Proceedings of the 30th Annual Conference on Deep Foundations, 2005, Chicago, Illinois, USA, (DFI)
Bearing Capacity of Post Grouted Drilled Shafts Using a Load Transfer Approach
Miguel E. Ruiz, Americo L. Fernandez, Miguel A. Pando
Post-grouted drilled shafts (PGS) are a deep foundation system in which grout is injected at high pressure throughout the tip after the drilled shaft has been installed. Pressure grouting of drilled shafts can significantly increase the ultimate capacity and stiffness of drilled shafts. Conventionally constructed drilled shafts usually require higher axial deformation to develop the available side friction and the tip resistance simultaneously. They also tend to have higher tip compressibility and reduced end bearing capacity due to disturbance at the bottom of excavation during drilled shaft construction. PGS foundations have been found to be a cost-effective solution to compensate for this capacity reduction and/or compressibility and end bearing capacity reduction. PGS construction procedure counterbalances tip compressibility and end-bearing capacity reduction. The tip grouting also has a pre-stressing effect over the shaft side friction since the drilled shaft tends to move upwards during the pressure grouting of the tip. After grouting, a bulb tends to form at the pile tip pre-compressing any disturbed material and, in certain cases, increasing the tip area, thus increasing shaft tip capacity. PGS load-settlement predictions using a load transfer approach are presented in this paper and compared with corresponding load test data. The paper also presents the main findings of load transfer analyses that were carried out to investigate the potential different factors that may be contributing to the observed improved performance of PGS over conventional drilled shafts. Results show that the town main factors, that may have a significant contribution on the overall performance of the foundation by increasing its ultimate capacity at any given level of settlement, are the stress-reversal of the skin friction and the formation of a grout bulb at the tip. The grout bulb tip contributes primarily through the increased tip area and to a lesser extent through increased stiffness and strength of the soil in the vicinity of the pile tip.
|article #1282; publication #72 (AM-2005)|