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Colorado forced to prioritize mitigation of rockfall danger

Colorado must prioritize mitigation of geohazards

Rocks tumbling onto mountain roads and heavily traveled highways is a hazard across Colorado, which has identified 750 areas where it is a chronic issue.

As a result, the Colorado Department of Transportation responds to between 50 and 70 geological-caused emergencies a year in addition to numerous calls to clear debris from roads after geohazard events, such as rockfalls, landslides, sinkholes and flooding, said Amy Ford, director of communications for CDOT.

Occasionally, travelers find themselves in the way of stones falling onto the roadway, Ford said. “We’ve been very fortunate this year, but we’ve obviously had some fatals over the years.”

This is where CDOT’s geohazards program comes in. It is charged with the evaluation and mitigation planning for hazardous corridors throughout the state as well as determining the priority for mitigating hazards, said Ty Ortiz, program manager.

Priority is determined by comparing the number of hazardous events in a given corridor and the average daily traffic though the affected area, Ortiz said.

This causes corridors such as U.S. Highway 550 at Bondad Hill to get a lower priority than higher volume roadways such as Interstate 70, which experienced large geohazard events in Glenwood Canyon on Feb. 14 and 15, said Edward Archuleta, program engineer for Region 5.The stretch of Interstate 70 affected by these February events falls into Region 3 under CDOT and is still undergoing repair work, which requires various lane closures.

A funding puzzle

With a total of five rockfall incidents in the Bondad Hill area since 2011, it raises the question of where this stretch of Highway 550 falls on CDOT’s priority list. The most recent slide occurred June 8 when a vehicle was stuck by debris but the driver was uninjured, The geohazards budget of $9 million is divvied up according to the priorities, Ortiz said. Nearly $5 million was spent on the mitigation and repairs at Glenwood Canyon this year.

This pushes Ortiz to find alternative ways to pay for mitigation efforts, such as incorporating them into large improvement projects across the state, he said. “If my program had to fund those things, well, we would not do very much each year but that one particular job.”

In the case of unforseen events, such as the latest Bondad Hill incident, there are limited funds that CDOT regions compete for, Archuleta said. “There is not adequate funding statewide to address all the geohazards in the state.”

Durango’s CDOT representative petitioned for a share of the money during a June 17 teleconference with members of the geohazards program in Denver, said Lisa Schwantes, communications manager for Region 5. They discussed the plans for Highway 550 at Bondad Hill but no decisions were reached.

Bondad did, however, make the list of potential projects for pre-construction funding through CDOT’s geohazards program, Schwantes said.

The incidents at Bondad Hill are caused by something called differential erosion, a process that precipitates rockfalls where two different geological materials make up a feature.

These materials can erode at different rates, causing large overhanging rocks that are prone to falling onto the road, Ortiz said.

“Given that this is happening frequently, we need to get it fixed,” said Archuleta.

If pre-construction funding becomes available, the agency plans to obtain topographical and right-of-way surveys of the area, Schwantes said.

Surveys would determine the scope of the project needed to mitigate this hazardous stretch of highway, Archuleta said.

These would be the first steps in a six-part process, ending with construction to stabilize the area after purchasing the land atop the cliff at Bondad, Schwantes said.

“There’s two property owners up there, the Southern Ute Tribe and then there’s a private entity, so we have to aquire right of way from both of those owners,” Archuleta said.

Region 5 officials are trying to perform as much preparatory work as possible so that surveying can begin right away if funds become available, he said.

Different causes

Some of the hazards in the area surrounding Durango have slightly different origins than the differential erosion at Bondad Hill.

In areas with hard stones, such as U.S. Highway 160 over Wolf Creek Pass and Highway 550 over Red Mountain Pass, rockfall is caused by something called ice-jacking, Ortiz said. Ice-jacking is where fractures in rock faces fill with water from rain or spring runoff that then goes through a freeze-thaw cycle, causing the fracture to expand until the rock separates from the underlying slope.

“We evaluate those differently, and we mitigate those a bit differently,” he said.

The fractures expanded by ice-jacking can be measured to ascertain if the geometry for a rockslide is present and if mitigation efforts need to be pursued.

Mitigation efforts fall into three basic categories: Avoidance, where construction is avoided or existing structures are removed from the hazardous area; protection, where physical barriers are used and stabilization, where the hazardous feature is modified to either strengthen it or physically remove it.

Protection can come in the form of rock walls, such as those present on Wolf Creek, the rock netting used on Colorado Highway 3 and construction of ditches alongside highways to stop falling rocks from reaching the road, Ortiz said. Stabilization can include placement of rock bolts, which effectively pin slabs in place, and “shotcrete” treatment to slow the effects of the elements on areas prone to erosion. Scaling is another form of stabilization where crews or machinery physically remove materials that are primed to fall, Archuleta said.

Scaling is typically the cheapest form of mitigation.

The geohazards program is trying to improve its evaluation of hazardous areas and identify features susceptible to failure through expanded scanning, Ortiz said.

The scanning will use a combination of methods to build three-dimensional models that can be evaluated over a series of years to track hazard areas, he said. The exact methods will be site dependent but can include such things as laser scanning of slopes and photogrammetry. They represent a potential new way for CDOT to improve the safety of travelers.

“It’s all about safety of our travelers on the road,” Schwantes said.

Aug 25, 2016
Cool temperatures, rain usher in rockfall season
Jun 15, 2016
Long-term mitigation plans for Bondad Hill in the works


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