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A high-rise building constructed in downtown Chicago was monitored for ground movements. The below-grade excavation consisted of two parts: the installation of drilled caissons through the underlying soft clay to hardpan, or in most cases to the limestone bedrock at a depth of 110 ft; and the excavation for a double-level basement to a depth of approximately 25 ft below grade. The monitoring of ground movements was considered essential at this site because it is located in downtown Chicago with adjacent buildings, utilities located in the streets and alleys, and an elevated transit system along the western boundary which is supported on shallow foundations. Monitoring was accomplished by two separate means. Ground surface elevations and lateral movements were taken at many points, including the foundations of the elevated structure and the streets, by surveying methods. Two inclinometer stations were installed and read on a periodic basis to measure the variation in horizontal movement with depth below grade. A correlation was established between the lateral survey readings and the inclinometer readings. The monitoring system was sufficiently accurate to record the observed settlement as a function of the construction activity. This paper is limited to a description of the instrumentation, the soil conditions, and the observed readings related to the various phases of construction.
excavation, inclinometers, surveys, measurement, construction, bracing, deflection, instruments, monitors, soils, caissons
Vice-president and principal engineer, Soil Testing Services, Inc., Northbrook, Ill.
General partner, The Engineers Collaborative, Ltd., Chicago, Ill.