2017-02-02_REVISION - M2003091 - Laserfiche WebLink

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MMM tries to segregate these materials in separate stockpiles. These stockpiles are designated"black" and"orange". Overburden stockpiles currently lie on the southeast perimeter and the bottom of the mined out Phase I and are currently being stockpiled on the south side of the Phase II mine cell. Plans are to strip parts of the Phase I1 cell and place it directly into the compacted slope liner. The mine site will be reclaimed as two ponds. MMM wishes to line one or both of the ponds to limit long term augmentation obligations. Field and Laboratory Investigations Field investigations by others(Lafarge and Olson Associates)were performed in 2002. The 2002 investigations included the drilling of exploratory borings,the installation of monitoring wells,packer permeability testing in the site bedrock, and the excavation of test pits at the site. Available logs are presented in Attachment A. Packer permeability test results are summarized on Table 1. Tetra Tech supplemented the 2002 investigation with a test pit field investigation in May 2013 and also collected samples of the stockpiled soils in January 2014. Approximate locations and logs of the Tetra Tech test pits are included in Attachment B. Geotechnical laboratory results are summarized in Table 2. Laboratory data are included in Attachment C. Review of the subsurface data and our knowledge of the site indicate the materials on-site can be broadly divided into three(3)units. The three(3)units are listed as follows:(1)overburden,(2)sand and gravel, and(3)bedrock. Where unmined,the overburden overlies the sand and gravel which in turn overlies the bedrock. These three units are more particularly described as follows: Unit 1 Overburden—As mentioned above,two types of overburden are broadly defined at the site. These are separated by their"black"and"orange"colors which are broadly stockpiled accordingly. These"black"and"orange"stockpiles are shown on the plans. The"black"and "orange"materials are locally intermixed with each other and are also locally intermixed with the underlying sand and gravel. After review of the available data,it appears that nearly all of the "black"overburden can be used as either Zone 1 a or Zone 1 b fill. Most of the"orange" overburden can also be used as both la and/or lb fill. However, some of the"orange"material will only meet the specifications for lb or miscellaneous fill because it contains too much sand. In some areas,the"orange"material appears to be"dirty pit run"in which some of the overburden appears to be mixed with underlying sand and gravel. The"black"overburden tends to range from a moderately plastic to very plastic, locally organic clay with sand to sandy clay. One stockpile sample classified as clayey sand. This unit tends to be medium stiff to stiff but is locally very stiff. Laboratory testing resulted in percent passing the 200 sieve ranging from 47 to 92. On the same samples Liquid Limits and Plasticity Indices ranged from 33 to 51 and 17 to 34 respectively. Moisture contents were typically 17 to 22 percent. Maximum Standard Proctor Density and optimum moisture contents on one tested sample were 111.0 pounds per cubic foot(pcf)and 16.0 percent respectively. The"orange"overburden tends to range from a low to moderately plastic, sandy clay to clayey sand. This unit tends to be medium stiff to stiff but is locally very stiff. Laboratory testing resulted in percent passing the 200 sieve ranging from 21 to 57. On the same samples Liquid Limits and Plasticity Indices ranged from 22 to 37 and 9 to 18 respectively. Moisture contents were typically 11 to 19 percent. Maximum Standard Proctor Density and optimum moisture Page 2 of 5