B.V.S. Viswanadham

Evaluation of Tensile Strength-Strain Characteristics of Fiber-Reinforced Soil through Laboratory Tests

AbstractTension cracking is a very important phenomenon in the case of compacted fine grained soils, which are commonly used in earth dams, embankments, and waste containment systems. The strength and permeability of earthen structures can be seriously affected by the formation of tensile cracks. The purpose of this paper is to evaluate the effect of discrete and randomly distributed fibers (DRDF) in improving the tensile strength-strain characteristics of bentonite amended natural silty soil. The authors conducted a series of direct tensile tests on unreinforced and fiber-reinforced soil by using a specially developed tensile test set up in the laboratory. They conducted tests by varying the fiber content and fiber length and studying their effect on tensile strength-strain characteristics and crack formation. The present paper indicates that the blending of fibers improved the tensile strength-strain characteristics and ductility of the soil. The authors carried out a digital image cross-correlation (DI...

Experimental Study on Flexural Testing of Compacted Soil Beams

This paper presents an experimental study on flexural testing of three different types of soils molded at three different water contents and two different compaction energies. For this purpose, a custom designed simple beam test setup was developed in the laboratory for inducing the controlled pure bending to the soil under evaluation together with a charged coupled device video camera mounted on to a moving triaxial base plate. Based on the analysis and interpretation of bending moment (BM)-curvature (κ) relationships of tested soil beams, effects of soil type, molding water content, and compaction energy on the flexural behavior could be obtained. For the varied molding water contents and for the range of plasticity indices varied, with an increase in plasticity index of the soil and molding water content, a trend of increasing tensile strain at crack initiation was observed. In comparison, flexural tensile strength at crack initiation was observed to decrease with an increase in molding water content. Further, flexural rigidity at crack initiation was found to be higher for soils having low plasticity index and moist compacted at low-molding water contents. The developed test setup is capable of inducing distortion levels of 0.105 and can be useful in assessing flexural characteristics of different types of fine-grained soils.

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

AbstractA 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 str...

Influence of slope geometry and nail parameters on the stability of soil-nailed slopes

Abstract Soil nailing is one of the in-situ soil reinforcement techniques that have been used for the past four decades. The stability of a soil-nailed slope is closely related to slope geometry and nail parameters. This paper presents a parametric study on various factors influencing the overall stability of soil-nailed slopes. Stability analysis was performed to study the effect of slope geometry, nail parameters and effect of rising water surface within the slope on the overall stability of soil-nailed slopes. The stability of soil-nailed slopes is considered in terms of a factor of safety. The slope geometry varied for the present study includes slope inclination and back slope inclination while the nail parameters include nail inclination, nail length, nail length pattern, and nail layout (horizontal spacing). The study shows that with horizontal back slopes, the optimal nail inclination (αopt) increases with the decrease in slope inclination but it increases with an increase in back slope inclination. Two nail length patterns: Pattern 1, nail length decreasing with depth of the slope and Pattern 2, nail length increasing with depth of the slope have been studied. Pattern 1 is found to be effective for a 10.2 m high slope whereas Pattern 2 is found to be effective for 7.2 m high slope. Rising water surface within the slope, increase in horizontal spacing and decrease in length of the nail reduce the overall stability of a soil-nailed slope. Contribution of nails to the overall stability of soil-nailed slopes is different and found to be dependent on their location and inclination with respect to the slip surface.

Development of a Motor-Based Differential Settlement Simulator Setup for a Geotechnical Centrifuge

This paper presents the design details of a motor-based differential settlement simulator (MDSS) setup for inducing differential settlement in a high gravity environment. The MDSS setup comprises of a motor, controller, screw jack, central platform, gear trains, connecting shaft, and bearings. Various features of the MDSS setup along with its design procedure are described. Calibration and performance tests were performed in a large beam geotechnical centrifuge facility available at Indian Institute of Technology Bombay for inducing differential settlement using the MDSS setup. A design chart was developed for obtaining prototype settlement rates from the speed of the motor and gravity level. The usage of this setup in assessing the deformation behavior of the clay barrier of landfill cover system under various settlement rates is briefly illustrated. The versatility of the developed setup for the applications involving ground loss or boundary displacement problems is highlighted.

Studies on the Performance of Geogrid Reinforced Soil Walls with Compressible Inclusion

In the present study, the concept of placing compressible inclusion behind the reinforced zone of geogrid reinforced soil walls was explored through finite element modeling approach. Centrifuge model tests on geogrid reinforced soil walls with two different vertical spacing of reinforcement layers were conducted in a geotechnical centrifuge by increasing gravity from 10g in intervals of 5g up to 75 g or failure, whichever occurred earlier. A wrap around technique was used to represent a flexible facing and in order to initiate rupture failure in reinforcement layers, ratio of the length of the reinforcement layer to the height of the wall was maintained as 0.85. Marker based digital image analysis was adopted to compute straining of geogrid reinforcement layers with an increase in gravity level. Further. finite element models were developed for these two centrifuge models and compared with centrifuge test results. Thereafter, the effect of compressible inclusions having low density and thickness ranging from 0.02 to 0.2 times the height of the wall on the stability and deformation behavior of geogrid reinforced soil walls was analyzed using a geotechnical finite element code. Geofoam inclusions were placed just behind the reinforced zone vertically. The analysis and interpretation of centrifuge and finite element analyses results indicate that the thickness of geofoam is vital in improving the performance of geogrid reinforced soil walls.

Performance of 2V:1H Slopes with and without Soil-Nails Subjected to Seepage: Centrifuge Study

This paper presents the results of centrifuge model tests on the behavior of a slope subjected to seepage with and without soil-nails. Centrifuge model tests were carried out at 30 gravities on model slopes with horizontal back slope having an inclination of 2V:1H and slope height of 7.2 m using a 4.5 m radius beam centrifuge facility available at IIT Bombay. Nail inclination of 25o with horizontal was adopted for reinforcing slope along with polyester fiber-blended plaster of Paris facing. An unreinforced slope was observed to fail once the seeping water reaches the toe of the slope after 5 days of seepage. In comparison, the 2V:1H slope strengthened with a soil-nail layout of 2.1 m x 2.1 m and inclined at 25o was found to significantly improve its stability and deformation behavior at the onset of seepage.The observations also indicate that the use of plaster of Paris strengthened with polyester fibers facing is viable option for modeling facing in centrifuge model tests. Further, the results on stability analysis of soil-nailed slopes with different layouts indicate that the horizontal spacing of soil-nails greatly influences the stability than vertical spacing and slope facing plays a significant role in improving the stability of soilnailed slopes. Finally, an attempt has been made to compare results of stability analysis of centrifuge model slopes with and without soil-nails.

Modelling the failure of a cantilever sheet pile wall

Abstract Cantilever sheet pile walls are routinely used to retain medium heights of earth in geotechnical practice. Earth pressures developed on either side of the sheet pile wall ensure its moment and force equilibrium. One of the mechanisms governing the failure of cantilever sheet pile wall can be described as structural failure of the sheet pile elements because of high stresses and/or loss of serviceability due to large deformation. This mechanism is particularly important with the advent of plastic bending in Eurocode 3 Part 5. Centrifuge tests were carried out on model cantilever sheet pile walls supporting granular fill by subjecting to varied g-levels (in steps of 5 g from 10 g onwards) up to a maximum set target g-level of 75 g or to excessive deformation of wall, whichever occurred first. The bending moment distribution, deflections of the sheet pile wall and the settlement of the backfill were monitored during the centrifuge test. The same problem was analyzed using the finite element method and closed-form solutions using Coulombs earth pressure theory. It was shown that the finite element analysis was able to capture the formation of the plastic hinge observed in the sheet pile walls during the centrifuge model tests. Location of the maximum bending moment obtained both from finite element analyses and closed-form solutions was found to be in agreement with the physically observed centrifuge model test results. This gives confidence in the numerical analyses of sheet pile walls where plastic bending is allowed in the design provided the deformations remain within the Serviceability Limit State.

Effect of Container Wall Friction on the Consolidation of Clay Specimen

The slurry consolidation (K0), one of the popular techniques producing specimens resembling field conditions, is usually carried out in large containers owing to large strains incurred during the process. The present studies investigate the influence of aspect ratio and wall friction of the container on the uniformity achieved in the consolidation of the soil specimens experimentally. Parametric studies are also carried out numerically so as to gain further insight into the consolidation behavior due to wall friction and aspect ratio (ratio of diameter-to-height) of the container. It is observed that the uniformity of the specimen is adversely affected by the wall friction especially at lower aspect ratios due to significant loss of applied pressure during transmission. However, even with the substantial wall friction the uniformity could be achieved within a certain zone if the aspect ratio is enhanced. The present studies also attempts to identify these zones and further suggests that any instrumentation for a physical modeling or the sample extrusion be carried out within this zone.

Centrifuge and Numerical Model Studies on the Behaviour of Soil-Nailed Slopes with and without Slope Facing

This paper evaluates the significance of slope facing on the stability and deformation behavior of soil-nailed slopes subjected to seepage. A short series of centrifuge tests was performed to study the behavior of 5V:1H soil-nailed slopes with and without slope facing by maintaining the model slope height (240 mm) with horizontal back slope, nail inclination of 10° with horizontal and model nail spacing of 60 mm x 60 mm constant at 30 gravities. The effect of flexible, as well as stiff, facing on the performance of soil-nailed slopes was also studied. All models were instrumented with displacement and pore water pressure transducers, and markers were digitized to arrive at displacement vectors with rising ground water table during centrifuge tests. It was observed that the slope facing prevents the local failure of the soil between the nails. Soil-nailed slope without facing was found to experience face failure due to bearing failure at slope surface and nail head surface, as well as due to build-up of excess pore water pressure at the toe region after 8.5 days of the seepage. Soil-nailed slope with woven geotextile (flexible) facing experienced 0.18 m of crest settlement and bulging of facing was observed in between nail heads. However, a soil-nailed slope with an aluminium facing was found to sustain large settlements and showed improved deformation behavior at the onset of seepage. Further, finite element analyses of the slope models - with and without slope facing - were found to be in good agreement with those of physically observed centrifuge test results.