Earthen Pond Construction Procedures
PRIOR TO CONSTRUCTION
As mentioned previously, the lagoon location should be chosen only after reviewing all ISDA, county and local siting requirements. The exact lagoon location and size should then be marked for the contractor. The local utility companies should also be notified at this time so they can mark any utility line locations, including electric, gas, water, sewer, phone and cable TV. The landowner should also mark any known utilities including the barn waste line, water trough lines, drain lines and electric.
CLEARING THE SITE
The first phase of construction is to clear the pond site of all existing vegetation. About the top 6" of soil and vegetation should be removed from the area.
This material cannot be used for construction of the soil liner, embankments or berms. Any existing manure or debris in the pond area must also be removed at this time. These materials are not used for construction because as the vegetation, manure or other debris decomposes it leaves small void pockets in the embankment or berm.
EXCAVATING THE POND
When the site has been properly cleared the pond excavation can begin. The contractor will usually start the excavation process by placing stakes in the pond at various locations to let the equipment operator know how deep to excavate. The excavation will usually be done using a scraper (Figure 6) or a dozer (Figure 7).
Figure 6. Scraper Excavating a New Lagoon
Figure 7. Bulldozer (Photos Courtesy of Austin Equipment Trader)
Larger lagoons are more efficiently constructed with a scraper, while a dozer or front end loader usually works better on smaller lagoons. Long and narrow lagoons can sometimes be built most efficiently with an excavator, backhoe or front end loader.
As the soil is excavated it will be stockpiled for later use or will be used to construct the embankments. If water is added to the soil to improve workability during excavation, care must be taken to insure that the soil does not become saturated or muddy. Rocks, gravel, and other unsuitable soil materials which are excavated should be disposed of away from the lagoon. If the pond bottom elevation is critical, such as 2' above groundwater or rock, the contractor should be instructed not to over excavate any areas. If any unexpected rock, gravel or groundwater are encountered during excavation, construction should be stopped and the NRCS or ISDA should be contacted to review the excavation.
PLACING THE FILL MATERIAL FOR EMBANKMENTS
The embankments may be constructed of imported soil material or the soil excavated from the pond area, however, the soil used for the embankment or liner must meet the SNTC-716 requirements discussed previously. The soil under the embankments should be cleared of vegetation as previously mentioned and then scarified or scratched with the ripper teeth of a dozer, excavator/backhoe bucket or tractor mounted box scraper. This will help the existing soil and the new soil bond together tightly. Some site conditions, such as larger embankments, may require a core trench or "key trench" as shown in Figure 8.
Figure 8. Core Trench Detail
The embankment material should be placed in 6 to 8" lifts (or layers). Each lift should be properly compacted as described in the following section. Each lift should be entirely completed before beginning the following lift. As each successive lift of fill material is added, the embankment should be narrowed to match the finished shape of the embankment. This will reduce the amount of final grading or dressing work required. The embankments should be placed approximately 10% higher than required by the design to account for the long term settlement of the fill material.
PLACING A SOIL LINER
If the on-site soils are unacceptable, a soil liner may be used to seal the earthen lagoon. This liner may be constructed of acceptable on-site soils or imported (hauled in) soils. The soil will usually be hauled in by truck or possibly a scraper or loader. This soil should be placed as evenly as possible initially to prevent having to work the soil into place after dumping. End dump trucks or trailers are best suited to this work as they can place a very consistent layer of soil. The liner thickness after compaction must be at least 1 ft. (2 ft. over ground water, rock or gravel). Placing the liner on the embankments can be done in conjunction with the embankment construction or after the embankments have been constructed.
COMPACTING THE SOIL
Compaction is probably the most important single phase of earthen pond construction because it will reduce the permeability of the soil and increase the strength of the embankments. The construction requirements previously mentioned for soil type and embankment size are based on a well compacted soil (90% of maximum density) on the bottom and embankments of the pond. The three key elements to obtaining 90% compaction are moisture content, lift thickness and compaction effort (See Figure 9).
Figure 9. Big Gun Sprinkler Adding Moisture to Embankment (Photo by Randy Elsberry)
The moisture content of the fill material is extremely important in developing 90% compaction. If the soil has insufficient moisture, 90% compaction is impossible to obtain without adding water. If the soil is too moist, it cannot reach 90% compaction until it has dried somewhat. A laboratory Proctor test is the only way to determine the exact moisture content that should be used for compaction. The data in Table 1 can usually be used to approximate the on-site moisture content, however. The correct moisture content for compaction, based on the table is the 75%-100% of field capacity range. Field capacity is the maximum amount of water that can be stored in the soil; this condition usually exists 1 to 3 days after irrigation.
|Loamy Sand||Sandy Loam||Loam/Silt Loam||Clay Loam|
|0—25% Field Capacity||Dry, Loose, Single-Grained; flows through fingers||Dry, Loose; Flows through fingers||Powdery dry, sometimes slightly crusted, but easily broken down into powdery condition||Hard, Baked, Cracked; Sometimes has loose crumbs on surface|
|25—50% Field Capacity||Appears to be dry; Will not form a ball with pressure1||Appears to be dry; Will not form a ball1||Somewhat crumbly, but holds together with pressure||Somewhat pliable; will ball under pressure1|
|50—75% Field Capacity||Appears to be dry; will not form a ball with pressure||Tends to ball under pressure, but seldom holds together||Forms a ball somewhat plastic; will sometimes slick slightly with pressure||Forms a ball; ribbons out between thumb and forefinger|
|75—100% Field Capacity||Tends to stick together slightly; sometimes forms a very weak ball under pressure||Forms weak ball, Breaks easily; will not stick||Forms a ball, is very pliable; slicks readily, if relatively high in clay||Easily ribbons out between fingers; has slick feeling|
|100% Field Capacity||Upon squeezing, no free water appears on soil, but wet outline of ball is left on hand|
1 Ball is formed by squeezing a handful of soil very firmly
Lift thickness is also an important component of the final density after compaction. Thinner lifts are used for better and more uniform compaction. Thick lifts, even at optimum moisture, cannot be properly compacted even with heavy compaction effort. The lift thickness to use varies based on equipment used and soil type. The best method of determining maximum lift thickness is to inspect an area after compaction to see if the lift was uniformly compacted. Digging a small hole with a knife or screwdriver often works well. If the lift was too thick the soil compaction will decrease deeper in the hole.
The last element of compaction is the equipment and method used. The best equipment for compaction is a sheepsfoot compactor or roller (see Figure 10). The compactor/roller applies the highest pressure to the soil during compaction. When a roller is used, at least six passes over the entire area should be completed prior to placing the next lift. The next best equipment for compaction is rubber tired equipment, such as scrapers, tractors, dump trucks and water trucks. When tired equipment is used for compaction care must be taken to insure that the entire embankment surface is compacted by the tires. The least favorable type of equipment to use for compaction is a dozer or excavator since the compaction pressure is the lowest. When using tracked equipment, be certain to compact all areas of the embankment.
Figure 10. Smooth Drum Vibratory Roller (Photo by Les Boian)