monkey drift

tunneling

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Last updated

June 30, 2026

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The term "monkey drift" refers to a specific tunneling technique used in soft-ground excavation, particularly in the construction of tunnels like those for the Chicago subway. This method involves creating small horizontal passageways ahead of the main excavation. These drifts are crucial for setting up wall plates, which then serve as a base for arch ribs. This process helps to support the roof of the tunnel and allows for the safe advancement of the excavation, especially in unstable ground conditions. The technique requires careful workmanship and thorough investigation of soil mechanics.1

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tunnels and underground excavations
Hand-mined tunnels
…preceded slightly by a “monkey drift” in which the wall plate is set and serves as a footing for the arch ribs, also to span over as the wall plate is underpinned by erecting posts in small notches at each side of the lower bench. As the ribs and…
Read More
…preceded slightly by a “monkey drift” in which the wall plate is set and serves as a footing for the arch ribs, also to span over as the wall plate is underpinned by erecting posts in small notches at each side of the lower bench. As the ribs and…
Read More