Proceedings of the 30th Annual Conference on Deep Foundations, 2005, Chicago, Illinois, USA, (DFI)
State-of-the-Art Rock Socket Analysis and Design for Laterally Loaded Drilled Shafts
Aly Mohammad PhD, P.E., Dongyi Yue PhD, P.E.
The design of rock sockets for drilled shafts and piles proves to be a challenge for the Design Engineer. The magnitude of loads on the shaft and the behavior of the rock are critical in determining the required socket length. Several acceptable methods are available in the literature to analyze a rock socket under a combination of axial loads, lateral loads and moments. This paper will present a state-of-the-art methodology to analyze rock sockets utilizing deformation characteristics of the rock. This design methodology was implemented in the design of a major project in New York City. The project discussed herein is a marine transfer station planned for construction at East 91st Street along the East River in Manhattan. In order to support the piers and platform structures at a site with relatively shallow depth of the bedrock, rock sockets were required for the drilled shaft foundations. Rock sockets were utilized to provide the required fixity to the foundations and the structures. Acting at the top of the rock sockets are larger shear forces and moments created by the lateral loads transferred from the structure. Upon review of the available techniques, a simplified method was proposed and used to analyze the rock sockets with a significant magnitude of lateral loading. The method recognizes two mechanisms through which the lateral shear and moment are transferred to the rock; a) load transfer through horizontal resistance of rock and b) the vertical shear resistance of the bond between socket and rock. The first load transfer mechanism was analyzed using the p-y curve method based on a rock model developed by Reese. The second was analyzed using a bond stiffness derived from the formula and curves utilized in the AASHTO Specifications for calculation of rock socket displacement. By comparing the two mechanisms, the main contribution to the large horizontal shear force that developed in the rock socket is identified. The paper will present a summary of the design methodology utilized in determining the length of the rock socket. It is believed that the presented method will provide substantial cost saving in selecting the most economical and efficient design length.
|article #1311; publication #72 (AM-2005)|