Originally published as “Comparative Life Cycle Assessment of Two Design Alternatives for a Geosynthetic Reinforced Bridge Abutment” in the GeoAmericas 2016 proceedings, Dr. Melissa Beauregard, Dr. Arunprakash Karunanithi, and Dr. Caroline M. Clevenger’s research provides an accessible, well written account of life cycle assessment with two major design alternatives. Specifically, they look at geosynthetic mechanically stabilized earth (GMSE) and geosynthetic reinforced soil (GRS), the impact on reinforcement layer spacing, and the “trade offs” involved when selecting one design approach over another.
Their attention to life cycle assessment and especially CO2 impact in construction makes this a great addition to Geosynthetica’s GeoAmericas series and this month’s content focus: 30 Days of Earth Day.
1.1 Life cycle assessments
In 2013, the American Society of Civil Engineers released an Infrastructure Report Card which awarded a D+ grade to the current state of our national infrastructure. For highways specifically, a grade of D was awarded, and it was estimated by the Federal Highway Administration (FHWA) that an annual investment of $170 billion would be required to repair the condition of our roads, taking into consideration both maintenance and expansion to reduce congestion. With such dire need for a large volume of work on US infrastructure, now is the time to assess which current building practices maximize the sustainability for the future. For large projects, there are some proprietary environmental rating systems such as Envision™ and Greenroads® (Soderland et al., 2008; Muench et al., 2011; ISI, 2012) that issue a rating to projects which is based on their sustainable practices during design and construction phases, similar to how the Leadership in Energy and Environmental Design (LEED) system works for buildings. While these tools are helpful for communicating to the public that a project has taken steps towards becoming more sustainable, they do not explicitly quantify environmental impacts and are not geared towards small projects such as single-span overpasses and the abutments that support them.
Although an individual abutment may not have a large impact on any one project, they are repeated in large numbers throughout highway corridors and often require replacement when roads are widened. It would be helpful, therefore, for designers to have a method for comparing various options during early stages of the design process in the context of environmental costs as well as financial. A comprehensive approach to quantifying the environmental impact of a project could be a comparative Life Cycle Assessment (LCA) (EPA, 2006). An LCA can be approached from a variety of perspectives and boundaries.
A popular approach is an input-output method which sums up the environmental impact of both inputs, such as energy and raw materials, and outputs such as pollutants into air and water as well as finished products and by-products which may have a separate usage elsewhere. LCAs may also focus on different environmental metrics.
One project may look at greenhouse gases emitted during manufacturing to determine global warming potential of different products, another may look into ecological impacts associated with construction techniques to determine how certain methods may affect local flora and fauna.
Since the purpose of this study is to compare two bridge abutment designs with similar performance requirements and design life the boundaries of this study begin with raw materials and end when the structure is completed. This boundary is known as cradle to end of construction. One limitation of this boundary type is that usage and end-of-life considerations are not included and may greatly affect the end result of the study if maintenance for the structures or recyclability of materials for the two options varies. As this type of analysis is still relatively new for geotechnical engineering applications the authors feel that this boundary is adequate since it accurately portrays the environmental cost of common materials and methods and also shows how environmental impact may be minimized in this early aspect of a project. Due to these structures generally being constructed within strict design codes, this boundary also highlights the phases that are most easily adjusted using current standards and results in a greater transparency of the environmental impact of the materials being used. For example, concrete has a notoriously high carbon footprint in production (Huntzinger and Eatmon, 2009; Sjunnesson, 2005) so limiting the use of concrete or incorporating recycled material to a concrete design could have significant environmental benefits.
What follows is a setup of the overall comparison, a summary of the design of the two abutments, and a discussion of those factors that lay outside of the established boundary as well as where a lack of available information makes a full LCA difficult to complete. The environmental metric chosen was carbon dioxide emissions quantified as KgCO2 equivalent (kgCO2e) for the functional unit of one abutment.
1.2 Comparison of GMSE and GRS
The two different abutment examples being compared are a GMSE and GRS design option for the same bridge abutment. A schematic of each of the two wall types is presented in Figure 1. The primary difference from a construction perspective is that the spacing of the GRS option (0.2m) is smaller than that for the GMSE option (>0.3m). There can also be differences in how the load is transferred to the reinforced structure as well as facing used. Due to the internal stability present in the GRS abutment, there are greater options available for facing as it is not a structural component. Often, CMU blocks are used as the facing for GRS due to being common, relatively inexpensive, and their ability to be lifted into place without needing heavy equipment such as a crane or boom truck. While GMSE abutments have historically been more commonly specified, GRS abutments have become more common due in part to the Federal Highway Administration (FHWA) choosing GRS abutments as an Everyday Counts Initiative solution and encouraging use of the technology in publicly funded projects.
Previous studies have investigated how modern geosynthetic reinforced soil walls compare to concrete gravity walls (Fraser et al., 2012; Frischknecht et al., 2013), and have indicated that the reinforced soil alternative is better in terms of sustainability due to the relatively higher environmental cost of mining aggregate for concrete. However, limited or no studies exist that compare one option of reinforced walls to another, which may be of more relevance to a designer of abutments. Although bridge abutments are the focus of this study, non-load bearing walls could also be compared by the same process. However, design of non-load bearing walls is often controlled by minimum values incorporated into design standards, such as a minimum reinforcement length of 70% of the wall height, and therefore the outcome of such a study may be more indicative of how factors of safety affect final design than the performance the structures.
In 2011, the FHWA published a guide to implementing GRS structures as bridge abutments (Adams et al. 2011). The design example in this guide was used as site for this study. One major difference is that no skew or super elevation was considered for this paper. Excluding this detail alters the placement of the beam seat which does not affect the outcome of this study. The model abutment is for a highway overpass that has 4.7 meters of clearance for traffic moving under the bridge and is 11.7 meters wide. The bridge requires one abutment on either side but is considered to be symmetric so only one side is designed. Seismic loads and water were not considered to be a factor in design. The GMSE abutment was designed using the same site conditions and assumptions, and was analyzed using the FHWA Method in MSEW Version 3.0 developed by ADAMA Engineering (ADAMA, 2014).
Sections of each abutment are presented in Figure 1, and a summary of major design elements follows. Spacing of primary reinforcement layers in each abutment was constant throughout the height of the structures, 0.2m for the GRS structure and 0.45m for the GMSE structure. The GRS abutment also featured layers of secondary reinforcement in a bearing bed to reduce deformation due to the load applied to the beam seat. The facing of the GMSE abutment is 1.5 m x 1.5 m panels that are 0.13 m thick. The GRS facing elements are 0.2 m x 0.2 m x 0.4 m CMU blocks. The foundation and retained soil are assumed to be the same for both structures and the backfill is a well-graded, freely draining gravel. The beam seat is precast concrete of the same dimensions for both. The cut angle behind each abutment is 45 degrees. The length of the reinforcing layers in the GMSE abutment is dictated by bearing capacity and pullout failure mechanisms according to the FHWA design standard (FHWA-NHI-10-024).
Since this project is a comparative life cycle assessment, some components of these structures that are the same in type and comparable in quantity are not considered since they equally impact both structures and, therefore, offset one another in the analysis. An example of this would be the rip-rap scour protection at the toe of each abutment. Additionally, some quantities can be calculated by determining the percent different between the two structures when the material is the same.
An example of this is water usage for moisture conditioning the backfill, which is 34% less for the GRS option. The quantitative portion of this study focuses on the environmental cost of producing the materials for the two abutment types since there is a lack of data available for equipment and labor in the United States. Table 1 is a list of the materials and quantities required for each abutment type, based on previously discussed design considerations.
From Table 1, it is possible to identify some major differences in material and quantity between the two structures.
Regarding the backfill, GRS requires 130 cubic meters less of premium backfill than the GMSE abutment. Regarding the geosynthetic polymer, GRS utilizes woven polypropylene (PP) fabrics with ultimate tensile strengths of 70 kN/m and 38.5 kN/m for primary and secondary reinforcement, respectively, while GMSE requires a stronger poly(ethylene) terephthalate polymer (PET) with an ultimate tensile strength of 300 kN/m. The material properties of the geosynthetics are presented in Table 2.
4. ADDITIONAL CONSIDERATIONS
Other than those reinforced soil structures which are anchored or nailed, the vast majority of reinforced soil walls are built from the bottom up with alternating layers of soil and reinforcement. Although the construction processes for these structures are similar, there are still some key components that vary that should be taken into consideration for this type of analysis. First, the equipment required for construction is different. GMSE structures with large panel facing require a boom truck or crane to lift the panels into place. Additionally, the relatively large spacing between reinforcing layers results in thicker lifts of soil being compacting and a machine with a greater compaction effort is required to ensure the reinforced soil is compacted to 95% of maximum dry density as determined compaction testing ASTM Standard D698 or D1557, which is the typical specification. When constructed with CMU blocks, construction of GRS facing requires only a one or two-person team and, while the compaction requirements are the same as for GMSE, the level of compaction required is obtained with less compaction effort when working with smaller lifts of soil. Sufficient information on compaction equipment in the United States was unavailable at the time of this study and therefore these factors are not included in this study.
Another major difference between the two design options is failure rate. The National Concrete Masonry Association NCMA estimates that failure rates for GMSE walls are between 2-8%. Additional studies calculate a failure rate up to 5%, which is within the NCMA range (Berg, 2010; Valentine, 2013). Although it is clear there is a difference in failure rates for the two structures, the variable is high enough that it would be difficult to quantify this impact and therefore it is omitted from this study.
5. ASSUMPTIONS & LIMITATIONS
As with any engineered structure, there can be many different designs that meet the design requirements. Assumptions were made to simplify the analysis performed during this study, and the authors recognize that choices left up to the designer could impact the results of the LCA comparison. Some examples of differences are the facing used. First generation CMU blocks were assumed for the GRS structure because it is a common facing material used and also because the ability to use dry-stacked CMU blocks is one aspect of GRS that sets it apart from GMSE. Due to the internal stability of these structures, a large number of materials could be used as facing. This includes, but is not limited to: concrete blocks with recycled content, timber, tires, large panels and natural or artificial stone.
Likewise, the large concrete panels were included in this study for GMSE structures because that particular system and finished aesthetic is commonly used in industry, though other materials can work when geosynthetic connection to the back of the facing is adequately designed. The PET reinforcement was chosen for the GMSE abutment because it is a likely choice for a designer due to relatively large tensile strength requirements associated with the larger spacing.
The closer spacing of the GRS reinforcing layers results in a lower required tensile strength and makes the selection of the less expensive and lighter weight PP fabric an economic decision.
Various aggregates could also be chosen that could meet design standards, and some walls have been constructed using less desirable, poorly-draining backfills. Freely draining well-graded gravel is assumed in this study because it was the material chosen by Adams et al. 2011 for their design example and also because it is a common design specification for both GMSE and GRS projects.
Water on site was not considered, but drainage systems designed for the structures would likely include an open-graded gravel behind the face of the each structure and a pipe to move water away at the bottom, which would likely be similar enough to off-set one another. No seismic load was considered in this study. The fasteners required in the back of the large GMSE panels were not considered for this study as their impact is expected to be minimal as compared with concrete, geosynthetic and volume of backfill considerations. The material considerations for each abutment ends at the bearing plate since approaches could be designed in a large variety of ways and the bridge structure would be the same for each option. On-site electricity and water usage was not considered in this study.
6. LIFE CYCLE ASSESSMENT RESULTS
Due to limitation in available information at the time of this study, the results of this project focus on the geosynthetic reinforcement as well as the aggregate used as backfill. The materials and their quantities were presented in Table 1. The environmental cost of producing each material was calculated using previously published data for the crushed aggregate (Sjunnesson, 2005) and the National Renewable Energy Laboratory Life Cycle Inventory database for the geosynthetics (U.S. Life Cycle Inventory Database 2012). Due to a lack of information on the manufacturing of the actual reinforcing fabric, the information for the geosynthetic includes only the manufacturing of the polymer resin. Since both the PP and PET fabrics are woven, the costs of creating the finished fabrics may offset. These material flows were subsequently converted into kgCO2e using standard emission factors.
Since the area of geosynthetic for the GRS abutment was 65% greater than for the GMSE abutment it may seem as though the cost of producing the reinforcements for the GRS abutment would be higher. However, since the weight of the PET fabric was higher and therefore required a greater amount of raw materials, the environmental cost of the reinforcements for the GRS abutment was ultimately lower than the GMSE, as shown in Table 3. As would be expected from the quantity of aggregate required for each structure, the environmental cost of the backfill materials for the GMSE abutment was 62% greater than for the GRS abutment.
Two interesting findings of this study:
- The environmental impact of production of the reinforcing geosynthetics similar despite large differences in quantities
- The environmental impact of the backfill aggregate dwarfs the impact of the geosynthetics
Specifically, the smaller reinforcement spacing minimized length of geosynthetic required to resist against pullout failure and the reinforced soil foundation as recommended by the FHWA allowed for a smaller area of excavation, have a large impact on the overall environmental impacts of the abutment design.
Ideally, a LCA would include materials and processes for construction, usage and end of life stages of a structure. However, this study and others similar are limited by the information currently available in regards to construction equipment and manufacturing processes. However, even with incomplete information this study shows how minor changes in design choices can have large impacts on the environmental cost of geotechnical structures.
One finding of this study is that the greater area of reinforcing geosynthetic required in a GRS abutment results in a relatively similar environmental impact in terms of CO2 emissions (1.5 kg) as compared with a GMSE abutment (1.9 kg) as determined using LCA analysis. In fact, the tensile strength required to design the larger reinforcement spacing in GMSE structures uses a geosynthetic with a substantially greater weight per unit area, which requires more material to manufacture. Additionally, findings indicate that a major contributor to CO2 emissions for these structures is the high energy cost of manufacturing select, freely-draining aggregates for the backfill soil, which is the typical requirement for all reinforced soil structures. This is not to say that a GMSE abutment could not be designed to minimize backfill volume, which might lead to a more sustainable project. However, the current state of practice as prescribed by FHWA standards results in the GRS abutment as a more sustainable option.
Much work remains to be done in this field, particularly in the United States. There have been several studies done to determine the environmental cost of manufacturing geosynthetics as well as the cost of running various construction equipment in Europe (Raja et al., 2015; Heerten, 2012). However, this data may not be applicable in the US due to differences in transportation and energy production and usage. This poses a “chicken or egg” problem for engineers in the United States since completed studies such as this may put pressure on industry to report energy usage, but studies cannot be complete until energy usage information is available.
An important component of a Life Cycle Assessment is the sensitivity analysis. A sensitivity analysis takes into account variability and unknowns and makes a report more transparent by showing how the uncertainty in the data may affect the final result. Unfortunately, the authors do not feel that there was enough information available to complete such an analysis at the time of this study and acknowledge the necessity of such an analysis as additional information becomes available. The effort to continue and grow this body of work is ongoing.
This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program as well as the Geosynthetic Institute Fellowship Program.
ABOUT THE AUTHORS
Dr. Melissa Beauregard, P.E., is Assistant Professor of Civil Engineering at the United States Air Force Academy. Dr. Arunprakash Karunanithi is an Associate Professor of Civil Engineering at the University of Colorado Denver. Dr. Caroline M. Clevenger, P.E., is Associate Professor and Assistant Director, Construction Engineering and Management, in the Department of Civil Engineering at the University of Colorado Denver.
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