Rock blasting is underway between Smiths Ferry and the Rainbow Bridge as road construction continues on ID-55.
The goal is to make this narrow, curvy mile-long stretch of state highway safer, by building wider shoulders, adding guardrail, and straightening the road.
140,000 cubic yards of rock need to come off the canyon, and it’s happening one controlled blast at a time. How do they do it? It takes a lot of work and careful planning.
Strategically spaced holes are drilled into the rock differing in size and depth depending on the situation, then the explosives are placed inside.
“We’re using a nitro glycerin based dynamite as a primer, and then we use ammonium nitrate as a blasting agent,” said Blast Operator Ryan Miller. “Each hole is timed, a 40th of a second. We shot that in a V, and the reason it stayed on the hill is because we shot the center first, and kind of sucked it uphill.”
Each blasting design plan is closely reviewed.
“The whole idea is to move the rock in the direction we want it to. We don’t want to launch into the river, fly rock is our enemy,” Miller added.
Through late November, weather permitting, the work zone is closed to traffic every Monday through Thursday from 10 a.m. to 2 p.m. so crews can safely blast and haul away the rock.
Drivers are urged to plan around the closures or use US-95 as a detour.
“We don’t want people or cars traveling through the work zone during blasting. It’s also an incredibly tight work zone with the river off to the east and the rock faces out to the west. So giving the contractor that four hours is really letting them be as efficient as possible,” said Project Engineer Alex Deduck.
Steel containers and temporary barrier keep the falling rock from getting in the travel lane, and protect the crews working in this steep canyon. When the road is not fully closed, one lane is open to alternating traffic.
“Now that we’ve reduced it to one lane it’s even tighter, and just a hard area to work in,” Deduck said.
This winter, construction will pause and both lanes will be open.
In the spring, blasting picks back up with full road closures Monday through Thursday from 10 a.m. to 2 p.m., and during the busy summer, one lane will be open to alternating traffic Monday through Friday morning. Both lanes will be open Friday afternoon through Sunday.
You can learn more about the project and sign up for updates at itdprojects.org/ID55smithsferry. The project is expected to finish in late fall 2022.
For the latest road conditions, be sure to download the Idaho 511 app or visit 511.idaho.gov.