Elastic Shortening of Concrete Piles 2

kapilgupt,

The basic formula is

deltaL = PL/EA

where P is the applied axial load, L is the length of the element, E is the elastic modulus of the member and A is the cross sectional Area.

If I recall correctly, you are using tapered precast concrete piles. This means that the area will change over the length of the pile. Moreover the skin friction will result in changing axial load over the embedded length. If you model a thin slice of the pile as a free body (with A equal to the average area over the slice, P will decrease on the bottom portion of the free body as skin friction takes up this load), you should be able to model the pile as a series of these thin slices (say in a spreadsheet) and determine the theoretical shortening for any embedded length.

A more detailed treatment can be found in Das' Principles of Foundation Engineering (single piles) or FHWA HI-97-013 (pile groups) for settlements of the soils inaddition to elastic shortening.

Jeff



Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

kapilgupt - Since jdonville has answered your question, I'll offer a few comments as a (former) Contractor who drove piling.

Elastic shortening is important on test piling where movement in measured in hundredths of an inch and the applied load is (intentionally) very high. It is less important when looking at a "production" pile group (I assume this is the same project that you are discussing here thread256-135311).

The service load (maybe 100 to 150 kips?), not the 600 kip ultimate load, is what the "production" piles will experience.

In a properly designed pile group the loading is concentric to the group; there should be no differential settlement (all of the pile deflect the same amount). The consequence being that some piles will carry more load than others - one reason (of several) for the relatively large safety factors used in pile driving.

In dense sand, the displacement piles you describe almost certainly develop significant skin friction (probably within the first 20 feet, or so).

Suggest that you visit my website (link below) and take a look at Section 15, "Interpretation of Load Tests", last paragraph on page 55, of "Bethlehem Steel H Piles". (I know that you are working with concrete piles, but in this case, issues discussed about steel pile are even more true for concrete pile).

In any event, (in a production pile group) the differential elastic shortening of a 70 ft. pile compared to a 100 ft. pile would be "lost in the noise" of all the other factors that are happening between soil directly under the pile tip and the applied load location. I agree that elastic shortening should be investigated, just be careful consider other factors too.

Best Wishes