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1 <br />' San Luis Mine Phase Q, Raise 2 Design Report <br />' 4.7 Post-Liquefaction Settlement <br />Following liquefaction of a deposit and dissipation of the induced pore pressures settlement will <br />occur. Tokimatsu and Seed (1984) present a method for estimating post-liquefaction settlement from <br />SPT N-value data. For an average (Nt)~ of 6.3 for the slimes and high earthquake induced cyclic <br />t stress ratios, this method predicts a volummetric strain of approximately 3.5 percent following <br />liquefaction. For average (Nt)~ values of 9.9 and 7.3 for the sand and mixed tailings respectively, <br />volummetric strains of approximately 2.5 and 3 percent are predicted. Although originally <br />developed for evaluation of settlements in clean sands, comparison of the results using the method <br />of Tokimatsu and Seed with those from Field studies of densification of silty tailings by blasting <br />' (Klohn, Garga and Shulkin, 1981) indicates that the results are reasonable. <br />The upstream crest of the dam raise will be founded on approximately 20 ft of saturated tailings <br />' while the downstream crest will be founded on approximately 16 ft of tailings. Accordingly, an <br />average post-liquefaction strain of 3 percent equates to 0.6 and 0.5 ft of serniement beneath the <br />' upstream and downstream crests of the raise, respectively. Settlements of this order of magnitude <br />are insufficient to compromise the integrity of the structure although some minor cracking may <br />occur. Minor cracking of the embankment poses no serious threat to the structure as the <br />' embanlmient does not dtrectly impound water. Post-liquefaction settlement of the tailings within the <br />impoundment is expected to be greater as the thickness of saturated tailings is greater. These <br />settlements will not compromise the impoundment liner system nor the function of the drainage <br />' ~ layer. <br /> <br /> <br /> <br />1 <br />1 <br />1 <br />u Y 4.10 ro~ect o. <br />1 <br />