EPA Comments on Picacho Final Closure/Post-Closure Maintenance Plan dated 10/27/10

2/10/2011

Page 1 of 3

The following comment reflect US EPA Region 9’s comments on the revised Picacho Final Closure/PostClosure Maintenance Plan dated 10/27/10. These comments can be incorporated into the Closure Plan 

or included as an addendum to the Closure Plan. 

1. Overall cover thickness of 3-ft vs. 4.5-ft proposed in the Closure Plan: EPA recommends an overall 

cover thickness of 3.0-ft vs. the 4.5-ft cover thickness proposed in the Closure Plan for the following 

reasons:

a. Modeling described in the Plan shows that a 3-ft cover would perform better than a 4.5-ft 

cover. 

b. Given the extreme arid nature of the site, a thinner cover (3-ft) is adequate to control 

infiltration versus the suggested 4.5-ft thick cover.

c. As discussed below, our analysis shows that a 3.0-ft thick cover is stable on 3:1 side slopes 

while a 4.5-ft thick cover is not.

The suggested final cover profile is as follows:

2. Side Slope Stability: As discussed in the EPA analysis below, the steep 2.25:1 side slopes are not 

stable under static conditions, however, are stable with 3:1 side slopes and a 3.0-ft thick cover. 

Therefore, EPA recommends regrading the side slopes during final closure to a maximum 3:1 slope 

and using a cover thickness of 3-ft. 

EPA analysis shows that under static conditions the existing slopes are stable, using the input 

parameters utilized in the Plan. However, the input parameters used in the Plan were not all 

conservative. The Plan’s write-up for the soils states that the borrow soil is “predominantly 

cohesionless”, yet soil cohesion values were utilized in the Plan’s stability analysis. Cohesion is a 

sensitive input parameter and has a significant impact on calculated factors of safety. 

Cover Soil

From approved borrow source 

Existing Interim Cover (free of waste/debris)

Verify thickness, Scarify and recompact

Waste Material

Surface Treatment–Rock/Soil Admixture

6”, min .

3’-0”, min.
EPA Comments on Picacho Final Closure/Post-Closure Maintenance Plan dated 10/27/10

2/10/2011

Page 2 of 3

Evaluation of the cover side slope stability was performed for veneer stability. Values from the 

Closure Plan for soil weight, friction angle, slope, cover thickness, and cohesion were initially 

utilized. Values for cohesion, cover thickness, and slope were then varied based on more 

conservative possibilities. The following table shows calculated factor of safety results along with 

accompanying input parameters.

With a cover thickness of 3-ft and conservative cohesion value of zero, a factor of safety of 1.7 

was estimated for static conditions. This is greater than the minimum factor of safety of 1.5. 

We therefore recommend that the side slopes be regraded during final closure to a maximum 

slope of 3:1 with a cover thickness of 3-ft.

3. Erosion Layer Admixture and Gradations for 3% slopes and 3:1 slopes: EPA calculated viable 

erosion layer mixtures and gradations for the slopes at the Picacho landfill.

1

 

3% slopes: The calculations for the 3% slopes show that the surface admixture should be composed 

of pea gravel (3/8-in – 1/2-in diameter) mixed with cover soil at the ratio of 25% rock to soil by 

volume with a minimum 6-in erosion layer. 

3:1 slopes: Calculations for the 3:1 slopes show that the surface admixture should be composed of 

gravel/rock (3⁄4-in to 1-in diameter) mixed with cover soil at the ratio of 50% rock to soil by volume

and result in a minimum 6-in erosion layer. Other rock size and percent content ratios can be used

depending on the availability of gravel and its size in the area, however, increasing rock sizes results 

in an increased overall admixture erosion layer thickness. As an example using gravel/rock (3⁄4-in to 

2-in diameter) mixed with cover soil at the ratio of 50% rock to soil by volume and results in a 

minimum 12-in erosion layer. Viable erosion layer options are shown in the table below.

Viable Options Available to Design Erosion Layer Admixture Gradations for 3% slopes and 3:1 

slopes

 

1 Based on the method described in Dwyer et al 2007.

Cover Thickness Slope Friction Angle Cohesion

Soil 

weight

Calculated 

Factor of Safety

4.5 ft 2.25:1 30 125 psf 118 pcf 1.9

4.5 ft 2.25:1 30 0 118 pcf 1.3

3 ft 2.25:1 30 125 psf 118 pcf 2.2

3 ft 2.25:1 30 0 118 pcf 1.3

3 ft 3:1 30 125 psf 118 pcf 2.9

3 ft 3:1 30 0 118 pcf 1.7

Rock/Gravel Size 

(in Inches)

Percent rock to soil 

(by volume)

Minimum Erosion 

Layer Thickness

3% Slopes 3/8-in to 1/2-in 25% Rock 6 Inches

3:1 Slopes 3/4-in to 1-in 50% Rock 6 Inches

3/4-in to 2-in 50% Rock 12 Inches
EPA Comments on Picacho Final Closure/Post-Closure Maintenance Plan dated 10/27/10

2/10/2011

Page 3 of 3

A spreadsheet with the utilized parameters is available and can be used to evaluate different gravel 

size and quantities.

4. Perimeter Berm Lining: EPA continues to recommend lining the perimeter berms discussed in 

Section 3.7.1.3 of the Closure Plan in a way that will minimize soil lose and infiltration. BAS argues 

that the proposed rock mixed into the perimeter berm’s cover soil will mitigate any erosion. We 

disagree. Soil/rock admixtures are appropriate to control sheet flow erosion, however, are not 

appropriate for concentrated flow such as conveying storm flow along berms or in channels. We 

recommend asphaltic lining for the perimeter berms.