J. David Rogers'
Professional Experience
Aldercrest-Banyon Landslide Kelso, Washington (1998-99)

The Aldercrest-Banyon neighborhood in eastern Kelso, Washington began experiencing gross ground movements in February 1998, following 3-1/2 years of above-average rainfall. The initial signs of distress were the breakage of underground utilities. In March 1998 some framing distress was noted on a few homes. On April 10, 1998 a noticeable crack, 2.5 to 6 feet high, developed above the natural crest of slope west of Banyon Drive and north of Cedar Glen Court. Two homes on Cedar Glen Court were evacuated. The City made a valiant effort to patch streets, fill cracks and provide above-temporary above-ground utilities to the affected neighborhoods so that people could remain in their homes as long as possible.

Movement of the upper block gradually increased, evolving into a continuous headscarp approximately 1500 feet long with a maximum vertical separation of 30 feet by June 1998. By the spring of 1999 this main scarp had reached a height of almost 125 feet height. 58 homes were situated in within the active limits of the landslide identified by June 1998 and another 3 homes were swallowed up during the winter of 1998-99.

The Federal Emergency Management Agency issued a Presidential Disaster Declaration on October 16, 1998 and some measure of assistance was rendered to the affected residents. The City condemned 137 homes in the immediate neighborhood and dispersed about $4.7 million in FEMA funds to the former residents. These funds were reimbursement to the City for the loss of public streets, utilities and infrastructure.

Dr. Rogers was retained by the home office of State Farm Insurance to make a review of the circumstances surrounding the catastrophe, which included a thorough review of the project history and existing documents relating to the development. The development area had been mapped for landslide hazards by the State of Washington in 1973, but this work wasn’t completed until a few months after the project gained initial approval from the City. In 1971 members of the City’s planning commission expressed concern about landslippage in the area, because Interstate 5 was being re-aligned to circumvent a large dormant landslide just downslope of the proposed development, on the opposite side of the same ridge. At that time (1971) the developer promised he would engage the services of a Seattle-based geotechnical firm to evaluate the site, naming the three largest firms of national reputation. Unfortunately, during the three phases of development (1971-79) none of these consultants were ever retained to evaluate the property.

The Aldercrest-Banyon landslide was one of the worst urban landslides in U.S. history, causing about $40 million in total damages in a small bedroom community with a population of less than 12,000 people.

Looking south at point where the crown scarp reached a maximum height of about 125 feet. This was behind 511 Banyon Drive. The steep face exposes sands and silt of the upper Troutdale formation overlying coarse cobble conglomerate of the lower Troutdale. The landslide appears to have translated along the contact between the lower Troutdale and the underlying Cowlitz formation.


Overview looking north from 40-feet high landslide scarp across Cedar Glen Court. Ground movements were initially noticed along Cedar Glen Court in February 1998. The headscarp graben varies between 40 and 175 feet deep, as measured from the crown above the slide.


This view shows the developed headscarp area where it severed Aldercrest Drive, as seen in March 1999. The home at right is sitting on a horst block that has translated about 100 feet from its original position.


Looking south along what used to be Banyon Drive, several hundred feet below the landslide headscarp. This area was underlain by back-rotated blocks which translated several hundred feet downhill.


About 500 feet below the headscarp, the slide material disintegrated into a series of shallow earthflows and debris flows, carrying structures and remnants up to 1500 feet downhill.


Another view of the debris field below what used to be Banyon Drive. Secondary debris flows coalesced from disintegrating earthflows, carrying fluidized material down to Bear Creek, a tributary of the Coweeman River (located about a mile north).


This map shows the properties affected by the Aldercrest-Banyon Landslide. The lots shown in red were those most severely damaged or destroyed. The homes portrayed in yellow and orange were subsequently condemned and removed from the area.


This is a geologic cross sectio through the headscarp of the slide where it reached its maximum dimensions. The slippage appears to have occurred along the contact between the Lower Troutdale and upper Cowlitz formations, a major permeability contrast.


This shows my proposed scheme for repairing the headscarp, using a Mechanically Stabilized Embankment, compacting soil between layers of geotextiles to increase the soil shear strength. This proposition came too late to change anyone's mind about abandoning the neighborhood.


In the spring of 1999 two more landslides reactivated on the opposite side of the same ridge, moving southwesterly. These were called the Haussler Road Landslides. The active slide areas are shown in red on this map. Interstate 5 is at lower left.


Questions or comments on this page?
E-mail Dr. J David Rogers at rogersda@umr.edu.