I’m having a new 3/4 acre pond dug and initially it looked great. As the basin was being dug, builder ran into sections where the surface would give under the weight of the equipment. Then water began to seep out and in a few spots bubble up.
Now the surface is all very good clay and the clay itself isn’t saturated, it just seems to be layered over something squishy.
This has limited his ability to drive the dozer over these sections and compact.
He came with good recommendations and is telling me not to worry, but this is keeping me up at night.
The effected section is perhaps a quarter of the total basin and in the shallow area, which would be 5-8 feet average. The deeper section, near the dam, is perfect and would be 12’ or so deep.
I visited the site yesterday and when walking on the bouncy surface, it moves under foot, so getting his D6 over it would be scary.
Attached a pic of pond. Interesting to note is the squishy section (circled in red) is higher in elevation than the depth at dam base. Also, he cored the dam and didn’t run into any water or similar conditions.
I am a geologist, NOT a dirt contractor. However, the only times I have seen or heard of a similar situation is where you had a water saturated area beneath the equipment, or a backfill of some material (old logs, a trash landfill, bog, etc.) that was later covered over with topsoil.
I assume there was nothing like the second option in that location before you started construction of your pond?
If not, then I think you have very saturated clay that is probably overlying a water-bearing layer of sand or gravel. The clay is so wet, that is now has high "plasticity" and can actually move since you have now stripped off the confining overburden.
It looks like there is a large excavator on location? If so, I would definitely dig a test trench. It doesn't even have to be within the pond boundaries (if the operator is worried about the bouncy clay). Go to just outside the pond edge adjacent to a good bouncy area and dig a quick test. Look at the material that is coming out in the bucket, but also look at the side walls of the trench. You should see some layering. (Lack of layering MIGHT mean backfill.) Do NOT let a person go into the trench since wet clay/sand underneath can lead to a trench wall collapse.
How deep is your projected pond depth? I would go at least 4' deeper than that in the test trench.
Let us know if you get down to "soupy clay". If you do, then keep digging deeper. I suspect you will find some type of water-bearing layer. That will probably change the construction plan for your pond. I don't know how the contractor bid your pond job. Do not let him refuse to find out what is underneath. IMO, a poor contractor wants to do the minimum job and get out of there when presented with a strange situation. A good contractor wants to know exactly what is causing the strange situation - so that it can be dealt with in the proper fashion to deliver a good final product.
Everything written above is speculation by a person that is NOT on location. Further, it was written by a person that does NOT have any local knowledge of the subsoil conditions in the region. A local contractor does have experience with local conditions.
Good luck with your pond project! I hope I have given you a little bit of helpful background as you continue work on your pond.
What are your two types of vegetation in the pic? Do you have lots of bare ground surrounding the pond right now? If so, I would get some groundcover vegetation planted ASAP that fits in with your current land usage.
You do NOT want your fall rains to wash that bare dirt down into the bottom of your pond.
All disturbed area was planted with grass and covered with straw. Already have quite a bit of germination, so hoping it continues for the reason you stated.
