Case History of Column Cracking Due to Rotational Footing Movement at a Site in Los Angeles, California

An existing building in California is being structurally modified. As part of the project, new foundations will be constructed adjacent to existing foundations. The level of groundwater is at the base of the foundations. During excavation adjacent to existing footings, cracks appeared on several columns. Survey data indicated that there was no measurable vertical movement of the columns or footings (within the tolerance of the survey), but the columns/footings moved 1/8 inch horizontally toward the excavation. A conclusion was reached that a small rotational movement of the footings occurred due to the excavation adjacent to the footings: the rotational movement induced shear stresses within the columns, resulting in diagonal cracks on the face of the columns. The excavation was completed and immediately backfilled with concrete. Crack gages were installed on columns and indicated that the movement ceased after the initial cracking. A comparison is made of the observed movement and the mechanism that would be expected in a conventional bearing capacity evaluation. In addition, the mechanism of translation of movement of the footing is evaluated relative to the observed deformation and cracking of the column. It is concluded that the observed movements are indicative of deformation consistent with conventional bearing capacity theory, despite a sufficient factor of safety being preserved during the construction activities.