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<br />Memo to Larry Oehler <br />2 <br /> <br />July 21, 1993 <br />Cone penetrometer testing of the tailing conducted during May of <br />1993 indicates that at least one potentially saturated zone has <br />developed in the tailing, within the foundation for tt~e proposed <br />upstream raise. Analyses provided by SRK demonstrate that <br />saturated zones within the tailing can be expected to liquify <br />during the design earthquake appropriate to this site. SRK also <br />provides stability analyses that indicate the embankment will <br />remain stable, with a factor of safety of 1.1 utilizing a seismic <br />coefficient of .15g, even if the potentially saturated zone within <br />the tails were to liquify. There are three problems with the <br />analyses as presented by the operator: <br />1. The SRK report states: <br />Visual observation of the tailings retrieved <br />indicates that deposition has resulted in complex <br />layers consisting of various mixtures of tailings <br />sand and slimes. <br />Despite the inhomogeneous and anisotropic nature of the <br />tailing, the stability analyses utilize a critidal cross- <br />section that is based on cone penetrometer soundings taken at <br />only six locations within the approximately 140,000 square <br />foot tailing surface that will serve as foundation for the <br />first raise. It is likely, or at least possible, that <br />additional potentially saturated zones exist or will develop <br />within the tailing. Due to this uncertainty, a factor of <br />safety of 1.1 is unacceptably low for the embankment under <br />seismic loading. The operator must incorporate design changes <br />into the proposal to raise the embankment that will increase <br />the dynamic factor of safety to at least 1.3. <br />2. The SRK report includes analyses of the amount of <br />deformation or displacement which could occur ander various <br />levels of seismic loading. The report indicates that <br />displacement along the upstream embankment fact failure <br />surfaces would be approximately four inches for a peak ground <br />acceleration of 0.4g arising from a M=7+ earthquake. The <br />displacement analysis presented does not consider deformations <br />that could result from the inability of liquified tailing to <br />support a load, which could result in slumping of the portions <br />of an upstream raise founded on tails. Also, the deformation- <br />type approaches presented are not applicable to mateffials that <br />experience increases in pore pressure due to cyclio loading, <br />such as the potentially saturated zone within the tailing that <br />failure surface 2, as depicted in SRK figure 4.1, intersects. <br />The operator needs to provide additional analyses of <br />displacements that may occur if saturated zones within the <br />tailing liquify, and incorporate adequate freeboard into the <br />