62-672.300. Construction of New Dams

(1) Design.

(a) Site investigation – The general area desired for use as a settling area shall be carefully inspected by the design engineer prior to selection of the exact location for a dam. Areas of uneven natural subsidence, sink-hole, pockets of organic matter, or other unstable soils shall be avoided, unless special provisions are made for their correction.

(b) Soil testing – A program of soil sampling and testing adequate to determine the characteristics of the foundation material which will support the proposed dam and of the material to be used for construction of the dam shall be performed. Sampling shall include borings or in-place samples from the exposed excavation face. All borings shall be logged using a recognized engineering soil classification system, (such as Unified System) with location and depths of all samples recorded on the log. Tests such as the determination of in-place densities, shear-strength; and permeabilities of the foundation and embankment soils shall be performed. Tests on foundation soils shall be performed on either undisturbed samples or on the in-place soil. Tests on embankment soils shall be performed on samples remolded to the densities to be used in construction.

(c) Cross Section design – There shall be a minimum freeboard of five feet (5') below the inside crest. The outside crest of the top of the dam shall be higher than the inside crest in order to force all crest drainage to the inside of the dam. Both inside and outside slopes shall be no steeper than two horizontal to one vertical. The design shall provide positive seepage control features, such as:

1. Cut-off trench in natural soil foundations.

2. Clay core.

3. Blanket drain.

4. Chimney drain and toe drain.

The top of the dam shall include a roadway which will permit wheeled vehicle traffic at all times. The design shall also incorporate an all-weather roadway near the downstream toe which will permit wheeled vehicle traffic around the perimeter of the dam for purposes of inspection of the slope, toe and natural ground beyond the toe, as well as maintenance.

(d) Stability analysis – A flow net analysis shall be made to determine the location of the phreatic surface, flow lines, and lines of equal head within the foundation and fill being designed. This analysis may be based on graphical construction, electrical or liquid analogs, soil prototype methods, or other accepted methods. The flow net and stability analysis shall use the maximum pool elevation with not less than five feet (5') of clear water, this elevation being five feet (5') below the inside crest of the dam. Possible fluctuations of the tail water level shall be included in the analyses.

(e) Design safety factors – The designing engineer shall use the following minimum safety factors: 1.75 for horizontal shear at base of fill; 1.5 for horizontal shear within the fill due to seepage through the outer face; 1.5 for bearing capacity of foundation soils; 1.5 for protection against shear failure of any circular arc in either inside or outside slope. It is imperative that water pressure distribution be included in the analyses.

(f) If a cast dam is to be constructed where adequate site preparation, as defined in subsection 62-672.300(2), F.A.C., below, has not been accomplished; or where the fill materials do not meet the requirements of subsection 62-672.300(3), F.A.C., below; then the design shall incorporate either of the following alternatives:

1. A portion of the material which forms the downstream slope shall be removed and the foundation thus exposed shall be prepared in the same manner as is prescribed herein for a rolled dam. Fill material shall then be placed as a rolled embankment which shall be of such design that the safety factor with respect to downstream movement of the rolled section shall be no less than 1.2 when assuming that the remaining cast material below the phreatic surface has become liquid.

2. Sand tailings shall be placed against the downstream slope to the extent that the wedge so formed shall have a safety factor with respect to downstream movement of no less than 1.2 when assuming that the cast material below the phreatic surface has become liquid.

(g) When the foundation for a cast dam meets the requirements of subsection 62-672.300(2), F.A.C., and the materials used for the fill meet the requirements of subsection 62-672.300(3), F.A.C., then the dam shall be designed in accordance with paragraphs 62-672.300(1)(a), (b), (c), (d) and (e), F.A.C.; except that the computations of all required safety factors shall be based on only seventy-five percent (75%) of the indicated strengths of the cast materials which are tested at the same density as will exist within the dam.

(2) Site Preparation – Ground which will become the foundation of earthen dams shall be stripped of all vegetation and organic detritus or residue, including muck, mud, slimes, or other material which would flow or undergo excessive consolidation under heavy loading. All earth foundation surfaces on which fill is to be placed shall be scarified or moistened and compacted prior to spreading of first course of fill material, and the dam base shall be well drained during construction, except when placing hydraulic fill.

(3) Material to be Used – Material used for earthen dams shall be free of stumps, vegetation, trees, palmettos, muck, and other extraneous matter which could affect the compactability, density, permeability, or shear strength of the finished dam. Tailings may be used for dam fill when such a completed dam will meet the seepage and structural requirements in subsection 62-672.300(1), F.A.C.

(4) Water level control – Sufficient water level control structures shall be installed in the impoundment area behind an earthen dam to maintain the minimum 5' freeboard and to accommodate the release of storm water resulting from heavy rainfall. Such structures shall be adequate to accommodate twelve (12) inches of rainfall on the watershed involved during any period of twenty-four (24) hours. All settling areas covering an area greater than fifty (50) acres shall have no fewer than two (2) water level control structures.

(5) Methods of Construction.

(a) Each new dam shall be constructed to meet or exceed the minimum safety requirements of the specifications and design for that dam. Draglines, drag scrapers, tractor or other appropriate earth moving equipment shall be used to place materials in dam construction. Materials used in rolled dams shall be blended prior to compaction. The soil shall be compacted and density tests shall be performed to ensure that the designed densities are obtained. During dam construction, quality control/quality assurance inspections shall be conducted by the engineer of record or a personal representative under his or her direct supervision. A third-party engineer or his or her representative shall be on site at all times during dam construction and during installation of all spillways. The department shall be advised 48 hours prior to construction or shaping of a new dam so that a department representative can inspect the site.

(b) Tailings may be deposited hydraulically on the upstream slopes of existing dams provided that the elevation of the discharge water is never higher than five feet below the adjacent crest of the dam. If water is impounded above natural ground level, tailings may be deposited on the downstream portion of a dam by either of the following procedures:

1. If the tailings are dewatered to not less than 50% solids by weight at the discharge point, the tailings may be deposited continuously.

2. If the discharge point is at or beyond the point at which the toe meets the foundation, or the discharge point is at least seventy-five (75) feet from the point at which water meets the dam, the tailings may be deposited continuously by hydraulic methods.

(c) Areas around any water level control structure pipe, any other conduit, or any surface of discontinuity between materials within the mass of the dam shall be carefully installed to avoid potential concentration of seepages. The design of spillway structures associated with earthen dams shall ensure that soils under and around a culvert are uniformly compacted and are in continuous contact with the external culvert surface. All conduits through dams shall have two or more seepage collars spaced in accordance with good engineering practices pertinent to the material used for the fill. Two collars will be installed within the core when there is a core within a dam. A third-party engineer shall evaluate the potential for piping around culverts and the engineering design shall reduce or eliminate such potential based upon site specific conditions. All pipes and joints in pipes extending through a dam shall be made leakproof and shall be constructed of materials suitable for the fluids carried and the load imposed. The elevation difference of any spillway pipe from its inlet to the outlet at the discharge ditch shall not cause supercritical flow conditions within the culvert. In order to avoid leaks associated with differential settlement, conduits through dams shall not be rigidly supported by piles or piers. Backfill around conduits shall be of a density that is equal to or greater than those of the surrounding embankment. Particular attention shall be devoted to the lower third of the conduit. The engineering design for the construction of a culvert shall require the use of a lean concrete cradle and gravel drain system or a design resulting in an equivalent level of protection.

(6) Documentation.

(a) After completion of construction and before the above-grade deposition of industrial waste behind the dam, the permittee shall contact the department to arrange for a department representative to inspect the facility in the company of the permittee. The owner of an earthen dam shall maintain in a permanent file the following construction records pertaining to said dam. The owner shall furnish a copy of the file and certification of completion of construction within 30 days after completion of the dam to the department.

1. Aerial photo of construction site after mining in the immediate area has been terminated and before shaping of the final dam.

2. Design drawings and calculations.

3. Design specifications.

4. Results of all soil tests on foundations and fill materials.

5. Logs of borings and engineering geology reports.

6. Certified copies of construction progress inspections pertinent to core trench, toe drain, internal drains, and other significant phases of the structure. Photographs of various structural items may be included in the file.

7. Aerial photo of completed dam taken within 30 days after construction is completed, weather permitting.

8. Description of and justification for all deviations or variances from the design plans or specifications.

Specific Authority 403.061(22) FS. Law Implemented 403.061(22) FS. History–Revised 12-8-72, Formerly 17-9.03, 17-9.030, 17-672.300, Amended 6-28-99.