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Wednesday, 27 January 2010 17:37

How to fix a mountainside

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In the first days following the rockslide on Interstate 40 last October, Jody Kuhne had the unenviable job of rappelling down the freshly scoured rock face and into a gaping chasm left in the mountainside.

As the DOT’s resident expert on landslides, his mission was to figure out the nature of the problem and begin plotting a fix.

A giant slab the size of a five-story apartment building had sheered off the mountainside. Most of it fractured on impact creating an enormous pile of over-sized boulders. But a large wedge was lodged at the base of the mountain like a stubborn bookend. Unless Kuhne could rappel behind it — a six-foot-wide fissure called the back crack — he wouldn’t know exactly what kind of slide they were dealing with.

With surveying instruments stowed in his pack, Kuhne harnessed up and lowered down the rock face on ropes to measure the angle of the fault line that caused the slide. Combined with aerial photography, he generated a 3-D map of the remaining mountainside and soon realized they were dealing with a worst case scenario.

The rock slide was known as a “wedge failure,” except only the lower half of the wedge had broken lose.

Imagine an upside down pyramid superimposed on the rock face. The tip broke off and slid down the mountain, but the wide base was left behind and now loomed 250 to 400 feet above the workers on the ground. A fault line — the same fault line that caused the lower part to slide — ran in a large vein all the way up the mountainside.

That fault line lurking below the surface left the upper half of the mountain susceptible to a slide. It was Kuhne’s job to figure out just how susceptible.

“If it came out to a certain factor of stability that was acceptable to us, we’d walk away. But it didn’t. It is on the borderline of stability,” Kuhne said.

Ideally, they could blast away what remained of the giant wedge to eliminate the looming threat.

“We tried that, but it was time consuming, expensive and extremely dangerous,” Kuhne said.

Short of a bombing run by the U.S. Air Force, that strategy seemed impossible.

“To get up there and start drilling and blasting, you risk a catastrophic failure of the whole thing. It would certainly kill anyone on it, around it or in front of it,” said Mike Patton, the lead DOT inspector on the slide site.

So if the mountain couldn’t be brought down, at least not until gravity was ready, Kuhne had to figure out how to stop gravity from eventually getting its way. The answer was bolts. Lots of them.

Theoretically, bolts drilled deep below the vein of weakness would apply enough torque to hold the mountainside in place.

“It has to be anchored below the failure plane and basically snug that thing to the slope,” Kuhne said.

Based on his modeling, Kuhne could calculate how deep the fault plane was. Along the outer edges, it ran about 40 feet below the surface. But in the center — the thickest part of the wedge — it was some 120 feet down.

Based on the force each bolt conveyed and the mass of the wedge being held in place, Kuhne came up with 590 bolts. They would be spaced every 10 feet creating a giant grid on the mountainside. Kuhne likens it to a blanket of force battening down the rock face.

The sheer number of bolts combined with the depth of the holes mean 9.5 miles of holes have to be drilled.

It took eight weeks to blast apart and haul away to pile of boulders created by the slide.

“If that was all we were facing we would be done and open right now,” Kuhne said.

But the process of drilling holes and anchoring giant bolts into the mountainside has proved time consuming, further hampered by snow, ice and record cold.

There are currently five drill rigs on the side of the mountain, each one about the size of a go-cart. The drill shafts come in five-foot sections. Every five feet, the operator has to stop and screw on another length of shaft as it bores deeper and deeper.

 

Installing the bolts

So far, nearly a third of the 590 holes have been drilled. This week, the first bolts will be installed, no easy task given their enormous length.

A helicopter will hover overhead suspending the bolts while men on the slope maneuver them into place and feed each one into its hole.

Since the holes vary in depth, each one is numbered. The bolts have a tag with a corresponding number — a piece of duct tape marked with a black Sharpie.

The bolts are only 1.5 inches in diameter, but the holes being drilled are roughly 3.5 inches. The space around the bolts will be filled with grout.

The job will take a lot of grout, about two tractor-trailer loads. Water to mix with the grout will be pumped from a stream cascading down the mountain near the slope. Pipes will carry the water overland to a giant holding tank at the top of the slope. The holding tank has a heater to warm the water up to 50 degrees before it can be mixed with the grout, Patton said.

Getting grout into the deep but narrow gap around the bolt is another challenge. Long tubes duct taped along the length of the bolts will carry grout pumped to the bottom of the hole.

“You are filling the hole up from the bottom up,” Patton said.

The bolt has a flexible plastic ring every 10 feet to serve as a spacer and keep it centered in the hole as grout fills up around it.

Further complicating the process, each bolt had to be cased in a large plastic sheath. Inside the sheath, the bolt is caked with grease. The bolt will stretch as it is tightened down, and the greased-coated sheath will allow the necessary movement without breaking the bolt.

“You put these thing under so much stress that, believe it or not, we will stretch that bolt about four inches,” Patton said.

Each bolt will be subjected to a force of 7,000 PSI (pounds per square inch) using a giant hydraulic jack to pull it tight.

Finally, a plate about seven inches in diameter will cap the bolt.

Given the roughly 400 holes left to drill, and all but 10 of the 590 bolts still to install, including five days to let grout dry in each hole before the bolts can be stretched and capped, it is optimistic to assume the DOT can meet its self-imposed deadline of getting the Interstate open again by the end of March. But Patton said that’s still the plan.

“When things really get rolling, that is not necessarily going to be impossible,” Patton said.

Hang-ups are inevitable, however, and not just from the weather. The contractor hoped to fire up additional drilling rigs on the slope, but for the past three weeks, crews have been waiting on an order of extra drill shafts to arrive from Italy.

“All the Italians went on extended vacation at Christmas time,” Patton said.

As the lead DOT inspector on the slide, Patton is tasked with assessing whether the holes are drilled right, the bolts are installed right and whether each one carries the right load.

“Only 590 good bolts achieves the safety factor we are looking for so you have to make sure all 590 are doing what they are supposed to be doing,” Patton said.

Only time will tell if the strategy will work. But one thing is certain: without the bolts, the upper half of the mountain would be a ticking time bomb given the past history of I-40 through the Pigeon River Gorge.

“Considering we get these every 10 years periodically and have had dozens since this was constructed, it will slide sooner than a million years,” Kuhne said. “If this decides to fail while we are standing here it will take about 10 seconds for it all to be in the road, and we will stand here and watch it all happen.”

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