Report: More stone could crumble from rock bridge

State park feature vulnerable; detour keeps visitors safe.
Thursday, August 17, 2006 | 12:00 a.m. CDT; updated 8:31 a.m. CDT, Friday, July 18, 2008

[Note: this story has been modified since its original posting.]

More blocks of stone could fall at any time from the natural bridge at Rock Bridge Memorial State Park, and the Missouri Department of Natural Resources faces difficult decisions regarding one of its most famous landmarks.

After a slab of rock collapsed and fell on a boardwalk beneath the bridge on June 9, park officials closed a section of the trail and asked mining engineering experts from the University of Missouri-Rolla to evaluate the stability of the rock bridge.

The engineers, Derek Apel and David Summers, submitted a report to the DNR on July 12 that suggests the agency must act soon to remedy the problem. Sections of the natural rock bridge — for which the park is named — must be scaled down, removed or reinforced if the trail that runs beneath it is to be reopened to the public. DNR officials and staff are considering their options and wondering whether it’s necessary to reopen the tunnel trail, which would cost about $28,000.

Apel is an assistant professor of mining engineering, while Summers is a curators professor and director of UMR’s Rock Mechanics and Explosives Research Center. Their report highlights three areas of “significant concern” regarding potential danger under the bridge and recommends the DNR remove entire slabs of rock, scale down others, reinforce existing formations and reroute the boardwalk that leads to the Devil’s Icebox cave.

Safely removing tons of rock requires significant work and technical expertise. To do this, according to the report, mining engineers would create vertical fractures by drilling holes in the stable part of the rock, then hammering wedges along the fracture until the rock is unstable enough to fall.

The three identified areas include the section that overhangs the west exit. It’s made up of small blocks of rock that, according to the report, are “only tenuously stable.”

“The blocks will, likely without warning, collapse at some future time, down onto the boardwalk,” the report reads.

Just below that area is the rock slab that partially broke off in June, destroying a section of the boardwalk. This “remaining rock slab is currently only held to the rock face by one side, which can break off anytime.” The engineers believe tree roots probably are the only thing holding the rock together and recommend it be scaled off with a pressurized water jet as soon as possible.

Finally, the report concludes that a nearby section of the bridge should either be removed or reinforced with steel rock bolts anchored to more stable rock behind it.

The engineers also suggest moving a section of the boardwalk away from the side of the bridge and closer to the creek that runs beneath it. This, they wrote, would “reduce chances of rock pieces rolling on the boardwalk.”

They also recommended that the stairs beneath the bridge, which now are closed to visitors, be relocated.

While the report outlines what needs to be done to reopen the trail beneath the bridge, DNR staff is wondering whether it’s worth it. Public safety is the most important factor.

“Right now, it’s safe” with the detour in place, park superintendent Jim Gast said. Visitors can still get a glance of the tunnel’s west end but never come close enough to be endangered by another collapse.

Gast said there have been few complaints about the boardwalk being closed.

Chrissy Jones of Columbia and her 7-year-old son, Alex, visited the park Tuesday and said they didn’t mind the detour. Jones had mixed feelings about the option of drilling and scaling off rock.

“If they could drill it and make it safe and still make it look untouched, it’s OK with me,” she said, but she wouldn’t want them to alter it too much just so people could walk under it.

Another fundamental question arises: Should the DNR interfere with nature just because a popular landmark is involved? While the DNR traditionally is reticent to tinker with nature, its policies would allow it to fix the bridge because it lies outside the park’s designated wild area.

Ken McCarty, chief of the natural resource management section with the DNR, said that while natural resources are always considered, public safety, viewing and interpretation would weigh more heavily in this case.

“I’m not sure how much (the potential) disturbance will factor in,” he said.

Gast said the cost is also a concern. The DNR is considering whether to apply for grants that could pay for the work.

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