Geologic Controls on Instability in WWI Excavations, Canadian National Memorial Site, Vimy, France
White, Maureen C.
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The Canadian National Memorial Site, near Vimy, Artois, France, commemorates the WWI Battle of Vimy Ridge; where all four divisions of the young Canadian Corps fought together with the British Forces to liberate the French ridge. Today, trench systems and an extensive subterranean network of tunnels underlie the gentle landscape of the park, which is visited each year by hundreds of thousands of tourists. Failure within these excavations is expressed as local subsidence and is potentially hazardous to the public. The following research identifies the geologic factors that influence instability in the excavations and how these factors vary both with depth and lateral extent. The Artois region of northern France is underlain by Upper Cretaceous chalk with a thin veneer of Paleocene sediments. Structure is dominated by the northwest-southeast trending Weald-Boulonnais anticlinorium. Three principle geological controls govern failure within the excavations at the Vimy site; lithologic variations, structural geometry and carbonate dissolution. An extensive stratigraphic study identified variable horizons such as chalk marls, nodular chalks, hardgrounds and flint seams, which affect the strength, permeability and structure of the rockmass. Structural geometry in the chalk varies with depth and clay content. Orthogonal fracture patterns are typical in pure carbonate rockmasses whereas inclined conjugate sets occur in clay-rich chalk. Three failure mechanisms were observed in the Vimy excavations that vary with structure and lithology. Beam failure via block fall-out is observed in pure chalk with subhorizontal and subvertical structures. Ravelling, the upward propagation of roof failure, is typical of closely spaced inclined jointing, and is also observed in shallow clay-rich lithologies. Finally, dissolution pipes occur at the intersection lineations of conjugate joint sets, and are also typical of clay-rich lithologies. Dissolution by meteoric groundwater is identified as the third geologic control and results in a decrease of intact strength, weakening of joint surfaces and overall loss of confinement in the rockmass, thereby initiating the failure modes described previously. These extensive geologic studies pinpoint the origins and variability of instability in the rockmass at the Vimy site.