Differential settlement prediction via reliability analysis approach

Evaluation of the immediate settlements in cohesionless soils by means of simplified methods is an effective way to predict settlements throughout foundations of common engineering structures. Therefore this approach relies on the hypothesis of the soil elastic behaviour which carries out a larger rate of settlements than the real one. Thus, in order to evaluate a more realistic immediate settlement rate the elastic-plastic soil model has been introduced. This constitutive law is employed here to verify the serviceable state of upper structures after differential settlements for shallow foundations in homogeneous and heterogeneous soils. Differential settlements frequently cause damage to the working level of upper structures which should be avoided by preventive assessment. These kind of assessments are carried out through a methodology which disregards the soil-structure interaction and focuses attention on the variability of soil behaviour under the footing loading system. To do this, an accurate soil description is necessary which reproduces the real mechanical behaviour and the physical characteristics of the medium investigated. Unfortunately few laboratory or in situ results are commonly available for a detailed characterization of soil conditions. A stochastic description of the physical and mechanical properties of soils is introduced in order to rationally deal with the uncertainties related to the natural variability and the scattering in SPT test results. As a matter of fact the NSPT values are affected with a large variability due to either the disturbance introduced by the technique adopted or the strength spatial distribution of soil. Here the authors interpret the variability of the formation processes of soils by means of the scale of fluctuation and the reduction variance function. Sensitivity analysis is carried out by the Point Estimation Method to estimate how much the uncertainty in input soil properties influences settlement resulting variability. Finally the results from a reliability analysis (Chebyshev’s inequality is employed) of differential settlement values for shallow foundations are presented and discussed.