Much of the metro’s miles of tunnels were constructed using cut & cover, digging down from the surface, pouring concrete and laying steel, then covering it back up. (Not the deep stations along the redline, those were bored into bedrock). Disrupt that much soil and there will be compaction that occurs years later, and that can damage infrastructure built above. So metro monitors surface elevations around tunnels for that problem.
the_real_dmac t1_j2a744q wrote
Reply to comment by LilGeographersRoom in Mystery WMATA/Metro survey markers: Can any intrepid city planners or engineers explain what these discs are (or were) used for? I seem them all over the place, even sometimes away from Metro stops. by LilGeographersRoom
Much of the metro’s miles of tunnels were constructed using cut & cover, digging down from the surface, pouring concrete and laying steel, then covering it back up. (Not the deep stations along the redline, those were bored into bedrock). Disrupt that much soil and there will be compaction that occurs years later, and that can damage infrastructure built above. So metro monitors surface elevations around tunnels for that problem.