Levee Open Cut - Emergency Closure

[nbr1]

For a levee constructed of cohesive fill (lean clay) over lean clay native subgrade conditions, should drained conditions be considered for an emergency sheet pile closure? It would be expected that a flooding event would be rather short; but at what point should drained analysis be considered in conjunction with undrained conditions?

 

[GeoPaveTraffic]

For critical structures, and levees certainly qualify, I would check undrained and drained conditions.

As far as how long it takes for conditions to change from undrained to drained, depends on the loading and how quickly the soil can drain. There are often thin horizontal layers of sand and silt in lean clay deposits in river valleys; this often leads to the clays draining much quicker than expected.

Mike Lambert

 

[cvg]

rapid drawdown is probably the worst case and should be analyzed.

 

[nbr1]

Not sure that rapid drawdown would apply. Again, this application is for a sheet pile system proposed for an EMERGENCY CLOSURE ONLY for an earthen levee; if needed during an open-cut activity.

Basically, the open-cut excavation in the levee is sloped on each side (i.e. V-Notch). If an emergency closure were needed, the contractor would close the notch by driving sheet piles the full length of the open-cut (about 125-ft) and support them with vertically driven H-piles.

 

[GeoPaveTraffic]

So you asking about whether the sheet piles and H-piles should be designed on the basis of drianed or undrained properties of the lean clay?

Again, I would say that it should be checked both ways.

How high is the levee? Also, you will need to check seepage beneath and through the sheet piles. Remember, a single line of sheet piles will leak like a sieve.

Mike Lambert

 

[nbr1]

Thanks Mike.

My intuition was that both conditions should be checked; although, I mostly see 'undrained' conditions referenced
for temporary shoring in clays. However, if installed in an emergency, flood waters could persist for days...
possibly weeks....so I like the idea of checking both conditions.

Not sure if leakage would be a 'serious' issue........I've not seen this in application.

 

[motorbiketocrank]

Off topic from your question but personally, I'd worry about trying to use sheetpiling and supporting H-piles for an "emergency" situation. Could take quite a while to install across a 125' reach. Plus, what about unknowns (ex. you start installing and hit obstructions, or the contractor's equipment can't drive/vibrate them to the depth needed, etc)......a real problem if there's an emergency and time is limited. What about other approaches? Could you stage the work to limit the amount of excavation open at any given time. Could you just hastily "re-build" the embankment to fill in the gap in an emergency and use riprap on the face for scour protection?