AUBURN — The 40-foot wall of sand and clay where a portion of Route 136 collapsed Thursday is so unstable that the motor of a crane that plunged from the road to the river is still running.

The operator of the crane left the motor idling when he scrambled out of the cab of the rig after it plunged four stories from the road level down to the edge of the Androscoggin River.

Both the crane operator and a welder standing nearby operating a pile-driver hammer, who also rode the avalanche to the bottom, escaped injury.

That was Thursday.

On Friday, engineers were still scratching their heads over the collapsed road. No one dared go near the crane’s cab to switch the key to the off position.

“It’s a big problem,” said John Linscott, co-owner of H.B. Fleming Co. of South Portland, which owns the crane. “The ground is so unstable that it’s almost unsafe to even go and take a picture of it.”

He said his company has hired an engineer to draft a plan to shore up the area well enough to retrieve its equipment.

The crane had been used to install sheets of metal along the riverbank in an effort to stabilize the road, also named Riverside Drive, which snakes along the river between Auburn and Durham.

The design likely will involve building a road from the north down to where the crane lies partly on its side, Linscott said. Once that design is complete, his company will seek state approval. He’s hopeful that might happen next week.

They considered working from a barge on the river, but that area is too shallow, he said.

“The DOT’s been excellent to work with on this,” Linscott said Friday. “Everybody’s been trying to solve the problem. It’s not easy.”

In his 38 years in construction, he’s never seen a collapse of this size and scope, he said.

“All of us are just shocked and surprised,” he said.

In addition to the crane, Linscott is hoping to retrieve the 40-foot tall sheets of metal weighing thousands of pounds apiece that also collapsed into the river when the road gave way.

Another goal of the operation at this point is to preserve a house that sits at the edge of the road at the point of the collapse, Linscott said.

That home belongs to retired U.S. Army Col. George C. Benjamin and his wife, Ruth.

She was writing a letter at about 11:30 a.m. Thursday when the crane plummeted along with the road that runs by their front yard.

“It came on so suddenly,” she said. “There wasn’t any warning . . . All of a sudden, the lights went out, the telephone went off.”

The crumbling road took utility poles and guardrails with it.

The couple didn’t realized what had happened until they left their house.

“When I went outside, all of a sudden I realized this looks different out here,” she said. “And when I crossed the road out here, of course there was a great big hole and it was absolutely shocking to see it.”

They had planned to go out to lunch to celebrate George’s 94th birthday, but changed their plans.

“This is a little frightening,” she said. “Now we only have half a road out there.”

Since then, they’ve received visitors from every utility company and state official overseeing the project.

“They have been the most caring people,” she said. “They’re all very understanding and very caring.”

The property, including 108 acres extending a mile behind the house, was purchased in 1930 by her parents when she was 11 years old. They deeded the 200-year-old farmhouse and land to her in 1951 when she was stationed in Japan in the civil service after a stint with the American Red Cross.

She and George moved in 40 years ago after George retired from the military.

At that time, there were four rows of trees across the road along the riverbank. But time and river water ate away at the trees one by one until there was only one left standing. State workers took it down down a couple of weeks ago, George said.

The couple feels like the river might not stop until it takes their house.

“It’s a possibility of course,” George said. “You don’t want to think about it.”

In the meantime, Route 136 is closed along that section. A 10-mile detour is marked by road barriers.

The Benjamins are bracing for the next order of business.

“It’s not impossible to think the road will go behind our house,” he said, through their property. “We wouldn’t like to have our farm split by a highway.”

“However,” she said, “we realize we need a safe road through here too. You have to face these things head on, I guess.”

“It’s no fun, though,” he said.

In 2008, a 120-foot section of riverbank along Route 136 slid into the river, taking a 20-foot portion of the shoulder of the road with it. A string of guardrails was left hanging in mid-air.

The road was under reconstruction when in June, another portion of the riverbank collapsed. The stabilization design involving metal sheets driven into the bank began earlier last week.

On Friday, another portion of riverbank dropped into the water, Ruth Benjamin said she was told.

Meanwhile, workers in buckets were busy planting new utility poles in a perimeter around the back of their house to string new power, phone and cable lines.

George and Ruth Benjamin spent Friday afternoon readying for the effects of Hurricane Earl, hoping the rain it brings won’t undermine the riverbank farther.

“It’s quite a stress on a couple of old guys,” George said.

“It’s certainly something we’ll never forget,” Ruth said.