Reliability-based design of spread footings on fibre-reinforced clay

The potential use of fibres in a number of geotechnical engineering applications is gaining more interest in the geotechnical community. A select application consists of the improvement of soft grounds to mitigate their problematic shear strength characteristics. Extensive experimental work has been reported on the response/behaviour of fibre-reinforced clay (FRC) and was recently complemented by several strength prediction models. The effectiveness of these models has not been thoroughly evaluated yet. The objectives of this study are to (1) quantify the model uncertainty of a newly developed FRC model that is aimed exclusively at predicting the “undrained” shear strength of FRCs, (2) combine the model uncertainty with other conventional sources of uncertainty to assess the reliability levels that are inherent in the ultimate limit state design of spread footings that rest on a top FRC layer underlain by weaker natural soft clay, and (3) recommend factors of safety that would ensure a target reliability level for these footings. Results indicate that the traditional safety factor of 3 should be used with caution as it may not be sufficient to yield the desired level of reliability, particularly for smaller footings, lower applied stresses, larger scales of fluctuation, and larger target reliability indices.