PILE PREDICTION EVENT IN RESIDUAL SOIL IN FEUP , PORTO -- Insight learning from the results of vertical loaded pile tests
Speaker:António Viana da Fonseca
DSc, Associate Professor
Faculdade de Engenharia, Universidade do Porto, Portugal
Date & Time:28 Mar 2008 (Friday) 17:00 - 18:30
Venue:Auditorium II (UM Library)


In the north-western region of Portugal residual soils from granite are dominant. The thickness of these regional saprolitic soils may some times exhibit more than 20m, with more common values of 5 to 10m. Due to their specific genesis such soils present specific, non-exclusive, complex characteristics conditioned by important factors, such as microstructure, cohesive-frictional characteristics, stiffness non-linearity, weathering and destructuration, permeability, condition of saturation, and rate dependency.

The current design practice of bored and driven piles in residual weathered formations is merely semi-empirical and based on bearing capacity analyses (in general, without deformation evaluation). Fully instrumented pile load test are very much informative for the elaboration of specific correlations between load-deformation behaviour and in situ tests results (and also fundamental soil mechanics parameters obtained from precise laboratory tests), for establishing well-based design criteria. The possibility of integrating very wide information in a typical granite residual soil profile enabled an innovative study of this subject

A fundamental base to the success of this study was the execution of an extensive site investigation campaign, and laboratory characterization tests, allowing a confident and flexible choice in input parameters of the pile prediction event. This will also enable the accumulation of experience on such on-textbooks?soils, vis-a-vis oung?residual soils (saprolitic) from granite.

The 2nd International Conference on Site Characterization (ISC'2) was held in 2004 at the Faculty of Engineering of the University of Porto (FEUP), Portugal, leading to project supported by 4 construction companies, which contributed to an experimental site with extensive site characterization including a large variety of in-situ tests and pile load tests. A pile prediction event was organized under the auspices of TC-16 and TC-18 of the ISSMGE. It was a class A prediction event, i.e., all the predictions were submitted before the conduction of static pile load tests, in January 2004. The event was part of a Special Session at the ISC'2 Conference.

In the experimental site (CEFEUP/ISC?), 3 different kinds of piles were executed: bored piles with temporary casing, continuous flight augered, CFA, piles with circular section - nominal diameter 600mm, and driven piles (with square section - with B=350mm). These 3 different types of piles were loaded axially side by side up to failure (piles E9 - bored, T1 - CFA and C1 - driven).

The compilation of submitted predictions (Santos et al., 2005; Viana da Fonseca & Santos, 2008) will be discussed, since they revealed surprising overestimations in the bored pile capacities, while in for the there was an underestimation of the gains due to pile installation effects (densification, etc.).

In this seminar a simple approach to quantify the locked-in toe residual loads will be addressed. A mathematical model will be presented- the Modified Two Straight Lines Method (MDRM) - allowing the interpretation of the pile head load-settlement curve and the determination of the shaft and toe resistances, apart from the toe residual loads.