Ariz. Admin. Code § R18-13-1003.01 - Additional Arizona Design Criteria for New CCR Surface Impoundments and Lateral Expansions of CCR Surface Impoundments

A. The requirements in this Section are additional to those in 40 CFR 257, subpart D, as incorporated in this Article, and do not replace any requirement of 40 CFR 257, subpart D, as incorporated herein.

B. Geotechnical Requirements. The owner or operator shall provide an evaluation of the static stability of the foundation, CCR surface impoundment, and slopes of the reservoir rim.

C. CCR surface impoundment Embankment Requirements.

1. Geotechnical Requirements. Table 1 states additional minimum factors of safety for embankment stability under various loading conditions not covered by 40 CFR 257.74(e).

a. The analysis of minimum factors of safety shall include the effects of anisotropy on the phreatic surface position by using a ratio of horizontal permeability to vertical permeability of at least 10. The Director may require ratios of up to 100 if the material types and construction techniques will cause excessive stratification.

b. The owner or operator shall use tests modeling the conditions being analyzed to determine the strengths used in the stability analysis. The stability analysis shall include total and effective stress strengths appropriate for the different material zones and conditions analyzed. The stability analysis shall use undrained strengths or strength parameters for all saturated materials.

c. If applicable, the owner or operator shall perform an analysis of the upstream slope stability for a partial pool with steady seepage considering the reservoir level that provides the lowest factor of safety.

2. Seismic Requirements

a. The owner or operator shall determine the seismic characteristics of the site as prescribed in R18-13-1002(B) and (C) and R18-13-1010.01(G)(3)(m).

b. The owner or operator shall determine the liquefaction susceptibility of the embankment, foundation, and abutments and may use standard penetration testing, cone penetration testing, shear wave velocity measurements, or a combination of these methods to make this determination. The owner or operator shall compute the minimum factor of safety against liquefaction at specific points and make a determination of whether the overall site is subject to liquefaction.

c. The owner or operator shall compute a minimum factor of safety against overtopping due to deformation and settlement in each of the following cases. The minimum factor of safety against overtopping can be no less than 2.5, determined by dividing the total pre-earthquake freeboard by the estimated vertical settlement in feet. The owner or operator shall determine the total vertical settlement by adding the settlement values of the upstream and downstream slopes.

i. An embankment, foundation, or abutment is not subject to liquefaction, has a maximum peak acceleration of more than 0.2g or a maximum peak acceleration of more than 0.35g and consists of clay on a clay or bedrock foundation; or

ii. The embankment, foundation or abutment is subject to liquefaction.

d. The owner or operator shall perform a liquefaction analysis to establish approximate boundaries of liquefiable zones and physical characteristics of the soil following liquefaction for an embankment, foundation, or abutment subject to liquefaction. The owner or operator shall perform an analysis of the potential for flow liquefaction.

e. Other analytical procedures may be required by the Director for sites with high seismicity or low strength embankment or foundation soils.

3. Miscellaneous Design Requirements

a. The design of any significant or high hazard potential CCR surface impoundment shall provide seepage collection and prevent internal erosion or piping due to embankment cracking or other causes.

b. The Director shall review the filter and permeability design for a chimney drain, drain blanket, toe drain, or outlet conduit filter diaphragms on the basis of unique site characteristics.

i. The minimum thickness of an internal drain is 3 feet.

ii. The minimum width of a chimney drain is 6 feet.

iii. The owner or operator shall filter match an internal drain to its adjacent material.

iv. The owner or operator shall design internal drains with sufficient capacity for the expected drainage without the use of drainpipes using only natural granular materials.

c. The use of a geosynthetic is not permitted in a design if it serves as the sole defense against CCR surface impoundment embankment failure. The use of geotextiles and geonets as a filter or drain material or a geomembrane liner is permitted only in a location that is easily accessible for repair or if its excavation cannot create an unsafe condition at the CCR surface impoundment. The Director may impose permit conditions, including monitoring appropriate to the hazard classification, inspection, and necessary repairs.

d. The owner or operator shall use armoring on any upstream slope of a CCR surface impoundment embankment. If the owner or operator uses rock riprap for armoring, it shall be well-graded, durable, sized to withstand wave action, and placed on a well-graded pervious sand and gravel bedding or geotextile with filtering capacity appropriate for the site.

e. The minimum width of the top of a CCR surface impoundment embankment is equal to the structural height of the CCR surface impoundment divided by 5 plus an additional 5 feet. The required minimum width for any CCR surface impoundment embankment is 12 feet. The maximum width for any CCR surface impoundment embankment is 25 feet.

Table 1. Minimum Factors of Safety for Stability

(Not applicable to an embankment on a clay shale foundation)

Embankment Loading Condition	Minimum Factor of Safety
End of construction case for embankments greater than 50 feet in height on weak foundations	1.4
Steady state seepage - upstream (critical partial pool)	1.5
Instantaneous drawdown - upstream slope	1.2

D. The requirements in this Section are based on Arizona dam safety standards additional to those in 40 CFR 257, subpart D, as incorporated in this Article, and do not apply to:

1. CCR surface impoundments with a maximum height of less than 6 feet, regardless of storage capacity;

2. CCR surface impoundments with a maximum height of between 6 and 25 feet and a storage capacity of less than 50 acre-feet; or

3. CCR surface impoundments with a maximum height greater than 25 feet and a storage capacity of 15 acre-feet or less.

Notes

Ariz. Admin. Code § R18-13-1003.01

New section made by final rulemaking at 31 A.A.R. 1363, effective 6/1/2025.