Testing a small red tagged dam 2

[petrosoft]

I have a project where the owner built a 0.3 Acre pond on his hillside property without a permit and got busted for it 10 years later. There was a civil designer for the project but there was no inspection of the work. The building department wants an engineer to to verify the compaction of the key and compacted fill areas in the dam. All the soils were from on site and were described as sandy loam. The key foting is 10' below the top of the dam. The dam has had no problems in storage or operation for 10 years.

Is there an easy way to determine that compaction of 90%ASTM D1557 has been satisfied in the key way and exposed areas of the dam face? I could use a backhoe to make a test pit for in-situ samples?

Thanks ---PS

 

[TDAA]

You could get Shelby tube samples, measure and weigh. You can then run a proctor to have a comparison. At that point, you can evaluate the existing conditions, as they relate to the compaction requirement. You could not say that the soils had been compacted to . . . since you don't know the condition at the time of compaction.

 

[cvg]

in most states in the US, this is not considered a regulated dam - it is much too small. Therefore design generally is not required to meet typical dam safety standards.

 

[petrosoft]

Thanks TDAA,

Is it possible to get the sample using a test pit?

PS

 

[boffintech]

Were you going to rent a trench box?

 

[petrosoft]

I guess what you are saying is that the general method is to take a shelby tube sample using an auger. I have a backhoe on site, but it might be difficult to take the sample in the trench.

I'm also thinking we could calculate the shear required by the dam and then check the soil using a triaxial test.

PS

 

[DRC1]

If the dam is stable after 10 years, shows no signs of significant distortion, piping or other problems, and if a failure would pose no significant hazard, I don't think I would go messing wih test pits. If it ain't broke, don't fix it.

 

[oldestguy]

I'd not use that back hoe. The reason is the apparent regulators appear impractical requiring the data and they are just as likely to then ask for proof that the test pits have not compromised the dam's integrity.

Either hand borings or machine borings with 3" or larger Shelby tubes seems to be the best thing short of telling the regulators to "bug off".

Don't forget to backfill these holes with pelletized bentonite to seal them.

Then comes this 90 percent compaction requirement. Why 90?.

Before you do any dam checking, go to the borrow area and test the natural ground. It is very likely that that area will be around 80 percent compaction naturally. Test the unconfined compressive strength. If it is anything above about 1.5 or 2.0 t/sf, check to see if this looks like a suitable figure for "dam design". Test deeply enough to get below surface dried zones to test representative material.

Then go back and ask the "regulators" if these strength figures will be acceptable, since the "design" shows it will hold, with some acceptable SF. I'd work on shear strength as the basis of a acceptance, not percent compaction.

If this line of reasoning appears OK, only then do the tests of the dam. It is very likely you find the dam properties are about the same as the natural ground, since it is unlikely that any drying was done to build it. Otherwise, I'd bet the tests won't meet 90 pecent. Ya gotta have some argument they might accept before you are stuck with a problem.

 

[TDAA]

I would avoid test pits too. The size of the dam is small, but why dig it up. For one, it is a death trap to get in a trench like that without protection, and two, you would never get it back in as good, and have a "bad spot" in the dam. You can grout the auger hole.

I have pushed tubes with a backhoe, in shallow test pits (Landfill cover). It is not a practice I recommend. The sampling is difficult, since the bucket has too much play, and the operator has too much control, and tries to "adjust" things as they push.

 

[jdonville]

I agree with TDAA and others who suggest sampling using an auger rig (3-/4 inch ID hollow stem auger to permit 3-inch Shelby tube sampling without removing the casing/augers) and grouting the hole using a non-shrink cement-bentonite grout as the auger is withdrawn.

I used to work on earthfill dams, and you defeintely DON'T want to test pit.

oldestguy makes an excellent point that you will want to clarify what criteria you will need to use to get acceptance from the regulator prior to taking samples from the dam itself. As at least one other has noted, this dam may not be subject to strict regulation at all - it sounds like you should check the applicable laws, and be prepared for legal action.

Good Luck!

Jeff




Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

The views or opinions expressed by me are my own and do not necessarily reflect the views or opinions of my employer.