Slip Sliding Away - a recap/field guide of the Landslides of Portland's Tualatin Mountains Field Trip
/Trip Date: September 28, 2024
Trip Leader: Scott Burns
Article by Carol Hasenberg
Photos, by Carol Hasenberg
Figure 1. Numbered field trip stops on a highway map of the West Hills.
A significant portion of the terrain of Portland’s West Hills between Burnside Road and US 26 is ancient and active landslides. Dr. Scott Burns, professor emeritus of Geology at Portland State University, took GSOC participants on a field trip exploring this terrain and observing the mitigation methods used over the years to try and arrest the sliding and damage. Dr. Burns has trained quite a few geologists about landslides over the years he has taught at PSU, and many of his students are now cataloguing and mitigating landslides throughout the Pacific northwest and other areas of the country.
Above is a map of the stops on the field trip superimposed over a road map of Portland west of the downtown area (Figure 1). The same stops are shown over the Oregon Department of Geology and Mineral Industries’ SLIDO map with the landslides depicted (Figure 2).
Figure 2. Numbered field trip stops on a DOGAMI SLIDO landslide base map. Bill Burns, who was once a student of Scott Burns at PSU (no relation), is now the lead landslide geologist at DOGAMI.
The first landslide visited on the trip was the large slide on which the Oregon Zoo is sited. Parts of this ancient landslide are active today, and mitigation measures have been installed to minimize the movement of the slide. This landslide, like all of the slides visited on the trip, is caused by water seeping into a paleosol, or ancient soil, layer between two flows of Columbia River Basalt. Construction of the Sunset Highway cut in to the toe of the slide, exacerbating the slippage.
In order to reduce the slippage, drainage tunnels were built under the zoo, and these drain into a large tunnel which takes the drained water under Highway 26, under the Providence Park Stadium, and eventually discharges into the Willamette River. Also, a pile of large cobbles was installed at the toe of the slide, and this can be seen from the zoo exit ramp for westbound lanes of Highway 26.
At Stop 2 the field trip vehicles were driven up to the head scarp area of the slide, adjacent to the bend in Knights Road above the Veterans Parking Lot.
Figure 3. A closer view of Stops 1-4 on a hillshade map.
Photo of Highway 26 at Stop 4, showing a small landslide with alder trees growing on it.
At Stops 3 and 4 along Canyon Court, Burns pointed out several smaller landslides along Highway 26. He pointed out how the landslides had alder trees growing on them, indicative of disturbed growth, whereas the more stable slopes were clad in Douglas Fir.
Figure 4. A closer view of Stops 5-6 on a hillshade map.
The group then headed up the hill to Stop 5 at the parking lot where the old Sylvan Brick Company once stood, where bricks were made of the same paleosol layer on which the slides occurred. Above this lot, the group could see a fairly large slide which had been arrested by dewatering, a retaining wall constructed at its base and weighted down with rock for a distance above the wall. This slide occurred during the notorious rain event of February 1996 and was caused by roof drainage piped into the ground from a house on the hill above the lot. Nowadays the City of Portland is very aware of the damage that can be caused by roof drains and does not allow homeowners to treat their storm water drainage in such a cavalier fashion in this and other steep terrain.
On a humorous note, Burns pointed out another smaller slide up the road from the parking lot which sported a For Sale sign above its retaining wall. The sign looked like it had been there for awhile.
Photo of the landslide at Stop 5.
At Stop 6 the group vehicles wound their way along Skyline Drive to a point higher up the slope from the Sylvan Brick site. An ivy-covered gabion wall held back another slide here. Gabion walls are stainless steel, heavy wire baskets filled with rocks. Burns said that the baskets are made by the Italian company Maccaferri, and he said “accept no substitutes” for this product. He can cite examples of failed gabions made from cheap wire mesh.
The group then proceeded along Skyline Road to Burnside, turned left onto Burnside, then right into Mt. Calvary Catholic Cemetery for Stop 7. A couple of landslides flanked the roads heading into and out of the mausoleums in this part of the cemetery.
Eastward facing landslide at the cemetery.
The first landslide, on the east side of the space, also occurred during the 1996 rain event. The slide proceeded down the hill until it hit the Audubon building along Burnside below. This slide is distinguished for being on the USGS national landslide database and is monitored by that agency. A solar panel powering the inclinometer could be seen in the brush down the slope. The landslide had the unintended benefit of clearing a path through the trees for a great view of downtown Portland and Mt. St. Helens and Mount Rainier.
On the west side, the existing slide threatening several of the graves along the roadside was reactivated by dumping debris onto its very steep slope.
Figure 5. Stops 7 and 8 with the hillshade background.
Figure 6. DOGAMI LIDAR image of cemetery area. Note the flattened hilltops and arrows pointing to the head scarps of the slides.
After viewing these slides, the group next visited another part of the cemetery, by heading further west on Burnside and then Skyline Blvd to Stop 8. in this loop, the cemetery ran into problems from another pile of dumped earth. In order to stop the sliding, they hired a contractor who promised he could make them some gabion baskets out of chicken wire very inexpensively. Well, after a few years, the chicken wire rusted away and there goes the rock. As you may have guessed, gabion walls and dewatering schemes are not cheap and some businesses cannot afford to pay for such improvements.
The group discussed another landslide mitigation technique used in Oregon along Highway 6 through the Coast Range to Tillamook. Some of the road washouts have been mitigated by using compacted wood chips retained by a concrete wall to rebuild the roadway. These rebuilds have been surprisingly successful and longer lived than originally expected. In states with few forests other lightweight materials are used. The use of lightweight materials in replacing landslide material is a third way to mitigate the problem.
Gabion wall photo from Stop 9. Note the young alders growing atop the slide, and Douglas firs growing farther back on more stable material.
The group had one more stop along Skyline Blvd before heading back towards town. Near the intersection of Skyline with Cornell Road is a 4- basket high gabion wall constructed to retain the toe of a small slide.
Figure 7. A closer view of Stops 10-11 on a hillshade map.
After viewing the gabion wall, the field trip group then headed east on Skyline to Burnside, and east to Stop 10 on Pittock Mansion Drive to view a slide which produced one of the most famous landslide videos ever taken. The reader is directed to two YouTube videos featuring this footage, Outrunning a Landslide (youtube.com) features Dr. Scott Burns of PSU, and Landslide Chases Father and Son (youtube.com) features Bill Burns of DOGAMI.
A photo of the tennis court fence with the dimple at the slide boundary shown below the arrow. The curb along the sidewalk was similarly dimpled.
The final stop on the field trip was at the Portland Rose Test Garden in Washington Park. The group parked on SW Marconi Avenue and walked up the hill through the Secret Garden and up to the Rose Test Garden. This whole area is an early example of landslide mitigation through land use planning, and the Oregon Zoo is as well. Early in the 1900’s, when the Washington Park Reservoir was being dug, this construction reactivated a large and ancient landslide. Burns noted that several mansions perched on the slide above the reservoir began to crack. Crews dug down through the slide to find the paleosol layer that was the plane of weakness causing the slide. The City of Portland then decided to convert the land enclosed by the slide into what is now Washington Park to prevent additional residences from suffering the same fate. Burns took the group to the fence enclosing the tennis courts above the rose garden to point out the deformation that is continuing today to the slide boundary.
So, that completed the field trip and the group headed back to Portland State University. During the trip the group discussed both causes of landslides and mitigation measures used to control them. The causes include:
Rainfall and channeling runoff into susceptible areas
Stratigraphic weakness
Construction activity which removes the toe of the slide
Dumping soil onto a slide or slide-prone area
The mitigation measures were:
Buttressing the toe of the slide with concrete retaining walls, installing cobbles and boulders, or constructing gabion walls
Dewatering the slide by providing a draining system
Replacing slide material with lightweight material
Using land use laws to direct construction away from slides and slide prone areas
If you missed this trip, you can do it as a DIY field trip with the data provided in this article. Here is a table of the stops we made for your use:
