This paper describes the design, contruction and monitoring of a complex urban excavation in stiff Oxford Clay. The project was novel for its complex curved basement excavation in stiff UK soil and the use of the observational method to optimise design and construction. One of the key challenges of the construction works was to prevent damage to adjacent structures, including the historic Savile House building built in 1897, which directly abutted the east wall of the excavation and the Civil War ramparts, which abut Mansfield College. Therefore, lateral wall movements, building settlements, heave and prop loads were closely monitored and constantly accessible to the site team through an innovative geotechnical monitoring dashboard manageable by way of mobile devices. The monitored behaviour of the piled retaining wall system proved to be very stiff and measured movements of both the retaining walls and nearby buildings were all within calculated design thresholds. The observational method enabled the site team to omit temporary steel propping for the double basement, which resulted in significant savings in costs, time and embodied carbon dioxide. The present monitored data provide a valuable frame of reference for future basement construction in Oxford Clay.
