Railroad Damage in the Alaska Earthquake

Permanent horizontal movements of wet unconsolidated water-laid sediments were a major cause of damage to the Alaskan Railroad. Sediments mobilized by the shaking moved toward the unconfined faces of topographic irregularities that ranged in size from small drainage ditches to wide valleys. The movements produced surface cracks in zones of distension on stream banks shortening the distances between stream banks. At stream crossings, bridge superstructures were compressed, and yielded by arching and horizontal deflection. Deeply driven piling in bridge substructures was shifted by the horizontal movements of mobilized sediments that extended out under the stream bottoms. Where the railroad crossed streams at an acute angle, streamward movement of the banks produced horizontal offsets between embankment center lines on opposite sides of the streams. The bearing strength of the unconsolidated sediments was so reduced by the shaking that embankments settled downward. Transient horizontal compression also occurred; some had considerable force. Linear surface irregularities localized surface cracks, and embankments produced cracks in adjacent sediments.