86th Street Cavern

Skanska is the managing partner of the jointventure awarded a $301 million project for construction of the 86th Street Station cavern portion of the new Second Avenue Subway. The major facets of the project include mining of the cavern via drill and blast methods, followed by construction of the reinforced concrete structural lining of the cavern. Additional work items include building demolition, installation of support-of-excavation elements, utility relocation and reconstruction, and underpinning of existing buildings adjacent to the work site. All work will be performed in the densely populated upper east side of Manhattan, making proper maintenance and protection of pedestrian and vehicular traffic through the area of paramount importance.

The station cavern, approximately 1,100 feetlong, seventy feet wide and sixty feet high, will be excavated utilizing controlled drill and blast techniques. Access to the work area will be via two access shafts, started under a previous contract and completed under this contract. These shafts will also be utilized for the removal of approximately 170,000 cubic yard of blasted rock. The joint venture will utilize a specially designed conveyor system to facilitate transfer of the excavated material to disposal trucks at street level. In conjunction with the excavation, initial support of rock faces will be accomplished by the use of rock-bolting and shotcrete installation. Following excavation and initial support of the cavern, a waterproof membrane system will be installed prior to construction of the final reinforced concrete structural lining. A custom-designed form system will be utilized to accelerate concrete wall and arch construction.

Other work on this project includes demolition of several buildings in the area to permit construction of the station entrances and utility ancillaries. The demolition will encompass both lead and asbestos abatement.

During demolition operations and construction of the station entrances and utility ancillaries, many of the adjacent building structures will require temporary support of existing slabs, walls and columns. The joint venture will design and install temporary supports and ground improvements to ensure the stability of both building structures and utilities during these operations.

Although much of the utility relocation is being performed under a separate early-works contract, the Joint Venture team will need to design and install temporary utilities to facilitate installation of station entrances and building support elements. This work will require detailed coordination with the numerous third-party utility owners as well as the adjacent building residents.

To mitigate the construction impact on local residents, businesses, the traveling public and emergency services, the joint venture will undertake extensive temporary detour and safety measures. These measures will include temporary sidewalks; relocations of surface features such as trees, benches, canopies and stoops; variable message signs; protective fencing and barriers. Throughout the duration of the contract, the team will be required to coordinate activities with numerous other contractors working in the area and to provide residents, community boards and local officials with information about work progress and significant activities impacting them.

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