Laserfiche WebLink
The following gradation, consistent with Colorado DOT Class C Filter Material, meets these criteria. <br />Table 11 Acceptable Filter Material Gradation <br />U.S. Standard <br />Sieve Size Percent Passing (Dry <br />Wei ht <br />3/4" 1 00 <br />#4 60-100 <br />#50 10-30 <br />#100 0-10 <br />#200 0-3 <br />Even using the high-plasticity clay sample from Trench 9 with a percent passing the #200 sieve of <br />99%, the required gradation range for the filter partially overlaps the gradation range of the material <br />described in Table 11. For this base material, the maximum D15 size is 0.2 mm because it falls in <br />Category 1. As mentioned above, the maximum D15 size for Category 2 materials is 0.7 mm. <br />Using Hazen's (1930) approximation for permeability of filter material, the minimum computed <br />permeability for the above filter material is 1.0x10"Z cm/s based on a D10 of 0.1 mm. This value is <br />higher than the values used in the modeling for the downstream drainage blanket and the trench <br />drain, which were 1.Ox10~ and 1.22x10-5 cm/s, respectively. Thus, the analysis is considered <br />conservative, and actual porewater pressures will likely be lower than those predicted from modeling. <br />8.6 Upstream Clay Blanket <br />A clay blanket will be placed on the upstream face of the dam to provide a barrier to seepage coming <br />from the filled reservoir. <br />The blanket will comprise colluvium from the borrow area, and quality control standards will be <br />imposed to ensure that sandstone cobbles and boulders are not present. Eliminating or selecting large <br />cobbles and boulders from being present in the clay blanket will allow for enhanced workability, <br />higher compaction densities, and the sandstone will not be pulverized into sand that might increase <br />the permeability of the clay blanket. <br />The permeability used in the modeling was 1.48x10"7 cm/s. This is the same permeability as that used <br />for the embanlanent material and the buttress. The high quality control standards imposed on the <br />blanket will probably decrease the permeability from the value above. In order to be somewhat <br />conservative in the design, however, the same permeability as the other two soil units was used <br />during modeling. <br />P:\Mpls\06 CO\26\0626067\WorkFiles\DesignReport\FINAL\DesignReportFINAL.doc 36 <br />