Performance of Long Rectangular Barrettes in Granitic Saprolites

In the last three decades, barrettes, namely, excavated rectangular piles, have been increasingly used as foundations to resist large vertical and significant horizontal loads. Empirical approaches are commonly used for the design analyses of barrettes, due to insufficient understanding of the fundamental behavior of the barrettes. In order to improve current design analysis, the behavior of the shaft resistance of long barrettes (ranging from 36 to 63 m long) under vertical loading is investigated in this paper. Fifteen well-documented and reliable tests on long rectangular barrettes founded in granitic saprolites in Hong Kong are selected for critical review and analysis. A database is compiled from these tests. The writers were heavily involved with the construction, testing, and analysis of three of the 15 barrettes. The barrette shaft resistance is quantitatively analyzed with respect to local displacements, in situ soil properties, and the effective stress principle. In addition, the effects of construction including grouting, construction time, side scraping, and excavation tools on the mobilization of the barrette shaft resistance are also investigated. For the barrettes considered, a minimum local displacement of 20 mm is required to mobilize the side resistance of barrettes fully. Grouting along the shaft of barrettes improves the side resistance. On the contrary, construction time has an adverse effect on the shaft resistance, possibly resulting in 45% reduction in capacity if there is a long period of standing time prior to concreting.