By Dave Tucker, January 10, 2010
Chuckanut Drive, or Washington State Highway 11, is famous for its almost annual landslide closures. The drive is a very scenic popular route that connects Bellingham to the Skagit River valley by traversing amazingly steep terrain along the west flank of Chuckanut Mountain. Westward views to the San Juan islands have been the subject of countless tourist photos. There were at least 4 slides in the fall of 2008 that required road closures, and two slides in late 2009. A large slide occurred on November 23, 2009. The last slide of 2009 was on November 27 just south of Spokane Street, within Bellingham city limits. Traffic was diverted to the Interurban Trail! I visited the site before it was cleared up, but alas, took no photos. The November 27 slide was very small. The rock wall remaining after the slide was festooned with tree roots, which had exploited a joint parallel to the road and probably played a role in loosening the block that fell. The patch that is visible now is much larger than the slide, due to the need to remove more loose material during the fix. The two slides in November 2009 made the Seattle TV news. When the road is closed, residents and restaurants at the south end are adversely affected. The road is occasionally closed for a month.
Rockslides occur from steep road cuts in the Chuckanut Formation. The sandstone and mudstones of this Eocene formation were originally alluvial deposits. Beds in the area of the slides dip (are tilted) downward steeply to the east- into the cliff, away from the road. During folding of these sedimentary rocks, after they were lithified, many joints, or fractures, formed, to accomodate the strain of folding in these shallow, brittle rocks. There are two sets of joints cutting across the tilted rock layers– a vertical set of fractures generally runs into the hillside, and the other runs along the hill face. Both sets, in tandem with the dipping rock layers, serve to dice up the rock into blocks. Construction of Chuckanut Drive, completed in 1913, undercut these rock layers- rockfalls generally slip along the jointing planes that cut across the layers, not along the bedding planes between layers. Rainfall of course plays a role, by lubricating joint surfaces. Is all this effective? In 1994 there was a very large slide just north of Pigeon Point. Your scribe went to check it out. I scrambled up the hill-side to talk to a fellow hanging on a rope, using a pry bar to knock off loose rock. I asked him if he figured the long bolts would work. He thought about this for quite a while, then came back with “Well”, you can’t stick shit to shit!” It hasn’t slid since.
When a rockslide occurs, the usual response by Washington DOT is to close the road, clear off the slide debris, cover the pavement with a layer of loose dirt to protect it, knock down threatening blocks on the cliffs, and haul off the rubble. Rocks are then bolted together with long 1″ iron rebar, and plastered over with ‘shotcrete’, a concrete that is sprayed on to help hold everything together. It also helps reduce lubrication of potential slide surfaces by water. PVC drain pipes are inserted during the cementing to allow water drainage from behind the impermeable shotcrete. Here are DOT photos of the work done to stabilize the November 23, 2009 slide, which occurred 0.3 miles north of Mile Post 13.
The cliffs above Chuckanut Drive are gradually becoming covered with shotcrete. The road cuts expose about the finest tilted rock layers I can think of in the western part of the state. It seems that several hundred square yards of lovely stratigraphy are covered up every year. Go see it while you can! Unfortunately, you take your life into your hands by walking along this shoulderless, narrow, curvy, and popular road. There are few spots to safely park a vehicle. The best way to appreciate the scenery, the wonderful dipping geology, and to see the scabbed over rockslide scars, is to ride a bicycle. Wear bright colors. Most of the action occurs between mile posts 11 and 13 (mileage gets smaller going south.
An added note on shotcrete- this method doesn’t always keep water away from the rock. In places along Chuckanut Drive, the shotcrete has pulled away from the rock, allowing water to get between the rock and the shotcrete. This can hold water within fractured rock that otherwise might harmlessly escape. This water, if originating from upslope, can be trapped by the shotcrete. When it freezes and expands as ice, the veneer of shotcrete as well as the containing rock can fracture. Doug McKeever, Geology Instructor at Whatcom Community College, notes “I have seen shotcrete placed in the 1980s along Chuckanut Drive disintegrate in this manner. In my opinion, all too often shotcrete is more cosmetic than functional, and it is “cut-rate cosmetics” at that!”
Getting there: Chuckanut Drive (Highway 11) is reached from Fairhaven at the south end of Bellingham, or by heading north out of the Samish flats through the community of Blanchard. The Chuckanut Formation is reached after rounding the sharp curve north of Oyster Creek. South of the creek are older metamorphic rocks, not prone to sliding, and the subject of a future story.