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AXOM <br /> Henderson 3 Dam Original Dam Regrade <br /> March 23, 2017 <br /> Page 3 <br /> RECOMMENDED DESIGN <br /> AECOM recommends regrading the entire original dam from a 2.9H:1 V on the southern half of the <br /> dam and from a 3.8H:1 V on the northern half of the dam to a uniform 4H:1 V slope along the entire <br /> original 3 Dam. The northern half of the regrade is not mandatory to meet stability requirements, it is <br /> only an opportunity. <br /> The plan, profile and details associated with the original 3 Dam slope regrade are shown on attached <br /> Drawings 1 to 3. These drawings were prepared to provide sufficient detail for Henderson to <br /> complete the work internally. <br /> A representative from AECOM will be on-site for two consecutive days during the first week of <br /> construction to observe implementation of the regrade in the field. Additionally, three one-day follow- <br /> up site visits have been assumed to observe construction progress. AECOM's representative will <br /> understand the design and can help facilitate design changes required, based on field conditions. <br /> AECOM will prepare daily field reports for each site visit completed along with a design construction <br /> summary letter complete with as-built drawings at the end of the project. Henderson will provide the <br /> survey for the as-built conditions. <br /> CONSTRUCTION CONSIDERATIONS <br /> During construction, there shall be no tailing deposition (spigot or leadoff) at the 3 Dam Stepback. <br /> Additionally, construction cannot occur until the completion of spring runoff. This will be determined <br /> by the following conditions: <br /> • Piezometers are no longer rising but have reached their peak and are starting to decline <br /> • Snow runoff is visually complete in the area of work <br /> Minor grading may be required around the horizontal drains to maintain the required pipe embedment <br /> and limit exposing of the stainless steel casing. The original 3 Dam along with the existing horizontal <br /> drains and a seepage collection ditch offset back from the toe of the original dam are shown in Photo <br /> 1. <br /> Photo 1 -Original 3 Dam with horizontal drains(left)and seepage collection ditch(right)-facing North(August 2016). <br /> M:\DCS\PROJECTS\WTR\22244514_HEN_TRB_PROGRAM\SUB 00\6.ODE LIVERABLES\3 DAM RESLOPE\HENDERSON-3DAM RESLOPE GEOTECH DESIGN <br /> LETTER REPORT VER O.DOCX\23-MAR-17 <br />