Centrifuge model studies on clay liners of landfills

Compacted Clay Liners (CCLs) made-up of soil of low permeability characteristics are generally prone to damage when subjected to non-uniform settlements. Non-uniform settlements lead to bending and are critical for the behaviour of clay liners. As a result, the clay liner could loose its integrity and sealing efficiency. In this paper, relevance of centrifuge modeling technique for studying affect of non-uniform settlements on the integrity of the CCL of a liner system is presented. The centrifuge tests were carried out at 50g by simulating non-uniform settlements of a landfill using a trap-door arrangement. In addition the possibility of using a geogrid layer within the CCL to retain the integrity is brought out

Studies on scaling and instrumentation of a geogrid

The scaling down of geosynthetic materials is essential in small-scale physical modeling studies in order to obtain the correct response of prototype structure. In this paper, similitude conditions concerning modeling of geosynthetic materials in Ig and Ng conditions are examined with an emphasis on geogrids. In order to model soil-geosynthetic behavior satisfactorily, it was observed that there is a substantial requirement of modeling geosynthetic materials for studies pertaining to the behavior of reinforced soil structures. Secondly, one of the selected model geogrids was instrumented with special type of tiny strain gauges adhered to base material formed with strengthened epoxy adhesive and calibrated. Guidelines are presented for selecting scaled-down geogrids along with demonstration of an application through a centrifuge model test on geogrid reinforced clay liner and calibration aspects of the model geogrid. The geogrid layer was used as a reinforcement inclusion within the model clay liner subjected to non-uniform settlements in a geotechnical centrifuge. The results indicated that the model geogrid is required to be calibrated several times by loading and unloading cycles till a reproducible response is accomplished. The measured tensile strength and observations made during the centrifuge test were reported. The results revealed that the strain Gauges can be used to measure in-plane tensile strength characteristics of embedded geogrid within the soil, however the influence of base material on the tensile strength-strain behavior of the aeogrid need to be considered. (C) 200

Modeling of geotextile reinforced highway slopes in a geotechnical centrifuge

The application of the polymeric reinforcement in the construction of steep reinforced slopes is becoming more common. The mechanisms of failure of these reinforced slopes from internal stability point of view are especially due to: (i) rupture of reinforcement layers and (ii) pull-out failure along the soil-geotextile interface. In this paper these particular failure mechanisms are investigated. Centrifuge tests were carried out on model geotextile reinforced sand slopes at the newly commissioned Beam Centrifuge Facility at Indian Institute of Technology Bombay. The lateral displacements of the geotextile reinforced slope and the surface settlement of slope were monitored using LVDT's. Further, the strain distribution along reinforcement layers was analyzed and interpreted. The centrifuge test results were found to give valid information about the behaviour of geotextile reinforced slopes at failure

Numerical Simulation of Geogrid-Reinforced Soil Barriers Subjected to Differential Settlements

A numerical simulation of centrifuge model tests was carried out to develop an understanding of the behavior of geogrid-reinforced soil barriers (GRSBs) of landfill covers subjected to differential settlement. The influence of the axial stiffness of the geogrid, soil-geogrid interface friction, overburden pressure, and thickness of the soil barrier on the overall performance of GRSBs was investigated. Results from the study indicate that unreinforced soil barriers (URSBs) experience tensile stresses and strains throughout their thickness at the zone of maximum curvature; however, with the inclusion of geogrid within the soil barrier, the depth of the tension zone was found to be reduced significantly. A significant reduction in the magnitude of tensile stresses and strains in particular below the location of the geogrid was noticed with an increase in the axial stiffness of the geogrid. The results demonstrate that the geogrid layer mobilizes higher tension and thereby transfers lesser bending stress to the portion of soil barrier placed below the geogrid layer. The magnitude of the mobilized tensile load of the geogrid was found to be directly proportional to the magnitude of the axial stiffness of the geogrid, overburden pressure, and thickness of the GRSB. The normalized depth corresponding to zero horizontal tensile stress was found to decrease with an increase in the axial stiffness of the geogrid, which indirectly suggests that the depth of tension cracks can be considerably reduced with an increase in axial stiffness of the geogrid. This study also suggests that the integrity of GRSBs subjected to differential settlements can be retained only when a geogrid having adequate tensile load-strain characteristics along with an adequate overburden pressure is provided. The results of numerical analyses of GRSBs subjected to differential settlements were observed to corroborate well with the physically observed centrifuge test results. (C) 2014 American Society of Civil Engineers

Centrifuge model tests on geotextile-reinforced slopes

This paper addresses the static response of geotextile-reinforced slopes resting on a firm foundation to the self-weight loading imposed in a geotechnical centrifuge at pre-failure and at failure. A series of centrifuge tests were carried out on model geotextile-reinforced sand slopes with different types of reinforcement, spacing and slope inclination. A wrap-around technique was used to represent a flexible facing. In order to initiate failure in the reinforcement layers, the ratio of the length of the reinforcement to the height of the slope was maintained as 0.85. Reinforced slope models were subjected to varied g-levels (in steps of 5g from 10g onwards) up to a maximum target g-level of 75g or to collapse, whichever occurred first. A digital image analysis technique was employed to arrive at displacement vectors of markers glued to the reinforcement layers. The displacements were used to compute and analyse the strain distribution along the reinforcement layers, and to identify the peak strain distribution pre-failure and at failure. The development of a particular type of failure mechanism was found to depend upon the tensile strength-strain characteristics of a reinforcement layer. Maximum peak strain in the reinforcement layers was observed to occur at mid-height of the slope. With an increase in slope inclination from 2V: 1H to 5V: 1H, the magnitude of maximum peak strain was observed to increase, and its location was observed to move downwards from mid-height of the slope. Stability analysis of the reinforced slope models was found to be in good agreement with physically observed centrifuge test results

Centrifuge model tests on the use of geosynthetic layer as an internal drain in levees

The objective of the paper is to examine the use of a geosynthetic layer as an internal drain in a levee subjected to flooding through centrifuge model tests. Three levee sections, having an upstream slope of 1V:1H and downstream slope of 1.5V:1H, were modelled at 30 gravities in a 4.5 m radius large beam centrifuge available at IIT Bombay. Out of the three levee sections modelled, one levee section was without any drainage layer (or clogged drain), while the other two had different types of horizontal drainage layers, namely, sand and nonwoven geotextile layer. The flood was induced with the help of a custom developed and calibrated in-flight flood simulator. At the onset of flood and subsequent seepage, pore water pressures within levee section, and surface settlements were measured using pore water transducers (PPTs) and linear variable differential transformer (LVDTs) respectively. Digital image analysis was employed to trace surface settlements, and downstream slope face movements at the onset of flooding during centrifuge tests. Levee section without any horizontal drain or clogged drain experienced a catastrophic failure. In comparison, the levee sections with an internal drain (sand/geotextile) remained stable at the onset of flooding. In the case of a levee with a sand drainage layer, the phreatic surface was observed to confine within the levee section itself, whereas it was found to migrate towards toe gradually in the levee section with a nonwoven geotextile layer. It is attributed to either due to suppression of drainage capacity of nonwoven geotextile layer or due to washing of fine particles into pores of nonwoven geotextile layer. Further, seepage and stability analyses were carried out numerically and compared with centrifuge test results. In order to address blocking of pores of nonwoven geotextile layer, a concept of sandwiching nonwoven geotextile layer with sand was explored. By sandwiching nonwoven geotextile layer with sand on either side, the thickness of drainage layer can be of the order of 0.05H

Modeling deformation behaviour of clay liners in a small centrifuge

In modern solid waste landfills, liner systems are essential structural elements that ensure that waste materials are safely separated from the environment. Many liner systems have been developed and used over the past decade. Among impermeable layers, clay liners are regarded as one of the very significant components of liner system and are being used worldwide as a waste containment system in landfills. One of the failures associated with clay liners is the occurrence of nonuniform settlements, resulting from the sudden collapse of waste, or the decomposition of waste materials, and (or) the subgrade over which the liner is laid. This paper deals with the use of a small centrifuge to model the deformation behaviour of clay liners. Model tests were performed in a small geotechnical centrifuge to investigate the behaviour of clay liners in landfills subjected to nonuniform settlements at sharp curvatures. A parametric study was conducted to analyze the influence of parameters like thickness, consistency, and overburden on the behaviour of clay liners. The clay liner without any overburden is observed to experience severe cracking in the form of deep and wide cracks at the maximum curvature zone. The depth and width of the cracks are found to decrease with an increase in clay liner thickness. The cracking failure pattern is suppressed by shearing for a clay liner with an increase in overburden pressure. Based on model test results, the clay liner with an adequate overburden is found to be free from cracking failure even when subjected to sharp curvatures. The effective usage of a small geotechnical centrifuge to model the deformation behaviour of clay liners is demonstrated adequately

Influence of geogrid layer on the integrity of compacted clay liners of landfills

The objective of this paper is to examine the influence of geogrid layer on the integrity of clay liners of landfills. A series of centrifuge model tests were performed on model clay liners subjected to non-uniform settlements with and without a geogrid layer embedded within the top one-third portion of the clay liner moist-compacted on the wet side of its optimum moisture content at 40 g. The model clay liner material has been selected in such a way that it envelopes the material characteristics of the clay liners, which are used for constructing an impermeable barrier in a lining system. By maintaining type and location of the geogrid within the clay liner as constant, the thickness of clay liner is varied to check the possibility of reducing the thickness of a geogrid reinforced clay liner. Digital image analysis technique was employed to ascertain the initiation of cracking and to compute strains both on the surface and along the cross-section of the clay liner with and without any geogrid layer. It was observed that clay liners compacted at moulding water content towards wet side of their OMC found to experience multiple cracking at the onset of non-uniform settlements. Contrary to this, geogrid reinforced clay liner was observed to sustain large distortions and experience only tiny cracks limited up to a location of a geogrid. With an increase in thickness of the clay liner reinforced with a geogrid, geogrid reinforced compacted clay liner was observed to retain its integrity and restrains cracking completely

Centrifuge model tests on the behavior of strip footing on geotextile-reinforced slopes

This paper examines the stability of geotextile-reinforced slopes when subjected to a vertical load applied to a strip footing positioned close to the slope crest. Vertical spacing between geotextile reinforcement was varied while maintaining a constant slope angle, load position, soil density and geotextile type. Small-scale physical tests were conducted using a large beam centrifuge to simulate field prototype conditions. After the model was accelerated to 40g, a load was applied to the strip footing until slope failure occurred. Digital image analysis was performed, using photographs taken in-flight, to obtain slope displacements and strain distribution along the reinforcement layers at different loading pressures during the test and at failure. Stability analysis was also conducted and compared with centrifuge model test results. The vertical spacing between reinforcement layers has a significant impact on the stability of a reinforced slope when subjected to a vertical load. Less vertical distance between reinforcement layers allows the slope to tolerate much greater loads than layers spaced further apart. Distributions of peak strains in reinforcement layers due to the strip footing placed on the surface of the reinforced slope were found to extend up to mid-height of the slope and thereafter they were found to be negligible. Stability analysis of the centrifuge models was found to be consistent with the observed performance of geotextile-reinforced slopes subjected to loading applied to a strip footing near the crest. (C) 200

Evaluation of tensile load-strain characteristics of geogrids through in-soil tensile tests

This paper evaluates in-soil tensile load-strain characteristics of geogrids with the help of a custom designed and developed in-soil tensile setup in the laboratory. Displacement controlled in-soil tensile tests were carried out to evaluate the effect of normal stress, soil type, and presence of sand-sandwiched layer, on the tensile load-strain characteristics of geogrid. Confinement of geogrid within the soil and application of normal stress were found to increase the mobilized tensile load and secant tensile stiffness of geogrid. Secant stiffness improvement factors were determined to quantify the improvement in tensile load-strain characteristics of geogrid under confinement, on comparison to in-isolation values. Geogrid was observed to exhibit lower secant tensile stiffness when embedded in marginal soil, moist-compacted at wet of optimum. However, the concept of sand-sandwiched geogrid was found to improve the tensile load-strain behaviour of geogrids embedded in marginal soil compacted at wet of optimum. (C) 2016 Elsevier Ltd. All rights reserved