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This project entailed the construction of a total of 17,200 LF (5,600 LF of rock & 11,600 LF of soil) of twin 20.4-foot-diameter tunnels that extend the existing Third Street Light Rail T-Line through a challenging, congested urban environment. Construction of these pre-cast concrete segmentally lined, twin-bored subway tunnels required working through varying geotechnical conditions and a variety of geologies ranging from soft ground to clays to hard rock formations.
The close proximity of existing adjacent structures, utilities, buildings, and crossing beneath the BART subway tunnels, with only 8 feet of separation, in the soft ground sections of the alignment added to the challenge. The tunnel alignment included two tight radius curves. The first of the two curves consisted of a 450-foot horizontal radius with a tight vertical curve crossing just 8 feet beneath the heavily travelled BART tunnel with zero registered structural movement above. The second of the two curves consisted of another 450-foot radius horizontal curve, which stopped at the Retrieval Shaft.