Month: August 2010

U.S. Environmental Protection Agency Proposes Transport Rule To Reduce Interstate Transport of Air Pollution

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On July 6, 2010 the U.S. Environmental Protection Agency (EPA) proposed a rule to address interstate transport of air pollution.  This proposed rule would replace the 2005 Clean Air Interstate Rule (CAIR).  The proposed rule, known as the Transport Rule, would require 31 states and the District of Columbia to improve air quality by reducing emissions of sulfur dioxide (SO2) and nitrogen oxide (NOx) emissions from electric generating power plants that contribute to ozone and fine particulate pollution in other states.  SO2 and NOx react in the atmosphere to form fine particulate matter less than 2.5 micron (PM2.5).  NOx also contributes to ozone formation.  The SO2 and NOx are then transported across states, making it difficult for states downwind to comply with National Ambient Air Quality Standards (NAAQS).

This proposed rule would clarify state obligations to reduce pollution affecting other states under the Clean Air Act by defining “significant contribution” and “interfere with maintenance.” In defining these obligations, the EPA proposes to consider the magnitude of a state’s contribution, the air quality benefits of reductions, and the cost of controlling pollution from various sources.

The emission reductions are scheduled to begin in 2012, within one year after the rule is finalized.  EPA estimates that by 2014, in conjunction with other state and federal programs, that emissions of SO2 and NOx from power plants would be reduced by 71 and 52 percent, respectively from 2005 levels.  Compared to 2005, EPA estimates that by 2014 this proposal and other federal rules would lower emissions by:

  • 6.3 million tons per year of SO2
  • 1.4 million tons per year of NOX, including 300,000 tons per year of NOX during the ozone season.

The proposed rule is expected to annually cost electric utilities and consumers $2.8 billion, but is expected by EPA to yield $120 to $290 billion in annual health and welfare benefits in 2014.  EPA also estimates that between 14,000 to 36,000 premature deaths will be avoided.

The rule specifies  that twenty-eight states would be required to achieve reductions in both SO2 and NOx emissions to assist downwind states in meeting attainment with the annual and 24-hour PM2.5 standards.  Furthermore, the rule requires twenty-six states to reduce NOx emissions during the ozone season to assist downwind states in reducing ground-level ozone concentrations in order to comply with the ground-level ozone standard.  The following map identifies the states subject to the rule and the emissions to be controlled.

US map of transport rule
Proposed transport rule coverage

EPA’s approach for reducing SO2 and NOX emissions in states covered by this rule is to set a pollution limit (or budget) for each of the 31 states and the District of Columbia. This approach allows limited interstate trading among power plants but assures that each state will meet its pollution control obligations.

EPA is also taking comments on two alternative approaches. The first alternative would set a pollution limit or budget for each state. This option allows trading only among power plants within a state.  The second alternative would set a pollution limit for each state and specify the allowable emission limit for each power plant and allow some averagingof the emissions.

To assure emissions reductions, EPA is proposing Federal Implementation Plans, or FIPs, for each of the states covered by this rule. The FIPs would put in place requirements necessary to reduce pollution in the covered states that significantly contributes to nonattainment of or interferes with maintenance of the national ambient air quality standards in other states.

States may choose to develop a State Implementation Plan (SIP) to achieve the required reductions, replacing its federal plan.

In order to achieve emission reductions outlined in the Transport Rule, power plants may be required to:

  • operate already installed air pollution control equipment more frequently,
  • use low sulfur coal, or
  • install control equipment such as low NOx burners, Selective Catalytic Reduction, or Flue Gas Desulfurization.

If your facility will be affected by the Transport Rule, or if you have questions regarding the rule, please contact CEC’s St. Louis office at 866-250-3679.

Avoiding Failures and Performance Issues with Mechanically Stabilized Earth (MSE) Walls and Slopes

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Over the past 20 years, the use of mechanically stabilized earth (MSE) walls and slopes has become very common in a large number of construction applications in the U.S. and around the world.  The technology used to build these structures is really quite simple:  reinforcing, typically metal or synthetic grids or sheets, are layered in with compacted soils, adding shear strength and allowing the soils to stand at progressively steeper angles.  Wall faces are typically constructed using concrete panels, split-face masonry blocks, or even vegetation that primarily provides erosion control and aesthetics. The faces provide little if any structural support to the retained soils.

The relatively low cost of MSE structures have made them quite prevalent in transportation and site development projects, and have also led to their use for waste management and environmental remediation projects.  MSE walls can often be constructed for less than half the cost of comparable concrete or steel structures.  This cost advantage increases as the height of the structure increases. This reduced cost has enabled the development of increasingly marginal projects, and pushed the limits of the technology, literally, to new “heights”.  For example, several recent airport expansion projects in the U.S. have utilized MSE wall and slopes well in excess of 100 feet tall.

However, this lower cost and increased use of the technology has come at a price.  While there are no specific published numbers available, the failure rate of these structures has been estimated by some to be as high as 5% to 7%, with 2% likely being a low-end estimate.  “Failure” in this case encompasses not only large-scale collapse or movement, but also settlement and performance issues.  In any case, the number of MSE walls and slopes exhibiting problems is alarmingly high for an engineered structure, and the cost to repair these problems can be many times the original construction cost.

So why do these failures occur?  Over the past 10 years, CEC has been involved with the specification, design, construction monitoring, and failure investigation of a number of MSE walls and slopes.  Published evaluations on MSE wall failures are also quite numerous.  Many studies have shown that, particularly in the private site development sector, engineering site layout, surface and subsurface drainage features, geotechnical engineering evaluations, and construction monitoring are often inadequate.  CEC’s experience investigating failures has identified a number of construction errors that have led to performance issues.  One re-occurring construction factor leading to failure is inadequate backfill compaction when clayey soils are used in the wall construction.

The published studies and our experience also indicate that the contracting methods used for both design and construction of MSE walls and slopes may be contributing to the high failure rate.  Most MSE walls are designed and constructed using a design-build contract where the contractor provides the detailed wall design and constructs the wall.  This process results in highly competitive “cut-throat” bidding among vendors, encourages overly optimistic design assumptions, and often hampers communication and review by the design team.  This process often places numerous risks unknowingly back on the owner.

How can you protect yourself and reduce the risk of failure for MSE walls and slopes on your project?  First, hire civil and geotechnical engineers with experience in the investigation, design, and specification of these structures and ensure that their services are carried through into construction.  If a design-build process is used, a detailed wall layout and performance specification must be prepared listing all wall design, testing, and construction requirements.  Full-time, on-site construction monitoring should be provided by either the wall designer or geotechnical engineer to ensure that the proper testing and site inspections are done. The contractor should not provide the construction monitoring services.   Finally, hire a contractor with experience and certification in MSE construction.

If you have any questions about the use, specification, design, or construction of MSE wall and slopes and how they may impact an upcoming project, contact Douglas Clark, P.E. ( or Jeffrey Woodcock, P.E. ( at 800-365-2324.