Laserfiche WebLink
.. ,.. <br />~~ <br />Memo to Larry Oehler <br />2 <br /> <br />July 21, 1993 <br />Cone penetrometer testing of the tailing conducted uring May of <br />1993 indicates that at least one potentially satur~ted zone has <br />developed in the tailing, within the foundation for the 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 critical cross- <br /> section that is based on cone penetrometer soundi ngs taken at <br />~ / <br />s^ <br />~", only six locations within the approximately 140 ,000 square <br />~ <br />da <br />~~ foot tailing surface that .will serve as founda ion for the <br />n,,.: ~~ °"f ~ <br />` first raise. It ~ is. likely, or at least po Bible, that <br />=~"` <br />' i';r~.~• additional potentially saturated zones exist or ill develop <br />-i„~% '~' within the tailing. Due to this uncertainty, a factor of <br /> safety of 1.1 is unacceptably low for the embam ]onent under <br /> seismic loading. The ..operator must: incorporate de sign changes C•' <br /> into the proposal .to raise the emban)mient that w ~ll 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 under various <br /> levels of seismic loading. The report indicates that <br /> displacement along the upstream embankment face failure <br /> surfaces would be approximately four inches for a peak ground <br /> acceleration of 0.4g arising from a M=7+ earth quake. The <br /> displacement analysis presented does not consider d eformations <br /> that could result from the inability of liquifie tailing to <br /> support a load, which could result in slumping of f ~he portions <br /> of an upstream raise founded on tails. Also, the eformation- <br /> type approaches presented are not applicable to ma erials that <br /> experience increases in pore pressure due to cycl ic loading, <br /> such as the potentially saturated zone within the flailing that <br />/ <br />~ failure surface 2, as depicted in SRK figure 4.1, intersects. <br />~ ,~i; .,,-.iJ ~~'" <br />/f~f The operator needs to provide additional analyses of ~ <br />~• <br />1 ~I/~/,~r displacements that may occur if saturated zones within the <br />~ <br />5 tailing liquify, and incorporate adequate freeboard into the <br />~gF~~~id' yer`t <br />5 <br />