The Pillar Mountain Landslide occurred in 1971 near the City of Kodiak, Alaska. The slide comprised 6 million cubic yards of material that failed from above the main highway to town and covered the roadway. Since the failure was progressive in nature it was not a catastrophic event, and it did cut off the City from the airport and shipping port for long. The roadway was rehabilitated with little other damaged noted. Of concern to the resident of Kodiak is the presence of ongoing rockfall that is occurring from the same slide area. It is not known with certainty if the rockfall is a precursor of further sliding activity and / or larger deep seated failure. It is also of concern that a large slide could result in a tsunami that could cause substantial damage to Kodiak beyond just closing the Highway into town. Many studies of the slide have been undertaken but none of them adequately explain the mechanism responsible for the 1971 failure.
This paper proposes an alternate failure mechanism for the 1971 failure and uses this to predict the potential for future failures. In general the structural geology in the vicinity of the 1971 Pillar Mountain slide is favorable with bedding dipping into the slope and some sub-vertical joint sets dissecting the blocky rock mass. It is postulated that the 1971 failure was initiated by oversteepening and loss of confinement at the toe of the slope as a result of quarrying activity. This may have resulted in dilation at the toe of the slope followed by flexural toppling failure. As a consequence of flexure of continuous columns of rock (separated by bedding) the tensile strength of the rock may have been exceeded. Once the tensile fractures started to coalesce at the base of the topping beds (failure perpendicular to bedding) it is postulated that slippage started on the rough but coalesced surface that dipped out of the slope in the oversteeped section of the slope. This mechanism and modeling undertaken to visualize and validate the mechanism are described in this paper. The results of modeling this mechanism are also described in the paper.