2.2.3 Settlement Analyses

If the foundations beneath an embankment consist of layers of compressible soils or soft rock or if the bedrock profile is very irregular, differential settlements could result from uneven loading or variable thicknesses in the compressible site conditions. These differential settlements may cause longitudinal or transverse cracks in the dam that could lead to subsurface erosion and dam failure by piping.

The magnitude of the anticipated settlement can be estimated from the results of laboratory consolidation tests on samples recovered from the compressible foundation strata and remolded embankment materials. The rate of settlement can also be estimated. However, the potential error in estimating the time for settlement to occur is appreciable, since settlement is influenced by soil drainage that is controlled by minute geological details that may not be detected during the foundation investigation. All predictions on the rate and magnitude of settlement and the change in pore water pressures need to be checked by field instrumentation. Predictions based on laboratory data can be modified by actual measurements to provide reasonably accurate longterm estimates.

If compressible soils are thick, it may be necessary to design the dam to absorb the anticipated differential settlements. If considerable total settlement is expected, the dam must be built higher to allow for the settlement.

2.2.4 Seepage Analyses

Seepage analyses evaluate the effects of seepage on the stability of the tailing dams and the rate of seepage through and beneath the dam and basin area. It is important that seepage pressures be controlled so that quick conditions and piping do not develop. Special design features such as impervious cores, cutoffs, impervious liners, a secondary collection system, etc., are needed to maintain the quality and quantity of seepage from the retention system within tolerable limits of water supply and pollution control requirements.

Seepage analyses-usually based on the steady flow of an incompressible fluid through a porous media-may use the graphical method of plotting flow nets, electric analogs, model studies, or mathematical solutions by digital computer using either finite-element or finite-difference methods. The graphical method of plotting flow nets is economically and easily performed, and it gives sufficiently accurate results for many seepage problems.

3. CONSTRUCTION METHODS

Construction methods for mill tailing dams are closely related to the planning and operation of the

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mill. Where a tailing embankment is constructed in a single stage of natural borrow materials or overburden and waste rock, conventional procedures for earth and rock-fill dams can be used.

Where a tailing dam is constructed in stages, one of the following three methods has been used: (a) upstream method, (b) downstream method, or (c) centerline method.

The upstream constructioethod is the oldest used by the mining indust. and is a naturally developed procedure for disposing of the tailing as economically as possible. An initial starter dike is constructed at the downstream toe of the ultimate dam with borrow materials. The crest of the dam is raised by placing fill materials in successive dikes located on the upstream side of the initial starter dike. The centerline of the embankment crest is shifted toward the upstream pond area as the height of the dam increases. The downstream toe of each subsequent dike is supported on the top of the previous dike, with the upstream portion of the dike placed over finer tailings (slimes) within the impoundment. These slimes, placed hydraulically, have a relatively low shear strength and remain in a loose and saturated state for many years after deposition (Ref. 24). As the height of the dam increases, the potential failure is located at an increasingly greater distance from the downstream face and through the slimes. As a result, the outside shell contributes less to stability as the height increases. The retained slimes are sufficiently loose and saturated that they could be liquefied to cause the failure of the dam if subjected to seismic shock or blasting.

With the downstream construction method, an initial starter dike is constructed at the upstream toe of the ultimate dam. The crest of the dam is raised by placing fill materials in successive dikes located on the downstream side of the starter dike. The centerline of the dam crest is shifted downstream as the dam is raised. Each subsequent stage of dike construction is supported on the top of the downstream slope of the previous section. All of the embankment section lies outside the boundaries of the sediment tailings. Materials incorporated in subsequent stages of the embankments may consist of the coarse mine waste or borrow materials from nearby pits. Downstream construction permits controlled placement and compaction to achieve high shear strength. It also permits the incorporation of drainage facilities to control the piezometric pressures within the embankment. Thus the dam can be designed and subsequently constructed to whatever degree of competency may be required, including resistance to seismic and blasting shocks.

The centerline method is intermediate between the previous two construction methods. The crest of the embankment is maintained in approximately the

same horizontal position as the embankment is raised to its final height. The dam is raised by spreading and compacting successive layers of materials on the crest, on the upstream shoulder, and on the downstream slope. The centerline method permits the downstream half of the tailing dam to be designed and constructed to conventionally acceptable engineering standards; however, certain portions of upstream slopes rest over the slimes and are therefore vulnerable to slope failure and seismic liquefaction.

These three construction methods lead to substantially different embankment cross-sections and produce different embankment material characteristics. Consequently, the embankment stability conditions are affected. In the upstream and centerline methods of construction, the stability of the ultimate dam is dependent, to a large degree, on the shear strength characteristics of tailings deposited upstream of the dam. The shear strength is governed by the gradation and density of the solids, the consistency of the slurry, and the distribution of the pore water pressures within the deposit. When initially deposited, the tailings have very low shear strength. The strength theoretically increases with time as drainage and consolidation take place under the weight of overlying materials. However, because of the very fine gradation of the tailings and the random nature of deposition, large variations in permeability and pore water pressure exist within the tailings, and the strength may not increase adequately to ensure the stability of the final slope (Ref. 25).

Downstream construction is the only method wherein all embankment sections lie outside the tailing boundaries, thereby permitting controlled placement and compaction of fill and incorporation of drainage facilities. Thus, for a given height and a given downstream fill slope, a tailing dam constructed using the downstream method will have a higher factor of safety than a tailing dam constructed by either the upstream method or the centerline method.

Based on the fact that the most important purpose of the tailing dam structure is to contain the radioactive waste materials and on the unsatisfactory performances of the hydraulically constructed dams and tailing dams (Refs. 6, 8, and 26), the downstream method appears to be the best of the stage construction methods to ensure the safety function of the tailing dams, especially in seismically active areas.

4. INSPECTION AND MAINTENANCE

Different conditions can develop throughout the whole active life of the retention system and could include unanticipated seepage conditions and changes in material characteristics. Such changes can drastically change the conditions governing the

stability of a dam from those provided for in the original design. Therefore, a continuous program of inspection of the retention system is needed, begin ning with the start of construction, through the tailing disposal, and continuing after abandonment of the completed system.

The main objectives of such a program are to ascertain:

(a) Whether the dam and its foundation are behaving as anticipated in the design, whether there are any unusual movements, settlements, cracks, erosions, sloughs, or leakages, and whether the waste and borrow materials being placed in the dam have the characteristics assumed in the design;

(b) Whether the tailing pond levels are rising as anticipated and whether the rate of dam construction is sufficiently rapid to keep the crest above the rising pond; and

(c) Whether embankment drainage is adequate, whether the capacity of diversion channels is adequate to pass experienced and anticipated runoffs. whether embankment soil is becoming saturated by seepage, whether piping or subsurface erosion is occurring in the tailing dam, and whether there is any unusual release of radioactive materials.

It is necessary that inspection be performed on a regular basis and that it include visual inspection of the abutments. A checklist similar to that used in water retention dams may be used to help the inspector in performing such a visual inspection.

Instrumentation needs to be installed to monitor dam and basin performances at regularly scheduled intervals. Instruments commonly used include piezometers to measure hydrostatic and pore pressure levels; weirs or flumes to measure seepage flows; wells to permit monitoring of water quality; and slope indicators, inclinometers, and settlement points to measure horizontal and vertical movements. The instrumentation should be simple, robust, rugged, reliable, and easy to read, repair, and maintain. It is important that recorded data from instrumentation and inspections be evaluated by competent personnel with delegated authority to take prompt action if remedial treatment is needed to maintain the safe I operation of the retention system.

C. REGULATORY POSITION

The following criteria reflect the latest general approaches approved by NRC. Information related to the investigation, engineering design, proposed conThe Nuclear Regulatory Commission announced in the Federal Register of June 3, 1976. (4) FR 22431) its intent to prepare a generic environmental impact statement (GEIS) on uranium milling operations. Management practices for uranium miil tailings may be subject to revision in accordance with the conclusions of that statement and any related rule making.

struction, instrumentation, and performance of the retention system should be presented in accordance with the applicable portions of Section 2.5.6 of Regulatory Guide 1.70, "Standard Format and Content of Safety Analysis Reports for Nuclear Power Plants." If an applicant wishes to use new information that may be developed in the future or to use an alternative method, NRC will review the proposal and approve its use, if found acceptable.

1. BASIC DESIGN CRITERIA

(a) Stability of the retention system, including the tailing dam, foundation, and abutments, should be ensured under all conditions of construction and operation.

(b) The magnitude of total and differential settlement should be within tolerable limits that will not result in harmful cracking and dam instability.

(c) Seepage through the embankment, foundation, abutments, and basin area should be controlled to prevent excessive uplift pressures, piping, sloughing, and erosion of materials by loss into cracks, joints, and cavities. The quality and quantity of seepage should be limited to the extent that the concentration of radioactive materials and other toxic materials at the site boundary is within the limits specified in applicable federal and state regulations.

(d) Freeboard should be sufficient at all times to prevent overtopping by wind-generated waves and should include an allowance for settlement of the foundation and dam. Adequate slope protection should be provided for the embankment against wind and water erosion, weathering, and ice damage.

(e) Either the surcharge capacity of the retention system should be sufficient to store runoffs over its service life or there should be an emergency discharge capacity capable of passing the probable maximum flood. The emergency discharge capacity may be obtained by constructing a spillway or by other means. The surcharge capacity should be adequate to store a probable maximum flood series preceded or followed by a 100-year flood, assuming a pool elevation equivalent to the average annual runoff.

2. METHODS OF ANALYSIS

(a) The probable maximum flood should be determined in accordance with applicable portions of Regulatory Guide 1.59, "Design Basis Floods for Nuclear Power Plants."

• Probable maximum flood series as defined herein comprises two floods: the Probable Maximum Flood and the flood equivalent to about 40% of the PMF about 3 to 5 days prior to the occurrence of the main flood.

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(b) The static stability of the embankment should be analyzed using commonly accepted detailed stability methods. Appropriate static soil and rock properties established on tested representative samples over anticipated in-situ and placement conditions should be used in the analyses. Results of a manual check on computer stability analysis results should be presented to illustrate adopted design procedures and criteria.

(c) Conventional pseudostatic analysis may be considered acceptable if the seismic coefficient appropriately reflects the geologic and seismologic conditions of the site and if the materials are not subject to significant loss of strength under dynamic loads. Liquefaction potential and the dynamic stability of the tailing dam and foundation should be assessed using appropriate state-of-the-art methods. The extent of the required dynamic analyses will be determined on a case-by-case basis. Appropriate dynamic material properties established on representative materials through adequate field and laboratory testing should be used in the analyses.

(d) The loading conditions to be evaluated in dam stability analyses and corresponding minimum factors of safety are:

(a) Conventional acceptable engineering practices of construction control for water retention dams (e.g., controls on foundation preparation, suitability of materials, proper placement, field moisture, and density) should be used for mill tailing dams. Where a tailing dam is raised in stages, the downstream construction method is preferred. Provision should be made to limit the concentration of radioactive and other toxic materials released from seepage and windwater erosion to within the limits specified in 10 CFR Part 20, 40 CFR Part 190, and applicable state regulations.

**Factor of safety is for pseudostatic stability analysis. In addition, liquefaction and excessive deformation should be assessed. *** Use shear strength for case analyzed without earthquake.

(b) The upstream and centerline construction methods will be acceptable only if extensive explorations and testing reveal the extent and characteristics of deposited tailings to have adequate strength under static and dynamic loading conditions for the stability and support of the added materials.

4. INSPECTION AND MAINTENANCE

(a) A detailed systematic inspection and maintenance program should be established to detect and repair damage that might tend to lessen the integrity of the retention system. Generally, visual inspections performed on a regular basis and supplemented by adequate instrumentation are acceptable. The safety inspection guidelines (Ref. 12) for earth dams set forth by the Corps of Engineers in response to the National Dam Safety Act should be used to develop a detailed checklist for performing field inspections. In addition, radiometric and water quality surveys should be included in the program.

(b) Instrumentation should be installed in the dam or its foundation to monitor changes that might be critical to dam stability or seepage conditions. Generally, instruments should be installed to measure piezometric levels, seepage flows, water quality, and embankment movements. The extent to which such instrumentation should be installed will be evaluated on a case-by-case basis.

(c) Results of inspection and instrumentation programs should be evaluated by competent and experienced engineers who have delegated authority to take prompt effective actions when necessary. Inspection and evaluation reports should be kept at the site and be available for staff review.

(d) The inspection and maintenance program should start at the beginning of construction and continue at least through the operation.

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