Undermining of Mortared Stone Dam

[dwolford]

A stone dam approx. 30 ft long and 4 feet tall has water seeping below the stone at each end. Scouring indicates stormwater flows over the top at both ends causing loss of soil at the downstream base of the dam causing weak points for water to undermine the dam. Assuming the side erosion caused by overtopping problem is fixed, what is an effective way to stop the undermining?

 

[cvg]

concrete apron with a cutoff wall below the estimated scour depth

 

[dwolford]

Is it practical to attach the apron to the existing (`20 year old) wall (mortared limestone blocks), and how would that be done to form a watertight seal?

 

[francesca]

You need a geotechnical engineer.

 

[msquared48]

Can you place stone rip rap at the toe of the dam where it is overtopping?



Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[cvg]

you really don't need to attach it, design it to be stable (against uplift, shear and settlement etc) and provide cutoffs at the upstream and downstream end to prevent erosion from undermining it. yes, you probably need a geotechnical engineer. 20 years is not old, we just got done doing a similar upgrade to a 97 year old dam.

 

[dwolford]

The dam was constructed on large boulders and is intended to create a permanent aesthetics pond. Currently the dam does not hold water. Water seeps below the mortared stone base. I could add an apron and cutoff wall at upstream end, but need to prevent water from seeping between the apron and wall.

 

[msquared48]

High Pressure grout dam?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[dwolford]

What would be appropriate method to apply a waterproof seal between the apron edge and the stone wall? See attached photo at previous email.

 

[francesca]

Is that limestone? You've probably got solution channels in spades there. Where is this? It reminds me of Dunbar Cave golf course in Clarksville, TN.

 

[dwolford]

Yup. Limestone, central Texas. I want to keep the dam, but attach an apron w/ water-tight seal. Any ideas?

 

[francesca]

Talk to a local geotech company. In a karst region, if you alter natural drainage patterns you tend to create sink hole fall-outs.

 

[beej67]

If the scour around the edges is caused by the dam being routinely overtopped, you should probably look at cutting a notch in it to allow peak flows to spill over the middle instead of going around the edges. How big a notch and of what depth will depend on the watershed's hydrology.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[dwolford]

Yes, there is an undersized notch (see photo). But my question is not about the notch.

 

[cvg]

there is no need to connect the dam to the apron

 

[francesca]

The photo does not resemble a scour condition. The overflow of the dam dissipates its energy on the limestone apron that is already there. It looks like there are two channels under the wall that have fairly consistent flow. You can grout these up and your pond will hold water for a while. But the water will find another way around or under and the result will be rather catastrophic for your client. You have a geotechnical issue and not a storm water issue. I guess you've never seen a 70 ft sinkhole fall-out in the attractive retention pond in an upscale residential subdivision.

 

[cvg]

unless you keyed the dam into bedrock or constructed a cutoff wall or trench, waterproofing the joint between the dam and the apron will do little good. Even if it is in bedrock, Francesca could be right about solution channels still causing leakage. A geotech visiting the site could tell you but impossible to know based on one photo. Bentonite or a pond liner might help. A cutoff wall might work, ask your geotech for help.

 

[Prepakt1]

You need to grout any voids and do permeation grouting to the ground around the dam. Not cheap, and probably too expensive for this small a dam.

Intrusion Prepakt /marineconcrete.com