Moment Reduction in Tied Back Soldier Pile Walls 4

[AlanCT]

Has anybody ever applied Rowes Moment Reduction Formula for a closely spaced (3'-8" to be specific)tied back 14" HP soldier pile (w/lagging) wall.

If so, do you know of a specific reference that allows one to do so?

Thank you in advance

 

[GeoPaveTraffic]

I've never heard of anyone reducing the moments in a soldier pile wall based on anchored bulkhead theory.

I just reread the infomration included in my copy of Peck, Hanson, and Thornburn (Foundation Engineering 2nd Edition)p. 453. Based on that discussion I do not believe that the reduction are asking about is appropriate for a relatively stiff wall like the one you are asking about.

 

[AlanCT]

Thank You GeoPav:

One additional Note. We would like to consider using the moment of inertia of the pile system on a per foot basis of wall. Thus, "I" of a HP14x117 is approx. 1200 and then divided by the effective width which equals 3'-8". "I" per foot then equals 327. Can it then be considered relatively flexible?

Thanks

 

[PEinc]

I've never seen a soldier beam wall with soldier beams spaced so closely. Why so close?

Terzaghi & Peck's earth pressure diagrams were envelopes intended for determining the maximum anticipated strut loads (brace and tieback loads). T&P say on Page 403 of their 1967 text, "In general, the bending moments in the sheeting or soldeir piles, and in the wales and lagging, will be substantially smaller than those calculated from the apparent earth pressure diagram suggested for determining strut loads."

It is not uncommon for soldier beams to be designed for between 66% and 80% of the moment determined from the pressure diagram. Various references acknowledge a reduction.

 

[AlanCT]

PEInc.

The contract drawings required a pile spacing of 6'-7", drilled in 30" diameter holes and filled with concrete. The contractor elected driving to avoid drilling as a matter of value engineering. Rock is shallow enough to require a tight pile spacing for passive resistance. In this case, the pile spacing matches the effective width of passive resistance (flange x 3.)

Can you please lead me to a reference that allows this reduction? This is critical for us. I've been surching hi and low!

Thanks in advance!

 

[GeoPaveTraffic]

PEInc,

I agree with your comment about the pressure diagrams from T&P. My approach has always been to use the T&P diagrams to design the moment requirements of the soldier piles, but I always used all of the moment capacity of the section. In other words instead of lowering the load, I increased the allowable moment that I would live with.

That said, with most of my work the sections were determined by maximum deflection near the ground surface. In that portion of the pile I used Ka, not the T&P diagrams.

AlanCT,

I have used a per foot section modulus for HP sections, however, I would not be looking at the passive resistence of the soil. The non-linier properties of the soil in this situation do not lend themselves to that analysis. You really need to look at the piles using a program such as LPile to determine how much deflection you are getting the piles.

 

[PSlem]

I'm not at work, but I believe the reference for a 33% reduction is in a 1948 Peck book. I'll poke around tomorrow.

 

[AlanCT]

PSlem:

Thanks, look forward to it.

Alan

 

[PSlem]

I found an old set of calcs that referenced Soil Mechanics in Engineering Practice, Terzaghi and Peck, 1948 and 1967 and Foundation Engineering, Peck, Hanson, and Thornburn, 1973. I only have the last book, but the section that says use 2/3 moment is on page 463.

 

[PSlem]

NAVFAC DM 7.2-102 says 80% except cantilever is 100%.

 

[AlanCT]

PSlem: Thanks so much. We have the 1948 Peck book, I am looking for it now.

We also have NAVFAC DM7.2. I think here what you are referring to is on Page 7.2-102. Can you verify? Our case is a timber lagging wall with steel piles and one tie back at the top.

Thanks again.

 

[PEinc]

Nice job with the quick references, PSlem.

 

[AlanCT]

PSlem:

Thanks for the references!, Alan CT

 

[SmokeyBear]

AlanCT,
I am a bit nervous about your methodology. It seems you are pushing towards using Rowe as a way of providing a cheaper solution and trying to increase the wall flexibility (so you can use Rowe) by considering I/foot run, etc. Being an Antidipodean, I have no idea on dimensions of your HP sections, but it may be drawing a long bow to "flexibleise" the wall by considering I/ft run.

As I recall, Rowe's moment reduction method (an empirical method based on MODEL test results)was developed to take account of the fact that the use of factored strengths, or factored passive pressures, in the calculation of bending moment is unrealistic. Because it is likely that passive pressure is fully mobilised near to excavation level even under working conditions.

Rowe's method should only be applied when the bending moment is calculated on the basis of earth pressure derived from factored strength parameters (or factored earth pressure coefficients.

With great respect to other contributors, I think we have newer tools available, such as FE programs like, WALLAP - Geosolve?? and FREW - Arups etc, which compute a pretty good deflected shape during construction stages and long term. L-Pile is also very good, GGU program suite; to name but a few.

I think these readily available tools should be used, in conjunction perhaps, with the Peck, Thorburn et al.

 

[AlanCT]

SmokeyBear:

Thank you. That was very informative. With the help of all these contributors and some indepedant researching, I've steered away from Rowe's Method. I did not realize though until now that that was based on factored loads.

What I gained mostly from this research was how to apply the active presures to the piles below the excavation line.

I was aplying it to the effective width (3 x Bf). Now applying to Bf. This was the discrepancy I was getting in results by others.

Thanks again to all.

 

[PEinc]

I don't believe that Rowe's Moment Reduction was based on the use of factored loads. His method, I believe, pre-dates load factor design. If you look at design examples of Rowe's Moment reduction in the very old U.S. Steel Sheet Pile Design Manual, now the Pile Buck Manual, all of the examples use allowable stress design.