FoodProduction101

How to Build an Earthen Pond and Dam Wall

It is extremely important that you choose your pond site wisely before bringing in the heavy equipment. I addressed how to do this in my pond planning article. In this article, we use the existing topography and soil types to make the best pond site choices.

 

Unfortunately for me, I really don’t have any site on my property that is a really good pond site. I picked the best site of my marginal choices, because I really wanted to have some open water on my property for diversity, wildlife habitat, fish, and amphibians. The main problems with my site is that I have a lot of slope, not a lot of catchment as I am at the top of the hill, and my soil is filled with silt, sand, and even some shale and rocks. So my site is pretty bad and complicated all around. Nevertheless I decided to go forward with the project.

 

I chose a site that was in a partial valley, and the slope was not too steep. It was definitely my best option for catchment as well.

 

1. Peg your site out with stakes and mark your levels with a laser level. I also wrote the levels down on the pegs as well. The pegs will get moved around a bit. We also re-measured a few times as we went along. 

 

Pond Marked and Ready for Excavation

 

2. Get your equipment and materials ready. For me, it was a sheepsfoot compactor, excavator, skid steer, and sodium bentonite to address my poor soil for ponds.

 

Mini Excavator and Skid Steer

 

3. Remove the topsoil from the site. Ideally, you should save this to dress the walls after so your plants have something nice to grow in.

 

Removing topsoil with skid steer

 

4*. Connect any diversion drains. When you do this really depends on your project. For me, it made sense to do it in the beginning as it made for less damage to my property, but you may want to wait until the end if you are going to have trouble keeping water out while you are working. 

 

Drain tied to swale

 

5. Build your keyway! This is extremely important. This must extend below the dam to lock it into the earth so it does not slide down and fail. We dug a 2 foot trench then mixed in bentonite with each 3 inch layer of soil that we applied. The process was to add 3 inches of soil, rake and pull rocks out, compact, add the bentonite, then rototill, rake and pull rocks out, then compact, and on and on and on until we got to the height we needed. This was unbelievably labor intensive. If I could do this over, I would have looked high and low to just truck in the appropriate clay. It took a week and a half just to get the wall up. Also, bear in mind that your bentonite amount is based on the layer of soil amount compacted. We put more than the recommended amount because we were applying the appropriate amount for the layer un-compacted. That made a big difference in the amount we used, but it does make me feel good about having a strong keyway and dam wall.

 

Building the keyway!

 

Keyway coming up

 

Keyway at grade

 

Recommended Application Amount of Sodium Bentonite

Clay: 1.0-1.5 pounds per square foot

Sandy silt: 2.0-2.5 pounds per square foot

Silty sand: 2.5-3.0 pounds per square foot

Clean sand: 3.5-4.0 pounds per square foot

Rock or gravel: 4.0-5.0 pounds per square foot

 

Building up the wall

 

6. Build up your inner and outer walls as you build up your dam wall for stability. Every 6 inches of soil you add should be compacted. It is very important that the soil is neither too wet or to dry to compact properly. The moisture level is something you have to constantly monitor and address. We kept a garden hose out and added water as needed. It was very dry so we were not worried about it being too wet. The inside of your dam wall should not be any steeper than 3 to 1, and the outside 2 to 1. However, I would recommend no steeper anywhere than 3 to 1. We had some problems getting our compaction equipment up a 2 to 1 slope.

 

Rain interrupting construction

 

7. Rain can cause some interruptions, so be ready with a pump if necessary to get you back on track. You still have to pay for the equipment if it is just sitting there.

The dam wall is up and compacted, now it is time to shape the bowl. It is important that you have no steeper than a 3 to 1 grade on the inside of the dam wall. The shore line can be 2 to 1, but I would not recommend anything steeper than 3 to 1 as it makes it difficult to compact unless you use a compactor attached to an excavator arm. If you are adding any soil layers to shape your bowl, make sure you compact every 6 inch layer added.

 

Wall height achieved

 

8. Get that bowl shaped up and compacted.

 

Pond ready for bentonite

 

9. Get you spillway squared away. The spillway should be an area where if the water goes above the freeboard, it can safely exit. This is very IMPORTANT to get right as it will determine your full water level, and it is your safety measure when it starts to overflow. You never want water to flow over the dam wall.

 

Spillway

 

Spillway links to swale downslope

 

10. For us it was time to mix the bentonite in the bowl. We already mixed it in the dam wall as it went up. We spread the appropriate amount of bentonite in the bowl by placing the bags out uniformly in the bowl then simply dumping them out evenly by hand at 3 pounds per square foot. 

 

Recommended Application Amount of Sodium Bentonite

Clay: 1.0-1.5 pounds per square foot

Sandy silt: 2.0-2.5 pounds per square foot

Silty sand: 2.5-3.0 pounds per square foot

Clean sand: 3.5-4.0 pounds per square foot

Rock or gravel: 4.0-5.0 pounds per square foot

 

11. After the sodium bentonite was spread, we used a rototiller attachment for the skid steer to till it in. This worked really well as we could get up the steep banks easily as it is mounted in the front. Also, we reversed the tines on the slopes to make sure we did not pull the bentonite off the sides down to the bottom as we tilled.

 

12. Now for the final compaction and this one is EXTREMELY IMPORTANT. Make sure the moisture levels are right and compact the hell out of the bowl. If you have a sheepsfoot compactor, that is great. If not, you can use the wheels of a skid steer, and excavators do have compactor attachments for the arm which are great for steep slopes. Some people do use the tracks of an excavator to compact, but it is less than optimal.

Now that everything is compacted and ready for water, we can get the detail work done.

 

13. Dress your outside walls and the freeboard with the topsoil you have been saving.

 

Pond Filling with a Rain

 

14. Add seed and straw. We used clover. I will be planting bamboo on the backside of the wall to add stability and shade in the spring. Do not plant anything with a deep tap root on the dam wall. This covers most trees except willows, although willows will drink from your pond lowering the water level.

 

Stone area where drain pipe exits to add aeration to incoming water to pond (

 

15. Add in any decorative or functional stone work that is needed. It is much easier to do this before it is filled. We added some stone along the spillway to slow erosion. We also added stone to the entry of the pond where my drain pipe exits to provide interest and aeration.

 

Fish Habitat

 

16. Add in any fish habitat you would like. It is important to have some features for spawning and places to hide for the small fish otherwise they get too stressed out. We used some old pallets weighted down with rocks and some ½” pvc bound together to create a cheap artificial reef.

 

Pond Filling Up

 

17. Fill your pond steadily and slowly. If you can, I would recommend filling your pond immediately with a pump or hose. I used a garden hose from my well and it was filled in 4 days. I propped the hose up on a rock so it would not erode the area. The benefit of filling yourself is that the sides don’t dry out, and you won’t have much erosion of the sides when the big rain comes. Also, if you have a clay or bentonite pond, you can have cracking if it dries out too much.

 

Pond

 

Pond

 

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