Geomechanics stability assessment of World War I military excavations at the Canadian National Vimy Memorial Site, France

The Canadian National Memorial Site, Vimy Ridge, France, commemorates the efforts and loss of life of Canadian troops in the First World War (WWI). The site comprises the monument, a Visitors Centre, the Grange Tunnels and restored trenches and several cemeteries. A substantial portion of the battlefield terrain remaining from WWI is preserved at this site, including trenches, shell holes and military mining craters. The ground surface is further disturbed by subsidence of the ground surface into underground excavations left from the war. A significant number of subsidence events have been recorded within the last decade. There is concern that ground surface subsidence could compromise the safety of visitors or workers at the site.The excavations of concern include dugouts, headquarters and hospitals, and were excavated in chalk. Possible excavation types, dimensions and support were investigated in the historical literature. To assess the potential for ground subsidence, the geological setting was investigated and the chalk was mapped and sampled, to determine the geomechanical characteristics of the material. The chalk was accessed in quarry excavations and in tunnels and declines that have been discovered and re-opened in recent years. Finally, the influence of excavation geometry and depth, chalk condition and effects of weathering on the excavation stability were assessed using numerical simulation, using a finite element program, and considering voussoir beam stability. Interpretation of the results of this analysis allow development of general relationships between span, depth and possible failure to isolate excavation classes that pose a long-term hazard, and to reduce the priority for those that are unlikely to fail to surface as well as those that are likely to have failed soon after construction.