REP10290 - Laserfiche WebLink

Home Browse Search

III IIIIIIIIIIIIIIII STATE OF COLG~.~ ,~~ MINED LAND RECLAMATION DIVISION Department of Natural Resources 1313 Sherman St., Room 215 Denver, CO 80203 303 866-3567 FAX 303 832-8106 DATE: February 4, 1992 T0: Shawn Smith FROM: Jim Pendleton pF COO F _' 9 .~ ;N?F&V~,> tiT (-'~~p .iC7. (?off Ly. Roy Romer. Governor Fretl R. Banta. Division Dncctor RE: "Southf+; ld Mine (5~8'f-014) Refuse Pile Stability Analysis" I have reviewed the above referenced report, prepared by Mr. Jerry Nettleton of ACZ, Inc., geotechnical consultant to Energy Fuels Corporation's Southfield Mine. This report presents the results of an analysis required as abatement for a violation issued by the Division. The report briefly summarizes the design assumptions and data sources from two earlier stability evaluations performed for this pile in 1981 and 1982. The report also documents that the pile is not being constructed as earlier analyzed and approved. The report states as follows: (1) The pile's sideslopes are 3:1 (h:v) or flatter, while 2:1 (h:v) was assumed in the earlier analyses. (2) Refuse 15 being placed in lifts (one layer of fine refuse to one or more layers of coarse refuse), rather than as a 3:1 coarse to fine mixture. (3) The proportion of the refuse material mixture varies with changes in geologic and mining conditions. (4) Strength testing performed in 1988 indicated a variation in fine material strength from 33 degrees at 45 pcf to 38 degrees at 64 pcf. Jerry Nettleton candidly observes that while 33 degrees may seem high for a "fine" material, the term "fine" was applied by Southfield to all refuse material passing the 28 mesh sieve, the large end of which is a medium grained sand.Further, he hypothesizes that the roof and floor of the Jack O'Lantern seam are primarily sandstones and shales. The refuse materials are also low in clay content and appear to perform granularly. My personal observations at the refuse pile during our recent field trip concur with Mr. Nettleton's hypothesis. ACZ, Inc. performed stability analyses on two "critical" cross sections, chosen to have the steepest facial angles with highest vertical relief. Using the XSTABL computerized slope stability model, the pile configuration was subjected to both modified Bishop's (rotational mechanism) and the Janbu (block gliding mechanism) failure analyses. In both cases the result determined minimum static slope safety factors for rotation of 2.7 and for block gliding of 2.5. ACZ, Inc. did not specifically analyze this model for near surface slope failure potential. However, their infinite slope failure model projected a static slope safety factor of 2.0 for 3:1 (h:v) slopes in a material with an internal angle of friction of 33 degrees.