Where driving meets geology
When a sinkhole emerged on Interstate 70 in Vail earlier this month, it closed the highway for an unprecedented four days. It was not, however, the first time geological problems have confounded drivers on the highway – and it’s unlikely to be the last.
An even larger problem occurred in 1976, shortly before the interstate was opened across Vail Pass. About a mile from East Vail, slightly above where skiers descending the East Vail Chutes are known to bail out, construction crews somewhat unexpectedly encountered a set of landslides from both sides of the valley. There, unstable soil sits on downward-tilting bedrock. The slides are about a thousand feet long.
Rick Andrew, a consulting geologist who has studied the work on Vail Pass, says construction was idled for several weeks as geologists and engineers huddled.
Their response was two-pronged. First, they buttressed both slides by raising the grade of boulder-strewn Black Gore Creek, and the constructed a small waterfall at the bottom of the buttress. Second, they shifted the alignment of the highway about 20 feet. A discerning eye can make out the shifted course, Andrew says, adding that such problems are not unexpected. “That’s just part of having highways in the mountains,” says Andrew.
Another landslide was found a half-mile west of the summit. Because geologists had previously identified it, engineers have surmounted it – for now, at least – by elevating the highway about 15 feet, to help buttress the slide.
Far more significant problems are found at Dowd Junction. There, construction crews in 1968 faced a major landslide when they began work on that segment of I-70. They stuck pipes into the hillside, the first of many, in an attempt to bleed some of the water that lubricates the movement. In the early 1970s, movement continued, however, forcing repairs to the abutment for the bridge there across Highway 6.
In 1984, the Dowd Junction slides gained great notoriety when, after two heavy snow years, the saturated soils slurped downhill, closing the east-bound lanes of I-70. A report completed several years later described the landslide as being among the three most dangerous in Colorado.
“If it moves, it really is the most serious landslide in terms of the economic losses,” says Andrew. “It has the potential of blocking the river, backing the water back up into Vail and Minturn. There is some evidence it has done so in the past.”
Last year, wells were drilled into Meadow Mountain to help study the interior of the mountain and the flow of water. Monitoring equipment is now in place to monitor the soil’s movement.
Partly because of this potential for a landslide that could block I-70, crews are now drilling a test bore through Dowd Butte, testing the potential for a quarter-mile tunnel that would divert I-70 from its existing alignment at Dowd Junction.
This landslide, called Dowd No. 1 Slide, is part of a larger complex of slides off Meadow Mountain. Also active is the Meadow Mountain Slide, which is pushing mud slowly but steadily onto the highway between Dowd Junction and Minturn.
The Whiskey Creek Slide, meanwhile, located to the west, once flushed mud all the way to the Eagle River. The River Run condominium complex is built on the toe of that slide, which geologists estimate happened between 20,000 and 24,000 years ago.
A study in the evolution of highway construction
By Allen Best
Construction of Interstate 70 has been a succession of lessons, with the knowledge gained from one segment applied to the next phase.
These phases can be seen on a drive from Denver to Glenwood Springs. Among the first segments completed was from Georgetown to the Eisenhower Memorial Tunnel. There, during the late 1960s, highway builders thrust the pavement onto the landscape.
For example, you don’t see the highway lanes terraced at Georgetown Hill. They might have been had the highway there been constructed a decade later.
Vail Pass came several years later, in the early and mid-1970s, demonstrating more subtlety in how a highway can be laid upon the landscape. For example, between Copper Mountain and Vail Pass, the highway lanes are bifurcated, or divided, across the valley, lessening the impact on each side. Creeks on both sides of Vail Pass, in fact, are bridged more gracefully – and more expensively – than creeks had been crossed in earlier phases of highway building.
This emphasis on grace – accomplished at significantly increased cost – was compounded in Glenwood Canyon, where work continued from 1984 through 1993. The cantilevered roadbeds testify to this new emphasis. Other evidence is in rock-sculpting, a technique first used at Vail Pass.
Rick Andrew, then a Colorado Department of Transportation geologist, says rock sculpting serves two purposes. First, by using the existing rock structure to control over-break and blast damage, rock-sculpting reduces the amount of rockfall onto the highway. Second, the sculpting diminishes the evidence of the blasting work, creating a more natural appearing rock cut.
“You’d be hard pressed to know we had rock excavations in some portions of Glenwood Canyon,” he says.
Now, in preparation for the next generation of the highway, Andrew has completed work on a study of the geology along the I-70 corridor from Denver to Eagle. If the highway is to be widened, as is considered likely, the study will guide engineers in how to steer clear of problems. While geologists have long been on hand to lend such advice, their understanding of the mountain geology is expanding.