Hello Jay,
This is a rather complex question as the techniques are different for each unique geology. There is no recipe or set way of naturally sealing a pond, this is one question where you really have to ask nature. Build experimental, to scale, models with the material on hand and observe how it functions. This way you can determine which materials will work and which won't, before you try it on a larger scale. Having a successful model is also a GREAT way to convince the other half, then she can see for herself that it will work before you try anything crazy. There are also some approaches to water retention landscapes that vary slightly.
Building a key way dam into a constrictive clay layer in the soil horizon is the most regenerative action you can take. This turns the soil body into a moisture storage reservoir, raising the water level on the landscape. When building this kind of dam with clay it is very important that it is not too moist, nor too dry.
If the clay is too dry, when the water comes and soaks into the poor space the clay will wash away. This is because you can't compact the clay enough when it is too dry, no matter how much you try. The material must be moist but not too wet. Test the material in your hands and form a shape with it. If the shape holds then it is a good moisture content. If it breaks apart it is too dry or the structure is too coarse. If water comes out when it is pressed, or if the structure sags, then the material is too wet. When the material is too wet it will shrinks during the dry season, forming cracks and a future catastrophe. Another thing with clay, even when the clay is at the perfect moisture and the dam is made with the proper proportions, if the dam is built too tall the weight on top of the clay forces the moisture out of the clay. All of the weight and pressure of the dam has squeezed the moisture out of the clay on the bottom. Then cracks again form and another catastrophe. For this type of situation you have to mix more stable materials into the dam to provide more stability.
If you want to get an engineer involved have them do a proctor test for your available clay(s). This test will tell you what the moisture content needs to be, and the compaction to create a stable water tight layer from your material. It will also tell you how water tight the end result is at this compaction and moisture content. For small projects this is overkill, but if you really want to be sure and have the money for an engineer it's not a bad way to go.
In your situation it sounds as though you will not be building a key way dam, but rather a small retention space. Such a small test has less severe consequences in the event of a failure. Here is where the excavator acting as a pig comes into play. Pigs are even better for this task, as their enthusiasm cannot be replicated with a machine. With this technique you are making use of the Brazil nut effect to settle the fine materials in the soil, compacting them and creating a seal for water retention. You can also sort the material while you excavate, as the large material will fall to the sides of a pile, and the finer material will stay on top.
Jay Angler wrote:1. How do you decide if you've got enough clay and do you sift it to keep rocks down to a minimum?
2. What action is the hoe and bucket taking - a hammering action? a stirring action? a vibrating action? a mixture of things?
3. Is there anything special about that back-hoe?
4. Is there anything special I should know as a relatively inexperienced operator?
5. How thoroughly does this action have to happen to every part of the bottom of the new pond? (ie overlaps and gaps recommended)
6. Does he put anything on top of the clay when he's done - like mineral soil - or just let organic matter build naturally?
1. This again comes back to the model building, the model will tell you if there is enough clay content, if the proportions are appropriate, and how things will react when you do it on a larger scale. Rocks aren't always a problem so long as there is enough clay in-between to make the pond tight. You can also sort the material to a certain extent while you handle it, as described above.
2. A pressing/vibration action. There are also great attachments, rollers, for the arm of an excavator. This attachment does an excellent job of providing the compaction. Bulldozers can also provide the compaction. If you are just using an excavator it requires a mix of working with the material and forming it with the back of the bucket, like super large scale pottery. You can bang away with an excavator all you like but in the end it doesn't provide the same level of compaction as a skilled press, and is going to cause some SERIOUS ware and tare on the machine. If you borrow an excavator from your neighbor and he see's you doing this I can guarantee he is never going to let you use it again.
3. In my opinion a back-hoe is a very difficult tool to create earthworks with. It is awkward and you have to drive all over the place compacting and destroying things to achieve the angles you want. Machines with tires as opposed to tracks have a higher psi exhibited on the ground, leaving behind more sever compaction in it's tracks. My tool of choice for earthworks is an excavator, or more elegantly said, a track-hoe. That being said if your goal is to build a few little ponds in a back
yard for your ducks the back-hoe should get the job done. For anything bigger though I'd rent a track-hoe, even if I already owned a back-hoe.
4. There is LOTS you should know before just going at a landscape with a machine. Built models till you are happy with your model. Only then get behind the controls of the machine.
5. If there are any leaks in the pond your water is going to drain out over the dry season. Be sure to seal the pond thoroughly, it's MUCH harder to work on a pond once it is full of water.
6. A thin layer of top soil is often spread upon completion. This is very dependent on the situation however, and is not the case in every situation.
I hope this helps!
