rehabilitation of ogie type dam 1

[roadspecialist]

the existing overflow type dam for an irrigation project spans 30m across the waterway with a height of 6m.
built without an apron slab on the downstream side, the dam is undermined and is hanging supported only by the abutments on both banks.
any suggestions on how to rehabilitate.
will filling up the gap under the existing dam be sufficient.any suggestions?

 

[BRIS]

6.0 m may not be a very high dam but it will still make an awful mess when it collapses. In the UK work on any storage reservoir exceeding 25,000 m3 must supervised by a panel engineer. Although this is UK legislation similar requirements are imposed throughout the world and I doubt that you have any other option than to consult a specialist dam engineer.

Filling the gap may well solve the symptom - the gap is a symptom of a problem – and you will need to also cure the problem. Don't make assumptions - I have investigated at least two dams where stilling basins, which were intended to be there to dissipate energy, had been filled in by a well meaning structural engineer with an affinity for pouring concrete.

Get specialist advice – Brian

 

[roadspecialist]

thanks brian.the irrigation system of which this structure is a part is ntended for rehab and i am just exploring the idea of maybe saving the existing structure due to economics.yes we will consult a specialist.

 

[BRIS]

I have found from experience that saving structures that are past their sell by date generally turns out to be a false economy. I was recently looking at some amazingly complex hydraulic structures, river barrages etc. in the former Soviet Union - all constructed in the 50's and 60's and all undermined or suffering settlement. In nearly every case it will prove more economic to demolish and start again.

Best of luck with yours Brian

 

[cvg]

how large is the impoundment behind the dam? If the impoundment is relatively small, the risk caused by failure is also small and remedial repair may be easier. However, if this dam is impounding a significant amount of water, you had best be very careful with the design and repair.

In Arizona, dam regulations state that a structure is not regulated as a "dam" if:

1) structure is lower than 6 feet from spillway elevation to lowest point at downstream toe.

2) structure is between 6 - 25 feet high with less than 50 acre-feet of water impounded

3) structure is over 25 feet high, regardless of how much water is impounded

As stated by BRIS, similar requirements exist in most developed countries. The risk from dam failure can be substantial if the structure is located in a urbanized area. If this structure is indeed a dam and in a high risk area, I would hire an engineer experienced in dam safety to evaluate and design any fix.

 

[Focht3]

[blue]cvg[/blue] -

You have a typo in your post. I believe that you meant to say (in 3) above) that all structures more than 25 feet high are regulated - and must be treated - as a dam.

I don't consider a 6 meter (almost 20 feet) high wall of water as simply "messy". If you are downstream and close to the failure, you're dead.

To me, the decisions on repair have everything to do with the value of the dam, the consequences of dam failure, and (to a lesser extent) the owner's willingness to accept risk.

Proceed with caution unless you think the dam might fail. If risk to life is great, act decisively to reduce that risk - lower the pool level ASAP. Attempt to fill in the void. Create an apron to reduce the risk of continued erosion. And grout beneath the dam as soon as the void backfilling is complete. You will probably have to engage in several methods of repair at once -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[flamby]

The efficacy of any repair work on dams, large or small, will ultimately be governed by site specific features which no one can see over internet. A very good diversion and dewatering arrangement will ensure that you can do geotechnical studies, grout the strata beneath and plug the gaps with concrete. That can be a good restoration. If the water upstream can not be diverted, any repair work may hover from useless to temporary arrangement.

If you think, a diversion scheme can be implemented, the chances of the dam surviving are very good.

Regards.

 

[cvg]

focht - you are correct, i did slip up. thanks for catching that.

your'e post is right on the mark, but I want to emphasize that if this is a dam, an engineer experienced with dam safety should be doing the design. My first recommendation would be to contact your local department of water resources, dam safety division and get their opinion. If it is a dam, I'm sure they would be more than willing to inspect the dam and recommend a course of action.

Try

http://www.damsafety.org

for more info.

 

[Focht3]

Agreed.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[roadspecialist]

thank you for the replies.
the ogee dam is a small water impounding structure at the elevated section of an irrigation system with a service area of about 150 hectares.the structure lthough recently concstructed was not provided with as Focht3 noted an apron slab,the cut-off wall wa also omitted and there is no trench core.the flow over the structure is about 5cms(cubic meters per sec.)there is not really any threat posed by the dam to lives and property as these structures are set up way up in the fluviall section of the waterway and is designed as for overflow.
due to deficiencies in construction, the scouring started at the toe of the dam and has progressed due to the absence of a trench core to carve a hole at the base of the dam.

 

[letrab]

I would recommend extreme caution in attempting rehabilitation of the structure. The dam was constructed without any consideration to energy dissipation, seemingly seepage and the associated risk of piping was not considered as the lack of core trench indicates.
Do you know whether the hydrology, spillway design, founding conditions etc. was adequately considered in the initial design?
It would be embarrassing to repair the damage caused by the omission of a stilling basin only for the dam to exhibit failure due to piping or deterioration of the spillway due to inadequate initial design.

 

[Focht3]

It sounds like replacement is the best option -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[cvg]

without looking at the design report, nor having any information on the geologic profile or subsurface data, you might be jumping to conclusions that the dam needs to be replaced. However, inspection by the state dam engineer and review by a qualified dam consultant can answer this question. Note, this consultant needs to review everything - geotech, geology, structural, hydrology, hydraulics, sediment transport etc. All must be reviewed within the overall framework of "dam safety". This is commonly referred to as a "phase 1 assessment" and should answer most of the questions for you. As far as what option would be cheaper/better, that can't be answered without the assessment.

 

[roadspecialist]

thank you guys for the replies.
for now the replacement option would tend to be the line we'll pursue.