Author: Douglas M. Clark, P.E.
Recently, the USEPA provided a prepublication copy of standards for the disposal of coal combustion residuals (CCR) in landfills and surface impoundment. Section 257 of the federal code will be modified to include new measures covering the location, design, operation, and closure of existing and proposed CCR disposal units.
Owners of existing CCR surface impoundments will be required to provide assessments of the “structural integrity” of their facilities shortly after publication of the final rules. Owners must obtain a certification from a qualified professional engineer stating that each of these assessments meets the requirements of the stipulated CCR regulations.
Within 18 months of publication, owners or operators of a CCR impoundment must provide the following initial engineering assessments:
Hazard Classification (§257.73(a)2) – The owner is required to document the hazard potential of the impoundment based on the potential loss of human life, economic loss, environmental damage, disruption of lifeline facilities, or other impacts resulting from failure or mis-operation. The hazard classification uses a scale of low, high, or significant, and directly impacts spillway capacity, emergency action planning, and response requirements.
History of Construction (§257.73(c)) – The owner must compile documentation of the physical condition of the impoundment, including information on foundation and abutment soils, embankment and staging information, detailed drawings of the impoundment (including drainage and outlet structures), and normal and peak pool operating elevations.
Structural Stability Assessment (§257.73(d)) – The owner must document the adequacy of the impoundment design, construction, operation, and maintenance. The assessment must document the condition of the impoundment foundation and abutments, slope protection, vegetation, spillway condition and capacity, and associated hydraulic structures.
Safety Factor Assessment (§257.73(e)) – The owner must document the calculated factors of safety for critical cross sections of the impoundment embankments. The safety factor assessment must include stability calculations for operating and loading conditions (such as seismic loading) typical for dam design and construction.
These engineering assessments must be updated every five years following the initial assessment.
Within 24 months, a written Emergency Action Plan (§257.73(a) 3) must be prepared for any CCR surface impoundment classified with high or significant hazard potential. The EAP must contain information on procedures to detect a safety emergency, define responsible persons and notification procedures, provide contact information, include a map delineating the downstream area potentially impacted by a failure, and provide for meetings between the owner and local emergency responders. The written EAP must be reevaluated, at a minimum, every five years.
Longer term (42 months after publication), owners must demonstrate that existing surface impoundments comply with location restrictions for fault areas (§257.62) and seismic impact zones (§257.63). Facilities located within these areas will require site-specific evaluations of the performance of the facilities under seismic (earthquake) events. These evaluations must demonstrate that all structural components (including liners, leachate collection systems, and surface water control systems) are designed to function under the impacts of maximum horizontal ground motion. This task will include performing liquefaction, slope stability, and deformation analyses of waste, foundation, and embankment soils.
Any existing impoundment not demonstrating compliance with these requirements must cease placement of CCR and non-CCR waste streams into the unit within 6 months and commence closure operations in accordance with §257.102.
If you have questions about the proposed changes to the federal rules governing CCR disposal and the engineering assessments discussed, please contact Doug Clark (firstname.lastname@example.org; 800-365-2324) or Steve Dixon (email@example.com; 800-365-2324). More information on the proposed rule changes is available at EPA’s website.
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. (firstname.lastname@example.org) or Jeffrey Woodcock, P.E. (email@example.com) at 800-365-2324.