Let’s start by saying all lakes—man-made or not—will leak. The goal of any lake construction project should be to have a lake that leaks the least given its site and the material it allows.
Although this post doesn’t focus on site selection, it is fair to say that lake-site selection is EXTREMELY important and can make or break a lake construction project. (Vollmar Pond and Lake Management can perform complete lake site assessments for you.) Here are some basic lake-site assessment questions to help you pinpoint your lake or pond location:
- How deep to bedrock?
- Is the bedrock fractured ? To what extent?
- How deep to sand or gravel layers?
- Is there enough suitable material to make a water tight layer of clay? The lining material should be at least 20 percent clay
- How much rain do you get?
- How much rain will runoff into the lake?
- How big is the watershed?
Assuming you have a decent lake site and you know how big the lake can be (not how many acres but how many acre-feet, a volume measurement), then it’s time to start digging.
As you begin digging you need to know which material or combination of materials will be used to make the water-holding clay layer (field test it and send to geotech lab). The most suitable material(s) need to be sorted from the less suitable material. The shape and depth profile of the lake should be taking shape as the material is excavated away and sorted. The best clay on-site should be reserved for the dam core; the second best for the water-holding clay layer that is applied over the entire lake surface and upstream dam face. A dam core is the inner water-holding structure of an earthen dam. The dam core should extend into a hard substrate; for instance, rock, clay or shale. The excavated hole should be about two to three feet deeper than the final depths of the planned lake because there will be two layers added to this excavated hole: 1) the water-holding clay layer and 2) the protective layer applied over the water-holding clay layer.
So the lake has taken shape, and the dam is being built and cored simultaneously. Before bringing in the material for the water-holding clay layer spread over the entire lake surface, the sub-grade of the lake needs to be watered and compacted (sub-grade layer is the ground in which the clay will be placed, and it should be two to three feet below final lake depths). The amount of water needed for compaction is determined by the material you are working with, but you don’t want it to dry or to wet. Think back to when you were a kid and where making mud balls-too much water and you have mud and it cannot be formed, too little water and you can’t get the dirt to stick. The sub-grade needs be watered and rolled with a vibratory sheep’s-foot roller about four to six times.
After the sub-grade is watered and compacted it should be ran over with a disc (plow) about two inches deep. This shallow plowing gives the suitable water-holding clay material which will be added to the sub-grade something to hold on to. The suitable water-holding clay material needs to be laid down in six- to eight-inch lifts and then watered and compacted. When using a sheep’s- foot roller, estimating proper compaction can be done by looking at the feet on the roller; if the feet barely sink in the material, proper compaction is close. The water-holding clay layer needs to be at least 12 inches thick once compacted, but final compacted thickness of the water-holding clay layer is a function of the proposed depth of the lake: i.e., the deeper the water, the thicker the layer needs to be.
Now that the water-holding clay layer is in place, it needs to be protected with non-select excavated material that was sorted earlier in the project. The non-select excavated material will help blanket the select excavated water-holding clay material you placed and compacted on the sub-grade. The water-holding layer should be ran over with a disc (plow) about two inches deep. This shallow plowing gives the non-select excavated material something to hold on to. Add the non-select excavated material in six-to eight-inch lifts as done earlier, plus these lifts need to be watered and compacted in the same manner as earlier. This protective blanket should be at least 12 inches thick. This protective blanket keeps the water-holding clay layer from being exposed to drying and cracking as well as erosional forces.
The above information mostly addresses the reservoir pooling area of the lake, but very special attention needs to be paid to the dam if your lake has a dam (some lakes do not, they are just “dug-out” holes). If a dam is constructed, the base trench of the dam needs to start at least two to three feet into solid suitable material (clay, shale or bedrock). This trench is sometimes called a cut-off trench or a key-way. The idea is this key-way doesn’t allow water to go under the dam. The key-way is the base of what is called the dam core. The dam core and key-way is usually the center section of the dam and is constructed with the best water-holding clay material available. The dam cores are at least eight feet wide. The dam is constructed in lifts of six to eight inches, but instead of just laying down one type of material, you lay down select material (makes up the core) and flank it with non-select material. These lifts are watered and compacted as described above. The core needs to be as high as the full water level.
Other Things to Consider:
- All material has a maximum compaction, and in lake building you want to hit at least 95 percent of the material’s maximum compaction. A geotechnical engineer can help determine this maximum compaction and tell you if you met it.
- Watering and compacting can improve most soil’s ability to hold water, which is why we recommend watering and compaction at all stages of construction
- Lake spillways are critical to the safety and longevity of your lake, so make sure the spillway is large enough to handle excess runoff in a manner that reduces the water’s erosive forces (slow the velocity). For larger lakes, an engineer should be involved in spillway sizing.
- This information listed above is basic and may not apply to very large lakes; however, any size lake project always benefits from water and compaction.
- If permeable material (fractured rock, gravel, sand) is found while excavating the hole, special attention needs to be paid in these locations. Generally you cut these materials out as far as possible and back-fill and compact with suitable material.
- Do not hire a contractor unwilling to water and compact as much as described here. By no means, hire a contractor who says they will compact with their bulldozer. When building a lake, you are paying for time and equipment-if the contractor is unwilling to compact at each stage of the process, they should not be considered.
This guest post is courtesy of Brad Vollmar, the owner of Vollmar Pond & Lake Management in Fredericksburg, Texas. Visit www.texaspondmanagement.com for more information regarding pond and lake construction on your property.