Publication Abstract

Proceedings of the 30th Annual Conference on Deep Foundations, 2005, Chicago, Illinois, USA, (DFI)

The Effects of Cylindrical Pile Insertion on the Vertical Stiffness and Strength of the Surrounding Soil
Emre Biringen, University of Wisconsin-Madison, DFI 2005 Student Paper Competition Winner

Results from a small-scale laboratory experimental program show the changes in the vertical stiffness and strength of a soft clayey soil adjacent to the shaft of an inserted pile (i.e. expansion of a cylindrical cavity). The experimental program provides a realistic axi-symmetric physical model of an element of a cylindrical pile and the surrounding soil. The study interprets the improvement and assesses its magnitude in vertical stress-strain-strength properties of a normally consolidated soft cohesive soil in response to preloading impact in the radial direction. The results verify that insertion of a cylindrical pile causes densification of surrounding soil in a radial direction due to change in stress state around the pile, and thus consolidating and stiffening of the soil. The test results indicate that most improvement in modulus and strength takes place within a distance of 4 times the radius of a pile, though significant improvement is realized to a distance of 8-10 times the radius, beyond where there is negligible effect. Settlement characteristics of the surrounding soil in a zone of 12 times the pile radius improve significantly after the formation of a cylindrical cavity. The miniature vane shear strength tests show a 1.5 times increase in a zone of 4 times the pile radius compared to the strength at 10 pile radii. The results also imply that the spacing of cylindrical piles should be more than 4 to 10 times pile radius, so that the surrounding soil matrix may improve the vertical bearing capacity under any foundation or embankment loading.

 article #1314; publication #72 (AM-2005)