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A=COM <br /> Henderson 3 Dam Original Dam Regrade <br /> March 23, 2017 <br /> Page 4 <br /> The horizontal drains and piezometers (F-Bench and G-Bench) shall be protected during regrading of <br /> the slope. The piezometer casings will also need to be adjusted following construction. Additionally, <br /> care around the seepage collection ditch should be provided to not allow material to block the flow in <br /> the collection ditch. <br /> The regrade plan provided does not expose or cut into the natural slope. However, the accuracy of <br /> the original topography could be off slightly. If natural material is encountered during the regrading, <br /> the Engineer should be contacted. The natural ground surface is shown on the drawings. <br /> There are several options on where to place material cut from the original 3 Dam to accomplish a <br /> 4HAV slope. The preferred path forward is to place the material in the area between the 3 Dam <br /> Stepback toe and original 3 Dam crest. <br /> Stockpile Guidance— Area between 3 Dam Stepback and Original 3 Dam crest <br /> Material placed between the stepback and original 3 Dam will need to consider the overall drainage <br /> toward the current collection box. The area between the original 3 Dam and the stepback will result <br /> in approximately up to an additional 10 feet of material in places. Analyses have been completed and <br /> confirm undrained stability conditions would still be met with this additional fill placement. <br /> A raise of 10 feet over this area is not possible without modifying the current stormwater collection <br /> system. It is understood that Henderson will extend the French drain to the collection box on both <br /> sides and would raise the collection boxes to get above grade including reconfiguration of the trash <br /> racks. Fill would need to be selectively placed in areas to maintain drainage towards the collection <br /> system. Additionally, the material shall be placed such as to not cause localized areas of ponding <br /> water. Once the snow melt in the area between the stepback dam and original dam has occurred; <br /> Henderson will survey the area and provide that data to AECOM. AECOM will then finalize a grading <br /> and drainage plan. <br /> MONITORING CONSIDERATIONS <br /> The slope shall be monitored continuously throughout construction. If signs of slope instability <br /> (tension cracking, sloughing, slope rotation, etc.) are observed, work shall be immediately stopped <br /> and the Engineer shall be notified. <br /> Piezometer responses in the area shall also be closely monitored through the use of Canary. <br /> Increased frequency of readings will be required. Piezometer readings in the construction area shall <br /> be collected hourly at the start of construction and once the pore pressure response can be <br /> understood, the frequency of readings can be decreased. Any signs of instability or if the pore <br /> pressure ratio begins to approach 1, the Engineer shall be notified and work shall remain stopped <br /> until approved by the Engineer. <br /> M:\DCS\PROJECTS\WTR\22244514_HEN_TRB_PROGRAM\SUB_00\6.0_DELIVERABLES\3 DAM RESLOPE\HENDERSON-3DAM RESLOPE GEOTECH DESIGN <br /> LETTER REPORT VER O.DOCX\23-MAR-17 <br />