Pin Pile Capacity and Spacing

[jdcullen]

I'm trying to help a colleague with a foundation repair on a house. They are looking at having a contractor install pin piles around the foundation to stop some settlement. I beleive these are the type where you attach a plate to the bottom of the foundation wall slab and jack the pile, using the foundation as your reaction point It's a shallow foundation ~40-in deep

In terms of analyzing the capacity of the pin piles, do you determine the individual pile capacity simply based on its allowable compressive capacity as a column assuming it's driven to a bedrock substrate? I'm trying to help them come to some reasonable (er.. maximum spacing to limit install costs) spacing, hopefully to compare to what their contractor says.

 

[structuralaggie]

Are these those little concrete cylinders that are jacked into the soil? If so, I think they use the weight of the structure to jack against to push the pile into the ground. A contractor told me they stop jacking when they pick up the house. Kind of a backwards way of figuring, but I guess you could figure out the capacity from the weight of the portion of the structure that is getting lifted.

 

[emmgjld]

For residential repair, the pile capacity usually is sufficient for spacings greater the capacity of the foundation to span. In my own experience, I usually see small diameter steel posts (about 3" to 4.5"dia.), pushed to resistance. The soil profile determines individual capacity. I have used 20 to 40 kips. CHECK YOUR CONDITIONS. If possible, I typically space the piers 12' to 17' c/c, depending on the building loads and the capacity of the grade beam. I tend to space them beyond the contractors comfort zone. But I get to be the engineer.

 

[jdcullen]

I think they will be using 3-in steel piles. In terms of estimating the capacity of the piles, do you then approach similarily to soilnails, micropiles, etc. and determine the individual capacity based on the pile diameter, length and allowable bond (shear) strength between the pile and soil? Like you mentioned, spacing outside the contractor's comfort zone will save some money for the homeowners, unless the house keeps settling of course.

 

[structuralaggie]

Good point on the capacity of the foundation to span. One I had was suppored on a perimeter 4X6 (built back in the 30's) that could only span about 5 feet. That pretty much dictated what the spacing of the piles needed to be.

 

[mdhshanwil]

There is quite a bit of information here:

http://www.seattlegeotech.citymax.com/page/page/2911351.htm

Including PDFs of a few related references for small diameter driven piles.

-M. Harney

 

[Donrolla]

The manufacturer of the push pile system (bracket and pile) should have ratings for the system based on assumed concrete strengths and near surface soil conditions. The ratings may be downgraded for poorer soil or concrete conditions. Push piers are normally used to bear in hard stratums below the zone of seasonal moisture fluctuation. The ratings may be based on actual field testing or other methods of analysis such as ASD method for columns using an assumed depth to fixity. The spacing is dependent upon the moment/shear capacity of the foundation system and the load rating of the pile. It is important to realize that there are eccentricities in the system, some of which will be taken by the bracket/shaft/soil interaction and some by the stem wall of the structure. The loading condition is also different if you are stabilizing the structure versus trying to lift the structure. The installation must be performed with calibrated jack/ram units so that a load test is performed at each location. If a lift is performed, it should be a simultaneous lift with the lifting pressure applied to each pile at the same time to distribute the load evenly among pile location. The important issue is to use a qualified installing contractor and a competent manufacturer.