Drilled shaft design for lateral load

[mike80]

We are designing a drilled shaft soldier pile wall, socketed into bedrock, to restrain a land slide area. Our geotech consultant has run the lateral load analysis and given us the maximum shear and moment on the drilled shaft.

We have designed the shaft for the maximum moment, reinforcing the 36" diameter shaft with #10 bars. However, we are having trouble designing for the maximum shear. Using the shear capacity of the spiral reinforcing, added to the shear capacity of the concrete, is not providing enough capacity. Instead of increasing the shaft diameter, the suggestion was made to drop a steel section into the drilled shaft to provide more shear capacity. This sounds reasonable, but what area of the steel section is used to calculate the shear capacity? Should only the web depth x thickness be used or the entire section?

Thanks for your help with this.


Mike

 

[AggieYank]

take it for what its worth, since I don't have experience with it, but I'd use the area of the web for one direction, and the area of the flanges for the other direction. I'd assume you'll line the steel section up so that the loading is about the strong axis, and the shear will be carried by the web.

I'm sure there is something that documents using composite sections.

 

[JAE]

Can you increase f'c enough to get the shear capacity you need?

 

[SlideRuleEra]

Web depth x thickness, based on an analogy with (obsolete) steel grillage foundations. Even though the steel members (usually I or WF sections) were completely encased in concrete, only the steel web was considered to resist shear forces.
Reference: "Foundations, Abutments & Footings" by Hool & Kinne, McGraw-Hill Book Co., 1943.

http://www.SlideRuleEra.net

 

[Structioneer]

Can you decrease spacing of the shafts?

 

[Lutfi]

Why not use an H pile in the center to increase the shear capacity? This is a quick thought. They use steel beams on thin slabs to increase the shear capacity near columns in flat plate systems. Then you would have an increase in the moment capacity as well.

Regards,


Lutfi

http://www.cdeco.com

 

[BigH]

is this a secant-pile wall?

 

[BigH]

Sorry - I hit the button too soon. I would take it that this is what I alluded too. Why not use tie-backs to take the shear and moments as well? I would suggest that you get a hold of recent Ground Engineering Magazines. The February issue has a couple of articles that you might find interesting (monthly topic "Slope Engineering": page 14 Step By Step; page 19 Sliding Scale; page 16 Cutting Costs). They also have a couple of articles on soil nailing. They use a lot of contiguous walls (secant walls) in UK.

 

[14159]

How much more shear strength do you need? If you're over by a couple of percent, then that's a lot different than if you're over by 2x.

I like the idea of increasing f'c. If I used the steel shape, then I'd conservatively use the web area only because I don't know of a reference for anything any better.

How tall are these walls? I'm a bit surprised that you can get flexure to work and not shear.

DBD

 

[jdonville]

mike80,

Many of the current publications dealing with slope stabilization with drilled shafts assume that the soil slope has failed and that the drilled shafts have to resist the entire wedge of soil above the failure surface.

The Ohio Dept of Transportation has been sponsoring research into this area. Unfortunately there is no hard and fast conclusion from the work as of yet - I understand that they will be undertaking instrumented drilled shaft testing as one of the next steps.

There is a slideshow (powerpoint) available online that illustrates the current state of thinking about shaft-stabilized slopes and their analysis. The following link is connected to the final report for one portion of the research.

http://www.dot.state.oh.us/research/2002/Structures/14705-FR.pdf

Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[jdonville]

mike80,

Sorry, the powerpoint is no longer online that I can find. I have posted it on my personal web site at

http://www.geocites.com/jtdonville/Stabilization_of_Landslides_with_Drilled_shafts.ppt

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[mike80]

Thanks to everyone for their helpful responses. To answer a few of the questions:

1. The shear exceeds capacity by about 150%, so increasing the f'c doesn't help enough.

2. Our geotech has proposed 36" diameter shafts on 6'-0" spacing, so would be possible to reduce spacing slightly, although it probably would be more economical to use the steel section, either an HP or WF.

3. This is not a secant-pile wall. The shafts will be completely buried and will depend on soil arching between the shafts. The shafts are to prevent further movement of landslide areas.

4. The shafts are about 28 feet long from surface of ground to top of bedrock. In this case the slip plane is at the surface of the bedrock, so the shafts are resisting all the shear forces. We have other walls on the project where the slip plane occurs several feet above the bedrock and the shear forces are significantly lower.

Thanks again for all the help. Based on the responses, we will use the web area for calculating the shear resistance.


Mike

 

[longhornse]

mike80, I kind of agree with DBDavis about the shear capacity not being satisfied. I would think for a wall this tall, moment would control. You said the shaft is 28 feet tall, how much of it is exposed? It puzzled me.

 

[jdonville]

longhorse,

No wall exposure. The application is the use of discrete drilled shafts to stabilize a slope.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com