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A=COM <br /> Henderson 3 Dam Original Dam Regrade <br /> March 23, 2017 <br /> Page 2 <br /> within UTEXAS4 were used for circular and noncircular trial shear surfaces to locate critical shear <br /> surfaces. <br /> Material properties are summarized in Table 1 for the undrained loading condition. The undrained <br /> loading condition is meant to simulate excess pore pressures that could develop during deposition. <br /> Undrained strength parameters were used for saturated tailings and drained strength parameters <br /> were used for the unsaturated tailings and foundation soils. <br /> Table 1 <br /> Summary of Material Properties <br /> Undrained Strength <br /> Material Type Unit Weight(pcf) Cohesion, c' V <br /> (psf) (degrees) <br /> Unsaturated Tailings 117 c'=0, �'=38° <br /> Anisotropic <br /> Coarse Tailings 119 Inclined (CIU') Su/p'=0.50 <br /> Horizontal (DSS) Su/p'=0.30 <br /> Upper Foundation 135 S /p'= 0.28 <br /> Lower Foundation 135 500 35 <br /> Starter Dam 1135 1500 35 <br /> Stability analyses completed for existing undrained loading conditions of original 3 Dam, indicate a <br /> calculated factor of safety (FS) of 1.24 for Study Section 2 on the original dam (with an existing slope <br /> close to 2.91­1:1V). The resulting failure surface is shown on Figure 3 and is below the minimum <br /> undrained FS of 1.3. This minimum undrained FS is consistent with the standard of practice and also <br /> agrees with the Colorado State Engineer required FS for End of Construction Loading conditions. <br /> Analyses have shown that resloping the southern half of the original dam face to no steeper than <br /> 3.6H:1V increases the FS above the target undrained loading FS. Based on discussions with <br /> Henderson, there is an opportunity to regrade the entire original dam to 4H:1 V for construction ease <br /> and potential long-term reclamation needs. The undrained FS for Study Section 2 increases to 1.41 <br /> for the proposed regraded slope of 4H:1 V as show on Figure 4. The northern half(Study Section 4) of <br /> the original 3 Dam has a current slope close to 3.8H:1V and has an existing undrained FS of 1.48 as <br /> shown on Figure 5, thus meeting the minimum design undrained FS of 1.3. <br /> Findings from the 2016 field and laboratory investigation along with geometry updates and stability <br /> results have been independently reviewed by a Technical Review Board (TRB) that Freeport initiated. <br /> The geotechnical TRB consists of Dr. Norbert Morgenstern and Dr. Roy Mayfield who are considered <br /> subject matter experts in tailings impoundments. <br /> Henderson has limited deposition on Tailings Dam No. 3 stepback, located upstream of the original 3 <br /> Dam, since Fall 2015/Winter 2016 with only spigot deposition in late May and early June 2016. <br /> Deposition is not planned to resume until the resloping effort on the original 3 Dam is complete. <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 />