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Numerical modeling of geosynthetic-encased stone column-reinforced ground
This paper presents the results of a comparative study on different finite element modeling approaches for modeling geosynthetic-encased stone column-reinforced ground for use in rapid embankment construction. The specific models considered include: (1) an axisymmetric unit cell; (2) a three-dimensional (3D) column; and (3) a full 3D model. The validity of the unit cell model was tested by comparison with the results from the 3D models. The applicability of continuum elements for modeling the geosynthetic encasement was also investigated. The results show that the 3D column model yielded practically identical results when compared with those of the full 3D model whereas the two-dimensional axisymmetric unit cell model tended to yield results that were 10 to 20% larger in terms of the vertical effective stress and lateral deformation of the stone column. It is also shown that a layer(s) of continuum elements can be used to model the geosynthetic encasement instead of membrane elements which are not readily available in commercial software for geotechnical analysis, provided that the axial stiffness of the geosynthetic encasement is taken into consideration. Based on the results of analysis, the effect of geosynthetic encasement on the performance of stone columns installed in soft ground under embankment loading is also discussed.
ID: 3725
Publication: Geosynthetics International
Vol: 16
Issue: 3
Pages: 116
- 126
Publisher: Thomas Telford
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Author: C. Yoo and S.-B. Kim
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Behaviour of two reinforced test embankments on soft clay
The paper presents the behaviour of two reinforced test embankments built on a normally consolidated soft clay deposit underlying a top sand layer. The embankments were constructed close to undrained conditions in about 60 days. The embankments were well instrumented, including measurements of tension forces in the reinforcement. The mobilised tension forces in the reinforcements were shown to increase with embankment height and larger values of tension forces were measured in the embankment built on a shallower clay layer. The critical failure surfaces obtained in limit equilibrium stability analyses were close to the observed field failure surfaces. These analyses used measured reinforcement forces and resulted in Bjerrum correction factors around μ = 0.60 for the two embankments. These analyses considered the measured reinforcement forces and three-dimensional effects. Consistent correlations were obtained between embankment loadings, factors of safety, measured reinforcement forces and inclinometer readings.
ID: 3726
Publication: Geosynthetics International
Vol: 16
Issue: 3
Pages: 127
- 138
Publisher: Thomas Telford
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Author: H. O. Magnani, M. S. S. Almeida and M. Ehrlich
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Design of prefabricated vertical drains considering soil disturbance
In the present study, the effect of soil disturbance on prefabricated vertical drain (PVD)-enhanced soft soil consolidation was studied using finite element (FE) analysis. Both square and triangular PVD installation arrangements were considered. The actual shapes of the PVD, disturbed zone and unit cell were preserved in the FE analysis. A parametric study for a wide range of possible hydraulic conductivity profiles with a sigmoidal shape indicated that the degree of disturbance in the immediate vicinity of the PVD and the extent of the disturbed zone are the two factors that significantly affect the consolidation rate. Proper evaluation of these factors is therefore necessary for satisfactory PVD design. A comparison was made between the degree of consolidation calculated using an analytical solution assuming a bilinear hydraulic conductivity profile and FE analysis assuming a sigmoidal hydraulic conductivity profile in the disturbed zone surrounding the PVD. The curves of degree of consolidation plotted against time factor that were obtained from these solutions were in reasonably good agreement. Based on the analytical solution, design charts for a variety of conditions and hydraulic conductivity profiles were developed. They can be easily used in practice to determine the required PVD spacing for the desired degree of consolidation and specified time. The design charts can also be used for conditions in which overlapping of disturbed zones occurs.
ID: 3728
Publication: Geosynthetics International
Vol: 16
Issue: 3
Pages: 147
- 157
Publisher: Thomas Telford
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Author: D. Basu and M. Prezzi
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Experimental investigation of infiltration ponding in one-dimensional sand–geotextile columns
The paper describes the results of laboratory tests that were carried out to quantify the geotextile water characteristic curves of a woven and nonwoven geotextile in new and modified conditions. The geotextiles were modified by contamination with kaolin powder. The specimens were placed in 2 m high, one-dimensional sand columns and subjected to constant head water infiltration loading. Conductivity probes and tensiometers were used to record the unsaturated–saturated response of the sand–geotextile layers and a control column with sand only. The test results showed a detectable delay in infiltration front advancement and ponding level, which increased with decreasing saturated permeability of the geotextile specimen. The use of geotextile–water characteristic curves from in-air laboratory suction plate tests is shown to underestimate water ponding heights in the columns, which means that their use for design may be non-conservative. Another practical implication of the work reported here is that for geotextile–sand systems under surface water infiltration loading, current filtration design criteria that call for the ratio of saturated hydraulic (conductivity) permeability of the geotextile to that of the surrounding sand be at least one, may need to be strengthened. For ratios of 1 and 10, the estimated ponding height above the geotextile was estimated to be 4 and 0.1 cm, respectively. Hence, ratios of saturated hydraulic permeability greater than one may be necessary to prevent lateral flow of water above a geotextile in sand fills used in wall, slope and road applications.
ID: 3729
Publication: Geosynthetics International
Vol: 16
Issue: 3
Pages: 158
- 172
Publisher: Thomas Telford
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Author: R. J. Bathurst, G. Siemens and A. F. Ho
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The performance of an embankment on soft ground reinforced with geosynthetics and pile walls
The construction of high embankments on soft ground faces several challenges, such as potential bearing capacity failure, global slope instability, local failure, large total and/or differential settlements, and large lateral movement during both construction and post-construction periods. Horizontal geosynthetic reinforcement combined with vertical reinforcement using piles or pile walls can provide an economical and effective solution to ground improvement to support high embankments. This paper reports the results of a numerical study on the performance of embankments on soft ground with three different reinforcement conditions: (1) unreinforced, (2) reinforced with geosynthetics, (3) reinforced with both geosynthetics and pile walls. The influencing factors, such as the tensile stiffness of the geosynthetic, the number of geosynthetic layers, the elastic modulus of the pile wall, and the pile wall distance ratio, on the interaction of geosynthetic-pile wall-soil systems were investigated and the distribution and dissipation of excess pore water pressure in the soil were examined. The numerical results show that a combination of geosynthetic reinforcement and the pile walls was very effective for the improvement of soft ground. The numerical results were shown to be in good agreement with the field data observed at the end of consolidation.
ID: 3730
Publication: Geosynthetics International
Vol: 16
Issue: 3
Pages: 173
- 182
Publisher: Thomas Telford
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Author: J. J. Zheng, B. G. Chen, Y. E. Lu, S. W. Abusharar and J. H. Yin
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Lifetime cost and performance of slope erosion control products for construction
This study investigated the relationship between cost and performance of slope erosion control products in the Texas Department of Transportation's Approved Products List. Using (1) the erosion control performance data, (2) product cost data collected from surveys and (3) predicted product longevity based on material composition, a lifetime product performance, which estimated the potential soil amount protected by the product over time, was quantified. The study also examined the relationship between product price, soil loss performance and predicted longevity. The results indicate that product price was highly associated with predicted longevity. Products that could offer the longest protection cost more than those offering short or temporary protection. Results also show that price did not correlate with initial soil protection performance. Rather, price significantly correlated with lifetime product performance, which indicates that marketed product prices well represent the lifetime worth.
ID: 3727
Publication: Geosynthetics International
Vol: 16
Issue: 3
Pages: 139
- 146
Publisher: Thomas Telford
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Author: M.-H. Li, Y.-J. Yi and J. McFalls
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Diffusion modelling of OIT depletion from HDPE geomembrane in landfill applications
The results of a diffusion modelling study to evaluate experimental data on oxidative induction time (OIT) depletion from high-density polyethylene (HDPE) geomembrane (GM) in accelerated ageing tests are presented. The paper provides: (1) results of diffusion modelling of OIT depletion from a GM immersed in leachate and in a composite liner with leachate above the liner at different incubation temperatures; (2) a comparison of the results of the diffusion model and the conventional first-order (exponential) antioxidant depletion model; (3) estimates of diffusion and partitioning coefficients at typical landfill temperatures based on Arrhenius-type relationships; and (4) an application of the estimated parameters to model a composite liner with 30 cm thick sand layer. The antioxidant diffusion coefficients ranged from 2.1 × 10−15 (at 26°C) to 1.6 × 10−13 m2/s (at 85°C) and the partitioning coefficients ranged from 720 (at 26°C) to 4 (at 85°C). The antioxidant depletion time obtained using the first-order model was similar to that predicted using the diffusion model for tests where the OIT was depleted during the test period. However the first-order model gave smaller predictions of depletion time than the diffusion model in cases where there was only limited OIT depletion and in these cases the diffusion model is likely to the give more accurate predictions. Arrhenius modelling provided a means of estimating diffusion and partitioning coefficients at field temperatures. At a typical landfill temperature of 35°C the calculated antioxidant depletion time for the geomembrane considered was about 130 years for a case where there was a 1.5 cm sand protection layer and 230 years for the case when 30 cm sand protection layer was used. Thus these results suggest that the use of a 30 cm sand protection layer in addition to the typical geotextile protection layer between the geomembrane and a coarser granular leachate drainage layer would provide potential benefits in terms of extending the geomembrane service life by reducing the rate of outward diffusion of antioxidants from the geomembrane (as well as providing good physical protection of the liner). This paper has also illustrated how diffusion modelling can be used for considering a range of situations different from those under which the basic experimental data was obtained.
ID: 3731
Publication: Geosynthetics International
Vol: 16
Issue: 3
Pages: 183
- 196
Publisher: Thomas Telford
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Author: S. Rimal and R. K. Rowe
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