRACT: Geosynthetic clay liners (GCLs) are effective barrier materials for liner and cover systems in waste containment applications. Exposure to non-standard chemical solutions can alter the chemical and mechanical properties of both the bentonite and geotextiles comprising a GCL. Considerable advances in laboratory testing and analysis of GCLs have occurred recently in regard to hydraulic conductivity, the existence and persistence of membrane behavior, and long-term shear strength of GCLs evaluated under stress-controlled conditions. The objective of this paper is to present a synopsis of advances in research related to GCLs that is focused on enhancing knowledge of GCLs used as hydraulic and chemical contaminant barriers.

KEYWORDS: Bentonite, Chemico-osmosis, Containment barrier, Enhanced bentonite, Geosynthetic clay liners, Hydraulic conductivity, Membrane behavior, Polymerized bentonite, Shear strength

DOI: 10.14456/seagj.2018.13

Vol. 49 No. 4 December 2018

Probabilistic Stability Analyses of Reinforced Slope Subjected to Strip Loading

Koushik Halder and Debarghya Chakraborty

ABSTRACT: The aim of the present study is to investigate the effect of uncertainty associated with soil friction angle (ϕ) and soil unit weight (γ) on the stability of both unreinforced and reinforced cohesionless soil slopes subjected to strip loading. The magnitude of CoV of ϕ and γ are varied to account uncertainties. The location of the footing on the top of the slope is also changed. Stability of both unreinforced and reinforced slopes is presented in terms of factor of safety (FoS). Deterministic FoS values are computed first by using a two-dimensional finite difference software FLAC. To perform probabilistic analyses, FLAC is combined with Monte Carlo simulations. The outcomes of the probabilistic analyses are presented in terms of probability of failure (pF) and reliability index (β). The value of β obtained from the present study is compared with the guidelines provided by USACE. It is found out that with the increase in the value of CoV, pF increases and β decreases. As expected, the failure probability of slope is found to be maximum, when footing is placed on the edge of the unreinforced slope. With the inclusion of a single layer of geotextile in the slope for the same footing position, pF reduces drastically, and β increases significantly. As footing position shifts from the slope edge, pF increases for a particular CoV value of ϕ and γ. The effect of uncertainty related to ϕ is found to be more prominent with compared to the uncertainty related to γ. The influence of cross-correlation between ϕ and γ is also studied. It is found that there is no significant change in the value of pF with the change in the value of cross correlation coefficient. Though the present study is related to a simple slope stability problem, but using the same methodology, probabilistic analyses of complex slopes can also be performed.

KEYWORDS: Probabilistic analyses; Reinforced slope ; Strip footing; FLAC; MCS

DOI: 10.14456/seagj.2018.12

Vol. 49 No. 4 December 2018

Performance of Ballasted Track under Impact Loading and Applications of Recycled Rubber Inclusion

Sanjay Nimbalkar, Sujit Kumar Dash, and Buddhima Indraratna

ABSTRACT: In this paper a review of the sources of impact loads and their effect on the performance of ballasted track is presented. The typical characteristics and implications of impact loading on track deterioration, particularly ballast degradation, are discussed. None of the procedures so far developed to design rail track incorporate the impact that dynamic loading has on the breakage of ballast and therefore it can be said to be incomplete. An intensive study on the impact of induced ballast breakage is needed in order to understand this phenomenon and then use the knowledge gained to further advance the design methodology. A stiff track structure can create severe dynamic loading under operating conditions which causes large scale component failure and increases maintenance requirements. Installing resilient mats such as rubber pads (ballast mat, soffit pad) in rail tracks can attenuate the dynamic force and improve overall performance. The efficacy of ballast mats to reduce structural noise and ground vibration has been studied extensively, but a few recent studies has reported how ballast mats and soffit pads reduce ballast degradation, thus obviating the necessity of a comprehensive study in this direction.

KEYWORDS: Impact loads, Recycled materials, Degradation, Rubber mats

DOI: 10.14456/seagj.2018.11

Vol. 49 No. 4 December 2018

Effect of Geobags on Water Flow through Capillary Barrier System

H. Rahardjo, N. Gofar, F. Harnas, A. Satyanaga

ABSTRACT: Capillary barrier is a two-layer cover system consisting of fine over coarse materials designed to protect slope from rainfall-induced failure. Previous studies have shown that the capillary barrier system (CBS) is effective for protection of gentle slopes, but the application of CBS on steep slopes requires further study. The fine materials are wrapped with geobags before laying them on top of the coarse materials. In this case, the bags serve as the separator between the fine and coarse materials. This paper highlights the effect of geobags on the effectiveness of CBS consisting of fine sand (Sand) as the fine material and reclaimed asphalt pavement (RAP) as the coarse material. Soil column tests were performed for two configurations (1) Sand overlying RAP (no-geo) and (2) Sand overlying RAP with geobags inserted at the interface (geo). The soil column was instrumented with tensiometer-transducer system, moisture sensors and electronic balance to measure pore-water pressures (PWP), volumetric water content (VWC) and outflow, respectively. Numerical simulations were carried out to support the findings from the soil column tests. Results of the soil column tests and numerical analyses on both configurations showed that the presence of geobags at the interface of Sand and RAP does not affect the effectiveness of CBS as slope protection from rainfall infiltration.

KEYWORDS: Geobags, Infiltration, Capillary barrier system, Soil column test, Numerical model

DOI: 10.14456/seagj.2018.10

Vol. 49 No. 4 December 2018

Finite Element Modeling of Embankment Resting on Soft Ground Stabilized with
Prefabricated Vertical Drains

B. Giridhar Rajesh, S. K. Chukka, and A. Dey

ABSTRACT: This paper presents the numerical modelling of embankment resting on soft soil improved by the use of prefabricated vertical drains (PVDs). The study has been validated with the field measurements of settlements and excess pore pressures for a trial embankment at the Krishnapatnam Ultra Mega Power Project (KUMPP) in Nellore, Andhra Pradesh, India. The paper elaborately highlights the intricate effect of various parameters such as the drain spacing, reduction of permeability due to smear, and the efficiency of floating drains. Two-dimensional finite element modelling was carried out using PLAXIS 2D. In the analysis, classical axisymmetric solution for consolidation by vertical drains has been converted into an equivalent two-dimensional plane strain analysis. The comparatives reflect the agreements and differences between the field measurements and the results obtained from the numerical model. Based on the results, the state of smear prevailing in the field has been identified. The numerical study suggests that the optimal length of the partially penetrating drains (75-80% of the full penetration) would be efficient in aiding sufficient vertical consolidation of the soft soil site, thus making its usage more economical.

KEYWORDS: Preloadng embankment, Prefabricated vertical drains (PVDs), Smear effect, Floating PVDs, Soil constitutive models

DOI: 10.14456/seagj.2018.9