Pile Design

[CPENG78]

Hello Everyone,
I am looking to design an anchor to withstand a lateral load created by two high temperature water lines. Due to high temperature and allowable distance that the pipelines are to experienced, the total lateral load to be resisted is in the order of 84 kips.

In withstanding such a load, I am thinking that a pile needs to be designed to transfer the lateral load to the soil. Can anyone point me in the right direction as to where to start. I am looking to give the client a rough estimate of how deep such a pile will be. Below is the information I currently have.

Total Lateral load = 84 kips
Lateral load resistance is allowed to be inscreased from the 0 to 300 pcf based on the soils' report.
fs = 0.3 per soil's report
Pipe diameter 24 inches (I would like to keep it around 24 inches due to restriction in space of other utilities)

Your assistance is greatly appreciated.

 

[dirtydude]

84 kips is a large latral load to be resisted by a single foundation element. You may need to consider an arrangement of piles or a larger element such as a drilled shaft.

Can you give more information, such as does all of this load have to be resisted at one location and the 24" width you call out, is that just for your water line or is that for the foundation too?

 

[msquared48]

I would look to at least partially eliminating the lateral load through detailing.

Is this load due to expansion and contraction of the piping?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[CPENG78]

Dirtydude,
Thanks for the quick response. To give you more detail, there are actually four pipelines in the same trench. Two of these lines create thermal stresses on themselves that equate to a lateral load of 84 kips. My supervisor and I both raised our eyebrows at such a large load and I have placed a phone call to the pipeline manufacturer to discuss such a load (they did the stress analysis on the water lines).

The available width from edge of trench to edge of the next adjacent pipeline (the third line after the two creating the thermal stresses)is 29.4 inches. I arbitrararily called out a pile of 24" diameter to start a calculation. Theoretically you could go up to a diameter of 29.4 inches without placing those water lines in their own trench. In response to your question, all load has to be resited at one location. The reason behind this, is that by allowing 0 displacement at this location, the system is forced to displaced at a different location where the system is designed as flexible to allow it to move.

 

[CPENG78]

msquared48,
Yes the forces are due to expansion and contraction of the pipe due to liquid temperature. There are a total of three anchors on the system with the middle anchor experiencing the largest load of 84 kips. The others I believe are 8 kips and 2 kips each. I don't believe the forces can be redistributed due to the way the system is designed and the requirement that displacement needs to be isolated to the area designated.

By the way, a pile was my first thought for a possible solution but I am open to other ways of poetentially solving this one. Thanks

 

[ishvaaag]

If I understand well you think of placing a single resisting pile out of the alignment of the load (in plan). Is it not possible to place a bigger number aligned with the pipes to be fixed UNDER the alignments in plan where the movement is wanted fixed? That way piles are also helped to restrict movement not only by their greater number and their more efficient position to stand the forces (no torsion), but by the degree of fixity provided by a solidary pile-cap.

What follows is with resistant piles and pilecaps aligned under the forcing pipes.

Then, to be clear, you only will be able to sustain any load, not only 84 kips, but at some displacement. So it is up to you to decide how small is your allowed displacement at the point to be "fixed". The smaller the allowed displacement, the bigger the piles and in bigger number.

Respect lateral resistance, as long as the geotechnical consultancy feels at ease with a lateral analysis of the piles on horizontal compression only Winkler's springs, you can analyze your model in a program like RISA or SAP 2000. Divide the piles in segments and allocate at each joint a lateral stiffness (spring) equal to the vertically tributary area (D·segment length, if segments are equal) multiplied by the lateral (horizontal) modulus of subgrade reaction. Dismiss initially the lateral stiffness in the upper node.

Now, you can't take additive this support stiffness for a number of piles bigger than 1. Use this stiffness once per alignment; for example, if you have in each forced alignment 3 piles, you would put a third of the above calculated spring constant value in each joint.

Model the pilecap as conjoint with the piles, for fixity, use P-Delta. Apply the 84 kip load (or loads if per alignment), see if lateral displacement remains under required value. Check also if, by the standig reactions, the lateral pressure then exerted on the soil, is low enough. When doing this, think again in "per aligment", that is, sum all the reactions at some joint level and then check as if this lateral compression was sustained by a single pipe, and not the actual number of pipes you have.

If you can´t place the resistant piles aligned with the forcing pipe, you are asking your setup to resist the force in torsion. A single round pile embedded in the soil is extremely inefficient to such task, it may rotate on its axis, so again use piles and pile cap to form a frame embedded in the soil and use the same principles than above to analyze your outfit.

 

[dirtydude]

I think ishvaaag has the right approach here in regards to efficiency. To expect a single element to handle this load with minimal deflection will be asking for trouble.

If it still has to be done at this one location, a 'super thrust block' if you will could be constructed. Use a couple 24" piles and a tieback anchor to resist the lateral force. Not sure what the work space restrictions are like or the budget constraints either, but it is a possibble solution.

 

[weab]

I don't know if this is a large yard where you have the room to do this but we have done this before. At certain intervals in a long run, add a 90 degree elbow with a run of a certain distance (maybe 10 feet or so), then a 180 and another 90 to get back in line. By doing this, you relieve the longitudinal stress. The pipe bends take the deflection and stress.

 

[DRC1]

You really need to run this by a geotechnical engineer. Soil response is non linearand will not be entirely on one side of the pile. There are programs charts and hand solutions available, but before any engineer solves this they would want o look at the data. However, for bending abblied at the top of a pile, it is generally aborobed in the the top 20 or so feet of the pile, so forty feet would be a conservative estimate for budgeting. again this is not a substitute for design.
another option would be driven sheet pile, precast or cast in place concrete deadmen tied to the structure, or grouted permant anchors. If rock is not too deeep, they may be considered. Batter piles can also resist lateral loads.

 

[cntw1953]

I would try a battered pile system with rigid cap to transfer the load to earth. But I would ask the pipe designer go back the drawing board re-valuate and refine the pipe system.

 

[dik]

I would be trying to minimise or eliminate the load as a first step.

Battered piles can be problematic for variable loads... collinear forces can often be non-collinear...

Dik

 

[CPENG78]

Thank you everyone for your insightful input.

Like I mentioned before, the pile was a preliminary idea. The more we think about it, the more we are leaning towards a "super thrust block" like dirtydude mentioned.

In response to jsdpe25684, the system is allowed to move in the section of the system that is designed as flexible with the bends and elbows like you mentioned. These restraints systems are to be placed near the face of the building and a third location were movement is not to be allowed to avoid the pipelines expanding and puncturing back into the building foundation.

DRC1: You are right, of course I am running this by the geotech to make sure we are using the right approach and the right parameters in order to appropriately design with the site contraints. At this time, I am just waiting for a few contractual items (ie money matters) to be iron out before the geotech can analyze the situation.

Unfortunately, I don't feel too confident that the pipe designers understand the concerns I have with regards to following the load path. Whether is a pile cap or a "super thrust block" the load needs to be transferred from the pipeline to the concrete. There is a welded pipe jacket or pipe anchor that would bear against the poured concrete, however the pipe manufacturer did not understand that there needs to be a mechanism for the force to be transferred from the steel jacket to the concrete. Right now, the design shows that the steel jacket only has 2.5 inches of steel plate around the pipeline bearing on the concrete surface to be designed by us. I am thinking of additional welded steel plate or rebar to transfer that load to the concrete. Otherwise, you could potentially puncture through the concrete and/or bend the steel plate bearing on it. Any input anyone? Thanks again.

 

[CPENG78]

To give everyone an update,
The original force of 84 kips was actually reduced by the pipe system manufacturer. They re-configure their system so that the load was reduced from 84 kips to 32 kips.

I ended up taking this 32 kip load and applied a factor of safety of 2.0 and designed a "super thrust block" to take on 64 kips. Like I thought originally and many of you also mentioned, I worked closely with the Geotech to obtain the correct soil profile for such an application.

At the moment, the drawings and structural analysis report are under review.