Static Loading Tests And Piled Foundation Design For A Three-Storey Building-PPT Presentation

PPT Presentation

Pdf Summary

The presentation describes the geotechnical investigation and piled-foundation design for a three-storey school in New Jersey. The site was raised with up to 5 m of clay and sand fill, creating expected ground settlement of 100–200 mm and downdrag on the piles. The design used 457 mm diameter bored piles embedded 22.5 m into dense to very dense silt. With 1,577 piles, the design load was 712 kN per pile, plus an estimated 445 kN drag force for axial-strength checks.<br /><br />Three instrumented test piles were statically load-tested. Their force distributions showed residual force caused by soil subsidence after the fill was placed. Accounting for this residual force produced more credible estimates of shaft resistance; once adjusted, the piles showed broadly similar responses.<br /><br />The authors advocate a “Unified Design” that considers pile-force and settlement distributions together. The neutral plane is established where pile and soil settlements equilibrate, and the pile-toe force and movement must be consistent with the applicable toe load–movement relationship. In this approach, drag force is relevant to pile axial strength, but should not simply be added to the structural load at the pile head.<br /><br />The presentation argues that the original code interpretation overstated the required pile length by adding drag to the applied load, neglecting resistance above the neutral plane, and failing to account for residual force. These assumptions led to overly conservative designs; approximately one-third of the piles were lengthened by 6 feet.

Keywords

geotechnical investigation

piled foundation design

New Jersey school

downdrag

settlement

bored piles

residual force

neutral plane

shaft resistance

Unified Design