Monitoring of tunnel deformation during their excavation is based on measurements of displacements of either single points or contraction of selected distances across the tunnel section. Such data, however, due to measurement errors and local effects may not be consistent with each other, and cannot describe precisely the real behavior of the ground/support shell. In the present study, a theory introduced to estimate the ground surface strain field on the basis of surveying measurements is adopted in order to estimate the average strain of tunnel sections. This theory is based on the assumption of uniform deformation across the tunnel section and of measurements affected by random errors only and uses repeated monitoring data (displacements or distance changes).

The proposed theory was applied to deformation data from representative sections of the Acheloos Diversion Tunnel (Western Thessaly, Greece), subject to nearly uniform strain. Mean strain

changes were estimated for various time intervals following the excavation. About one year after the excavation, maximum horizontal strain of ~1.3%, accompanied by practically negligible vertical strain, was computed.