Rowe's Moment Reduction

[AlanCT]

For anybody that replies to my previous questions, the wall is anchored only one tier towards the top and is a 30 foot excavation. Piles are embedded 22 feet.

 

[PEinc]

Well, that answers why your piles are so close together. Why are you anchoring the soldier beams near the top of the beams? This gives very large soldier beams (or very close spacings) and very long embedment depths. Can't you lower the tiebacks down to better balance the bending moments? I usually try to locate the tieback, in a one tier wall, about 0.35 x H down from the top of the soldier beam. Also, 30 feet is just about the normal limit of a one tier system.

 

[DRC1]

Your toe to cut ratio is .73, which is high. I don't know if Rowes moment reduction would apply to your situation. I agree with the previous post (Big H or Geopave- sorry forgot which) the spacing and pile size does not really allow the sytem to be flexible. I believe Peck allowed a 20% reduction, but that was for a system with multiple brace points. I also agree with PE Inc that 30 ft is pushing the limits of a single tie back. It seems that you are trying to get a system to work that is warning you it is marginal at best. I would try a design with 2 rows of anchors. This would allow you to reduce anchor size, your pile toe (significantly) which would reduce your pile length and allow you to open your pile spacing which would reduce the number of piles. You may find that by doing this you may have a sytem with a higher factor of safety and a lower cost.

 

[AlanCT]

Thank you for your responses. Our anchors are located 5 feet down. We are trenching across an existing highway to place tie backs to an anchor block. The anchors will be prestressed. We do not want to sheet for the trenches. Its a tricky area and are limited by the site conditions. Its a bridge replaceent in a onjested area in New York.

 

[DRC1]

Without running any numbers I would guess you are running about 10 kips per linear foot for your wale loads. Are you going to be able to develop sufficent reaction from a deadman beam 5 feet down? Are you using blocks or a continous beam? With such close spacing, you might be better off with a continous beam.
I would still look at drilled in anchors. If you need to shorten the anchors, multi stage anchors can give you a desired capacity in a much shorter length than the traditional drilled in anchors.

 

[PEinc]

Tie rods, especially those at only 5 feet deep, need to be checked for clearance with existing and future utilities. I assume that the sheeting and tie rods are temporary. Also, for a 30 foot high wall, the tie rods will be fairly long. As DRC1 said, consider drilled and grouted tieback anchors. They can be installed at a lower elevation and at a downward angle to avoid utilities, they can develop high capacity (if the soils are conducive to anchors), and you do not have to dig up and replace the roadway paving.

 

[AlanCT]

DRC1/PEInc.:

We are around 15 kps/ft. We're using a continuous block and have it back far enough with anchors every other pile to a waler. We want to avoid drilling for cost purposes. We're trenching, installing PVC sleeves and pushing the anchors through later.

Yes, this is only temporary to remove 1/2 of a bridge's abutments. Logistically, this is the way the contractor wanted to go. Dollars!

One more question if you don't mind. Would one apply the active earth pressure below the dredge line to the pile flange width or the effective passive resistance width (3 Bf)?

Thank you.

 

[PEinc]

I try not to add active pressure to the back of temporary soldier beams, below subgrade. AASHTO, 17th Edition, Page 106, Note 1 (below Table 5.6.2A) indicates what active and passive pressures should be applied to a TEMPORARY wall embedded in GRANULAR SOIL or ROCK. Read the note carefully and do only what it says. My experience shows that most engineers never read this note and always add the active pressure. The extra added active pressure doesn't usually hurt the design too bad, but it does hurt. Most soils will arch well enough to not apply the extra active load below subgrade. Also, Kp is usually 10x Ka. Given the "accuracy" (or lack thereof) of the soil properties, why worry a lot about reducing Kp by Ka which is only about a 10% reduction?

HOWEVER, I believe that the newer LRFD AASHTO may not say the same thing. Somewhere, for no apparent reasons, AASHTO "fixed" something that wasn't broken.

 

[AlanCT]

PEinc.

Thanks for the useful insight.

Its much more than a 10% reduction I beleive because the active wedge above the embedded portion is calculated using the height of excavation as opposed to the passive resistance which is based on only embedement depth. Would you concur?

Thanks again.

 

[PEinc]

The height is much greater but the active width of the back of the soldier beam is only b while the passive width on front of the soldier beam is 3xb. I never really quantified it. I don't add the active for temporary walls but do for permanent walls.

Another thing - when designing by hand calculations, it has been very common for many, many years to assume a hinge at subgrade, find the "ground reaction", and then calculate a passive depth with a safety factor. When computers came along, the programs didn't assume a hinge at subgrade and automatically solved for the embedment depth. When a program solves for the embedment depth, you get larger soldier beam bending moments and sometimes longer embedment lengths. This results in a more conservative design. Don't be afraid to assume a hinge at subgrade and then solve by hand for the required embedment depth. This could help you greatly with the design you are now doing.

 

[PSlem]

I've kept an old article from some mag like Foundation Drilling that quantified it. It looked at the passive zones and assumed a common line when they overlapped. This is one area I've wanted to explore as I have never seen a toe failure, so my old assumptions of 3x width for driven piles or 2x width for drilled ones are very conservative. More when I get in.

 

[AlanCT]

Thanks to both of you. This is an interesting sublect. After switching from adding the active below the dredge line from 3B to B, my embedements went down from about 22' to 15'. Also, the moment is reduced. The Temporary works guide from AASHTO illustrates a 3B distribution for the active (and ofcourse passive), thats what we've been using.

The following results for this 32.6 foot deep excavation with piles at 3'-8" oc(also 3B) are as follows:

Embedment: 15'
Pile: W14x120
Anchor Load: 13.2 kips/foot.(located 5 feet below grade)

Does this embedment seem reasonable now?

Thanks

 

[PEinc]

AlanCT,

AASHTO and PADOT's design manuals do show 3b on back of the toe. If you look at the equations for the actiove and passive pfrces, you can derive each one except the active one with the 3b. I spoke to Jerry Dimaggio of FHWA about this active 3b being a mistake. He told me it was wrong but that I should be using the LRFD method for sheeting design. So, in the end, he wasn't much help and basically ignored the problem. I never uses anything more than b on the back of the soldier beam toe, and only for permanent walls or sheet pile walls.

Re run your design with a hinge at subgrade. You will get even better results. Are you using any particular sheeting design program?

Where is ground water? Are you using buoyant weight below subgrade? Does your 15' toe include any safety factor? 15' doesn't sound too bad for a one-tier system with the tie so high. Who established deflection criteria? Are you supporting any critical utilities or structures - or just roadway traffic?

 

[PSlem]

Analysis of Passive Pressure on Soldier Pile Embedment was in Dec/Jan 1986 edition of FOUNDATION DRILLING. It was written by Alan Macnab, but his new book doesn't address it. He instead references a program ct-SHORING Suite Plus. However the original article looked at the force to plow a strip in front of the soldier beam and significantly reduced embedment. Too detailed to type, though.

 

[AlanCT]

PEInc:

I like the hinged pile at subgrade approach. I will try it. Also, ground water is at excavtion bottom and yes, I am including bouyant weight. Just supporting roadway traffic. What do you think?

 

[PEinc]

I still think you need to use drilled tieback anchors, smaller soldier beams, and a wider soldier beam spacing. Depending on the tieback load, I would also look at 2 levels of anchors.

Are you running the calcs by hand or are you using a computer program? If so, which program?

Very rarely does anyone use FE analysis to design sheeting walls. Specialty contractors who design and build earth retention systems almost never use FE analysis.