Mount Dora C&D Landfill

Mount Dora C&D Landfill
Slope Stability and Settlement Analyses

Madrid CPWG performed slope stability analysis and settlement analysis for an additional 40 feet of landfill elevation above the previously permitted elevation of 169 feet. The landfill area is approximately ½-mile across with undulating terrain and a liner was already installed over the central area of the landfill area when we were introduced to the project. Madrid CPWG completed a review of the original geotechnical investigation. Our team determined that there was a paleo-sinkhole (ancient sinkhole) in the middle of the landfill in the same area where the liner had recently been installed and covered with drainage sand. One previous boring encountered an approximate 140-feet thick layer of peat and the boring was still in the peat layer when the boring was terminated at 195 feet depth.

Madrid CPWG Approach

Madrid CPWG completed additional borings outside of the lined area and combined that information with the previous boring data to analyze settlement in two long cross-sections across the site using finite element software. Images showing examples of the cross-section and settlement graph are shown below. The estimated settlement was analyzed by using

• Historical data of the original ground surface elevations at the site
• Reported current elevations (the site was previously excavated as a sand mine)
• Proposed landfill loading up to the original design height
• Data for a proposed additional 40 feet of landfill height increase

The results indicated as much as 3 to 4 feet of settlement with the original proposed design height and possibly 5 feet or more settlement in the center of the paleo-sinkhole; however, previous settlement estimates appeared to have grossly underestimated potential settlement in the peat layer and Madrid CPWG recommended geogrid stabilization to help smooth out differential settlement across the site.

Madrid CPWG Benefit

Lake County officials did not want to remove the liner that was already installed due to the additional cost required but we had concerns that the liner might fail under these extremely variable soil conditions.

To meet the County’s needs, Madrid CPWG designed a multi-layer geogrid system to be installed above the liner to help reduce differential settlement from the landfill loads above.  The geogrid system would effectively eliminate “point” loads that could cause the liner to fail.

Madrid CPWG also showed that the maximum liner deflection would be reduced to no more than 1%, which was within the tolerance of the liner system.