Levee Stability - Riverside or Landside Excavation

[nbr1]

Which would produce the least favorable conditions for slope stability and/or seepage
(see attached); assuming all reasonable loading considerations with the exception
of earthquakes and all other things considered equal?

- a riverside excavation OR a landside excavation near a levee toe

 

[cvg]

first of all, seismic is a reasonable load consideration

the stability is related to the slope of the embankment and phreatic surfaces. you have not indicated what is happening with interior drainage but assuming it does not impact the land side of the embankment, than rapid drawdown would probably be the worst case on the river side.

Depending on the details, excavation 50 feet away is probably not a big deal. excavation on the river side is fairly commonly done as borrow for construction and to increase the channel capacity

 

[GeoPaveTraffic]

I generally agree with cvg. Effect when no flood is on the levee will be minor. Also, this looks to be a homework problem; if it is you should so indicate.

Slope stability affects are likely to be fairly limited.

Underseepage affects may be significant. How significant will depend on the specifics. For example, are there any penetrations of the clay between the levee and the river? Does the ground elevation change between the levee and the river? If the excavation is on the landside, will it be full of water during a flood event? What happens to the ground elevation as you get further from the levee? Is the sketch to scale?

Also, it appears that the ground surface is lower on the riverside of the levee. If this is the case it will affect which side is more critical.

If this is a homework problem, then say so and give us your opinion as to which will be more critical and then we can prod you along a little. If it isn't a homework problem, then answer the questions above.

Mike Lambert

 

[nbr1]

Just to clarify, this is not a homework problem; nor is it a specific project....it's just conceptual, so no need to be get too precise.

I'm just curious as to which side of a levee would be more favorable (in terms of stability/seepage concerns) for an excavation;
say for a large pit or quarry which is desired to be near a levee....water filled and otherwise.

 

[cvg]

from a pure stability standpoint, probably on the land side. in-stream gravel mining causes multiple problems with both rivers and levees

 

[Erdbau]

An excavation on the landside is risky. During a flood, the clay acts as a confining layer, because it has lower permeability than the sand. The water wants to punch through the clay, i.e. there is an uplift gradient on the base of the clay layer. The resistance of the clay layer to this uplift is a function of its thickness. So if you reduce the thickness of the clay by more than half, the uplift gradient acting on the clay more than doubles (assuming the excavation is dry). High gradients can cause sand boils and can initiate piping under the levee.