2006-10-26_PERMIT FILE - C1996083

• Permit No: C1996083
• IBM Index Class Name: Permit File
• Doc Date: 10/26/2006
• Doc Name: 10/26/2006 Stability Evaluation of proposed South Coal Wastepile Buckhorn Geotech
• Section_Exhibit Name: Volume XI Coal Mine Waste Disposal Area No. 3
• Media Type: D
• Archive: No

foot wide intermediate bench. The maximum height evaluated for the gob pile was 90 feet.
Table 1 summarizes the cross - sections evaluated with their respective total heights.
Table 1. Cross - Sections Evaluated
6.2 Shear Strength Parameters
Shear strength parameters for the coal waste material were estimated based on unconsolidated
undrained (UU) and consolidated undrained (CU) triaxial shear testing previously performed on
"undisturbed" and remolded samples collected from the existing gob pile. Detailed discussion of
the testing on coal waste is presented by Buckhorn (2006). Shear strength parameters for the
foundation material were estimated based on subsurface conditions and triaxial shear testing
conducted by Lambert and Associates (2001) for the railway south of the proposed new gob pile
site. Shear strength parameters used in the stability analyses are presented in Table 2. The shear
strength parameters were varied to assess their sensitivity to the factor of safety. A site specific
investigation is proposed to verify the foundation conditions and material strength parameters
used in the stability model. Additionally, on -going shear strength testing of the coal waste is
recommended.
Total and effective strength analyses were conducted. Total stress parameters for the coal waste
were represented by undrained shear strengths determined from UU triaxial testing. The
undrained shear strength for the coal waste was assumed to (i) be constant with depth, and (ii) to
increase linearly with depth. An average undrained shear strength of 1,650 psf was determined
from four UU triaxial tests on undisturbed and remolded samples. Due to this small database of
undrained strengths and the uncertainty of these values, a reduced undrained shear strength of
1,200 psf was also used in the analyses. This value represents a reduction of 25% from the
average tested undrained strength. Effective stress parameters for the coal waste were
represented by Mohr- Coulomb failure criteria (�' and c') based on CU triaxial shear testing. The
moist unit weight of the coal waste used in the analyses was 104 pcf. This is an average moist
unit weight obtained from density testing of compacted material at the existing gob pile and
proctor compaction tests.
Total stress parameters for the foundation soils were estimated based on UU triaxial shear tests
conducted on foundation soils obtained from borings along the railway south of the proposed
new gob pile site by Lambert and Associates (2001), and by our professional engineering
judgment. These total stress parameters are represented by Mohr- Coulomb failure criteria (i.e.,
and c) on samples that are assumed to be partially saturated. Testing of foundation soils by
Lambert and Associates (200 1) indicates the foundation soils to consist of 40 to 60% fines (i.e.,
silt and clay) with variable sand and gravel content. Description of the soils also indicates that
Bowie No 2 South Gob Pile preliminary stability evaluation
Project #05- 126- GEO -08
Page 6 of 10
Total Height
Cross - Section
Location
Evaluated
NW -SE trending section at the maximum section
70
A -A'
of the gob pile.
90
B -B'
NE -SW trending transverse section
42
62
C -C'
NW -SE trending section south of A -A'
68
88
6.2 Shear Strength Parameters
Shear strength parameters for the coal waste material were estimated based on unconsolidated
undrained (UU) and consolidated undrained (CU) triaxial shear testing previously performed on
"undisturbed" and remolded samples collected from the existing gob pile. Detailed discussion of
the testing on coal waste is presented by Buckhorn (2006). Shear strength parameters for the
foundation material were estimated based on subsurface conditions and triaxial shear testing
conducted by Lambert and Associates (2001) for the railway south of the proposed new gob pile
site. Shear strength parameters used in the stability analyses are presented in Table 2. The shear
strength parameters were varied to assess their sensitivity to the factor of safety. A site specific
investigation is proposed to verify the foundation conditions and material strength parameters
used in the stability model. Additionally, on -going shear strength testing of the coal waste is
recommended.
Total and effective strength analyses were conducted. Total stress parameters for the coal waste
were represented by undrained shear strengths determined from UU triaxial testing. The
undrained shear strength for the coal waste was assumed to (i) be constant with depth, and (ii) to
increase linearly with depth. An average undrained shear strength of 1,650 psf was determined
from four UU triaxial tests on undisturbed and remolded samples. Due to this small database of
undrained strengths and the uncertainty of these values, a reduced undrained shear strength of
1,200 psf was also used in the analyses. This value represents a reduction of 25% from the
average tested undrained strength. Effective stress parameters for the coal waste were
represented by Mohr- Coulomb failure criteria (�' and c') based on CU triaxial shear testing. The
moist unit weight of the coal waste used in the analyses was 104 pcf. This is an average moist
unit weight obtained from density testing of compacted material at the existing gob pile and
proctor compaction tests.
Total stress parameters for the foundation soils were estimated based on UU triaxial shear tests
conducted on foundation soils obtained from borings along the railway south of the proposed
new gob pile site by Lambert and Associates (2001), and by our professional engineering
judgment. These total stress parameters are represented by Mohr- Coulomb failure criteria (i.e.,
and c) on samples that are assumed to be partially saturated. Testing of foundation soils by
Lambert and Associates (200 1) indicates the foundation soils to consist of 40 to 60% fines (i.e.,
silt and clay) with variable sand and gravel content. Description of the soils also indicates that
Bowie No 2 South Gob Pile preliminary stability evaluation
Project #05- 126- GEO -08
Page 6 of 10