Abstract

The Broadway Tunnel is a pair of highway tunnels side by side, near Oakland, California. The geological conditions were not accurately determined prior to the driving of the tunnels, yet the contract was arranged on a firm unit-price basis. Construction was handicapped by unequal pressures exerted on the tunnels, general instability of a large portion of the rock, local running ground, irregular distribution of rock units, and plastic dikes. Two cave-ins occurred, expensive methods of construction had to be adopted, and a large financial loss resulted. Litigation inevitably followed.The formations penetrated by the tunnels include Miocene sandstone, shaly sandstone, and shale; the Claremont formation (Miocene chert and shale); and the Orinda formation (Pliocene mudstone, sandstone, and conglomerate). Some of these rocks are cut by many brittle sandstone dikes and by plastic, hydrothermally altered diabase dikes.The southwest portal of the Broadway Tunnel is in a terrain of unsystematic structure, but the greater part of the tunnel is in an overturned limb of a syncline, where dips are moderately uniform (about 60 degrees ). Many faults of very small displacement cut the Miocene formations, and indefinite fracture zones locally add to the instability of the rock mass.

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