Performance Monitoring of a Bridge Abutment Spread Footing From Construction Through Service

The use of spread footings over compressible soils is becoming more common for Minnesota Department of Transportation bridges as technologies improve to better predict, mitigate, and evaluate settlement. In August of 2011 the north abutment of a new bridge crossing I-494 was constructed over compressible soils following a soil fill preload, designed to reduce the foundation settlement from several inches to less than one inch, to meet project requirements. Spread footing foundations are seldom outfitted with instrumentation; adequate performance is frequently assumed based on the decision to use shallow foundations. Here, a monitoring plan was developed to validate the preloading technique for mitigating otherwise unacceptable deformations, assess the efficacy of shallow foundation monitoring methods, and gain a better understanding of shallow foundation behavior with time. Instrumentation consisted of two earth pressure cells, a horizontal MEMS SAA deformation monitoring array, and four optical survey reflectors which were installed during the construction of the foundation and abutment wall. During the course of construction, portions of the abutment backfill soil volume were placed and removed to accommodate the construction of the bridge deck and the adjacent wall footings. The effect of the various loading and unloading conditions was observed on the sensors. The abutment foundation performance over the construction timeline is discussed, including apparent loading, deflection, and rotation. The data from the manually observed survey targets is compared to the automated data from the SAA and earth pressure cells