2006-06-07_PERMIT FILE - C1996083

• Permit No: C1996083
• IBM Index Class Name: Permit File
• Doc Date: 6/7/2006
• Doc Name: 02/15/2006 Gob Pile #2 Stability Evaluation TR-42 Buckhorn Geotech Inc
• Section_Exhibit Name: Volume IX Coal Waste Disposal Area No. 2
• Media Type: D
• Archive: No

➢ The slope above the first 50 vertical feet to the ultimate height (approximately 210 feet)
of the stockpile was varied. Slopes of 5H:1 V (existing configuration), 4H:1 V, and
3H:1 V were evaluated.
Shear strength parameters for the coal waste material were conservatively estimated based on
triaxial shear data (presented in Section 4.2) and are summarized in Table 7. Shear strength
parameters for the foundation material were assumed as used by Lambert and Associates (2003).
Although no shear testing was conducted on the foundation materials, the values used by
Lambert and Associates are considered to be reasonable and conservative. The moist unit weight
of the coal waste used in the analyses was 104 pcf. This is an average moist unit weight from
density testing and proctor compaction tests performed to date. The simplified Bishop and
Spencer procedures were used in the analyses. Seismic loading conditions were not evaluated.
Table 7. Strength Parameters Used in the Stability Evaluations
Notes:
1. d = depth in feet
2. Failure selected at maximum pore pressure from CU triaxial tests
3. N/A = Not Applicable
Selection of the factor of safety for stability analyses depends primarily on the uncertainty of the
analysis conditions and the consequence of failure. The greater the uncertainty or consequence
of failure, the larger the required factor of safety. For these analyses, a minimum factor of safety
of 1.50 is desired for long -term stability and a factor of safety of 1.30 is desired for the short-
term stability (i.e., end of placement or dissipation of excess pore pressures). Results from the
stability evaluations are discussed herein. Output from critical stability analyses is presented in
Appendix G.
6.1 Short -Term Analyses
Short -term stability analyses are intended for projects when excess pore pressures are anticipated
to dissipate slowly relative to the rate of material placement. At Bowie, excess pore pressures
are expected to dissipate slowly for the fine- grained materials in the gob stockpile. As pore
pressures dissipate, the factor of safety with respect to stability will increase to the long -term
factor of safety. Short-term stability analyses were conducted using total stress analyses using
undrained shear strengths (Su) from the UU and CU triaxial shear tests (Table 7). Total stress
analyses were conducted assuming the undrained shear strength to be constant with depth and
assuming the undrained shear strength to be a function of the vertical effective stress (i.e., Su
Bowie No 2 Gob Pile Stability Dec2005
Project #05- 126 -GEO
Page 14 of 18
Undrained
Shear
Strength, Su
Cohesion, c'
Analysis
(psf)
(deg.)
(psfl
Short Term Su constant with
1,650
N/A
N/A
depth
Short Term Su increases with
Su = 53d'
N/A
N/A
depth
Lon Term Effective Stress
Long Drained Condition sZ
N/A
28
100
Notes:
1. d = depth in feet
2. Failure selected at maximum pore pressure from CU triaxial tests
3. N/A = Not Applicable
Selection of the factor of safety for stability analyses depends primarily on the uncertainty of the
analysis conditions and the consequence of failure. The greater the uncertainty or consequence
of failure, the larger the required factor of safety. For these analyses, a minimum factor of safety
of 1.50 is desired for long -term stability and a factor of safety of 1.30 is desired for the short-
term stability (i.e., end of placement or dissipation of excess pore pressures). Results from the
stability evaluations are discussed herein. Output from critical stability analyses is presented in
Appendix G.
6.1 Short -Term Analyses
Short -term stability analyses are intended for projects when excess pore pressures are anticipated
to dissipate slowly relative to the rate of material placement. At Bowie, excess pore pressures
are expected to dissipate slowly for the fine- grained materials in the gob stockpile. As pore
pressures dissipate, the factor of safety with respect to stability will increase to the long -term
factor of safety. Short-term stability analyses were conducted using total stress analyses using
undrained shear strengths (Su) from the UU and CU triaxial shear tests (Table 7). Total stress
analyses were conducted assuming the undrained shear strength to be constant with depth and
assuming the undrained shear strength to be a function of the vertical effective stress (i.e., Su
Bowie No 2 Gob Pile Stability Dec2005
Project #05- 126 -GEO
Page 14 of 18