The project site is a nearly 400-acre fly ash disposal basin for a very large, coal-fired electricity generating facility in Michigan. The fly ash disposal basin is enclosed by a 3.5-mile long, 40-foot high, compacted clay embankment. For more than 20 years, the exterior face of the embankment has experienced distress in the form of tension cracking and sloughing. Previous slope mitigation measures proved unsuccessful at stabilizing the slopes. The client retained Geosyntec in 2009 to evaluate the cause of the sloughing and to design slope remediation measures. The client later expanded Geosyntec’s scope of services to include preparation of construction documents, performance of construction quality assurance monitoring, and related engineering services.
Geosyntec’s Scope of Services
Geosyntec conducted a complete records review and a thorough visual inspection of the ash basin and the perimeter embankment. Geosyntec implemented an initial site exploration consisting of field and laboratory testing of the in situ embankment fill and concluded that the sloughs were caused as a result of the steep slope angles (2H:1V) combined with the weathering of the upper two-to-three feet of embankment fill, causing surface water infiltration and weakening.
A mitigation plan we developed was accepted by the state that defined the slope mitigation measures, the priority of those improvements, and the implementation schedule. Geosyntec also designed periodic inspection and instrumentation monitoring that could be implemented by the client’s personnel to monitor embankment conditions. Geosyntec was then retained to prepare detailed construction documents for slope mitigation performed in 2009, 2010, 2011, 2012, and 2013. Geosyntec was also retained to provide engineering and full-time, on-site construction quality assurance monitoring and documentation services fora number of years.
Geosyntec also conducted a potential failure mode analysis (PFMA) to assess the overall stability of the facility. The analysis was conducted with all stakeholders from the client and resulted in action items to reduce the risk of failure. The client addressed all of the issues by the end of the year.
Rather than provide a single mitigation design for the entire site that would have been over-designed for certain areas, Geosyntec’s multiple mitigation design was tailored to match site conditions. It is estimated that by providing four solutions instead of one, we reduced the potential construction costs by around 50%. It was critical for the client to accomplish and complete each phase on an expedited schedule to be able to keep their permit active for the power plant. Geosyntec was able meet all deadlines and worked within the requirements of the project, allowing the client to keep their permit.