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. The right embankment has an impervious core with adequate transitioning zones into rolled rockfill, 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 piezometers on the right embankment 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 (entirely impervious) 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 (4-8 ft)over the last 28 years slowly approacing summer pool. Any thoughts please? Thanks in advance. Also see cross section for better visual.

 

[hydrogeology21]

One of the left embankment PZs response versus time.

http://files.engineering.com/getfil...558e-4990-a055-55f241bdd501&file=PZ,_Pool.jpg

 

[BenJohnson]

I wouldn't bet my license on this answer, but my opinion is...

This looks like a dam which is maturing over a long time. As you know, all soils are different and even an imperious material can have degrees of exactly how impervious the soil is at a certain location. For example, let's take for example a cubic foot of soil at P17. It probably took a long time for water to penetrate the upstream embankment and finally reach P17.

The first time water entered our cubic foot of impervious soil, the extra water content could cause the overburden pressure to further consolidate the soil at P17. But who can guess what the internal water pressure should be? Initially, it may respond to lake water level changes (ie, it feels the annual pressure changes caused by seasonal water level variation. Later, as the cubic foot of soil reaches a new equilibrium, it becomes more impervious and no longer feels the seasonal water level variations. That is, it keeps the piezos at a more constant level.

However, other shifts, consolidations, etc in other parts of the dam can cause our cubic foot of soil to change again. Decades of drought or heavy rainfall can also change the average water level in the lake. All of this can affect the piezos quickly or slowly. Soil is not homogeneous, so the piezos are not going to consistently show pressure according to textbook theory.

In conclusion, I think you are seeing pressure variations not unusual to man-made soil structures. I don't see any glaring indication of dam failure. You should carefully monitor the volume of water that seeps through the dam. Any significant changes to the seepage rate plus a sudden change in one or more of the piezos could indicate piping. Then take action.

One option if you are really worried is to bring in a experienced and knowledgeable engineering consulting firm who may have some additional equipment or other techniques for analyzing the dam. I am thinking it may be possible to put some back-pressure (or suction) on the piezos and measure the recovery rate (like a well test). That is beyond my level of experience.