Use of Probabilistic Analysis in Design of Shallow and Deep Foundations

The paper deals with some aspects of reliability analysis, with special attention drown to Point Estimation Method, in foundation design. The aim of calculations is to establish reliability index beta and probability of failure P-f for two main types of foundations - spread footing and pile system. Most certainly foundations are the most important part of any structure. The correctness of its design determines the durability and safety of the facility at every stage of construction and service life. The resistance of the selected type of foundation depends on its geometry and to a large extent on the soil parameters, which have a strongly stochastic character. In most publications, authors rarely or almost never deal with the problem of deep foundations. For this reason, in the paper equal attention is paid to both types of foundations - shallow and deep (focusing on piles in the second case). The probabilistic analysis is conducted in accordance with the recommendations included in the EN-ISO 2394 standard titled General principles on reliability for structures. Subsoil bearing capacity of the spread footing is determining in relation to both Eurocode 7 and Polish standard It should be noted that in the reliability of the foundations the applied safety measures take into account the randomness of both: geotechnical parameters and loads (i.e. statistical analysis), such approach is not fully possible according to the standards used in the design of foundations. On the other hand, probabilistic analysis is much more sophisticated and requires the understanding not only of engineering but also mathematics. For example the difficulty of defining correlation coefficient (rho(R,S)) between resistance R and loads S forces most authors to assume that this variates are not correlated, which is correct in case of pile foundation, but can be very disputable in case of footing. Thus in the paper the influence of correlation coefficient on the reliability index is also shown.