Assessment of long-term structural movements in a historic cliffside construction through Lomb-Scargle spectral analysis of unevenly spaced time records: the Punta Begoña Galleries (Getxo, Spain)

Long-term monitoring of structural movements in historic buildings and heritage sites allows assessing their stability and recognizing damages that require intervention. The Punta Begoña Galleries, built in the earlier part of the twentieth century, present pioneering techniques in the use of reinforced concrete in building construction. They stand directly over a coastal cliff, and their recovery requires first to guarantee their stability, while maintaining their historic and patrimonial values. Thus, with the goal of analyzing their global stability, as well as the extent of the observed damages, we implemented a motion monitoring network that includes three boreholes for extensometric control, an inclinometer, and five crack gauges (crackmeters). This monitoring was complemented with the recording of hydrometeorological variables at the surface and in four piezometers. The spectral analysis of the signals of movements was performed by introducing the use of the Lomb-Scargle (LS) periodogram, which is particularly well-suited for the analysis of unevenly spaced time series. This analysis allowed us to differentiate the reversible seasonal elastic components of the records and to recognize the irreversible long-term plastic displacements, which highlight the sectors with active instability. In our case, the identified damages are related to two local problems of building support. Even though the irreversible component of the displacement after seasonal sinusoidal detrending is small (with maximums up to 0.12 mm/year), it does imply a dynamic plastic deformation, which calls for the need to adopt structural stabilization measures.