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https://nap.nationalacademies.org/catalog/27432/critical-issues-in-transportation-for-2024-and-beyond

TUNNELS AND SUBWAYS ON THE SAN FRANCISCO BAY AREA RAPID TRANSIT SYSTEM

APPROXIMATELY 20 MILES OF THE 75-MILE LONG SAN FRANCISCO BAY RAPID TRANSIT SYSTEM (BART) IS UNDERGROUND. FOUR MILES OF THIS ARE UNDERWATER TUBE, CONSTRUCTED BY SINKING STEEL AND CONCRETE UNITS INTO A TRENCH EXCAVATED IN THE MUDS OF THE BAY FLOOR. OF THE REMAINING 16 MILES, 4 MILES ARE IN MEDIUM SOFT ROCK, 6 MILES IN SOFT GROUND, AND 6 MILES ARE CUT-AND-COVER CONSTRUCTION. TWO MAJOR ACTIVE FAULTS, THE SAN ANDREAS AND THE HAYWARD FAULTS PASS THROUGH THE AREA. THE SAN ANDREAS FAULT IS NOT CROSSED BY THE BART SYSTEM. THE BERKELEY HILLS TUNNEL PENETRATES THE HAYWARD FAULT AT RIGHT ANGLES. THE HAYWARD FAULT RUNS NORTH-SOUTH ON THE EAST SIDE OF THE BAY. MOVEMENT ALONG THIS FAULT IS OF THE ORDER OF 1/8 INCH PER YEAR. THE ROCK IN THE AREA IS HETEROGENEOUS CONSISTING OF SANDSTONES, SILTSTONES, SHALES, AND CONGLOMERATE, WITH VOLCANIC INTRUSIONS. THE BERKELEY HILLS TUNNEL CONSISTS OF TWIN BORES EACH 3 MILES LONG, AND OVERSIZED TO PERMIT A SEISMIC CREEP OR LOCAL SHEAR OF 2 FT HORIZONTALLY AND 1 FT VERTICALLY. TWO MAJOR CONSIDERATIONS INFLUENCE THE HORIZONTAL ALIGNMENT: (1) THE NEED TO KEEP THE TUNNEL UNDER THE STREETS AS FAR AS POSSIBLE IN ORDER TO MINIMIZE THE COST OF EASEMENTS AND RIGHT-OF-WAY, AND (2) THE NEED FOR FLAT RADII TO PERMIT THE NORMAL MAXIMUM SYSTEM SPEED OF 70 MILES PER HOUR. IN MOST PARTS OF THE SYSTEM, A MINIMUM RADIUS OF 1500 FT WAS SET DOWN. ALTERNATIVE DETAILED DESIGNS OF TUNNEL LINERS FOR THE SOFT GROUND WERE PREPARED FOR UTILIZING CAST IRON, DUCTILE IRON, FABRICATED STEEL, AND FOR THE SHALLOWER TUNNELS, PRECAST CONCRETE. ADJACENT STRUCTURES WERE UNDERPINNED DURING CONSTRUCTION. HOWEVER, SUBWAYS INTERFERED WITH EXISTING FOOTINGS. EXCAVATION METHODS HAVE VARIED WIDELY BETWEEN CONTRACTS, AND INCLUDED THREE DIFFERENT TYPES OF MOLE, SEVERAL HAND-MINING SHIELDS, AND CONVENTIONAL ROCK BLASTING. TWO OF THE MOLES ARE SOFT GROUND MACHINES WHOSE DESIGN WAS AFFECTED BY THE SPECIFICATIONS OF THE CONTRACT ON WHICH THEY WERE USED. OF THE TWO ROCK TUNNELS, THE LONGER BERKELEY HILLS TUNNEL WAS EXCAVATED USING CONVENTIONAL DRILL AND SHOOT METHODS, WITH A TWO-TIER JUMBO. THE SHORTER TUNNEL IN SAN FRANCISCO USED A JAVA MOLE WHICH UTILIZES TUNGSTEN CARBIDE INSERT CUTTERS WHICH UNDERCUT THE FACE REMOVING CHIPS OF ROCK. STEEL TUNNEL LININGS WERE CAST IN ROCK TUNNELS IN A CONVENTIONAL MANNER, USING FORMS WITH TWO HINGES TRAVELING ON RAILS. FOR SOFT GROUND TUNNELS, CONTRACTORS WERE GIVEN TWO ALTERNATIVE METHODS OF FILLING THE VOID FORMED BY THE SHIELD BETWEEN THE LINING AND THE GROUND. MOST OF THE TUNNELS ARE BELOW THE GROUNDWATER TABLE. ACTUAL SETTLEMENTS OCCURRING DURING THE BART CONSTRUCTION HAVE BEEN LESS THAN EXPECTED. TUNNELLING COSTS AND PROGRESS RATES ARE PRESENTED.

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  • Supplemental Notes:
    • Vol 2, No 1, PP 13-17, 9 FIG
  • Authors:
    • Thon, J G
    • Amos, M J
  • Publication Date: 1970-1

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  • Accession Number: 00235642
  • Record Type: Publication
  • Files: TRIS
  • Created Date: May 19 1970 12:00AM