Pile Wall using Slope W 2

[pelelo]

Hello,

I am working on the stabilization of a 8 m cut using Slope W. 2 m off the cut there is a 2 story building.

I am not sure what forces on the pile I need to take into account. I guess I need to consider the pile weight. I read somewhere I need to take into account the shear force But Not sure how to compute it.

I was thinking to use the passive force as shear force.

Anyone has feedbacks about this?, Are there any other parameters I need to consider.

Thanks

 

[FixedEarth]

Pelelo

Can you attach a sketch showing section + soil properties?

In general a pile or a pier can have two kinds of lateral loading-one kind is a lateral load + moment applied at the top of a free headed pier. The other is a pile close to a strip load or an embankment, where there is a continuous lateral pressure that looks like a question mark resulting from the strip or embankment loading. Either case, the pile or pier will have an induced shear load along its depth and a bending moment along its depth. The resulting lateral deflection will be at its maximum on the top of the pile or pier.

You mentioned about the passive force. The induced deflection is multiplied by the soil modulus to obtain a lateral pressure. This is the pressure that the pile or pier pushes along its length on to the soil. This value is best kept to 1/2 to 2/3 of the passive resistance of the soil.

So when one says "Shear force", you must clearly distinguish the lateral load or the induced shear in the pile. For example, a Geotechnical engineer may say the lateral load but a structural engineer may say the shear reaction.

I have not used SLOPE/W, however, I suspect they are asking the lateral capacity of the pile. Remember a pier in a slope has to have some kind of facing so as to distribute the soil pressure on to the piers.

As far as references, anyone involved in slope stability and lateral load analysis should have a copy of "Engineering Design in Geotechnics" by Azizi, 2007 or later edition. I also found the two geotechnical books by John Cernica (1995) to be very useful. This is involved topic, and you may need to read several chapters (lateral earth pressure including lateral stresses from surcharges, slope stability, lateral load analysis of single piles or piers and forces induced by a slope creep).

 

[pelelo]

Fixedearth,

Thanks for your reply. The sketch is attached.

This case is a pile wall closed to a strip load.

I ran an excavation software called PYWALL in order to check pile lenght and spacing for this pile wall design.

Of course, I got bending moment and shear force values acting on the pile.

In my global slope stability analysis, using SLOPE W, I guess I need to enter somehow the shear (lateral) force acting on the pile, maybe as a point load?. I am not sure what to do with the bending moment on SLope W. To my knowledge I would use the bending moment only to determine how many bars will need on the pile section.

That passive force I mentioned I read it from a sheet pile wall analysis on slope W. What I found courious was It was included the passive force but lateral (shear) force acting on the pile was not.

Slope W refers to "shear force", but since it is more a geotech software than a structural software, they should refer as "lateral force" instead.

 

[FixedEarth]

Attached is my approach to the lateral stress analysis from the strip load. 8 m excavation is a lot to cantilver, specially given the building loads that is only 2 m away. Similarly sheetpile is also considered too felxible. Iam thinking that even a tieback wall would deflect too much. Have you though of secant pile wall or a slurry wall option? Check with the shoring designer of other options.

 

[pelelo]

FixedEarth,

Thanks for your reply. I will try to obtain your software as it seems to be a very quick way of computing lateral earth pressures. You are right, We will analyzed this wall using secant or tangent piles depending on the loads.

Now I got some questions for you:

1) You assumed an embedded depth of 40 feet. I guess you computed your lateral earth pressure (103 kips / ft) considering the whole area of the diagram (psf vs depth). Shouldn't you compute the lateral pressure ONLY for the embedded portion (40 ft) instead of (66 ft), which btw I don;t know why your software extends to 66 ft, does it do that randomly?. The concerning part for this purposes from the top to the bottom of the pile wall.

2) On my slope stability analysis, the lateral pressure acting on the pile against the sliding mass will be the one I computed on part 1), applied at whatever depth from the top of the pile. (it is not 1/3 as it is not a triangular distribution).

3) The depth at which I will apply this lateral load has to do anything with the sliding mass wedge at which I will install my secant or tangent piles. I mean, the applied force can be outside the failure wedge, does that make sense? or it needs to be IN the failure wedge?

4) It seems there are other ways of computing the lateral force, which is by using p-y curves, according to Reese and Vam Impe (2001), seems to be are more tedious way of dealing with this.

 

[PEinc]

Have you considered just underpinning the building and using tieback anchors (or bracing) to laterally support the underpinning?

http://www.PeirceEngineering.com

 

[FixedEarth]

Answers for you:

1- If you use the equivalent beam method, then yes, we can take the earth pressure to 10% below subgrade. However, in this instance, I used the earth pressure + surcharges on the cut + the embedment. It can be refined a bit, but you have to decide the Free earth method or the equivalent beam method of anlaysis. See Azizi text for more details.

2- Correct, it is not at 1/3rd from the base. Take your area and that is the thrust force. Find the centroid of that area and that is your point of application.

3-If you use the equivalent beam method, it will be at much higher level than if you use the free earth method. So decide on which method you feel is more appropriate.

4- I like the simplest method the beast. We are dealing with heteregenous soils that can vary in any direction and are working with very simplified earth pressure geometry. The surcharges are based on Bousinnesq's formulas and the final earth presure is an aggregate of the surcharges + lateral stresses. I am sure there are other ways, I just like this simpler approach.

 

[pelelo]

PEinc,

Using tiebacks might be an option, I need to run the numbers. Using secant pile wall I get very high moment values which leads to too big pile diameter.

FixedEarth,

I agree with you about keeping it simple. Another way of doing it is by using Finite Element Analysis (Sigma W) and use the vertical and horizontal stresses on Slope W. I haven;t try yet, I just read through. Seems to me it is somewhat more time consuming.