Numerical modeling of behavior of railway ballasted structure with geocell confinement
Ben Leshchinsky a,Hoe I. Ling b a Oregon State University, Department of Forest Engineering, Resources and Management, 280 Peavy Hall, Corvallis, OR 97331, USA b Columbia University,Department of Civil Engineering and Engineering Mechanics, 500 West 120thStreet, New York, NY 10027, USA
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Railroad foundations are geotechnical structures that are highly dependent on quality ballast to dampen impact loading and railway vibration, facilitate easy construction, distribute stresses more evenly, reduce long-term settlements and provide a competent base under low confining pressures. However, there are various instances where the use of ballast alone may not be completely adequate or could be prohibitively expensive, i.e. costly transport of select materials, weak subgrade, etc. One possible method of managing these issues is the use of geosynthetics, primarily reinforcements that utilize a confining mechanism to enhance the strength of a soil by utilizing its own internal friction: a mechanism where geocell is applicable.Based on prior large-scale laboratory tests of ballast embankments with geocell confinement and relevant numerical modeling, an acceptable material model was validated for a parametric study using finite element analysis. The purpose of the parametric study is to investigate the effects of geocell confinement on ballasted embankments when encountering a soft subgrade, weaker ballast, or varying reinforcement stiffness. This analysis suggests that based on numerical modeling, geocell confinement can have a significant benefit when used on a wide range of subgrade stiffness,when using weaker ballast and that mechanically, most polymeric materials commonly used for geosynthetic reinforcements are adequate. The composite effect of the confined ballast selected as infill also demonstrates a “mattressing”effect, distributing stresses more uniformly to the subgrade, which can provide higher bearing capacities and possibly less settlement, all while preventing significant lateral spreading. In certain situations, the benefits provided by behavior of the geocell-ballast composite may be economical by allowing for use of weaker/inferior ballast, less embankment maintenance upon problem soils, improved bearing capacity and reduced foundation settlement.
In the past few decades, geosynthetics have been increasingly popular in the construction of different geotechnical structures, including earth retention, slopes, roadway construction, landfill lining, and coastal protection, due to its ease of use and cost efficiency. To cater to this broad variety of geotechnical functions, geosynthetics have been developed in a multitude of forms and material combinations. These include geogrids, geomembranes, geotextiles, geonets,geocomposites and geocells (Koerner, 2005). Geocell has long been used as means for improving soil conditions.
It was originally developed by the US Army Corps of Engineers (USACE) to increase vehicular mobility over loose, sandy subgrade through cellular confinement (Webster and Alford,1977). Geocell has been shown to increase soil strength by confinement,reducing lateral spreading and causing the confined composite to behave as a more rigid mattress (Zhou and Wen,2008). The higher stiffness of the geocell system reduces the stress applied to the subgrade due to bending stiffness oft he mattress composite, similar to a slab (Pokharel et al., 2011). Several studies have shown that utilization of the cellular confinement mechanism significantly improves the strength and stiffness of a granular material; however a lack of generic design methodology has inhibited its implementation (Han et al., 2008). Geocell is generally sold in folded form, whereupon it can be outstretched into its three-dimensional shape and infilled with soil. The granular soil, generally weaker at lower confining pressures has added strength due to the confinement effects of the reinforcement cells surrounding it,providing a higher bearing capacity and stiffness. Geocell significantly increases the shear strength of the soil as shown by past triaxial tests (Koerner, 2005). The geocell
also prevents excessive displacements of the infilled soil because of the cell confinement and the redistribution of stresses to the underlying soil. The composite action of the geocell and its fill is known as the “mattressing”effect and allows the reinforced soil to distribute loads much more uniformly to its subgrade, contributing
to the aforementioned increase in bearing capacity, stiffness and reductions in displacements. These benefits are especially pronounced when used on soft subgrades (e.g., Zhou and Wen,2008).Despite the use of geocell reinforcement in a variety of geotechnical applications for decades, there is limited study on its use in railway engineering, possibly due to a combination of the conservative nature of the field and a lack of design methodology for such an application,specifically for railroad embankments.
Although the reinforcement has shown to improve performance under static and cyclic loading, optimal placement of geocell and its performance in a challenging environment such as train ballast is not well-studied, but has significant promise. Further insight into geocell and ballast behavior in arailroad application could provide incentive for the development of relevant design methods. Such an application could have economical and environmental implications for future railroad design and track rehabilitation. Ballast functions as a base that absorbs energy, drains easily and resists forces acting vertically and laterally, providing a stiff, competent foundation for the repeated loading exerted by train passes (Selig and Waters,1994). However, these important roles face significant technical issues that challenge the function of a working railroad. The pressures resulting from train loading can result in rearrangement and degradation of ballast over many loading cycles, reducing grain interlocking and facilitating lateral movement of particles (Lackenby et al., 2007). Track stability can decrease with the lateral spreading of ballast particles due to decreasing frictional strength (Seligand Waters, 1994). Vertical and lateral deformations as a result of spreading or foundation problems result in loss of track geometry. Retention of ballasted foundation geometry is important; the cost of track maintenance due to geotechnical issues is significant when compared to other track expenses (Indraratna et al., 1998).Ballasted railway foundations are supposed to be thick enough to ensure uniform loading of the subgrade at an acceptable intensity(Indraratnaet al., 2006). Geocell confinement increases strength and stiffness of the infill, which in turn distributes the stress to a larger area, especially upon soft subgrades(Chrismer,1997; Zhou and Wen, 2008; Yang, 2010). It is possible that geocell-ballast composite action could enhance this mechanism, which is especially advantageous under the high loading intensity of moving trains. In addition to the redistribution of vertical stresses (Chrismer, 1997), the shear behavior provided by other reinforcements has been shown to reduce and/or re-disbribute shear stresses at the subgrade interface (Giroudand Han, 2004).Since ballast is generally a highly frictional material while the subgrade is often inferior, the reduction of shear stresses is highly beneficial. Some studies have suggested that use of geocell can improve ballast performance and stability, including a reduction in deformation (Raymond, 2001), sustained track geometry (Chrismer, 1997) and an increase in strength and resilience under cyclic loading (Indraratna et al., 2006). The increase in the confinement in the ballast due to geosynthetics would reduce the strains encountered in the foundation as well (Indraratna et al.,2010).