CUSTER (AP) - The National Park Service is suspending guided tours at Jewel Cave National Monument in southwestern South Dakota to make improvements along the half-mile long scenic route.

Tours will be shut down from mid-October until about April, as workers upgrade structures built in the late 1960s and early 1970s, including sidewalks, stairs, guardrails, handrails and lights.

The structures were built in an "ad hoc manner" and are difficult to maintain and repair, Superintendent Michelle Wheatley said in a statement. The work will address safety concerns, including slips and trips, and will eliminate the possibility of falls for the 80,000 people who tour the cave each year, she said.

The National Park Service awarded the nearly $5 million contract to MAC Construction of Rapid City. Company co-owner Brett Oleson said the job presents some challenges.

When some of the concrete walkways were built in the cave decades ago, workers cut tire inner tubes in half, filled them with wet concrete, slung them over their shoulders, and lugged them into the cave, he told the Rapid City Journal. MAC Construction plans to use equipment that will make it easier for his workers, including electrically propelled carts and dollies.

"We're fairly confident in our ideas," Oleson said. "But I do imagine there is going to be some adjustment once we get down there."

Precautions will be taken to prevent damage to the sensitive cave environment. Blankets will protect cave surfaces, and dust will be captured by filters on tools and by plastic enclosures built atop work spaces. Workers will use electronic power tools and equipment, to prevent damaging the cave with exhaust fumes or other fuel-related contaminants.

Jewel Cave National Monument, known for its caverns with jewel-like calcite crystals, is 13 miles west of Custer in the southern Black Hills. Its visitor center, park store and surface trails will remain open during the rehabilitation.

"It's such a great landmark for our area, and to be able to say you had some part working in there, I think it's going to be pretty rewarding for me, our company and our guys," Oleson said.