BILLINGS — Crews contracted by the city of Billings were making progress driving soil nails into a hillside near Park Place and North 15th Street to stabilize it from slipping, said Debi Meling, Billings public works director Wednesday.
“Really what we wanted to do was make sure that material didn’t continue to move. That material does provide stability for the upper slope, which of course provides stability for the canal. So we want to make sure that every part along the way is stable. And that’s what we’re doing with these soil nails," Meling said.
Staff with GeoStabilization International are responsible for driving 150 metal soil nails along 200 feet of the hillside south of the Billings Bench Water Association canal. The nails are driven in along with grout to ensure they stay in place. Meling said the work is expected to be complete in a week and a half.
The city was first made aware of the earth movement on May 27 and after assessments were made, Meling said it was important to act quickly. As part of the stabilization effort, staff with SK Geotechnical placed markers at various points around the problem area so they could monitor earth movement, Meling said.
"That gives us a really good indicator of whether the slide is actually moving, whether it’s slowing down, whether it’s stopped. It just gives us that indication of knowing what it’s doing," Meling said.
As evidence of the movement of the hillside, Meling pointed to a section pavement that shifted inches south due to the soil behind it pushing down.
Meling said the hills beneath the Billings Rimrocks are prone to natural movement due to erosion and the yearly freeze-thaw cycle.
“Any time you have as steep a slope as we have here you get some separation of those materials. A lot of times it’s caused by water or moisture in that slope and that can cause some of the material to slip off of the material that is stable," Meling said.
Staff with Castlerock Excavating also removed top sections of the soil, called overburden, to lessen the total weight pushing down on the hillside. Meling said the combination of the soil nails and overburden removal should prove a fix for the 200-foot section of hillside, but more work could be needed further west to remove overburden in the future.
"The next phase may actually be moving further down the slope and taking some of the weight off that slide area. If this material isn’t actually working to stabilize, we actually want to pull that off so it doesn’t contribute to the slide. We’ll be looking at that under the next phase to see if we need additional work," Meling said.
The cost to stabilize the hillside came out to about $500,000, Meling said.