Soil flow mechanisms around and between stiffeners of caissons during installation in clay

Abstract

Structural stiffeners placed inside caissons have created significant uncertainty regarding soil flow mechanisms, side friction, and end bearing, and consequently in the prediction of the underpressure required for installation. This paper describes centrifuge modelling of the penetration of stiffened skirts in overconsolidated clay, identifying the soil flow mechanisms around and between stiffeners and the penetration resistance. The effects of stiffener width and spacing as well as soil strength ratio are discussed. The present guidelines for predicting the point of soil backflow in the gap that forms between the stiffeners are shown to be invalid and a new expression, based on a stability factor (su/γ′B, where su and γ′ are the undrained shear strength and effective unit weight of the soil, respectively, and B is the combined width of the skirt and stiffener close to the caisson tip) is proposed. End bearing at the skirt tip and base stiffener are seen to dominate installation resistance. The normalized installation resistance stabilizes at a value of ∼17.5 at deep penetrations and is shown to be independent of whether the cavity between the stiffeners remains open or is backfilled with a soil–water mixture. This factor of 17.5 is shown to be in good agreement with reported field data and centrifuge test data and hence may be used to obtain a first-order estimate of the installation resistance of a caisson with stiffeners.