Analysis of Effect of Reinforcement on Stability of Slopes and Reinforcement Length optimization
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
