Manhattan schist rock formation under the station. A three-level structure was being built in each cavern, with one train platform and two tracks each on the top and bottom levels. A passenger concourse was being constructed between each cavern's upper and lower levels. An upper-level concourse, accessed by a series of stairs, elevators, and escalators, would replace ten tracks on the west side of Metro-North's Madison Yard. The cavern structure is located under the Park Avenue Viaduct, which surrounds Grand Central Terminal, while the storage tracks are located under the Park Avenue Tunnel, which is located south of Grand Central Terminal. The track connections from the new station to the 63rd Street Tunnel were excavated using tunnel boring machines. The first machine was delivered in May 2007. Dragados/Judlau created a launch chamber for the tunnel boring machines under Second Avenue at 63rd Street in Manhattan using a controlled drill-and-blast method, then assembled and launched each 640-ton machine. The first tunnel boring machine was launched westbound then southbound from the 63rd Street Tunnel in September 2007, and it reached Grand Central Terminal in July 2008. The second machine began boring a parallel tunnel in December 2007 and had completed its tunnel at 37th Street on September 30, 2008. Geocomp Corporation was hired to monitor the boring, using a battery of instruments to record vibration, ground settlement and any tilting or drift suffered by the tunnel boring machines. The instruments include inclinometers, extensometers, seismographs, observation wells, dynamic strain gauges, tilt meters and automated motorized total stations with prismatic targets. The tunnel boring machines bored an average of per day. Cross-connections between the tunnels were created under Park Avenue, between 49th and 51st Streets, by controlled drill-and-blast. The work began in mid-July 2008 and required between six and eight months to complete. In April 2008, the MTA awarded Dragados/Judlau another contract. The $506 million contract was for excavating caverns for the three-level platform structures, mezzanine, escalators, passageways, and track crossovers. The MTA gave Gramercy Group Inc. the $38.9 million contract to reconfigure part of Madison Yard in March 2009. In September 2009, the MTA awarded Yonkers Contracting Company a $40.8 million contract to demolish a building at 47 East 44th Street and construct a ventilation plant & concourse entrance on the building site, along with constructing a station entrance at an existing office tower at 245 Park Avenue (at East 46th Street). Smaller