Gravity dam with stilling basin - Uplift pressures

[Adeel7]

Dear Sir/Madam,

I am calculating uplift pressure for a gravity dam with stilling basin. In design guidlines, uplift pressure at toe of the dam is considered as equal to tailwater depth x gama. But if tailwater is present at the toe or above stilling basin it should reduces the uplift pressure as it also apply weight of water over the stilling basin? I am using Geostudio for seepgae analysis. Kindly guide me which is the right way to calcuate uplift pressure for mentioned case.

Thank you

 

[graybeach]

When you say stilling basin, do you mean a concrete apron? Assuming that is so, and if you do not have pressure relief drains on the apron, you will actually have more uplift at the toe of your dam. Generally you would draw a straight line from the uplift at the downstream end of the apron to the headwater pressure.

As for what the pressure is at the downstream end of your dam, I think you would take the tailwater depth. Then separately you would add the weight of water and the dead load of the water and concrete downward for your overturning analysis.

 

[cvg]

remember you also need to evaluate different scenarios to determine the critical one for design and perhaps use different factors of safety for different scenarios. They may all have different amounts of tailwater and uplift.

Possible scenarios to analyze include:

1) Normal operating condition, sunny day and lake is full, no flow over the spillway

2) Sudden drawdown, the lake has been drained quickly, saturated conditions may exist still (applies mostly to earthen embankments and slope stability)

3) Normal flood condition, the spillway is in operation at a normal level - up to about a 5- or 10-year flood

4) Severe flood condition, the spillway is in operation, perhaps at 50- or 100-year flood level

5) Extreme event, the spillway is in operation and passing the Inflow Design Flood which might be 1,000 year or greater flood

6) Dam overtopping or full to the top

For some, especially sunny day and perhaps normal flood condition, you may also want to include seismic forces

 

[Adeel7]

Thank you very much.

I am doing seepgae analysis of Concrete dam with stilling basin (concrete floor on which their will be some tail water)

@ graybeach. By stilling basin i mean reinforced concrete floor. By apron generally it is provided for scour protection on which no water rests. If i am not wrong you mean first carry out seepage analysis and considering tail water depth at toe of the dam for uplift pressure, but if i calcualte seperately for water depth it will automatically balances the pressure at toe of the dam as water head at toe and tail water is same?. At toe of the dam net pressure will be zero because of tail water?.

Secondly i think that pressure will not dissipate as quickly as flood level.Flood may last for 2-3 hours and uplift pressure may not relief in 2-3 hours.

Which approach is right way to model such case with and without drainage?

@cvg

For seepage analysis i assumed that critical case will be when reservoir is at MCL (Maximum Conservation level/Normal Pool level)and no tail water as i thought that tail water will add positive effect by adding weight on concrete floor.
Some person told me that critical case will be when dam is full up to flood level + tail water.

I am confused in these two cases.

 

[cvg]

maximum hydraulic gradient is when the lake level is at it's highest point without going over the spillway

 

[fattdad]

draw a flow net for the dam (the concrete to subgrade interface will represent a flow path).
Interpret the head losses that occur along the concrete to subgrade interface.
Calculate the pressure from the flow net for some portion of the dam "footprint" (i.e., downstream third, downstream 10 percent, etc.)
Consider your boundary conditions (i.e., tailwater elevations).
Calculate uplift pressures.

Bear in mind if there is ever a need to pump out the stilling basin (i.e., for concrete repairs or such), you should account for this in your uplift calculations.

Have fun too!

f-d

¡papá gordo ain’t no madre flaca!