Construction methods Second Avenue Subway

1 construction methods

1.1 phase 1
1.2 phase 2
1.3 phases 3 , 4

construction methods

tunnel @ 64th street

the construction of 8.5 miles (13.7 km) of second avenue subway underneath densely populated manhattan require use of several construction methods, depending on section of line. line s tunnels largely consist of twin tunnels diameters of 23.5 feet (7.2 m). 90% of tunneling performed tunnel boring machines. rest done using cut-and-cover method, or through use of mined drill-and-blast, sections averaging 275 meters (902 ft) in length, namely station boxes. methods used construct sections of line confirmed in 2003, modification of section north of 120th street announced in 2016.

street-level work @ 83rd street



phase 1

phase 1 of second avenue subway constructed between lexington avenue–63rd street station on 63rd street lines , existing tunnel segment between 99th street , 105th street, terminal station @ 96th street. in phase 1, tunneling completed between east 63rd street , east 92nd streets through use of tbms. tbm launch box 814-by-75-foot-wide (248 23 m), , part of 96th street station. 2 access shafts constructed 72nd street station. slurry or diaphragm walls, 1.1 meters (3.6 ft) wide , 6.1 meters (20 ft) long , 35 meters (115 ft) deep, built alongside sections between east 93rd , 95th streets. since rock shallower between east 91st , 93rd streets, 1.1-meter-diameter (3.6 ft) secant piles did same work @ shallower depths. earth excavation conducted between walls once installed, , box structures built using bottom-up construction method. temporary decking constituted top of boxes, , decking both braced excavation , supported walls , second avenue traffic.

the stations @ 86th , 72nd streets mined. challenging, given number of expensive high rise properties in vicinities. 96th street cut-and-cover station 15 meters (49 ft) deep, making 1 of shallowest stations being built on line; shallowness new line align preexisting piece of subway tunnel built in 1970s between 99th , 105th streets. stations @ 2 mined stations between 25.9 , 27.4 meters (85 , 90 ft) deep in rock. construction method used supposed ease concerns buildings above station sites, because 2 shafts required excavation.

of below-ground obstacles, arup director of construction david caiden stated: s spaghetti of tunnels, utilities, pipes , cables—i ve never seen it. complicating process, project must go over, or under, subway lines, amtrak railway lines, , queens-midtown tunnel linking manhattan , queens, in later phases. in addition, there geological anomalies along way of phase 1. manhattan s geology changes along subway s length, passing through rock , soft ground, consisting of sands, silts, , clays on manhattan schist, , there faults , shear zones fractured rock. hard-rock tbms 6.7 meters (22 ft) in diameter, 450 feet (140 m) in length, , 485 short tons (433 long tons) in weight used tunnel during first phase, progressing @ rate of 20 meters (66 ft) per day. tunnels near 125th street station need go through soft soil in addition diving underneath existing irt lexington avenue line. soft-soil tunnels in contrast hard-rock bored tunnels south of 92nd street , cut-and-cover tunnels north of point (necessitated because manhattan s rock profile drops sharply north of 92nd street).

phase 2

phase 2 extend line north 96th street station station @ 125th street , lexington avenue. north of 120th street, constructed through use of tbms. tbm launch box located between 121st street , 122nd street on second avenue. tbms head north under second avenue 121st street before turning east curve under east river houses, turning west on 125th street, crossing lexington avenue, before ending 525 feet (160 m) west of fifth avenue accommodate storage tracks. south of 120th street, line utilize tunnel section built during 1970s, located between 110th street , 120th street. section have tracks , other essential equipment installed, of rest of line. since deemed uneconomical use tbms, cut-and-cover used connect existing tunnel section bored section north (at 120th street) , portion of line in operation south (at 105th street). bellmouth constructed allow future extension bronx. storage tracks west of 125th street station replace storage tracks north of 96th street station, used in revenue service part of phase 2.

the 125th street station, planned part of phase 2, contain intermodal transfer metro-north trains @ harlem–125th street (pictured).

a transfer constructed @ eastern end of 125th street station connect irt lexington avenue line s 125th street station. new lower-level mezzanine house connection between 2 stations, directly connecting downtown platform lexington avenue service. direct connection staircases upper level rebuilt. @ western end of station, transfer built connect harlem–125th street station of metro-north railroad. elevator, stairs, , escalators connect 2 stations.

phases 3 , 4

phases 3 , 4 extend line south 63rd street houston street , hanover square, respectively. part of phase 3, connection ind 63rd street line built, allowing non-revenue moves queens. connection constructed through underground drilling , blasting. bellmouths exist connection east of lexington avenue–63rd street station. section, rest of line, consist of two-track line. however, between 21st street , 9th street, 2 additional tracks constructed on either side of main alignment allow storage of 8 trains. location selected due sufficient depth of area.

like phase 1, sections between stations largely constructed through use of tbms, while stations constructed through cut-and-cover , mining, allowing construction of station caverns, shafts, , entrances. 5 transfers planned connect stations on second avenue line , nearby stations on adjacent lines, increasing travel options passengers. transfer @ grand street require construction of mezzanine below existing station, allowing vertical transfer constructed. existing station have rebuilt accommodate increased volume of passengers using station. houston street station s transfer second avenue station of ind sixth avenue line require construction within existing station. remaining 3 transfers being proposed, , constructed, baring increased cost of construction. transfer bmt canarsie line s third avenue station consist of passageway 200 feet (61 m) long. complicated of transfers connection between 42nd street station , grand central station on irt flushing line. allow connection built, 900-foot long tunnel built under 42nd street west side of third avenue second avenue. flushing line station might have reconstructed in anticipation of increased volume of passengers , due americans disabilities access requirements transfer. allow necessary vents , emergency exits built, cut-and-cover used. final transfer between 55th street station , lexington avenue–53rd street station on ind queens boulevard line. either shielded mining or cut-and-cover used complete connection. existing station have modified allow transfer passageway built.

as part of phase 3, station built @ second avenue , 23rd street (pictured).

three construction options evaluated during project s environmental impact study portion of line between 11th street , hanover square. 1 option known shallow chrystie option use cut , cover, while deep chrystie street , forsyth options use combination of tunneling earth pressure balance machines (epbms) , cut , cover. shallow chrystie option have used existing confucius plaza tunnel section between canal , division streets, , plan 1970s, there have been cross-platform transfer existing grand street station, transfer expected heavily used. however, option require digging sara delano roosevelt park east chrystie street not wide enough fit 4 tracks. under option, track connection built allow trains second avenue run via manhattan bridge north tracks allow service brooklyn. forsyth option curve below park forsyth street , station built under forsyth street, requiring 200-foot transfer passageway less convenient other options. deep chrystie option have second avenue subway run deeper underground, running underneath existing grand street station, mezzanine in between 2 stations. in order allow sufficient room stairways transfer second avenue line, grand street station widened have twenty foot wide platforms. no track connection built under option, , confucius plaza tunnel section not used subway service, instead might used ancillary subway facilities. currently, preferred option.

south of terminal @ hanover square, 2 tail tracks constructed through use of tbm allow storage of 4 trains. tracks built @ depth of 110 feet (34 m) under water street, allowing line deep enough tunnel under east river possible future extension brooklyn. cut-and-cover used build vent facility @ traffic island located @ water , whitehall streets.