International Journal of Geoengineering Case Histories ©, Vol. 4, Issue 2, p. 113 St. Isaac Cathedral (St. Petersburg, Russia): A Case History Anna Shidlovskaya, Associate Professor, Department of Hydrogeology and Engineering Geology, Saint Petersburg Mining University, Saint-Petersburg, Russia; email: shidanna2013@gmail.com Jean-Louis Briaud, Professor and Holder of the Buchanan Chair, Zachry Department of Civil Engineering, Texas A&M University, College Station, Texas, USA; email: briaud@tamu.edu Mehdi Mohammadrajabi, PhD, Design Engineer, Hayward Baker Inc., Nashville, USA; email: mohammadrajabi@HaywardBaker.com 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 underlying 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. KEYWORDS: Monuments, St. Isaac’s Cathedral; Engineering properties of soil; Foundation capacity; Settlement analysis; Microbial activity; Numerical simulation SITE LOCATION: Geo-Database INTRODUCTION The historical monuments in downtown St. Petersburg including St. Isaac’s Cathedral are listed as World Cultural Heritage and have been protected by UNESCO since 1990. The current St. Isaac’s Cathedral is the fourth church at this site dedicated to St. Isaac. The first church was made of wood and was built in 1707 in honor of St. Isaac. Peter the Great made the decision to construct the Cathedral in part because his birthday (May 30 th ) coincided with St. Isaac feast day. The first church was too close to the Neva River bank and was soon destroyed by floods. The second bell-tower church was built out of stone by Georg Mattarnovi in 1717 at a location between the first Cathedral and the current Cathedral but was finally dismantled. The third St. Isaac Cathedral was built in 1802 by architect Rinaldi and completed by architect Brenna but was later partially replaced. The fourth and current St. Isaac Cathedral was built in 1858; it has a Greek architectural cross ground plan with a large central dome and four smaller domes at each corner. The engineer in charge was August de Montferrand. The main problems with St. Isaac Cathedral is cracking of the columns due to excessive differential settlement and a slight overall tilt of the structure to the southwest. The purpose of this paper is to analyze the behavior of St. Isaac Cathedral based on existing historical documents, on site investigations performed in 1954 and 2009 including in-situ and laboratory tests, on settlement calculations and observations, and on a series of numerical simulations of the Cathedral foundation and soil by the finite element method using ABAQUS. Submitted: 29 December 2016; Published: 3 November 2017 Reference: Shidlovskaya, A., Briaud, J.L. and Mohammadrajabi, M. (2017). St. Isaac Cathedral (St. Petersburg, Russia): A Case History. International Journal of Geoengineering Case histories, http://www.geocasehistoriesjournal.org, Vol.4, Issue 2, p.113-133. doi: 10.4417/IJGCH-04-02-03