anchored bulkhead supporting excavation

[ketanco]

Hello,

For the question #86 on the latest version of Six Minute solutions of Geotech, 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.

For the ones who dont have the book, 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.

 

[PE_Guy]

Typical bulkhead cross section

See attached geometry & apply in your problem.

 

[ketanco]

okay thanks now i understand the geometry.

based on that understanding, i looked at the solution again. this time i saw that 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.

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 and find pivot point like that?

 

[PE_Guy]

That is really confusing. Why can't they calculate tie rod force by summing all horizontal forces to zero? That is static equilibrium to me. i.e. T + total active pressure Pa + total passive pressure Pp = 0

 

[MikeMcC215]

PE guy you are correct. Sum forces in X dir. = 0 and solve for unknown tie rod tension force.

 

[PE_Guy]

Does it mean that procedure and answer given in 6 minute solutions book is incorrect?

 

[ketanco]

i think they were trying to treat passive force as reaction force and mobilize it only as much as it is needed, not more. not like a regular passive force calculation for that