Piezometer Response in Impervious Embankment

[hydrogeology21]

For your consideration and input. I am performing an evaluation for a dam which utilizes a concrete gravity spillway structure with monoliths placed on firm bedrock, an installed double line grout curtain to control seepage and relief drains to collect the seepage that passes through the grout curtain. The right embankment has an impervious core with adequate transition zones with filter drains adjacent to the impervious core to collect seepage and prevent piping. The entire left embankment is constructed of impervious material with chimney and blanket drains to collect seepage. The left embankment also uses a cutoff trench excavated to firm rock and a grout curtain along the trench. Several piezometers monitor the hydrostatic pressures in the foundation and embankment (right and left). The data goes back to the early 80's approx. 5 years after an extensive grouting program was undertaken for the right embankment due to high heads before first impoundment. The program was successful and the %pool reflected reduced greatly from before to after grouting. The piezometers on the right embankment( which is entirely impervious material) generally appear to be functioning as expected except one (sensing the impervious core) which has historically read dry up until the last 5 years in which it has slowly risen 14 ft. just above winter pool. However, there is no response to summer, winter or high pool so this could be malfunctioning piezometer. The left embankment has 10 piezometers installed. 5 sensing rock or top of rock and 5 sensing the impervious core. All pz's sensing the core have slowly increased with time (see attached). Each pz is responsive to normal pool fluctuations as expected however as in the case attached each pz has risen over the last 28 years (4-8 ft) approacing summer pool. Any thoughts please? Thanks in advance.

 

[hydrogeology21]

Correction, it is left embankment which is entirely impervious not the right.

 

[GeoPaveTraffic]

It is hard for me to follow your discussion and to visualize the locaion of the piezometers. Also, the entire history of the embankment and pz response needs to be reviewed to make any conculsions.

Analysis of this kind of data is a slow tedious process. There is simply no short cut to reviewing each piece of data individually and then in groups to develop ideas as to what is going on.

Good luck.

Mike Lambert

 

[hydrogeology21]

Mike,

Thanks for the comment. Your correct, the data has been thoroughly reviewed and analyzed and the evaluation and conclusions are nearing completion. My goal is to elicit your initial response as to what could be going on keeping in mind I am not asking for an in depth analysis. I'm confident in my conclusions I only wish to account for all possibilities and likelihoods to ensure I haven't overlooked something, as this is my first evaluation of this nature. If you're still interested in looking further, you mentioned trouble visualizing the project, here is a section view if thats any more helpful. You can see from my initial post that there are 5 pzs sensing rock or TOR and 5 sensing the core ( P-19, P-54, P-55, P-1, P-17). The ones sensing the core exhibit the upward trend as evidenced in the previous attachment of pool and pz level vs time with the measurements starting in 1985. The ones sensing rock or TOR respond as expected. Thanks again.

 

[dgillette]

"Impervious" isn't exactly impervious, but the permeability is low, so piezos in the fill can take many years to reach a consistent reading for a given reservoir elevation (and the reading may not vary much with cycling reservoir level if the piezo isn't real close to the reservoir). The fill was probably not fully saturated at the end of construction, and it takes time to saturate all the voids, which fits with the gradual rise.

I don't think you mentioned the type of piezo. Vibrating wires do go bad, typically showing a major increase when they do. Standpipe piezometers have less that can go wrong.

That's about all I can say about it without doing a detailed study myself.

 

[aeoliantexan]

Please show us the downstream toe and tailwater level.