Fiber-Optic Monitoring of a Twin Circular Shaft Excavation: Development of Circumferential Forces and Bending Moments in Diaphragm Walls

This study investigated the behavior of a 38-m deep twin circular peanut-shaped cofferdam interconnected with a rectangular section for cut-and-cover tunnel construction using distributed fiber-optic sensors (DFOSs) based on optical frequency domain reflectometry (OFDR). The distributed sensors revealed that temperature changes on the two sides of the diaphragm wall were different upon its exposure by excavation, while the measured strains were used to evaluate the wall deflection and bending moments. The high spatial resolution achieved by DFOS measurements revealed unique aspects of the wall response, which are difficult to obtain by conventional types of instrumentation. In particular, the strains along vertical and lateral directions of the wall panels were measured, the latter of which indicated eccentric compression in the concrete panels that arises from the distinctive peanut-shaped geometry. Developments of hoop forces and circumferential bending moments in the panels at various construction stages are discussed, with particular focus on the release of such during partial demolition of a temporary cross wall to facilitate the assembly and launching of tunnel boring machines. The mechanisms of stress developments and release were simulated using three-dimensional finite-element models that, together with the field measurements, enhance the understanding of the behavior of multicell cofferdams.