Like a loose tooth that refuses to come out, "Monkey Face" and an adjacent large rock known as "Shark Tooth" clung to the Rims on Friday afternoon despite a pair of explosions and inflatable devices trying to bring them down.
Crews from GeoStabilization International of Grand Junction, Colo., joined with the city’s consultant, Terracon; Greg Rogers, an explosives expert from Stevensville; and city officials working on the Zimmerman Trail rockfall remediation project for Friday’s expected destruction.
The goal was to bring all three rocks — which weigh up to 800 tons apiece — down safely.
By mid-afternoon Friday, only one of the three large rocks had fallen.
Crews continued to work at dislodging the rocky behemoths Friday afternoon and planned to resume their efforts Saturday morning.
Geology and the proximity of the homes were working against creating a blast strong enough to more readily bring down the large rocks, explained Dan Nebel, a Terracon geologist.
Too many explosives would have resulted in flying rocks, he said. Too little and the big rocks don’t even budge.
The sandstone is soft enough, he said, that it absorbs part of the concussion of the explosive blast. That also works against efforts to blast them off the face of the Rims.
GeoStabilization crews also turned to using air-filled pillows, which are inserted into cracks or fissures and then inflated. Normally, the air pressure is enough to wrest the rocks loose and send them cascading, but technicians were having a tough go of it Friday.
Later on Friday afternoon, workers were inserting pillows behind Shark Tooth in an effort to bring it down. Only one piece was dislodged as of about 4 p.m.
During the run-up to Friday’s efforts, city officials evacuated a half-dozen homes closest to the danger area and kept onlookers back about 900 feet from the blast and the expected slide area.
Other nearby residents were also encouraged to leave their homes, which were protected by an earthen berm. The berm was also designed to protect Zimmerman Trail from any further damage.
Dianne Peterson and her daughter, Trish Tuell of Denver, came out to watch the activity from behind a police barricade. They said they preferred to take in the action from the back yard of Peterson’s Edmond Street home, but came out when a siren blast indicated an explosion was five minutes away.
As it turned out, both explosions — one shortly after noon and the other about two hours later — brought down only one of the three rocks. The first blast fractured the base of Monkey Face’s skull, but failed to bring the old simian down. The second blast brought down a rock face that weighed about the same as Monkey Face, Nebel said.
Peterson’s neighbor, Margaret Rogers, who’s lived in her house for 44 years, said her father used to visit from Butte. When he’d look up at Monkey Face, he’d invariably tell her, “That rock is going to fall.”
But not Friday.
“I hate to think of all that scenery coming down,” she said. “It’s nature, and maybe they’re helping it along today.”
Public Works Director Dave Mumford said he was hoping for what engineers call sympathy failures. As one large rock crumbles, the adjacent rock fails and crumbles too. But that was not the case Friday.
City Engineer Debi Meling said despite Friday’s setback, Zimmerman Trail is still on track to reopen in June. But before that can happen, crews must repair the damage to the road and replace guardrails damaged during the March 25 rock slide, which closed the road.