The public is invited to comment on a proposal to stabilize and reopen Spruce Tree House, a premiere archaeological site at Mesa Verde National Park at risk of collapse.

The impressive village nestled inside a massive west-facing alcove was constructed by Ancestral Puebloans between 1200 and 1278, and features 120 rooms, eight kivas and two towers.

But the millennia of geologic forces, wind, rain and freeze-thaw that formed the sandstone cave has reversed course toward its impending demise.

Invisible to most tourists, the alcove roof and upper face of Spruce Tree House form a natural arch slightly offset from the adjacent mesa top.

In 2015, increased rock fall and an inspection indicated the 270-foot sandstone bridge could fall, and the park’s most popular cliff dwelling has been closed since. Occasional rock fall and spall issues have continued at the site.

As part of the draft environmental assessment, three alternatives were studied regarding the stabilization.

The park’s proposed preferred alternative is to stabilize the arch and reopen the cliff dwelling for public visitation similar to the access available before the 2015 closure, which allowed self-guided tours into the site, according to the environmental assessment.

Proposed stabilization work would be limited to the sandstone geologic formation in which the cliff dwelling was constructed. None of the ancient, human-made structures would be stabilized as part of this alternative, although the National Park Service would work with the construction contractor on temporary measures to protect the cultural site during construction.

The other two options are to keep the site closed for safety concerns or to stabilize the arch but not reopen it to the public.

The park has been consulting with Native American tribes that have ancestral connections to the park regarding the future of Spruce Tree House.

Upon conclusion of this environmental assessment and decision-making process, one of the options will be implemented.

To comment online and view the environmental assessment, go to parkplanning.nps.gov/MEVE Comments must be received by April 10.

Before it closed, Spruce Tree House was the most visited cliff house dwelling, and is one of a few where self-guided tours were allowed. The year it closed, 266,351 people took self-guided tours, or about half the parks annual visitors. The site can be reached via a short paved trail from the Chapin Mesa Museum in the heart of the park.

“When it went out of commission, it really affected the visitor experience to the park,” said Kristy Sholly, chief of interpretation and visitor services for Mesa Verde National Park., during in May. “Spruce Tree House is a powerful, immersive experience, so there has been disappointment because of the closure.”

At first, there was a lot of surprise that it was closed, especially from people who had visited it before, Sholly said Thursday.

Now that it has been closed for seven years, the word has gotten out about the situation, aided by information from online planning.

“We don’t get as many questions about it, but people are still curious what the plan is and when it will reopen,” Sholly said.

People have been understanding, added Park Ranger Eric Sainio, especially “when we tell them about the rock fall danger.”

The National Park Service noted sandstone degradation from the alcove and arch as early as 1908, two years after the park was established. The narrow gap between the alcove and arch drains water from the upper mesa into the back of the cliff dwelling and causes erosion.

In the early 1900s, park archaeologist Jesse Fewkes tried to solve the problem by using dynamite to construct a canal on the rock mesa above the cliff dwelling alcove to drain water away from the space.

While it worked to a degree, it did not have the proper grade and created standing water that seeped into the cliff dwelling, said Park Project Manager Allan Loy. The century-old trench above the alcove is clearly visible from the Chapin Mesa parking lot.

In 1962, engineers drilled a row of 12- to 15-foot bolts straight into the southern portion of arch to secure it to the alcove. They filled the gap with concrete and burlap from bottom to top.

“They did a good job, but it has reached the end of its life cycle,” Loy said.

Burlap and chunks of concrete are falling out of the back of the alcove, and water continues to flow into the site.

Detailed stabilization designs have been intensely studied to secure the arch and possibly reopen the site to the public.

The new stabilization plan, if approved, would use 76 20-foot bolts drilled in at different angles to tie the arch into the alcove and secure individual arch rock blocks.

“It mimics nature in the way tree roots fan out to secure the tree into the ground,” Loy said. “The reticulated bolt pattern stitches it together and increases strength.”

The bolts would be fitted with wireless monitoring devices to alert engineers when there is movement. Crack gauges will be installed to determine whether they are growing.

To stop the water flow, the crack would be filled with specialized mortar and silicon.

A type of brace, called a haunch, may be fitted under the arch where it meets the abutment. The bolts and brace will be camouflaged to match the surrounding rock.

Rock scaling also is in the plans, a process that peels unstable sections from the arch and alcove. Vegetation that grows into arch crevices would be removed because it weakened the geologic structure.

The complexity of the problem was greater than anticipated and has led to years of exhaustive geotechnical analysis on the best stabilization strategy, Loy said.

Itaska Consulting Group, of Minnesota, has been the lead engineer. Photogrammetry and lidar were used to create detailed computer models of the arch and alcove.

The park has spent about $500,000 studying the issue and coming up with design solutions, Loy said.

If the stabilization alternative is chosen, a construction budget of between $2 million and $3 million has been earmarked through the Federal Lands Recreation and Enhancement Act.

The work would take six months and be completed in fall and winter. A crane would be positioned on the mesa, and platforms would be lowered for workers to reach the site.

“We think we have a design that has good longevity and stability if the decision is made to go that way,” Loy said. “The whole reason to stabilize this arch is to protect the site.”

jmimiaga@the-journal.com