2006-06-12_REVISION - M1982015 (14)

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Global Stability Analysts and Impact Assessment for Existing Manmade Structures Teriacon Bayahore Development -Weld County, Colorado Terracon Project No. 25045300 The following table presents the results of the global stability analyses along wtlh the required Factor of Safety recommended for State of Colorado lur+sd~ctional dams Graphical outputs from the scope stability Computer software are attached at the end of this report Table 2 Scenario Required Factor of Satety Actua~ Factor of Safety Static Condition (Upstream) t 5 2.039 Static Conditon ~ 5 1 689 (Downstream) t Rapid Drawdown (Upstream) 1 2 1 397 Impact to Existing Manmade Structures: Impact to existing manmade structures within 200 feet of the proposed lake improvements was also considered as part of this study in accordance with comments submitted by Division of Minerals and Geology (OMG) officials Two existing structures and several utility crossings were identrhed which required assessment per OMG. The exiting structures identified were Weld County Road 9-1/2 and also the bridge crossing the St. Vram River on Weld County Road 9.1/2 (I-25 Frontage Road). Based on the d+stance from ;he proposed lake improvements, +t ~s our opinion that these structures will not be negatively impacted by the construction of the proposed improvements. The ut+lity cross+ngs will be located and protected by the slurry wall contractor with assistance from the utility owners Furthermore, it is our understanding that notahzed "no negatwe +mpact' letters Gom the utility owners will be provided separately, Conclusions and Recommendations: As outlined above m Table 2, ilia ait+cal lake embankment sect+on exceeds ilia required Factors of Safety. Therefore. is our op+nion that the proposed configurations are stable provided that material placement earthwork recommendations are followed per the previously mentioned supplemental geotecnn:cal engineering report As previously outlined, rip rap eroswn protection will be required along the lake embankment on the St. Vra+n aver side in order to provide eros+on protection and overall global stability of the take embankment. Furthermore, flattan+ng of ilia existing slope to a maximum inclination of about 2.712 to 1 will t>e required. it +s our recommendation that a non-woven geotextite filter fabric wdh sufficient weighUSirength to withstand rip rap placement be installed behind the rip rap erosion protection to provide confinement of the underlying soils and prevent loss of fine materials migrating through the rip rap during prec~oitationlrunoff events. Based on the visual observation of the ex+stng natural vegetation along portions of the exist+ng embankment, speual permitting may be required by regulatory agenc+es m order to prate the required rip rap n some areas. Acquiring or assessing the need for special permitting is beyond the scope of this study