A refrigerator-size sinkhole in the paved Bank of America parking lot at West Stockton and South Green Street has opened up, collapsing into the City of Sonora’s ancient main drain system.

It’s the latest example of flood damage to aging infrastructure due to pounding hail-and-rain thunderstorms back on March 6.

A barber tried to park his pickup in the lot about 8:30 a.m. Saturday when he felt the pavement give way beneath his 2007 Dodge Ram 2500 diesel truck, and in three to five seconds he realized he had to hit the gas hard to get out of whatever was collapsing and opening up underneath him.

Randy Petersen wasn’t injured but the sudden acceleration out of an actively expanding sinkhole ruined his front-end alignment, cracked his rear left-side rim, and bent his rear axle.

‘Scared the bejeezus out of me’

Petersen said he’s waiting for an estimate for how much it will cost to repair all the damage to his truck.

“It scared the bejeezus out of me,” Petersen, 24, who works at Gold Line Barber Shop on Stockton Street, said Monday. “Both my tires were inside the hole. It’s 8 feet wide, a good 6 feet deep.”

Doug Kennedy, the developer who is trying to sell the multimillion-dollar Sonora Armory entertainment venue he recently built just uphill from the parking lot, looked at the sinkhole Monday morning and said, “This is a mess.”

His Trado Restaurant Corporation office building, at 55 W. Stockton St., is the nearest above-ground structure to the sinkhole and it remained open Monday. Kennedy said he can access part of the city’s old main drain system through a manhole cover, but he said it looks dangerous.

City of Sonora workers put barriers and tape around the sinkhole Saturday, Tim Miller, city administrator, said Monday. Jerry Fuccillo, city engineer, examined the gaping hole in the parking lot pavement on Saturday.

“I t’s over the city’s main drain and the drain does appear to have collapsed,” Miller said. “We’re bringing in a crew to secure and excavate the site. Once we determine the extent of the problem we’ll proceed with repairs.”

The excavation crew is expected on-site by Tuesday morning, Miller said. They will likely cordon off a larger area above-ground when they begin work. The city has no estimate for costs on the excavation, assessment and repairs for the sinkhole and the damaged portion of the old main drain system.

“We won’t know if there’s any impact to the Trado building until we excavate,” Miller said, “but at this time we don’t believe there’s any hazard there.”

Old main drain

The main drain system is underground and it follows a low elevation line draining from a point behind Gold Country Gas and the slope of South Washington Street down toward Green Street, Sonora Creek and Lower Save Mart. In places it dates to the late 1800s and parts of the system are rock-lined tunnels with iron doors, Fuccillo said March 7, one day after historic thunderstorms as tall as 40,000 vertical feet unloaded on a narrow corridor that focused on and drained toward downtown Sonora.

“It goes under the Opera Hall and the vault of Bank of Stockton,” Fuccillo said Monday. “When they built that in 1972 they realized there was a big ditch under the building site, and they built concrete box culvert, 4-by-4 or 5-by-5, right under their vault.”

Fuccillo said he knows about it because he remembers one of his first engineering jobs back then, working for George Reed Construction, was to come up with an engineered plan to shore up the Opera Hall and the Bank of Stockton for installation of the culvert.

He said he went down in there to look at the old main drain system and it was four feet wide in places and narrower, as close at two-and-a-half feet wide, and it scared him.

“I got into 10 feet and I said ‘No way’ and turned around,” Fuccillo said. “It’s too claustrophobic. You can’t turn around in places.”

The old main drain system carries storm runoff from the city’s streets, sidewalks and other above-ground surfaces, Fuccillo said. It is not connected to any sanitation or sewage system, but storm runoff is polluted with everything from gasoline and motor oil and even human waste from above-ground surfaces, so the interior of the old main drain system should not be considered sanitary.

The main drain goes under the old Bourbon Barrel and the new Armory, and under the Trado building, Fuccillo said. Kennedy said the Bank of America parking lot flooded with runoff and hail on March 6, like so many other low spots in downtown Sonora did that day.

Other problems

The sinkhole is the City of Sonora’s second storm drain problem due to high volumes of water and runoff that took place March 6 that city staff are aware of so far, Miller said Monday.

The City Council for Sonora has two March 6 storm-related items on their agenda Monday evening, and one deals with a storm drain line failure in the Sunrise Hills area north of Morning Star Drive.

During the March 6 storm, runoff overwhelmed, collapsed and blocked with rock the storm drain system for the Sunrise Hills commercial development, causing a backup that eroded a bank or berm at 800 Morning Star Drive, city staff said. More rainfall could increase and expand bank erosion, impact an adjacent parking lot, and compromise more storm drain piping.

Repairing the storm drain is expected to cost more than $100,000 and it should be considered an emergency because it was a “sudden, unexpected occurrence that poses a clear and imminent danger,” Fuccillo and other city staff said in a resolution that was be put to the city council Monday evening.

The March 6 thunderstorms brought several inches of hail followed by several inches of rain in a three-hour period and resulted in widespread flooding in Sonora, with locations including commercial and public buildings and private residences, the proclamation says. Local creeks overflowed, roads became impassable and were compromised by excessive water, flooding and erosion, infrastructure was damaged, debris accumulated and phone service and emergency communications were lost.

Forecasters say a weak storm system is approaching the Mother Lode and the rest of the Central Sierra, with possible light rains and snow levels above 5,000 feet late Tuesday to early Thursday. Sonora could get a half-inch to 1 inch of rain, with the heaviest rains expected Wednesday. A faster-moving system is possible Friday into Saturday.

Contact Guy McCarthy at gmccarthy@uniondemocrat.com or 588-4585. Follow him on Twitter at @GuyMcCarthy.