Stability and strength analysis of leaning towers

Many ancient towers are afflicted by stability problems. Shallow foundations are typical components of such structures, and their behaviour strongly influences the overall safety of the tower. This paper is concerned with two major, geotechnically related, failure modes of tower foundations: (a) bearing capacity failure, due to lack of strength of the supporting soil; and (b) instability of equilibrium, due to lack of foundation stiffness. Both of these hazards are tackled in a unified way in the paper, using a strain-hardening plasticity model for shallow footings from which a non-linear rotational stiffness (moment–rotation curve) for the soil–foundation system can be derived. The stability of equilibrium analysis is augmented by the inclusion of rotational creep, which is new, and an important feature of the behaviour of real towers. The practical utility of the analysis resides in its simplicity, input parameters being derived from conventional geotechnical data. The analysis provides not only a complete framework within which both failure mechanisms can be assessed but also a prediction of which of them is eventually most likely to be critical. The methodology is demonstrated in relation to the Pisa Tower.