STABILITY ANALYSIS OF WELL FOUNDATION DURING CANTILEVER CONSTRUCTION OF SEGMENTAL PSC BOX GIRDER BRIDGE

Abstract

Well foundations (also known as caissons) are commonly used to support major bridges, particularly when building across rivers where the ground is very deep before reaching solid, stable layers. When segmental prestressed concrete (PSC) box girder bridges are built using the cantilever method, these foundations face a range of complex and changing loads that are quite different from what they will experience once the bridge is complete. The uneven forces that occur during the step-by-step construction process create important stability challenges that engineers need to consider carefully when designing and building the foundations. In this thesis work, a complete stability analysis of the well foundation for segmental PSC box girder bridge construction using free cantilever technique has been carried out employing the STAAD.Pro structural analysis software. The analysis has been carried out for a pier-well foundation where the superstructure is built up using the balanced cantilever technique from the top of the pier. This analysis includes various cases of unbalanced loads such as (i) additional erection of one segment on one side, (ii) difference in concrete densities (left and right cantilever arm - 2.4 t/m³ and 2.6 t/m³ respectively), (iii) wind load on the partial cantilever, and (iv) live load on the additional segment erected on the unbalanced side only.The structural model comprises the pier cap, pier shaft, and well foundation members. Members 2 through 7 represent the diaphragm region, member 1 represents the left side cantilever segment, and members 8 and 9 represent the two right-side segments creating the unbalance condition. The maximum segment weight is 84 tonnes with maximum depth of 5.5 m and soffit width of 5.5 m. The traveler (launching equipment) weight of 8.5 tonnes is also incorporated in the loading. Base pressure, resultant eccentricity, overturning moment, and factor of safety against overturning and sliding are considered to analyze the structural stability of the footing. Data for the analysis is provided based on different load cases according to IRC and IS standards. It is found that the maximum eccentricity and critical state of overturning arise during the condition where right-side segment erection takes place along with density difference and wind loads.