REP34115 - Laserfiche WebLink

Home Browse Search

1 ' San Luis Mine Phase n, Raise 2 Design Report ' The derived SPT N~ values were normalized to 1 ton/ftz overburden pressure to correct them [o (N,)~-values which were then averaged for each type of tailings (sand, slimes, or mixed). The data indicates an average (N,)~ of 9.9 blows/ft for the sand tailings, an average (N,)~ of 6.3 blows/ft ' for the slimes, and an average (Nl)~ of 7.3 blows/ft for the mixed tailings. This data is presented in Appendix C.2. ' For the tailings sand, the CPT data indicates relative densities which are generally of the order of less than 40 or 40-50 percent and friction angles of 30 to 34 degrees in non-saturated tailings and ' less than 30 degrees in saturated tailings. The slimes and mixed tailings generally exhibited an undrained shear strength of 1 to 2.5 tsf and do not show any consistent trends with depth. ' The pore pressure dissipation data can be used to determine the coefficient of horizontal consolidation (Ch) from which the coefficient of vertical consolidation (C~) can be estimated. The ' Ch data is included on the pore pressure dissipation plots presented in Appendix C.1. The pore pressure dissipation data resulted in a Ch range of 2.3 to 94.8 cm2/min (1.7 x 10-' to 4.1 x 105 ' fTz/sec) and an average on the order of 20 cmZ/min (3.6 x 10' ftz/sec). Depending on the micro- layering within the tailings at the specific test location, the ratio Ch/C~ will tylpically vary from 2 to 10 in tailings, i.e. tailings have a horizontal to vertical permeability ratio of 2 to 10 (Vick, ' 1983). For the San Luis tailings micro-layering does not appear to generally be significant and these ratios should be neaz the lower end of the range typical for tailings. Thus the data obtained is consistent with previous determinations of C~ by CPT and laboratory testing pedformed for Raise 1 . design evaluations and the Reclamation Evaluation. The pore pressure dissipation data can also be used to estimate the horizontal coefficient of permeability (K,J. The time for 50 percent pore pressure dissipation (tbo) from piezocone CPT data ' has been correlated to laboratory derived values of Khby Robertson, et. al. (1992). Based on the upper and lower bounds presented for the correlation, the 1995 CPT pore pressure dissipation results for the San Luis tailings indicates a Kh range of 9 x 10'5 to 1 x 10-' cm/sec. Based on the average ' Robertson, et. al. correlation, the tailings exhibited a Kh range of 1 x 10.5 to 7 x 10' cm/sec and an average of approximately 3 z 10-6 cm/sec. For a horizontal to vertical permeability ratio near the ' lower end of the range typical for tailings (2 - 10), this data is consistent with previous tailings vertical permeability test results. ' 2.3 Auger Borings ' During May 1995, a series of three hollow-stem auger borings were drilled in the tailings which will underlie the embankment raise. The borings were conducted by Layne Environmental Services, Inc. of Denver, Colorado using a CME 75 drill rig. The borings were conducted pn three of the four ' earthen access tamps constructed on the beach tailings, as shown on Figure 1. The borings were u y 2-5 rorect o.