Analysis and strengthening of caisson foundations for uplift loads

Abstract

Many existing self-supporting towers are built with constant-width caisson foundations. Due to the increased demand to add more antennas to the towers, and more stringent strength requirements in recently revised design standards, many existing caisson foundations require significant strengthening for additional uplift resistance. Although a number of retrofit design solutions are frequently used in practice, there is a lack of literature providing guidelines for the proper analysis of retrofitted foundations. This study proposes a detailed analysis and design methodology to significantly increase the uplift capacity of existing caissons through the use of helical micro piles and reinforced concrete cap beams. Strut-and-tie models are developed and nonlinear finite element analyses are undertaken to verify the behaviour of the proposed design. The overall design methodology is presented in a case study involving an existing tower. The proposed design has a general applicability and is suitable for applications where there is limited space around the existing foundations.