MEASURING DAMAGE IN THE LOWER PARKING LOT AS A PROXY FOR SLOPE MOVEMENT AT THE OREGON CAVES NATIONAL MONUMENT

The Oregon Caves National Monument (ORCA) is surrounded by steep slopes. The lower parking lot (LPL) area used by daily park visitors overlies argillite and some serpentinite, both weak rock types susceptible to slope failure. There is a history of slope failure at ORCA. In the LPL specifically, there are two recorded slope failure events in 1942 and 1996 both of which damaged park structures.

Given the continued risk of slope failure, this project was designed to provide a way to monitor the rate of slope movement from year to year. In 2012 GeoCorps Intern Aspen Schindel designed a method for gridding and then measuring the cracks and potholes in the lower parking lot area. The 2012 grid could not be accurately repeated, so a modified pattern was used, but the measuring methods remained the same. Potholes were measured by length, width and depth. Cracks were measured by linear length (the straight distance between ends of the crack) and orientation. Width of the crack was recorded as a range only when the crack was wider than 0.25 inches.

For 2013, 185 cracks with a total linear length of 3097.59 ft were measured in a 46,306 sq ft area. Not all of the cracks measured could be confidently matched to cracks measured in 2012 due to the changes in the gridding pattern. Matching only cracks longer than 15 ft., 24 cracks were compared. In 2012, 657.48 linear ft were measured vs. 710.53 linear ft in 2013. On average, the length of each crack increased by 2.21 ft over the course of one year.

By comparing the density of cracks in different parts of the LPL, the areas at highest risk of failure can be determined. The portion of the LPL directly over a slope down the mountain has the highest concentration of cracks at 8.27 ft/100 sq ft compared to the 6.69 ft/100 sq ft average for the entire LPL. The effectiveness of a buried slope retaining wall can also be determined with this method. The buried wall is located in the south portion of the LPL with the highest concentration of cracks, yet the area directly north of the retaining wall has a crack density of only 1.60 ft/100 sq ft.

Continued monitoring of the cracks and potholes in the LPL can reveal if this rate of movement is constant, or if rates change after wetter or drier years. This information can also be used to support the installation of technologies to prevent further movement, such as additional buried retaining walls.