Design of Solider Pile Wall 1

[t230917]

Hello

I am in the process of designing a soldier pile wall for an access road. The max height of the wall is 12 feet and the SGR recommends drilled soldier pile walls as the best alternative. I have not designed a soldier pile wall before and needed help with how to get started. I am starting off using the AASHTO LRFD Chapter 11.8 for Non-Gravity Cantilever Wall design.

The SGR notes that the soils are dense to very dense sand, with bedrock 15 feet below finished grade of the access road (26 feet from the top of wall). There is also a soil layer of silty loam at the top 5 feet, so I will have to use multiple different values of "Ka" to get the soil pressures. The wall will also have a 250 psf live load surcharge on it, with a 1:2.5 (V:H) slope behind it.

Can you please help me with any insight and or design examples that I can follow?

Thank you.

 

[PEinc]

AASHTO addresses earth pressure for stratified soils. Dealing with a cantilevered soldier beam wall in stratified soil should be easier than a tiedback wall in stratified soils. You just need to be creative in how you deal with the rock that is below the excavation subgrade.

http://www.PeirceEngineering.com

 

[t230917]

@PEinc

Thanks for your input. Any suggestions on how I should deal with the rock?

 

[BridgeSmith]

If you're dealing different layers of embedment materials, I suggest using a P-y pile analysis program, such as All-pile or LPile. The approach in AASHTO is not too bad for a single embedment material, but it would be a PITB for multiple layers.

Getting the loading of multiple layers of retained soil is just a matter of calculating the pressure at the top and bottom of each layer, based on the soil properties and overburden from the layers above, and summing the effects from each layer.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[PEinc]

t230917, just draw a section through the wall at a soldier beam. calculate and sketch out the theoretical, triangular, active and passive earth pressure along the length of the beam. When you get to the top of rock, use AASHTO's uniform, passive pressure for embedment in rock. The magnitude of the passive pressure is a function of the rock's shear strength. With many rocks, your calcs will show that you need very little embedment in rock. Therefore, the questions are, "How little embedment in rock are you comfortable with?" and "How much embedment is reasonable and affordable?" You probably won't need to use more rock shear strength than the cohesion value for a very hard clay.

http://www.PeirceEngineering.com

 

[BridgeSmith]

You probably won't need to use more rock shear strength than the cohesion value for a very hard clay.

Around here, much of what is considered "bedrock" would have shear strength values well below that of hard clay (weathered shale is a prime example). I advise caution in making assumptions about the shear strength of any material without at least something more descriptive than "bedrock". In lieu of a geotechnical investigation, there are presumptive lower-bound values that can be used, based on a characterization of type of rock and the level of exposure (weathering).

Rod Smith, P.E., The artist formerly known as HotRod10

 

[t230917]

@Bridgesmith

Thanks a lot for the input. Yes, I was also thinking of getting something like LPile to help with the load calculations and will try to see if I can get the software for our company. Thanks for also explaining how to go about it in case I have to do it manually. Also, the SGR done for this shows that the bedrock has a modulus of rock mass of 300 ksi, a uniaxial compressive strength of 7.7 ksi and a RQD of 27%. I dont see a shear strength noted on there, so I will check with them if they can provide it to us.

@PEinc

Understood. Thank you for the detailed explanation. Just this morning, I was looking at Fig. 3.11.5.6-2 on AASHTO for the bedrock and thinking how to incorporate it into the calculations. Thanks for explaining this. I will try to get the shear strength from the geotech folks who did the SGR and hopefully that will help me get all the loads.

The top 5 feet of soil is a silty loam and the SGR notes that this has an undrained shear strength of 2,500 psf. I will try to use 2,000 psf of shear strength for the bedrock till I hear back from the geotech folks.

 

[t230917]

I mis-spoke. The SGR recommends to use the drained shear strength in the long term conditions, which is 100 psf (for the silty loam). I will go with this value for now.

 

[BridgeSmith]

I don't think Lpile will calculate the loads for you; just the resistance (from the dredge line down). It's made to analyze piles with soil at the same level on both sides, not retaining walls. You'll have to calculate and input the applied moment and shear from the retained soil, and set the ground surface elevation as the surface in front of the wall (dredge line).

Don't forget to multiply the soil pressure load effects by the pile spacing. (Been there; done that; looked like an idiot)

Rod Smith, P.E., The artist formerly known as HotRod10

 

[t230917]

@BridgeSmith

OK understood. That helps, so I know what to do if/when I use LPile.

To your second point - Yep I will do that. I saw the AASHTO had the "l" term for this, and noted that this was the pile spacing; but only for the active pressure calculations.

Thanks.

 

[BridgeSmith]

Yes, the resistance calculations use just the width of the pile. If the piles are close together, there could be some overlap of resistance zones that may reduce the soil resistance. In AASHTO, there's no reduction in capacity for center to center spacing equal to or greater than 5 times the pile width. At a spacing 3 times the pile width, the capacity would be reduced to 80%. If you need to have your spacing is less than the 5X, we can explore how to handle any reduction for your application, but I'm not going to hurt my brain trying to figure that out if it's not applicable.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[PEinc]

Passive resistance for soldier beam retaining walls is very commonly considered to act on 2 to 3 times the width of the soldier beam flange or, for drilled-in soldier beams backfilled with at least lean, low strength concrete or flowable fill, to act on 2 to 3 times the diameter of the concreted soldier beam drill hole. The total width of the passive resistance should not be greater than the soldier beam spacing.

http://www.PeirceEngineering.com

 

[BridgeSmith]

Passive resistance for soldier beam retaining walls is very commonly considered to act on 2 to 3 times the width of the soldier beam flange

If doing a hand-calculated approximation, that may be true. However, the equations in the AASHTO spec are calibrated for use with the actual pile width.

If the pile is grouted into a larger hole, for the grouted portion, the diameter of the concrete shaft would be used.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[PEinc]

"If the pile is grouted into a larger hole, for the grouted portion, the diameter of the concrete shaft would be used."
AASHTO allows 3x flange width or 3x concrete diameter.

Per AASHTO C3.11.5.6, "The effective width for passive resistance of three times the element width, 3b, is due to the arching action in soil and side shear on resisting rock wedges."

In AASHTO Figure 3.11.5.6-1, Pp = 1.5xKpxgammaxD[sup]2[/sup]xb = 0.5xKpxgammaxD[sup]2[/sup]x3b. The 1.5 comes from multiplying 0.5 x 3. AASHTO is therefore allowing the use of 3b for calculating passive resistance.

http://www.PeirceEngineering.com

 

[tk78]

California Trenching and Shoring Manual is a good reference for soldier pile design.

 

[jdonville]

t230917,

You know you could just pawn this off on a specialty contractor as a design-build project, right?

 

[BridgeSmith]

I've seen and used those equations in AASHTO, PEinc. It's what I was referring to when I stated "the equations in the AASHTO spec are calibrated for use with the actual pile width." I wanted be sure it was clear that it would be incorrect to multiply the actual pile width when using the equations, because the AASHTO equations already account for effective resistance width.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[bridgebuster]

This is a link to a NYSDOT manual for flexible wall design. It explains how to design a soldier pile lagging wall.

Link

 

[PEinc]

BridgeSmith, thanks for your clarification. Designers can get into trouble if they blindly use the AASHTO equations for the earth pressures without knowing how to derive those equations. I remember one of the previous editions of AASHTO's design manual that also had incorrectly multiplied the embedded pile width by 3 when calculating the active pressure acting on the embedded length below subgrade.

http://www.PeirceEngineering.com

 

[t230917]

Thanks all for your input and guidance.

Yes, I have reviewed the NYDOT guide as well as AASHTO in detail.

I have attached a spreadsheet PDF that I have come up with for this design. Can you review it and let me know if it looks OK? I have the equations used right next to the final values, so hopefully it is clear.

I have not taken a look at the california trenching and shoring manual, but will review it now.

Thanks.