MTA No. 7 Line Extension along 41st St. from 8th to 11th Avenue Contract C-26503, New York, N.Y.

The Challenge:
GZA was retained to evaluate ground conditions and design the rock support system for the 34th Street cavern, the Tunnel Boring Machines (TBMs) and three access shafts. During construction, GZA performed geologic mapping and observed installation of ground support for compliance with the design drawings. GZA also provided geotechnical and structural instrumentation monitoring to evaluate performance of the ground support system and confirm anticipated ground behavior during construction.

Our Solution:
GZA completed supplemental subsurface explorations and field testing to provide a more detailed geologic interpretation of ground conditions and rock mass classification required to design the support system. GZA performed 3D modeling of major rock discontinuities.

The Result:
Based on real-time interpretation during construction, GZA determined less ground support was required than originally anticipated, which allowed the main cavern to be completed five months ahead of schedule.