Leak in Raw Water Reservoir for Potable Water Treatment Plant

[TJH2161]

I am an engineer for a local water system in south Louisiana. We have a raw water reservoir that was constructed out of gunnite. It was constructed about 40 years ago. We did some testing recently to determine our low level draw down of the reservoir and during that test we noticed quite a few spots where there were holes in the gunnite surface. Looks like there has been some pretty severe erosion of the soil behind the gunnite as I could not even see the soil behind the holes in some spots. Earlier this week an operator informed me the level is very soft at one spot and it is clear that the water is starting to seep through the levee and causing some maintenance issues with cutting grass. I was curious if anyone has ran into similar problems and a possible low cost solution to fix it. I've dobne some research to try to find some products to patch these areas but am not having much luck. Any suggestions on products that could be used to patch these holes? Ideally the product would be able to be placed in wet conditions. Appreciate any help I can get. Thanks

 

[Ron]

Is this a prestressed tank? There are not that many contractors who do that type of tank, so it might be one that uses a patented process in their construction to seal the joints in a corrugated steel diaphragm that sits between two layers of shotcrete (Probably not Gunite...that's a nozzle mixed approach). Those joints might now be leaking, or it might be just cracks in the shotcrete. In either case, probably repairable by epoxy injection. You should drain the tank and do a thorough inspection, both inside and out, included sounding the shotcrete for delamination. Let us know the tank builder if you can find it. Do you have original plans?

 

[JedClampett]

You don't say the size of the impoundment. But I fear that any local repairs will just set off a series of fix and chase.
I would recommend building up the eroded spots and installing a liner. I'm normally not a huge liner fan, but you can't beat their bang for a buck. And the installers are pretty good in my experience. They'll help with materials and details.
You might need to make plans for a total replacement in the future. Forty years for a gunite containment is pretty good, especially in the Louisiana environment.

 

[LittleInch]

To be honest I'm surprised that when you find a serious breach in your reservoir you haven't immediately lowered the water level to below the hole.

This is I assume a soil mound reservoir faced with gunnite / sprayed concrete to seal it. Do you have any details of the cross section or sketch or photos?

But now you have a set of holes and soft patches. This can only get worse, possibly very rapidly. The fact that the soil has already washed away behind the holes and the ground is so soft that you can't mow the grass tells me you're in for some major reconstruction, not a simple patch. Your options then are many and various, but first lower the water level is my advise.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[TJH2161]

@ Ron, This is a 350'x 500' pond with a layer of gunnite around the perimeter and bottom of the pond.

@JedClampett, the problem with a liner is the gunnite surface is very unlevel and I think you'd have issues trying to get the liner to adhere to the surface. I may still try to reach out to someone in the business and see what they recommend. Thanks for the feedback!

 

[TJH2161]

@LittleInch lowering the water level that low for an extended period of time will cause algae issues in the reservoir. We can lower the reservoir to low level when necessary but it should not be done for extended periods.

 

[cvg]

your best option is probably remove and replace

 

[LittleInch]

TJH,

I can't see your reservoir so can't judge it, but you clearly run the risk now that your reservoir will lower the water level all by itself.... how deep is this reservoir / pond by the way?

History has shown that if this is an earth dam, that once leakage starts it is very difficult to fix in such a away as to be secure enough. Earth dams also fail on a regular basis

But we have very little to go on here in terms of information on the number and size of the holes - a 1" hole wold be quite different to a 6" hole.

If the ground has gone "very soft" in that area I can't see any responsible action other than cut out and replace myself. I suppose you could try using sheet piles to create a line of security around the area(s) in question, but installation could cause failure if you're not very careful.

I can only suggest you consult a locally based civil engineer with experience of earth dams, if indeed this is what this reservoir is. someone like this lot?

http://www.ecmconsultants.com/dam-inspection.html

Doing Nothing is not an option I feel.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Ron]

@TJH....thanks for the clarification.

If you can lower the water level below the holes/patches, you could apply a layer of shotcrete over the whole area to reinforce the existing. Could be a quick process and the water level can be returned as soon as the shotcrete has set, which doesn't take very long. You could then inject foam or chemical grout to stabilize the soil below the perimeter.

 

[cvg]

dump bentonite pellets into the holes and patch with grout. cheap, works in water and might slow the leak temporarily

https://southwesternmaterials.com/pond-sealer.html

 

[TJH2161]

All very good information. Thank you all. I'll be discussing with our General Manager and will update.