Dam Inspection

[geomalf]

Hi all

I’m not an expert in specification and construction of the dam, but I got a situation with an existing small dam. The existing dam is an earthfill that is about 140’ x 28’. The height of the dam is approximately 9 feet tall. During the previous repair, the top of the dam was covered with about 18” of concrete. Sheet piles appear to be driven into the ground on the upstream side of the dam, and backfilled with Portland Cement Concrete materials.

The client has asked us to inspect the dam because the locals are concerned about the safety of the dam since water is observed seepage out of the downstream side of the dam. During the construction of the sheet pile structure, a spillway structure was constructed on the upstream of the wall, and is used to direct the flow of the water into downstream

During the field exploration, it is discovered that water leakages are noticed along the edges and crack lines of the existing Portland Cement Concrete cover on the downstream side of the dam. We did some careful drilling at the edges of the dam, and found out that the existing clayey soils are soft and weak (SPT ranges from 0 to 10 within top 20 feet). The other bad sign is that the water levels are not consistent on both edges of the dam; the difference is almost 10 feet. All these findings indicate that the seepage is not consistent, wet and low shear strength of the soil, existing Portland Cement Concrete structure is imeffective

Neither we have the dam construction notes nor the sheet pile information. Apparently, they don’t have it. At first, the client wants to know what’s the situation of the dam, and now they want us to figure some way to repair the dam. I thought of increasing the size of the existing spillway structure, new intake structure, remove the existing Portland Cement Concrete cover to repair the cracks, but I’m not sure whether this addition will work on the existing dam.

Any ideas? Thanks.

 

[PEinc]

Yes, post your question in the Dam & Earth Retention Engineering Forum.

 

[geomalf]

PEinc, Thank you for reminding me that. It seems that I forget to place my questions.
My questions are there any way to save the existing dam structure? The client doesn’t want to spend too much money. At minimum, I want to lower the upstream water level by installing a new intake structure at a lower elevation to reduce the water flow into the existing spillway structure and therefore may limit the water seepage along the cracks.
And possible install two monitoring wells on both edges of the dam to monitor the groundwater level after the construction of the new intake structure.
What do you think?

 

[PEinc]

I'm not a dam expert but it seems to me that you are not fixing the dam with the solution you are considering. You are only trying to reduce the hydrostatic head and seepage.

The dam is not very high or long. Maybe you should consider a grout curtain or a new, improved cut-off sheet pile wall.

I suggest you post in the dam forum for comments from others who have more damn dam experience.

 

[GeoPaveTraffic]

If the dam is only 9 feet high and 140 feet long, just rebuild it. Nothing you can do, with any confidence, is going to cost less than rebuilding it.

 

[PEinc]

I agree.

 

[dgillette]

How about a weighted filter blanket over the seepage areas? Put down 2 feet of ASTM C33 concrete sand or similar, covered by 3 or more feet of relatively clean gravel that is filter-compatible with it, to act as drain and weight (or 2 feet of clean gravel covered by several feet of AOD* for weight). That should provide a lot of protection against piping - C33 sand will filter darn near any soil. A blanket can also provide some improvement in slope stability by providing weight on the toe.

What was the function of the concrete and what areas does it cover? (Crest and both u/s and d/s slopes?) Was the concrete intended to be slope protection in place of riprap, or the main seepage barrier, or what? Does it tie in with the sheetpile, or is there seepage coming over the top of it? PEinc mentions a grout curtain. Is the geology suitable for that? (Probably not, unless there is rock right below the sheetpile.)

The inconsistency between the piezometric levels in the two abutments is not strong evidence of a problem. Large differences can often be explained by differences in geology, without concluding that there is a problem.

On second thought, there are a lot of issues here (geology, original embankment and cutoff design, seepage quantities, piezometric levels, etc.), and I don't think they can be addressed properly in this setting.

*AOD: Any Old Dirt, also known as "miscellaneous fill."

 

[Prepakt1]

Possible fix is to permeate grout with ultrafine grout.
You will need to drill holes at 3 ft centres probably up to about 20 ft deep to ensure you seal the ground below the dam to stop piping.
You will not really know the depth required untill you start drilling and see what type of material is in the dam and the base soils.Ground conditions may vary as you move along the dam.
Cost of doing this type of work varies on the grout needed at each hole, but your probably
looking at around $ 5000/ hole.


Intrusion Prepakt /marineconcrete.com

 

[dgillette]

Prepakt1 - How fine can the soil be for permeation grouting, maybe D15 = #40 sieve? Do they need set-retarding admixtures so the grout has time to travel through less-pervious foundation soil?

Roughly US$300,000 to treat 140-foot dam and a little way into the abutments might be more than the owner can afford.

 

[Prepakt1]

I am not a soil expert but ultrafine cement particle sizes are much finer than any soil I have ever come across.
The idea is, that the fine slurry will flow into any area water can permeate and set up. There are retardents in the mix anyway.The set up time of the grout can be varied . Expected time to a solid 1- 2 days.
The usual proceedure is to grout each hole and return a few hours later and top up the mix as it slowly permeates. Sometimes it will be topped up again the following day.
The drill casings are sealed with a packer and the grout injected under pressure but you really do not usually need to much pressure. 20 ft of slurry head can be quite sufficient for most permeation grouting.

Intrusion Prepakt /marineconcrete.com

 

[dgillette]

Thanks. I haven't cranked any numbers, but it sounds like it would be limited to pretty pervious material (clean sand)if it has 1-2 days to travel several feet, under a pretty small gradient if there is a high water table.