The proposed Red Line project, in preliminary engineering design for the Maryland Transit Administration, includes two tunnel segments: the 0.8-mile long Cooks Lane Tunnel through the Mount Washington Formation of the Baltimore Complex, and the 3.4-mile long Downtown Tunnel through the Druid Hill Amphibolite, Carroll Gneiss, and previously unmapped schist, amphibolite, and marble, as well as overburden. Subsurface investigations have included extensive soil sampling and rock coring, surface-based seismic refraction surveys, borehole acoustic televiewer logging, rock laboratory testing, and petrographic analyses.
Mineralogy and texture of the various lithologic units along the alignment profoundly affect their engineering properties. The generally massive amphibolite and epidote amphibolite of the Mount Washington Formation exhibit exceptionally high unconfined compressive strength, tensile strength, and abrasiveness. In contrast, the layered gneiss, schist, calc-silicate schist, and more foliated amphibolite to the east exhibit much lower strength and a tendency to break along foliation planes.
Characteristics of fault zones along the two proposed tunnel alignments are variable, depending on rock type, amount of displacement, reactivation or secondary displacement, post-faulting alteration, and proximity to surface weathering. Mylonitization at ductile fault contacts has produced localized rock of high strength and durability, presenting potential challenges for tunnel excavation. In other cases, extensive microfracturing has altered the rock’s properties to produce a more favorable excavation condition for TBM tunneling than similar rock without microfracturing. Brittle fracturing, hydrothermal alteration, and chemical and physical weathering processes have also affected the mechanical and drillability properties of rock along the proposed Red Line tunnel alignments.
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