1 INTRODUCTION The suspension bridge that spanned the Peace River, south of Fort St. John, British Columbia, was erect- ed in less than nine months. Dufferin Paving Com- pany (Toronto, Ontario) and John A. Roebling’s Sons Company (Trenton, New Jersey, USA) con- structed the bridge under contracts from the Fort St. John Division, Alaska Highway District, Public Roads Administration, and the US Federal Works Agency (Carrigan 1943). Construction started in November 1942. The bridge consisted of two ap- proach spans of 38.1m, two side spans of 141.7m, and a main centre span of 283.5m, for a total length of 640m (Fig. 1). Figure 1. Alaska Highway Bridge (1943-1957) over the Peace River (photograph looking south-west). In October 1957, the north abutment of the Peace River Bridge was caught in a landslide which culmi- nated in the collapse of the north approach span and a side span (Thomson 1958). The loss of this bridge resulted in a complete disruption of all traffic to communities along the Alaska Highway as far north as Whitehorse, Yukon, located some 1475km away. On the north side of the Peace River, 45m above river level, is a terrace which is capped by up to 25m of clean, sandy gravel. The terrace overlies nearly flat lying marine shale of the Cretaceous Shaftes- bury Formation (Okulitch et al. 2002). This for- mation is characterized as flaky to fissile, rusty weathering shale to mudstone (Stott 1982). A road excavation through the gravel into the shale had been made to the elevation of the bridge deck. The general area prior to the failure is shown in Figure 2. The massive gravity abutment, founded on shale, acted as the anchor for the suspension cables. The road excavation allowed ingress of water to the shale, which in 15 years, weathered to high plasticity clay. The reduction of strength of the shale, since the construction of the bridge, was the major cause of the landslide leading to the loss of the bridge. The collapsed spans and part of the scarp are shown in Figures 3 and 4. A moderate velocity landslide with immoderate consequences D.M. Cruden, D. Martin, & S. Thomson Dept. Civil & Environmental Engineering, University of Alberta, Edmonton, Alberta, Canada B.G.N. Miller BC Ministry of Transportation and Infrastructure, Prince George, British Columbia, Canada ABSTRACT: At approximately 23:00 on 15 October 1957, the Alaska Highway was closed by movement of the north anchor block of the Peace River highway suspension bridge, at Taylor, BC, Canada. The two north spans of the bridge collapsed thirteen hours later. There were no injuries. Ten photographs depicting the col- lapse occurring are reproduced here. The north anchor block slid 3 metres downward on shale of the Shaftes- bury Formation. One million cubic metres were displaced in a moderate velocity compound slide. Weak bed- ding planes, candidates for the rupture surface of the slide, have been identified in upstream outcrops. Back analysis using a wedge/sliding block suggests a friction angle of 11º on a bedding surface is consistent with the observed movements. The estimated cost of this landslide, at about $146 million dollars, without casual- ties or legal activity, is amongst the most expensive in Canada.