Analysis of piles in a residual soil - The ISC'2 prediction

Abstract

The Second International Site Characterization Conference (ISC2), Fall of 2004, included a seminar for prediction of pile capacity organized by the Faculty of Engineering of the University of Porto (FEUP) and the High Technical Institute of the Technical University of Lisbon (ISTUTL). The prediction event involved three, six metre embedment length, test piles, one 350 mm square driven concrete pile and two 600 mm diameter, strain-gage instrumented, bored piles. Invited predictors were provided with the results of in situ tests and laboratory studies, records from dynamic tests, and construction notes. This paper summarizes the test layout, and soil information, and pile data, and presents the results of calculations of pile capacity and load distribution, submitted prediction, and results of the static loading tests. Six CPT methods and one CPTU method were applied to the CPTU data. The results show considerable scatter in calculated resistance distributions total CPT-predicted capacity values ranged from 500 KN through 1,400 KN for the driven pile, and from 1,000 KN through 1,900 KN for the bored piles. CAPWAP analysis on companion piles to the test piles indicated capacities of 1,380 KN, for driven pile and 1,200 KN and 1,500 KN for the two bored piles. The static loading test on the driven pile showed an offset limit load of 1,200 KN and a plunging capacity of 1,500 KN. Despite pile movements of 100 mm for 1,200 KN of applied load, neither of the bored piles showed sign of having reached an ultimate resistance value. The offset limit load for the bored piles was 700 KN and 900 KN, respectively. Effective stress analysis of strain measurements for the bored piles showed the data to correlate to a beta coefficient of 1.0 and a toe coefficient of 16. The tests show the bored pile to have rather small toe resistance values, which could indicate some construction induced softening. Most submitted predictions underestimated the capacity of the piles.