Tieback soldier pile wall built from bottom up (in fill)

[cwrugrad]

I know these walls are more usually built by excavating, but...

Does anyone know how to analyze a wall like this for the temporary condition just prior to installation of the 3rd tieback, which is when the problem becomes indeterminate?

I used the approach described in Braja M. Das's Foundation Engineering text to analyze the wall as a cantilever wall, prior to installation of the lowest tieback, and as a wall with one row of tiebacks, prior to installation of the 2nd tieback from the bottom. But parts of the wall are tall enough to need a 3rd row of tiebacks.

The two approaches described in the 2004 AASHTO LRFD Section 11.9.5.1 Commentary (Tributary Area Method and Hinge Method) give very different results for the single tieback condition if the tieback is below the center of the pressure diagram, and the way they ignore everything below the lower groundline makes me uneasy.

 

[cwrugrad]

BigH, thanks for your suggestion about deadman anchors. We will leave construction methods up to the Contractor, but I suspect he will be very likely to use deadmen for the earlier stages of the backfilling, because the horizontal forces at the early stages will be much smaller than the forces in the top-row permanent tiebacks.

 

[BigH]

cwrugrad - no, I'm a Cornell grad but growing up east of Cleveland I was knowledgeable about Case Western.

The reason I suggested the deadmen is that a few years ago, we were building a new two lanes of highway adjacent to existing - on the one side was an irrigation canal. Designer wanted to use piles to resist the 7m of new fill overturning moment on the wall. The piled option had 1.2m dia bored caissons some 20m deep in two rows at 3m c-o-c. That's a lot of piles and the pile cap was huge. I looked at the cross sections and thought that a few widely spaced bored piles with a grade beam across them supporting a thin wall using tie-backs was quite doable. Having only 10 bored piles rather than 60, a small grade beam instead of a huge pile cap, thin walls and the like. Elegant, in my view. I lost out. Why? Likely because the contract didn't have an "item" for the tie-back rod and supporting plates on the wall. Too, it would have caused a real design to be done rather than scalping a design from another project. Finally, it might have to do with certain specialties only "knowing" certain solutions. Highway engineers seldom know about wharf construction or large oil storage tank design and construction. We are, as some say, a prisoner of our own experience.

 

[Panars]

cwrugrad-
"If they'd just let us raise the bike path 10' or so ... But I'll go out on a limb here and guess that they will politely decline that idea..."

Don't underestimate the pursuasive power of money. If there is a significant savings to the owner, they may reconsider what to do with the bike path.

 

[dgillette]

Hey! CWRUgrad!

Just noticed your handle. Anybody with a name like that can't be all bad.

Me? CWRU Civil Engineering, class of 1981. Took mostly structural classes from Dario G. and Art H., but went into geotech, perhaps influenced by Adel "Toni" S. Then, left Cleveland for grad school in Boulder.

 

[cwrugrad]

dgillette, I came through a little after you (class of 1985). Same profs, but as you can see I stayed with structures. Ended up in bridge design on the West Coast.

UPDATE ON THE WALL:

We ended up calling for the Contractor to build a temporary bench of soil up to the right elevation for installation of the top-row tiebacks before he starts wall construction (before he installs the soldier piles).

It's the simple, safe solution. More costly than some other ideas a clever Contractor could come up with, but in a low-bidder-wins environment, better to start with something that's hard for them to mess up. If a reputable, expert Contractor gets the job, he can always put in a CRIP (Cost Reduction Incentive Program) request.

Thanks to everyone for all your help and advice!