CONTRIBUTION OF NEARBY ROAD AND RAIL VIBRATIONS TO CRACK GROWTH IN A NATURAL ROCK ARCH

Recent collapses of culturally important rock arches and towers have highlighted the need to better understand the mechanics of these failures in support of conservation management, as the specific reasons behind these failures remain largely unknown. Rainbow Arch (span: 4 m), located only 175 m from US Highway 191 near Moab, UT, collapsed unexpectedly in the winter of 2017–2018. Here we show that in addition to crack growth from daily and annual environmental cycles, ground-borne energy from nearby road and rail traffic contributed to a sustained increase in vibration of the arch, potentially leading to accelerated crack propagation and collapse. We developed a new analytical approach where we compute the magnitude-cumulative frequency distribution of continuous peak ground velocity data in order to assess the range and frequency of occurrence of vibrations felt by the arch. We then related vertical vibration amplitude to longitudinal strain and thus stress for a hypothetical tensile crack, to predict rates of crack growth using kinetic fracture mechanics principles. We found that an arch close to busy roadways can experience specific vibration amplitudes ~1000 times more often than an arch in a remote area, and that elevated vibration levels can cause a theoretical crack to grow faster than in conditions with no added anthropogenic energy. Our analysis provides quantitative outputs that may be used in conservation management applications, specifically in assessing the impacts of anthropogenic vibration on the rates of structural degradation of natural landforms.