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~~~c~ f,~p ~~~rracon <br />June 2, 2006 c~,•.,~ o,~u e~~~~..~. a s~ .~,~..~. <br />JUN ~ y 11,E ., .:,., ~,Y>,~ ri,H, N<~,, ..,.,. , <br />Ohl ~, , n,a:~,:ux.~ <br />Carma floe of hfiwralun0 r{~4 ~; ~ , :z? .r ~t ~ <br />Highland Place t t •°~~~"^^~~~ • ^ ~ ~ ~° <br />9110 East Nichols Avenue. Swte 180 <br />Greenwood Village, Colorado 80112 <br />Attn Mr Tyler Packard <br />Re: Global Slope Stability Analysts For EmbankmentlSlurry Wall Construction and <br />Impact Assessment for Exlsting Manmade Structures <br />Proposod St. VralnlBayshore Development <br />Carma Weld County Sito <br />Weld County Road 28 and Weld County Road 9.112 <br />Terracon Project No. 25045300 <br />At your request, Terracon has completed global stability analysis and impact assessment for <br />existing structures for the proposed slurry wall construction and grading around the existing <br />lakes at the St VrarNBayshore Development. This report is intended to supplement our <br />supplemental geotechnical engineering report dated November 9. 2005 (Terracon Protect <br />No 25045300}; all previous recommendations from the prevwus report remain valid and <br />applicable. <br />Global Stability Analysis: Stabtlrty analyses were performed in order to determine the <br />global stability of the -worst case" embankment profile in venous water IeveUdrawdown <br />scenarios The worst case section was determined based upon review of the most recent <br />grading plans and information pertaining to the expected mean water levels and high water <br />levels expected fora 100-yr storm event provided by Carroll and Lange. Inc (C8L). Stream <br />flow velocities were also provided by C8L along the St Vram Rwer channel at venous <br />locations (during the 100-year storm event) which also contnbuted to the selection oI the <br />critical section. It was determined that the southern boundary o} the lake adjacent to the St <br />Vrain River would present the critical section for the purposes o! performing stability <br />analyses (correspcnds to the approximate location of Bonng No t9 from the previously <br />referenced Supplemental Geotechnical Englneertng Report) The cntical section was <br />selected based on the maximum head differential between the water impounded by the <br />lakes and the St Vrain, the narrowest width of the proposed embankment. and the <br />maximum stream vetoaty and proximdy of St. Vrain River <br />UUI¢~ng GeoSlope Studio software. SlopeW. limd equtlibnum stab~lrty analyses were <br />performed on the cotical section using eNective stress parameters with user selected pore <br />pressures. The following table presents the effective stress parameters used for the stability <br />