If seepage appears localized, dig out the suspect area and cover that section with a 1-foot-thick layer of compacted clay soil. Form the clay blanket with two layers of soil, compacting each layer separately.
When repairing leaks in dams or levees avoid making vertical cuts, as they are difficult to seal. Dam or levee cuts should be made in a broad V shape so the new fill will bond with existing soil as it is compacted.
If spot treatment will not work or if leakage cannot be localized, the entire pond basin will require treatment. Methods for complete pond basin work-overs include simple compaction, compaction aided by additives, or the addition of a liner.
Compaction:
For many soils, compaction is critical to reducing leakage. Compaction works well when the soil has a wide and well-graded range of soil particle sizes. With soil containing sand, silt and at least 10 percent clay, water acts as a lubricant and the force of compaction squeezes air from the soil and locks the different size particles into the smallest possible arrangement.
This leaves little space for water to move through the compacted layer. For adequate compaction be certain to:
- compact thin layers;
- use sufficient compactive force;
- make sure that the soil has the right amount of moisture.
Compact no more than 8 to 9 inches of loose soil at a time.
This will give a compacted layer about 6 inches thick. This is critical, as the force of compaction diminishes quickly with soil depth. Compacting only the top of a pile does little to reduce the permeability of the soil underneath.
Using specific equipment is essential.
A bulldozer alone will not compact adequately, as the broad tracks on a dozer are designed to spread out the weight of the machine to keep it from sinking into the soil. A sheepsfoot roller behind a bulldozer compacts much better (Fig. 1).
As the roller makes repeated passes, compacting the soil and making it firm, the roller begins to “walk” out of the soil on the feet extending from the drum. Adequate compaction will require at least four to six passes of the roller over all of the pond bottom and sides.
Tractor drawn wheeled dirt pans also can be used over the dam or levee, although they do not compact as well as a roller. The amount of water in the soil when it is compacted affects the resulting permeability, especially for clay soils.
Though determining the optimum moisture content for a soil requires laboratory tests, a good indication in the field is soil that is too wet for good tilth but not so wet that water oozes out when it is compacted. If the soil is too wet, it can be disked to speed the drying process. If the soil is too dry, spray water over the site and allow it to soak in before proceeding.
When compacting an existing pond, the bottom must be dry enough to support the weight of the equipment. Remove any vegetation, tree stumps, roots and large stones. Disk the pond bottom to a depth of 16 to 20 inches, mixing the soil and ensuring that the soil moisture is near optimum before compaction. Seepage increases proportionally with water depth.
For ponds less than 10 feet deep, a 1-foot-thick layer of compacted soil is usually sufficient. This layer should be composed of two separate layers, each compacted separately. Soil will need to be stockpiled to one side or “borrowed” from a nearby site in order to apply a second layer to a compacted pond bottom.
For each foot of water depth beyond 10 feet, an additional 2 inches of compacted fill is recommended.
Clay Blanket:
Compacted blankets of clay soil have been used successfully to seal areas of exposed, fractured rock or other permeable material. The best soils are those with a wide range of soil particle sizes and with 20 percent or more clay.
Again, a minimum thickness of 1 foot is recommended. When suitable soils are available nearby, it is relatively inexpensive to cover the pond bottom with compacted soil layers. Hauling in soil from a distant site adds greatly to the cost.
The failure of clay blankets is often associated with on-site application problems. If the soil is dry, clods should be broken apart and water worked into the soil to achieve proper moisture content.
Bentonite:
Sodium bentonite is a highly plastic clay that expands 8 to 20 times in volume when wet. It is used to fill in voids in porous soils. Bentonite has been used successfully on relatively sandy soil (at least 10 to 15 percent sand) where there is adequate support for the bentonite-treated layers. Calcium bentonite does not swell to the same extent as sodium bentonite and should not be used.
The amount of bentonite needed varies with soil type and laboratory testing is recommended to determine optimum application rates. Given its high cost, bentonite is more suited to spot treatments or small ponds.
Typical application rates are:
For best results, spread bentonite over the pond bottom at the recommended rate and mix it in with a disk or tiller to a depth of 6 inches. Moisten the soil if necessary, and compact the pond bottom as described in the section on compaction. Seepage will not decrease immediately because the clay swells slowly.
Once treated, the pond bottom should be kept moist to prevent the clay from cracking and shrinking. If a pond cannot be drained, granular bentonite can be used as a spot treatment over a suspected seepage area.
The granular form allows the bentonite to sink to the pond bottom before dispersing. The proprietary compound ESS-13 (Seepage Control, Inc.), a resinous polymer emulsion, also can be applied to undrained ponds to reduce seepage.
Author:
Nathan Stone
