Experts still don’t know what caused an ancient landslide site to collapse and close Highway 87 March 21, but they have discovered three similar sites nearby that could endanger Payson’s connection to the Valley.
The March 21 landslide that closed Highway 87 for nearly a week and may cost $18 million to repair, could have been triggered, in part, by heavy rainfall, according to a study by the Arizona Geological Survey (AZGS).
The study also says the site of the landslide is on an ancient landslide deposit that occurred several thousands of years ago, one of four such landslide deposits in nearby hills.
Crews began looking at the hill’s stability after the March landslide broke nearly 45 feet of ground, leaving the southbound lane of Highway 87 near milepost 224 broken, uplifted and buckled. The highway was closed for six days and southbound lanes were reopened in late May.
Crews have already installed a 4-foot-wide drainage pipe, concrete pilings and monitoring devices to measure any future movement in phase one of the project.
During phase two, which is expected to begin in February or March, a 20-foot-high retaining wall will be created using the pilings, said Arizona Department of Transportation Public Information Officer Bill Williams.
ADOT is expecting contractor bids on phase two beginning Dec. 26. The transportation board will look at the bids and approve a contractor in January, Williams said.
Phase two is expected to cost between $4 million and $5 million.
Williams said there would be a phase three, but he doesn’t know what that will entail.
“Phase one was stabilizing the road and removing some 200,000 cubic yards of dirt,” Williams said. “We are currently discussing the Arizona Geological Survey findings and will present those to the contractor.”
According to AZGS, four days before the landslide, a quarter-inch of rain fell in less than two hours on nearby Mount Ord. The heavy rainfall and saturated soils may have contributed to the landslide, but the investigation is ongoing. AZGS recommends further study to identify the cause of the landslide.
Mapping by AZGS revealed that the site of the landslide is on an older and larger paleo-landslide deposit that is about 1,500 feet wide and a mile long. AZGS survey crews also unearthed three more paleo-landslide deposits south of the March 21 landslide known as the Iron Dike landslides.
AZGS also recommends study and mapping of the new landslide deposits to determine their stability and potential for failure.
The report from AZGS is available free at www.azgs.az.gov.
ADOT wins award
ADOT and construction contractor Royden have received a partnering excellence award for their collaborative efforts in rebuilding State Route 87 after March’s landslide.
The Arizona Transportation Partnering Excellence Awards competition recognizes partnership teams that demonstrate a high degree of achievement through their practice of the partnering principles and application of the partnering processes related to the transportation industry.
Royden Construction of Phoenix will receive its award at a ceremony on Wednesday for its partnering excellence on the construction work that stabilized the landslide zone and allowed ADOT to re-open the highway within six days of emergency closure last Easter Friday, Williams said.