Performance of Tied-Back Excavation at Water Tower Place

Abstract

The earth retention system used for the construction of a four-level underground garage at the Water Tower Place project, located in Chicago, Ill., is described. The excavation covered an area of 530 by 212 ft and extended to a depth of 44 ft below ground surface. It was supported by a cast-in-place reinforced concrete wall, 30 in. thick, 62 ft deep, installed by the slurry trench method. The wall spanned between the shafts of perimeter foundation caissons spaced 30 to 31 ft on centers, bearing on “hardpan” at a depth of 88 ft. The shafts were designed to carry part of the lateral load and were encased in a steel shell ⅝ in. thick to a depth of 72 ft. The slurry wall was supported at two levels: by a row of grouted earth anchors at the upper level 11 ft from the top, and by inclined rakers at the lower level 27 ft from the top. Performance of the earth retention system was monitored by load cells on tiebacks, strain points on rakers, inclinometers, and by surface measurements. The load carried by each tieback at the completion of substructure was measured by a hydraulic jack. Measured loads are compared with those predicted by the commonly used earth pressure theories. Terzaghi and Peck's pressure diagram appears to fit the observed data. Lateral displacements were generally comparable to those recorded elsewhere for excavations supported by concrete walls and tiebacks. Vertical displacements contiguous to the cut were much less than the predictions based on settlement data reported by Peck for basement excavation in the downtown Chicago area.