Effects of loading obliquity on field performance of rocking shallow foundations in cohesive soil

Abstract

This paper presents a field study of obliquely loaded rocking shallow foundations resting on cohesive soil. Lateral snap-back and cyclic loading tests at an oblique angle of 45° with respect to the footing axes were carried out. During the snap-back tests, an initial drift ratio was applied to the deck and then the system was released to enable the free vibration. The cyclic loading consisted of five packets containing three to four cycles of similar displacement amplitude. The rocking system consisted of a 1·5 m by 1·0 m concrete footing, steel column and deck. The factors of safety against bearing failure were varied from 4 to 20. It was observed that the system primarily rocked in plane. The moment capacities about footing axes deviated from the calculated values at the orthogonal loading conditions. A method of estimating the rocking moment capacity of a footing subjected to oblique loading was developed and validated by the tests. Natural periods, damping ratio, re-centring ratio, settlement and stiffness degradation during the tests are discussed and compared with the results from previous studies with orthogonal loading. The soil–footing contact area was approximately triangular. A method of calculating the critical contact area was developed based on the bearing capacity theory with two-way eccentricity and then confirmed by observation.