An inconspicuous free-standing arch that spans the famous Spruce Tree House cliff dwelling at Mesa Verde National Park is secured by 60-year-old bolts and is threatening to collapse.
Plans are proposed to stabilize the 270-foot sandstone bridge hanging above the park’s third-largest cliff dwelling, which is also its most popular.
The site was closed in October 2015 because of increased rockfall and an inspection that suggested the arch could fall.
“Nobody has been allowed in, not even park staff,” said Kristy Sholly, chief of interpretation for the park. “Rockfall has continued to happen.”
The park has more than 500 cliff dwellings that were built and occupied by Ancestral Puebloans who lived in the area from 550 to the late 1200s.
Spruce Tree is the most preserved and most visited of those dwellings. It features 120 rooms, eight kivas and two towers. It had a population of 60 to 90 people and was constructed between 1200 and 1278.
Sholly said visitors love it because exploring the spectacular ancient village year-round does not require a ticket or guide and it can be reached via a short, paved trail from the Chapin Mesa Museum in the heart of the park.
Half the park’s 500,000 to 600,000 annual visitors come to see it. The year it closed, 266,351 people took self-guided tours.
“When it went out of commission, it really affected the visitor experience to the park,” Sholly said. “Spruce Tree House is a powerful, immersive experience, so there has been disappointment because of the closure.”
The closure has delayed archaeological studies, and the popular luminaries staged in the cliff dwelling over the Christmas holidays have not happened for seven years.
A preliminary range of concepts are being considered. They include leaving it to nature’s whim and permanently closing the site, stabilizing it to reopen it, and stabilizing it and keeping it closed.
The park is developing a set of alternatives under the National Environmental Policy Act and anticipates releasing the document in May. After additional public comment and analysis, a final decision will be made.
Detailed stabilization designs have been intensely studied to secure the arch and possibly reopen the site to the public, said Park Project Manager Allan Loy.
He explained the problem and engineering solutions during a tour a Journal reporter and a Durango Herald photographer.
The free-standing arch spans above and slightly offset to the upper alcove of the cliff dwelling.
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, Loy said. 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 can be seen falling out of the back of the alcove, and water continues to flow into the site.
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.”
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 weakens 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.
Various bolt configurations were tested on the models to determine breaking points, ideal positioning and longevity.
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.
“It is a real different, intellectual problem in a unique setting. You have a prehistoric dwelling under a geologic stability issue. The engineering (solutions) are similar to mining and slope stabilization,” Loy said.
The park has spent about $500,000 studying the issue and coming up with design solutions, he said.
If stabilization 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 the fall and winter offseason. A crane would be positioned on the mesa, and platforms would be lowered for workers to reach the site.
The project would draw a lot of interest, Sholly said. Staff would be prepared to explain the highly visible activity to visitors.
The dwelling itself would not be disturbed or altered as part of the project. Preserving the aesthetics of the site also is a priority, so there will not be netting or support beams.
“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.”
In project documents, the park stated “action is needed to prevent collapse of the arch and resulting damage to Spruce Tree House cliff dwelling in adherence with the park’s enabling legislation and National Park Service Policies.”
Public comment has been gathered, and 26 Native American tribes with ties to the Ancestral Puebloans are being consulted. After the final environmental analysis is released with various alternatives, there will be more opportunity for public comment before a final decision is made.
About 40 public comments have been submitted. They range from stabilizing the arch so the site can be reopened to letting nature take its course, Sholly said.
“That is the preservation paradox – return to nature or preservation,” she said. “People see the value in these limited sites to learn and understand the Pueblo culture that is still here today. They are a great connection between the past and present.”