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Benefit/cost analysis as it relates to service life of geosynthetics

August 1st, 2020 / By: / GSI News

On the surface, performing a benefit/cost (B/C) analysis is a straightforward process. However, ensuring that such an analysis is accurate and thorough is a much more nuanced task. In 2008, Bob Koerner laid out the procedure for geomembranes in Geosynthetic Research Institute (GRI) White Paper #12 “GS Selection Based on B/C Ratio.” This article expands on Koerner’s methodologies, identifying the necessary steps and considerations for a project manager to create an effective B/C analysis when considering geosynthetics. Hopefully, it will shed some light on a process that many industry professionals use daily. 

A B/C analysis is used to evaluate the total anticipated benefit of a project compared to its total expected cost to determine if the proposed implementation is economically worthwhile. If this comparative evaluation method suggests that the benefits associated with a proposed action outweigh the costs, then a business or project manager will often choose to follow through with the implementation.

The first step of a B/C analysis is to identify and quantify all benefits anticipated as a result of implementing the proposed action. These benefits can include direct profits, decreased costs due to technology improvements, increased production capabilities, and increased revenue due to factors such as customer satisfaction, standardization, efficiency, sustainability, reliability and product durability. Some of these factors are more difficult to directly translate into monetary values than others; therefore, it is important to use available market data when possible to determine the impact that certain benefits have on the project’s profit margin. Along with costs, the service life of the project greatly affects the magnitude of many of these benefits.

The next step is to identify and quantify all relevant costs associated with the proposed action. In order to successfully identify such costs, one must follow a standardized procedure in compiling all monetary and nonmonetary costs incurred by the implementation of a particular project. Fixed costs (installation equipment), variable costs (labor, raw materials) and recurring costs (site maintenance) must all be considered. Additionally, the project’s opportunity cost, which stems from losing potential benefits obtained from using alternate project solutions, must also be accounted for.

The cost-identification process also requires projecting costs throughout the useful service life of the implementation. This projection will not only map out important lifetime maintenance, depreciation, risk and uncertainty patterns, but will also provide important insight when financing a project long-term. The optimal project solution from a cost-projection perspective would have a long service life and minimal recurring costs throughout its life.

The final step is to weigh the benefits and costs to determine if the proposed action is worthwhile. The expected benefits must be divided by the identified costs to produce a B/C ratio. If the ratio is greater than 1, it can be concluded that the proposed action is potentially a worthwhile investment and should be further evaluated as a realistic opportunity, compared against other alternatives with similar B/C ratios.

Engineers are often consulted in quantifying technical matters on both sides of the B/C ratio; being asked to predict the service life of a geosynthetic in various environmental, transportation, hydraulic or geotechnical applications is commonplace. Regular discussions around service life of exposed versus buried geosynthetics are common in the industry. It is refreshing to hear project managers acknowledge that plastics undergo first-, second- and third-order degradation mechanisms over time. We now have educated customers in our industry. Important questions are being asked: How long will it last? When will this need to be repaired or replaced? What environmental conditions (temperature, elevation, exposure orientation, latitude and longitude) can be controlled to protect this infrastructure?

It is freely admitted that the focus of this discussion over the past 30 years has been general in nature. Engineers typically looked at worst-case scenarios and concentrated on a few key parameters of the geosynthetics to make gross estimates (i.e., a few years, 10–30 years, more than 100 years) of service life. Such gross estimates make it difficult for project managers to produce effective B/C analyses due to the huge variability that each of those ranges contain. However, I believe that it is time for our industry to become more transparent about the key indicator properties for durability-challenged geosynthetics. Listed below, in no particular order, are key “tells” that your geosynthetic is reaching the end of its service life:

  • Material thickness decrease of 30% (Stark, Choi and Diebel 2005)
  • Density increase in excess of 0.04 g/cc (Haxo 1982; Hsuan and Koerner 1998)
  • Oxidative induction time (OIT) decreases to 1/3 of specification (33 standard and 133 min. high pressure) (Hsuan and Koerner 1998; Koerner 2016; Rowe, Rimal and Sangam 2009)
  • Stress crack resistance less than 150 hours (Hsuan, Lord and Koerner 1991)
  • Change in melt flow index of 30% (Benson and Dwyer 2006)
  • Tensile elongation at break, decrease of 50% (Apse 1991)
  • Initial or break modulus, 30% stiffening (Zanzinger and Martin 2018)
  • Change (pass versus fail) of impact resistance (Rollin, Mlynarek and Zanescu 1994)

Additionally, decreases in geosynthetic performance over time can also result in observable surface degradation (chalking, flaking or cracking), as well as affect the feasibility of maintaining or repairing the material (seaming the material may no longer be possible after a certain amount of degradation occurs).

In conclusion, lifetime assessments for B/C ratio evaluations are part of an essential process to all geosynthetic projects. Definitive answers can be elusive. However, armed with decades of case histories and experimentation, the geosynthetic community is now better equipped to make estimates on service life based on specific geosynthetic materials performance. These newer estimates are usually adequate for economic predictions such as B/C. Finally, it is important to note that a material reaching the end of its service life does not mean immediate failure of the system due to redundant safety and design considerations implemented by the project engineer. However, to ensure dependability, safety and reliability of the solution, replacement before end of life is encouraged.

References

Apse, J. I. (1991). U.S. EPA computer code on Flexible Membrane Liner Advisory Expert System (FLEX), Cincinnati, Ohio, EPA Risk Reduction Laboratory.

Benson, C. H., and Dwyer, S. F. (2006). “Material stability and applications.” Chapt. 3 in Barrier systems for environmental containment and treatment, ed. C. C. Chien, H. I. Inyong, and L. G. Everett, CRC Press, Boca Raton, Fla., 143–201.

Haxo, H. E. (1982). “Effect on liner materials of long-term exposure in waste environments.” Proc., 8th Annual Research Symposium: Land Disposal of Hazardous Wastes, EPA-600/9-82-002, U.S. EPA, Cincinnati, Ohio.

Hsuan, Y. G., and Koerner, R. M. (1998). “Antioxidant depletion lifetime in HDPE geomembranes.” Jour., Geotech and Geoenvironmental Engineering, 124(6), 532–541.

Hsuan, Y. G., Lord, A. E. Jr., and Koerner, R. M. (1991). “Effect of outdoor exposure on a high density polyethylene geomembrane.” Proc., Geosynthetics 1991, IFAI, St. Paul, Minn., 287–302.

Koerner, R. M. (2016). “Lifetime predictions of exposed geotextiles and geomembranes.” Proc., GeoAmericas 2016, vol. 1, Miami Beach, Fla., 1–46.

Peggs, I. D. (2004). “Geomembrane liner durability: Contributing factors and the status quo.” Jour., Environmental Science, https://doi.org/10.1680/gpte.32347.0001, Corpus ID:138299534.

Rollin, A. L., Mlynarek, J., and Zanescu, A. (1994). “Performance changes in aged in situ HDPE geomembranes.” In Landfilling of Waste: Barriers, E&FN Sons, ed. T. H. Christensen et al. Chapman Hall, London, 915–924.

Rowe, R. K., Rimal, S., and Sangam, H. (2009). “Ageing of HDPE geomembrane exposed to air, water and leachate at different temperatures.” Jour., Geotextiles and Geomembranes, 27(2), 137–151.

Stark, T. D., Choi, H., and Diebel, P. W. (2005). “Plasticizer retention in PVC geomembranes,” Proc., GeoFrontiers 2005, ASCE, GSP, 130–142.

Zanzinger, H., and Martin, A. (2018). “Evaluation tests on PE-HD geosynthetic barriers after exposure tests,” Proc., 11th ICG, Seoul, Korea.