St. Isaac Cathedral (St. Petersburg, Russia): A Case History

Abstract

St Isaac’s Cathedral in St. Petersburg was completed in 1858 after 40 years of construction; it is today the fourth largest domed Cathedral in Europe. The soil is a relatively soft saturated sediment and carries this 3155 MN structure that is 100 meters high with an imprint of 92 m by 100 m. It is founded on a 7.5 m thick mat of granite and limestone blocks resting on relatively short timber piles of different lengths. The Cathedral has progressively experienced significant deformation including differential settlement causing cracks in the pillars and tilting of the porticoes. The paper summarizes the available geotechnical engineering and engineering geology aspects of the soil on which the Cathedral is built as well as classical issues such as foundation ultimate capacity and settlement analysis through simple calculations and numerical simulations. It also includes some less classical issues such as the influence of microbial activity on the behavior of the Cathedral through the changes the microbes and their activity create in the engineering properties of the soil and in the groundwater composition. The paper concludes with the results of a numerical simulation of the soil and the foundation under load and a comparison with the limited measurements that have been collected.