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Ultimate (ULS) and serviceability (SLS) limit state design of rigid foundations on layered soils

Abstract:

The design of shallow foundations has been of interest to Man over the past 4 millennia. It is also one of the classic problems in geotechnics and especially soil-structure-interaction. Although many structures have been built successfully, and a great deal of research work has been carried out, the basis of design still tends to be for highly idealised cases in respect of foundation and subsoil geometry and the associated soil properties and behaviour. There are two key conditions, which must be considered: ultimate limit state (ULS) at failure and the serviceability limit state (SLS) under working loads, for which settlements must remain within pre-specified tolerable levels. Even national design codes for determining the ULS vary enormously for these simple cases, despite being based on similar mechanisms of failure, as do the designs that are developed from them. In Switzerland, soil conditions in the post-glacial valleys, are anything but ‘ideal’ since layers have been deposited containing soft clays sandwiched between sandy/silty layers. The failure mechanism is one of extrusion and hence current ULS design methods are inexact and may prove to be either too safe or unsafe, dependent on the foundation geometry. Similar layered conditions have been found at locations where the upper layer of the soil is artificially densified or replaced by more competent material, typically in cities near to lakes. Currently SLS design is assuming ever increasing importance as possibilities for more detailed and accurate design and analysis are improving and the city and industrial environment is becoming more built up. However the analyses tend to be based on elastic theory and adopt a totally decoupled mechanism from that assumed for ULS design. For cyclic loading on foundations due to traffic or industrial machines, only some empirical assumptions are available. The aim of this research project is to investigate the behaviour of foundations under vertical and inclined loading, initially just imposing static loads, but developing onto analysis of the effects of cyclic loading, to cover the full range of load application. For these layered soils, the mechanism leading to settlements and thus to failure is not well constrained. Physical and numerical modelling will be carried out to establish what influences the development of these mechanisms and whether they follow a similar form from the development of the early deformations through to failure. Parametric studies using physical modelling will be carried out in the new ETHZ Geotechnical Drum Centrifuge, with associated innovations in measurement techniques and observation methods. Advanced numerical simulations will be carried out before the physical tests to predict the foundation response and afterwards to validate the numerical method against ‘real’ data. Design methods will be recommended for use by the industry for predicting the settlements at the SLS as well as the failure loads at ULS.

Authors:

Nater, Philippe

Index Terms:

SoilGroup; centrifuge modelling; numerical modelling; layer; Nater, Philippe

Remarks:

PhD Project

Further Information:

Date published: 2000