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ARCADIS <br />correspond to approximately the first 5 to 6 feet from the toe of the slope (in the case of ponds NHN -001 <br />and -003). In the case of pond NHN -002 the entire embankment could be susceptible to swelling <br />pressures. The swelling soils may also impact the structural stability of the principal spillway due to the <br />differential heave. In addition, the areas which swell can exhibit shrinkage and this can lead to reduced <br />strength. Swelling at the toe of the embankment will reduce its resistance to surface erosion and can also <br />exhibit reduced strength against seepage force, potentially leading to piping. It is well identified that <br />spillways may have reduced capacity because of the volume change of an embankment. It is proposed <br />that rip -rap be added to the areas along the toe in order to add weight, to overcome swelling and protect <br />the soil loosened by swelling from erosion and hydraulic force from seepage across the dam. No <br />swell /settlement analysis was conducted as part of this project. <br />Summary <br />Results of the stability analyses indicate that the stability of the proposed embankments will more than <br />meet the minimum required FOS of 1.3. As discussed, Case 6, the "most likely' scenario, achieves FOS <br />greater than 2.0 for all three of the proposed embankments. This scenario is based on average soil <br />strengths for the foundation base and somewhat conservative soil strengths for the embankment soils. <br />Tables 1A, 1113, and 1C summarize the results of all the stability analysis cases. Case 6 is adequately <br />stable without the use of a key trench (cutoff trench); however, the use of a key trench is recommended. <br />As discussed, Case 1 is probably an unrealistic case in that the soil strengths used for the foundation <br />base are the lowest recorded strengths based on laboratory testing. <br />It was assumed that similar material will be used to construct the new embankments for ponds NHN -001, - <br />002, and -003. If this is not the case, then the slope stability models should be revised to account for the <br />use of different materials. <br />No stability analysis was conducted with regard to the cut slopes in the native /fill soils that will form the <br />remaining perimeter of the ponds. If it is deemed necessary, this analysis could be conducted using the <br />base files that the current modeling was conducted with. <br />No soil swell /consolidation analysis was conducted as part of the project; however, it is noted that the <br />subsurface soils present at this project site have the potential to swell and cause differential heave of the <br />embankment and structural damage to the principal spillway. It is proposed that rip -rap be added to the <br />areas along the toe of the embankments to provide weight above the soil that could swell, protect against <br />potential piping issues, and protect the inside slope of the ponds against scour. <br />Bearing capacity of the underlying soils should be adequate to handle the loads imposed by the <br />construction of the embankments; however, no analysis was conducted to verify this assumption. <br />Page: <br />yr ssnhmgtoW1537 0001_ sedcaddelmablesldraR _repatlsbpestabiRyanaysislsbpe staMMy anaF sis and recommendation memo 11-17 -11 d= 4/4 <br />