Gravity RCC Dam founded on Drilled Shafts 3

[donatz68]

I am working on a conceptual design for a reservoir dam. The dam will be roughly 1600 meters long and consists of a gated spillway, an RCC section designed for auxiliary overtopping (roughly 460m long), and an earth embankment section (835m) strictly to contain water and not designed to be overtopped. The water table is at 7 meters below existing ground surface and competent rock is at roughly 30 meters below ground excavation. I am proposing to excavate to just above water table and place drilled shafts to competent rock (the dam will be roughly 30 meters in height from water table elevation). I am looking for any insight or supplemental references on drilled shafts foundations supporting gravity dams. If anyone can help, it would be greatly appreciated. Thanks.

 

[civilperson]

Drilled shafts are deep foundations used when poor soil require them. If poor soils exist, then the piping characteristics of the soil beneath the RCC will cause the flow beneath the dam. A staged embankment built on the poor soil will consolidate in time and be like 90% of earthen dams on alluvium soils.

 

[BigH]

expanding on civilperson - please advise the nature of the soil between ground and "competent rock".

 

[donatz68]

Thanks for the input guys. The soil between the competent rock (mostly high TCR and RQD Breccia and Meta-Siltstone) and the existing ground elevation consists of relatively low blow count alluvium material (mostly low permeability clay material) that cannot support a dam. A grout curtain or cutoff wall will be included in the design. The reason for such a choice in foundation is to eliminate massive dewatering and reduce the risk of a flood washout during construction. There is not a lot of literature on drilled shaft foundations supporting gravity dams so any references or additional input would be greatly appreciated. Thanks.

 

[GeoPaveTraffic]

I would be very concerned about seepage beneath the dam where it is supported on drilled shafts. A grout curtain/cutoff wall is all well and good but they will not be perfect. With time the soil beneath the shaft supported dam will settle under it's own weight and a gap will form at the top of the drilled shafts. As soon as this process starts the amount of seepage will start to increase and may lead to a piping failure of the dam through the foundation materials. I expect that this is the reason you not finding information about this system.

The better method of construction is to either excavate to compentent material to start construction (with the required dewatering and coffer dams) or to construct slowly allowing the materials to consolidate and gain strength. There are probably many additional ways to construct a dam at this location but those are the one that come to mind off the top.

 

[dgillette]

I don't get a warm fuzzy feeling from this one (and I've been involved in a lot of dam-safety projects, having done almost nothing else for 19 years, and some before that). Some of the earlier literature on piping was based on concrete dams constructed on alluvial foundations. Part of the problem was partially penetrating cutoffs and gaps between the bottom of the concrete and the top of the soil, due to settlement. GeoPaveTraffic is right to be concerned.

A couple of thoughts:

You wouldn't be able to grout the alluvium, so you would probably need some form of cutoff wall, such as plastic concrete placed under slurry, depth depending on materials in the fndn - $$. (A fully penetrating c/o is, of course, preferable to partially penetrating c/o, in general.)

You would need a pretty massive structural connection between the piers and the RCC in order to transfer the weight. The clay would carry practically none of it. Again, $$.

You would need to prepare for large differential settlements where the conventionally constructed embankment on soft clay abuts the RCC on piers.

Overall, if it's not considered feasible to exc clear to bedrock, you may be better off with embankment all the way across, with some other means of passing the water, such as a fuse-plug auxiliary spillway, overtopping protection on embankment, etc. Allow for consolidation settlements by either staged construction or just overbuilding it if fndn conditions allow w/o concern for stability during construction.

Let us know what you decide on.

 

[muuddfun]

I very much aggree with dgillette, RCC does not seem like the best bet for this condition. Go with an embakment dam, and just acount for the setteling, put a lot of camber in the crest, widen out your filters and drains, stage the construction etc. One thing you may look into for the foundations is soil cement mixing for your cutoff etc. If you want to go with the RCC, then I would consider the soil cement mixing under a majority of the dam section. Expensive though it may be, I would want something more substantial than just drilled shafts alone and the soil cement option could address both seepage and foundation support.