After construction was completed in 2005, it was noticed that the central pier of bridge 815 presented differential settlements of about 2 to 2.5 inches between the North and South edges of the pier footing. The resulting deck deformations and potential for foundation failure required a stability assessment and an underpinning solution to reestablish the bridge deck cross slope. A geotechnical investigation that included borings through the existing seven foot thick footing was completed. A geotechnical three dimensional model of the pier foundation was developed using a finite element computer program. This Combined Analysis model developed for the project allowed to perform all calculations following the LRFD design considerations and to include the contribution of the existing spread footing in the final underpinning solution. The model also allowed determining the design capacity required by the micropiles. The design solution, which considered locating the micropiles within the existing footing footprint, reduced the impact of the repair on the normal functioning of the road and reduced the number of piles required. The recommended solution has been constructed and four of the micropiles have been instrumented and then monitored for a period of 20 months. A comparison of the loads predicted by the design model and the actual loads on the piles is presented. The results validate the application of the Combined Analysis model for bridge foundation design.
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