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1 <br />' The value proposed by Robertson for the critical Qc, referred to as Qc~ is as follows: <br />Qc, = Qc " (Pa/a„')0.5 = 68 tsf <br />' It should be noted that this value is based on analyses of relatively young, uncemented, well rounded, <br />medium compressibility quartz sands. <br />' S.3 State Based on Vs Criterion <br />Using a recently developed empirical correlation (Robertson et. al., 1992), it was demonstrated that <br />the contractive-dilative boundary can also be expressed in terms of shear wave velocity, V,. The <br />critical shear wave velocity, V„ is defined as follows: <br />' V,, = V, * (Pa/a~)o.25 = 525 ft/s <br />Recent laboratory data using bender elements in triaxial tests (Sasitharan et al 1993) lave confirmed <br />' the empirical Endings of Robertson et al, 1992. <br />' S.4 Discussion of Liquefaction Assessment <br />Since cone bearing is a high strain measurement, it is highly susceptible to factors such as drainage, <br />compressibility, as well as sort layering. The critical Qc approach that is presented was developed for <br />clean sands. The interpretation results presented in Appendix B demonstrate that most of the tailings <br />are silty sands and sandy silts so the critical Qc approach must be viewed with some caution. High <br />compressibility, significant fines as well as other factors will have a tendency to make the critical Qc <br />approach overly conservative. <br />' As penetration becomes undrained, the resistance to penetration (cone bearing) decreases rapidly. <br />Undrained penetration can be recognized by excess positive or negative dynamic pore pressures <br />(non-hydrostatic). Clean sands generally do not generate excess pore water pressures during <br />' penetration. SoII mineralogy and grain angularity can also affect the penetration resistance of sands <br />and silty sands. Highly angular grains and the presence of platy minerals such as mica increase the <br />compressbility of the soil skeleton Hence for a given initial void ratio a high compressibility sand will <br />' have a much lower resistance to penetation than a medium or low compressibility sand. The relative <br />density values that were generated in the interpreted output of Appendix B assume an average or <br />medium compressibility sand. <br />' Soil layering can also cause an underestimate of the in situ density. When the conk is advanced <br />through a thin high density layer which is underlain by a weaker layer the dense soil cannot fully <br />' develop its penetration resistance. The weaker loose soil layer underlying the dense layer begins to <br />deform and ultimately fails before the dense layer does. The result is that the tip response becomes <br />subdued. <br />' Ca+el~c, wc. <br />om„Q, corm <br /> <br />