0201_Alexander_CDLF_C&DOnTopSlopeStability_DIN27431_20170216

February 16, 2017

Mr. Wayne Sullivan, PLS
Municipal Engineering Services Co. PA
PO Box 97
Garner, NC 27529

RE: Report of Geotechnical Engineering Services
Alexander County Landfill Slope Stability
Taylorsville, North Carolina
ECS Report Number: 09: 27484

Dear Mr. Sullivan:

ECS Southeast, LLP (ECS) has completed the geotechnical slope stability analysis for an
expansion of the existing Alexander County Landfill, located in Taylorsville, North Carolina as
requested by Municipal Engineering Services Company, PA (MESCO). The work was
performed in general accordance with ECS Proposal Number 09:25008-P dated February 8,
2017.

Project Information

The Alexander County Landfill is located at Taylorsville, Alexander County, North Carolina.
Based on the information provided by MESCO, it is our understanding that the existing landfill
has a maximum height of approximately 52 feet and side slope of approximately 3 horizontal to
1 vertical (3:1). The new Alexander County landfill will be constructed on top of a municipal
landfill that was previously closed in 1994. The proposed expansion consists of raising the
landfill vertically with side slope of 3:1 using demolition debris. Preliminary Plans for the
expansion to be approximately 30 feet. However this analysis is extended to determine potential
expansion height. Based on the plans provided by MESCO, the existing grades around the
landfill are between elevation 1000 and 1022 feet and the surface elevations in the landfill area
are between elevation 1022 and 1074 feet. We understand that the purpose of this phase of the
project is evaluating the feasibility of such an expansion based on the parameters used for
slope stability analyses of the existing Alexander County Landfill. It is our understanding that
these analyses will not be used for construction.

ECS has been provided with the results of a slope stability analysis of the land fill that was
performed at the time that the landfill was previously closed (1994). No additional subsurface
information was provided. The Alexander County Landfill lies in the Piedmont Physiographic
Province of North Carolina. In the Piedmont Physiographic Province, soils typically consist of
residual materials that have formed in place from chemical and mechanical weathering of
parent bedrock materials. The residual soils typically consist of clays or clayey silts (CH, CL,
MH, ML) near the surface, where soil weathering is more advanced. The soil profile generally
transitions with depth to silts or clays (ML, CL) or silty/clayey sands (SM, SC) in zones where
soil weathering is less advanced.

Report of Geotechnical Engineering Services
Alexander County Landfill Slope Stability
Taylorsville, North Carolina
ECS Report Number: 09: 27484

2

Engineering Analyses

Global slope stability Analyses were to determine feasibility of the proposed expansion. These
analyses were based on the parameters used in the previous report, provided by MESCO.
Analyses included static and seismic stability of the completed expansion to determine the
maximum height of the expansion. The analyses were performed by engineers specializing in
geotechnical engineering and copies of the slope stability analyses are attached in Appendix B.

The profile used for the analysis assumed that the existing landfill was covered with a protective
soil cover that will be removed between the existing waste and the new waste, to provide
adequate drainage for new landfill. Similar to the previous slope stability analyses, the profile
assumes that the waste is underlain by a 20± feet think subgrade soil.

The waste fill was evaluated for slope stability analysis using a circular potential failure
mechanism. One section was selected for the stability analysis, which is considered
representative of the most unfavorable conditions and is consistent with the section used for
previous analyses. The location of the analyzed section is shown on the Slope Stability Location
Diagram in Appendix A. The slope stability analysis was performed using the proprietary Slide
7.0 computer program. The modeled slope configuration was based on the topographic
information and site grading plan provided to us by MESCO, while the soil engineering
properties used in these analyses were assumed similar to the parameters used in the previous
report. For the pseudostatic analysis of the slope, we used an earthquake ground motion having
a 2-percent probability of exceedance within a 50-year period (2,475 year return period).

Based on our previous experience with the similar projects in the same region, we considered a
Seismic Site Class “D” for seismic stability analysis. According to the ASCE 7-10 Standard, the
seismic acceleration at the bedrock level based on the probabilistic earthquake (2,475 year
return period) for this site is 0.1g. The mean peak ground acceleration for this site is 0.16 g due
to attenuation through the soil to the ground surface. Therefore the seismic coefficient, ks, for
the site is 0.08g, by assuming an acceleration multiplier of 0.5 for slope stability analysis, as
recommended by Duncan and Wright (2005)1.

The Factors of safety were determined for static and seismic loading for various heights of
expansion. Typically a minimum factor of safety of 1.5 is desired for landfill slope stability under
static loading and 1.1 for the seismic loading. Assuming that the expansion is built with slopes
of 3H:1V, the maximum height of the expansion that would provide suitable factors of safety is
60 feet. The resulting factors of safety for the slope of 3H:1V were computed to be 1.52 for
permanent slopes under static loading conditions and 1.19 for seismic conditions.

In conclusion, for the assumed materials properties, the results of the geotechnical analysis
indicate that the 3H:1V slope configuration will have a factor of safety which is acceptable for
the proposed vertical expansion up to 60 feet above the existing landfill. If the expansion moves
forward into design, we recommend additional analyses be performed including collection of
subsurface data for the site.

1 Duncan, J. Michael, and Stephen G. Wright. Soil strength and slope stability. John Wiley & Sons, 2005.

APPENDIX A

FIGURES

SITE
SITE VICINITY MAP
Alexander County Landfill
Taylorsville, North Carolina
SOURCE: Google Maps
FIGURE NO.
1
SCALE
NTS
DATE
2/16/2017
DRAWN BY
SM04
CHECKED BY
MBO
PROJECT NO
09:27484
N
FIGURE NO.
2
SCALE
NTS
DATE
2/16/2017
DRAWN BY
SM04
CHECKED BY
MBO
PROJECT NO
09:27484Slope Stability Location Design
Alexander County Landfill
Taylorsville, North Carolina
N
SOURCE: Stability Analyses report , date 11/3/1994 ; Provided by Municipal Engineering
Services Co. PA
A
A

APPENDIX B

SLOPE STABILITY ANALYSES RESULTS

Material Property Legend

Soil
Layer/Material
Unit Weight
(pcf)
Friction
Angle (deg)
Cohesion
(psf)
New Waste 40 25 0
Existing Waste 40 25 0
Protective Soil 125 25 250
Subgrade 120 25 0

Project Summary
Static‐60ftFile Name:
7.011Slide Modeler Version:
Alexander County Landfill ExpansionProject Title:
Static‐ 60 ft ExpansionAnalysis:
SM04Author:
ECS SoutheastCompany:
2/9/2017, 8:55:07 AMDate Created:

Analysis Options
VerticalSlices Type:

Analysis Methods Used
Bishop simplified

50Number of slices:
0.005Tolerance:
75Maximum number of iterations:
YesCheck malpha < 0.2:
YesCreate Interslice boundaries at intersections 
with water tables and piezos:
1Initial trial value of FS:
YesSteffensen Iteration:

Surface Options
CircularSurface Type:
Grid SearchSearch Method:
10Radius Increment:
DisabledComposite Surfaces:
Create Tension CrackReverse Curvature:
Not DefinedMinimum Elevation:
Not DefinedMinimum Depth:
Not DefinedMinimum Area:
Not DefinedMinimum Weight:

Material Properties
New WasteCover SoilExisting WasteSubgradeProperty
Color
Mohr‐CoulombMohr‐CoulombMohr‐CoulombMohr‐CoulombStrength Type
4012540120Unit Weight [lbs/ft3]
025000Cohesion [psf]
25252525Friction Angle [deg]
NoneNoneNoneNoneWater Surface
0000Ru Value

Alexander County Landfill Expansion: Page 1 of 2
SLIDEINTERPRET 7.011
Static-60ft.slim ECS Southeast 2/9/2017, 8:55:07 AM
Global Minimums
Method: bishop simplified
1.519990FS
30.232, 670.760Center:
629.874Radius:
57.993, 41.498Left Slip Surface Endpoint:
381.208, 147.733Right Slip Surface Endpoint:
8.94963e+007 lb‐ftResisting Moment:
5.88794e+007 lb‐ftDriving Moment:
5565.85 ft2Total Slice Area:
323.214 ftSurface Horizontal Width:
17.2203 ftSurface Average Height:

Alexander County Landfill Expansion: Page 2 of 2
SLIDEINTERPRET 7.011
Static-60ft.slim ECS Southeast 2/9/2017, 8:55:07 AM
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Project Summary
Seismic‐60ftFile Name:
7.011Slide Modeler Version:
Alexander County Landfill ExpansionProject Title:
Seismic‐ 60 ft ExpansionAnalysis:
SM04Author:
ECS SoutheastCompany:
2/9/2017, 8:55:07 AMDate Created:

Analysis Options
VerticalSlices Type:

Analysis Methods Used
Bishop simplified

50Number of slices:
0.005Tolerance:
75Maximum number of iterations:
YesCheck malpha < 0.2:
YesCreate Interslice boundaries at intersections 
with water tables and piezos:
1Initial trial value of FS:
YesSteffensen Iteration:

Surface Options
CircularSurface Type:
Grid SearchSearch Method:
10Radius Increment:
DisabledComposite Surfaces:
Create Tension CrackReverse Curvature:
Not DefinedMinimum Elevation:
Not DefinedMinimum Depth:
Not DefinedMinimum Area:
Not DefinedMinimum Weight:

Material Properties
New WasteCover SoilExisting WasteSubgradeProperty
Color
Mohr‐CoulombMohr‐CoulombMohr‐CoulombMohr‐CoulombStrength Type
4012540120Unit Weight [lbs/ft3]
025000Cohesion [psf]
25252525Friction Angle [deg]
NoneNoneNoneNoneWater Surface
0000Ru Value

Alexander County Landfill Expansion: Page 1 of 2
SLIDEINTERPRET 7.011
Seismic-60ft.slim ECS Southeast 2/9/2017, 8:55:07 AM
Global Minimums
Method: bishop simplified
1.194470FS
30.232, 670.760Center:
629.874Radius:
57.993, 41.498Left Slip Surface Endpoint:
381.208, 147.733Right Slip Surface Endpoint:
8.73622e+007 lb‐ftResisting Moment:
7.3139e+007 lb‐ftDriving Moment:
5565.85 ft2Total Slice Area:
323.214 ftSurface Horizontal Width:
17.2203 ftSurface Average Height:

Alexander County Landfill Expansion: Page 2 of 2
SLIDEINTERPRET 7.011
Seismic-60ft.slim ECS Southeast 2/9/2017, 8:55:07 AM
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