Analysis of Sheet Piling with Multiple Tiebacks

[chrisrosebud2001]

I am attempting to analyze sheet piling and size for an estimate. The piling is to be used for a construction excavation. The piling will be approximately 26' deep and must resist very large pressures due to groundwater. I have referenced the US Steel sheet pile design manual and have determined that only using 1 tieback near the top will not be sufficient. I am considering 3rd or quarter points. Does anyone have any good references for this scenario or have any suggestions on how to model this in a FEM? I have access to STAAD, but I'm not sure how to model how the pile is embedded.

 

[handex]

If it is new to you then definitely don't try to model it in FEM. Far too many inputs and you will generally just get garbage out. Doing a basic analysis of the wall depends on the soil properties and number of anchors (cohesive/non cohesive, drained/undrained etc). If there was a soils report provided then in my experience they typically give guidance as to a pressure distribution for preliminary design. Sheet piling design requires experience so I would recommend going through it with a senior engineer in the office.

As for anchor locations, if the excavation is for a building basement we position the anchors just above the FFL of the slabs because on most jobs we do the anchors are temporary only and need to be destressed and removed(council requirement). Being just above the finished levels of the slabs allows easy access for the rig to remove the anchors once the slabs have been poured and are supporting the sheets. Also, have you considered dewatering the site for the temporary excavation?

 

[chrisrosebud2001]

Dewatering is not recommended in the geotechnical report. There are existing foundations and settlement would be expected.

 

[dcarr82775]

I don't normally write this, but: Hire someone who knows how to do this. It isn't hard, but you need to understand all the things that go into the simple sheet pile design. If you are not familiar enough with the concepts to extend it to multiple rows of tieback, find someone who is.

 

[GeoPaveTraffic]

In addition to what has already been posted, you should consider that sheetpiles are permeable. Depending on the site soils, your excavation supported with sheetpiles may very well effectively dewater the site.

Mike Lambert

 

[chrisrosebud2001]

I have the geotechnical report including the passive and active pressure coefficients. The soil is non-cohesive. I am familiar with the concepts of sheet piling design, but I have not tackled this particular situation. I was just wondering if someone had an example and suggestions for perhaps modeling the soil conditions in STAAD. I attempted modeling the soil as lateral springs with stiffness increasing as a function of depth, but the results did not compare well with the results of an analysis based on the US Steel sheet piling manual.

 

[FixedEarth]

You can model it using lateral subgrade modulus values but the soil pressures you enter should be distributed (passive loading). If you have access to the books by Tschebotarioff or Jumikis, you can get their classical design methodology for anchored sheetpiles. However, the books will show you "final" design values, in other words, when the sheetpile is already stressed with the 3 anchors in its final stage. In reality you have to check for adequacy of the sheetpile in all stages of construction. Further, there are other geotechnical checks that should be calculated like bottom heave, piping, rotational sliding and deep seated settlement. So when you add all the above checks, it comes out to be a bit more cumbersome than modeling it as an equivalent beam.

You also mentioned that there are other foundations with settlements likely - in such a case sheetpiles are too flexible for use and you should consider more rigid retaining structures. Then comes the comparisons of classical method to SSI or Finite Element method. What if all 3 methods agree with each other, but you have no "feel" for anticipated loads and maximum moments? How or when would you say, "these values can't be right"?

Finally, it is nice to start off with a simple cantilever pile, then on next job a one level tieback sheetpile and so on. If this is your very first sheetpile job, it is a bit advanced for a starter project. I come to the same conclusion, get an experienced designer to assist you. Good luck.

http://www.soilstructure.com/

 

[EngineeringEric]

I would recommend the CalTrans Trenching and Shoring Manual 2011, free PDF. They have an example 7.3.5 multiple tie back sheet pile wall. And they have lots of other useful info as well.