ABSTRACT
The aim of this paper is to provide general background on the design methods of axially loaded piles according to Eurocode 7, and to implement the results of the pile load tests within this framework.
A theoretical development of the procedure according to EC 7 will be given, focussing on:
– The procedure to derive characteristic and design values of the pile compressive resistance from static pile load tests and from the results of in situ tests;
– The calibration of the calculation rule, to account for the variability of the prediction of the pile bearing capacity compared with the measured ultimate bearing capacity.
– The method to account for variability of the pile compressive resistance throughout the site.
When the design of the foundation is based on the results of static pile load tests, it is straightforward. When however semi-empirical calculation rules are used to predict the compressive resistance, account shall be taken of the reliability of the calculation rule. This is dealt with through a “model factor” deduced from the comparison of the predicted and measured ultimate pile compressive resistance.
Another source of uncertainty is the spatial variability of the pile compressive resistance throughout the site. This is tackled through the “characteristic value of the pile resistance”, i.e. a cautious estimate of the pile compressive resistance governing the behaviour of the structure. It is deduced from the measured or calculated ultimate compressive resistance by applying the so called ξ factor relating the characteristic value of the pile compressive resistance to the mean or the lowest of the measured or calculated compressive resistances.
Finally, the “safety” is introduced through the partial safety factors applied to the shaft resistance and to the base resistance.
The Eurocode 7 methodology is applied to the results of the pile load tests in Sint-Katelijne-Waver. In a second step, other static pile load tests in overconsolidated (OC) clay are included too. This allows to propose a value for the model factor corresponding to the calculation rule applied.
The value of the model and partial factors is validated by comparing the safety obtained to an equivalent deterministic global safety factor (i.e. validation by comparison with existing practice) and by assessing reliability of the design value of the pile resistance (validation by probabilistic approach).
An example of a pile foundation in overconsolidated clay illustrates the design procedure.
