Pond Boss Forums POND CONSTRUCTION Building a dam coring question

Well the games started today!!!
Day one land is cleared and tomorrow begins the digging.
My question is..... if I have good clay approx. one foot below surface do I have to put a core trench in my dam? Btw, its really wet.

MakeDo (love the name!), where my dam is the dirt guys found some great veins of clay only 3 to 5 feet beneath the surface. However, they still dug the core trench down 15 feet to make sure the dam would hold water even if the shallower veins occasionally played out. Which they did in certain spots.

Personally, I'd be nervous about a 1 foot core trench. Maybe it will work great in your situation and I'm just too cautious, though.

Of course, the dirt guys like Mike Otto are the true authorities on this subject. Hopefully they'll be along soon.

PS Congratulations! I know how exciting it is to get started...

I can't give you a qualified answer on exactly how deep it should be.

But the reason for a core trench is to keep water from flowing beneath the dam. As long as there is a continuous clay bottom you might be ok but during excavation if you run into anything other than the clay (like a rock or sand layer) water could follow this layer under the dam. So the core trench needs to go deep enough to prevent this from happening.

Unless building a pit pond directly into the clay, all dams should be cored - why take the risk? Congrats on your new project, keep us apprised and post photos please!

In our part of the world it's common to find sand veins within heavy clay, they can be from 2" to 10 feet wide. So yes core trench.

Howdy MakeDo!

Like was said, a core is almost a must! What I noticed, was your saying the clay is "really wet". Clay HAS to have moisture in it to compact and seal well, but too much moisture is just as bad as none.

Building a pond and dam is NOT just pushing dirt. It is an art and science if you want to save money in the future. If the clay is too wet, it will need to be spread and thinned, tilled, and allowed to dry some before being brought back in and well compacted.

Another consideration is that a mix of loam and clay is ideal. A 100% clay dam will crack when it dries.

My builder just told me he averages a core trench 10 feet deep. He said it will be his blade length wide 12'.

Fast, the core isn't so much about depth, rather, it is getting into good clay that cuts off anything upstream flowing. It can be as little as a couple feet, or as deep as 20-30 feet.