This work proposes a method for predicting the behavior of shallow footings bearing on an upper layer of processed cemented soil that overlies a lower layer of weakly bonded residual soil with a high void ratio. The paper describes the results of a series of field plate tests and numerical simulations. The results lead to a semi-empirical method for designing shallow foundations on a double-layer system. The method has been validated by comparison of predicted values with results from a separate series of plate-loading tests. For engineering practice, the proposed method provides acceptable predictions of bearing capacities and load-settlement curves.Key words: footings, cemented layer, layered system, weakly bonded, high void ratio.Résumé : Ce travail propose une méthode pour prédire le comportement de semelles superficielles portant sur une couche supérieure de sol-ciment fabriqué qui repose sur une couche inférieure de sol résiduel faiblement cohérent avec un rapport de vides élevé. Cet article décrit les résultats d'une série d'essais de plaque sur le terrain et de simulations numériques. Les résultats conduisent à une méthode semi-empirique pour la conception de fondations superficielles sur un système à double couche. La méthode a été validée en comparant les valeurs prédites avec les résultats d'une série séparée d'essais de chargement de plaques. Pour la pratique de l'Ingénieur, la méthode proposée fournit des prédictions acceptables des capacités portantes et des courbes charge-tassement.
Plate load tests were carried out on un-reinforced sand and sand reinforced with fibres, compacted at relative densities (DR) of 30%, 50% and 90%. For the reinforced sand, 24 mm long polypropylene fibres were added, a concentration of 0·5% by dry weight. The analysis of the results indicates that the soil load-settlement behaviour is significantly influenced by the fibre inclusion, changing the kinematics of failure. The best performance was obtained for the densest (DR = 90%) fibre-sand mixture, where a significant change in the load-settlement behaviour was observed at very small (almost zero) displacements. However, for the loose to medium dense sand (DR = 30% and 50%), significant settlements (50 mm and 30 mm respectively) were required for the differences in the load-settlement responses to appear. The settlement required for this divergence to occur could best be represented using a logarithmic relationship between settlement and relative density. The overall behaviour seems to support the argument that inclusion of fibres increases strength of sandy soil by a mechanism that involves the partial suppression of dilation (and hence produces an increase in effective confining pressure, and a consequent increase in shear strength).
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