Lateral Capacity of Rigid Caisson

[ghgeo]

I need to analyze a reinforced concrete caisson that will support a sound barrier wall that is subjected to wind loads. The caisson is to be 3.0 ft in diameter; the top of caisson is located 1.5 ft below ground surface; and the caisson length is 12.5 ft. I’ve attached a sketch showing the applicable geometry.

The caisson width and length come from approved DOT design drawings. However, these drawings are only applicable when the ground surface is level. Since the adjacent ground is to be sloped, I need to do a site specific design.

I have the basic understanding of how to run a Com624 lateral analysis, but based on the depth to width ratio of this caisson, I would think it would act like a “rigid” pile and the Com624 analysis isn’t really applicable?

All of my available references explain the difference between a flexible pile and rigid pile, discuss how to perform a lateral analysis of flexible piles, but skip over the lateral analysis of a rigid pile.

So, I’m looking for any suggested references, methods, hints, tips, etc., on how to approach this analysis.

Thanks!

 

[FixedEarth]

In the following link, there is a great program called POLEFDN. Use it solve your required embedment after entering soil data and loading data.

http://www.steeltools.org/STEELTOOL...?UserKey=bd227887-bcb8-4012-baf5-6621c8ac8032

If you want more analysis like shear, moement and deflection with depth, "Foundations and Earth Retaining Structures" book by Budhu has a free software called APILES.

 

[rscassar]

ghgeo

I would completely ignore the top 5' where the slope is located. The piling design code I use ignores the top 1.5*pile diameter to allow for soil shrinkage away from the pile.

For your example, I would have something like this:

*see attachment*

Mr. Tomanovich excel spreadsheet abilities are just awesome.

The forces I have used in the attachment are estimates based on what a pole of similar dimensions would be designed in my area. I also used metric units so there may be some rounding errors as I convert back and forth between units.

thread507-278480
thread507-254169

The above are some similar threads that have occurred recently.

Important things to note on your drawings will be the lateral load on the caisson. The geotechnical engineer will need to confirm whether the soil will have the capacity to develop the passive pressures either side of the pile. I am not a geotechnical engineer, but what I have done in the past is asked the geotechnical engineer to confirm the undrained shear strength of the soil (Cu or Su depending on region), multiplied this by 9 (i.e. 9*Cu) to calculate the passive resistance provided.

All the best.

 

[VoyageofDiscovery]

What Code are you required to design to?

 

[six06]

kikflip. That assumption of not considering the first 5 ft is usually normal to do if you have cohesive soils. Also that way of estimating/approximating the passive pressure is for cohesive soils also.

ghgeo. There is a very good method called strain wedge model (SWM) for analyzing laterally loaded piles. Its very good documented and there are two versions of a software based on this method that you can use (one free the other is not). The creator of the method are M. Ashour and Norris.

This method allows you to analyze any type of piles (rigid or flexible) and you can include the slope effect to. Because what happens is that you probably dont have enough soil to generate all the passive resistance. But, considering the size of the pile, maybe you wont really need a lot. Remember that piles the problem of piles under lateral loads is a soil structure interaction problem that depends on both, the soil and pile properties.

One good thing about the method is that you dont need to deal with P-Y curves and the method takes into consideration the flexural stiffness of the pile which will give you more realistic results.

 

[six06]

i forgot to mention the names. One is SWM 6.1 (Payed) DFSAP (free).

 

[VoyageofDiscovery]

Chances are if you have any kind of decent soil, deflection/rotation of top of footing will govern.

Typically, you need at least 0.02 radians to mobilize full passive pressure which is at a ultimate state.

Try to limit your top of footing rotations to 0.005 to 0.01 radians in service.

HTH

 

[paulyshore]

AASHTO 1989-with 1992 and 2002 interim, "Guide specification for structural design of sound barriers" ($60 at the AASHTO bookstore) gives a procedure which 'may be used'.
Basically you construct the net pressure diagrams for each side (uphill or flat and downhill, using the graphs to determine reduced kp on the downhill side), then assume the embdment depth and hinge point to balance the lateral force and moment.