calculating pivot point at excavation wall and tierod tensile force for bulkhead

[ketanco]

Hello,
In a question I tried to solve when preparing for PE exam, there is an anchored bulkhead supporting an excavation wall. I can not construct the geometry of the problem in my head.. so i couldnt understand the solution.

The question says, an anchored flexible bulkhead, which is fixed from rotation and horizontal movement, is supporting a large excavation. the bulkhead is installed 35 feet deep. The excavation is 20 feet deep. The tie rod will be installed at 5 feet depth, and at a sufficient distance behind the active wedge. it asks for the tensile force in the tierod per unit width of bulkhead. the question gives parameters for the soil also.

Can someone help me to establish the geometry of this problem, or if possible draw a sketch of it and show it? because of not understanding the geometry, i also didnt understand how the pivot point is calculated in the solution. i know what bulkhead means but i dont know what they mean by anchored flexible bulkhead in the first place.

 

[oldestguy]

Any soils engineering text probably has this situation discussed. Have you drawn your cross section and then showed the pressures and anchor loads? Follow with a bending moment diagram and that should answer the question.

Here is a reference with plenty of good stuff for use when you are on the job doing these.

http://www.wbdg.org/ccb/NAVFAC/DMMHNAV/1025_4.pdf

 

[oldestguy]

One more handy reference.

http://portal.tugraz.at/portal/page/portal/Files/i2210/files/eng_geol/NAVFAC_DM7_02.pdf

These may answer many of your questions.

 

[ketanco]

thanks and i will now review the reference you gave. but I couldnt draw anything, as it is the problem. I can not visualize the geometry of this question in my head... i am not very familiar with bulkhead installations.

 

[Kuhuh]

I might be wrong here but I believe they misused the work bulkhead and should have used soldier pile or retaining wall. The way the question is written says its buried deeper than the excavation and is fixed from rotation so automatically regardless of what it calls it it you should be able to tell its cantilevered something with a tie back into the active side of the soil.

 

[ketanco]

okay thanks now i understand the geometry.



based on that understanding, i looked at the solution again. this time i saw that they subtracted the active pressure from passive pressure at the portion below the dredge line.



so they made



Kp x gama soil x height below dredge line (minus) Ka x gama soil x total h (as active pressure starts from the top of wall, not dredge line)



so they found a passive net pressure at the BASE level of the wall. (passive is bigger as its coefficient is much bigger, although its height is smaller)



to this point i am ok



but then , in order to calculate the pivot point, for a reason that i can not understand, they said "using static equilibrium, the pivot point can be calculated by comparing areas" and



the made passive force below the dredge line equal to active force below the dredge line, separated at the pivot point height. so essentially, they divided the section below dredge line into a passive and active part, separated at pivot point, and made those parts equal to each other, to determine the height of pivot point.



why did they make the active and passive pressure below dredge line equal to each other separated at the pivot point? this wall is bigger than that portion below the dredge line. it is continuing above the dredge line. above the dredge line, there are active forces and the force from tie rod. to use static equilibrium, by common sense, you should take the whole thing, or introduce a moment where they cut it, in this case the dredge line shouldnt they? so how can we say active and passive pressure is equal to each other below the dredge line at the pivot point and find pivot point like that?

 

[Kuhuh]

Try this and see it helps you out. Its from Cal Trans Shoring Manual 2011. My understanding of the question is to find Pivot point which is to be D' in the example. Regardless, download the whole manual its a great source for understanding the basics of shoring calcs.