Repair crews accessed a blockage in downtown Sonora’s Gold Rush-era underground main drain this week, removing a 6-foot mountain of slate rock, dirt and gravel from underneath the paved Bank of America parking lot at Stockton Road and South Green Street.

City Administrator Tim Miller said in the next few days crews will install a 3-foot in diameter surface-level manhole to allow continual access to the main drain, which was badly damaged by a rock collapse during the intense hail and thunderstorm on March 6.

“It depends how long it’s going to take to clear the remainder debris and clear the drainage structure, and that is weather dependent,” Miller said.

A refrigerator-sized sinkhole revealed itself on Saturday in the Bank of America parking lot behind the Trado Restaurant Corporation office building on 55 Stockton Road, prompting the emergency excavation and repair operation.

Miller said a preliminary cost estimate provided to the state was $25,000, but it would likely be double once the project was completed.

“After we were able to excavate the area the extent of the damage and the amount of drain failure and erosion was more than what was first evident,” Miller said.

Workers from Mozingo Construction Inc. were inside of the main drain this week — approximately 2 feet wide and little more than 3 feet tall — removing watermelon-sized rocks and debris that collapsed into it during the storm.

Mozingo is a company from Oakdale specializing in underground utility projects, according to their website.

Miller said the hypothesis for the sinkhole was that the intensity of the storm dislodged a large rock from drainage structure, causing a blockage which diverted water from its intended path.

“I suspect that drain filled up pretty much to capacity,” Miller said.

The water eroded a substantial amount of soil around and above the drain, creating a void.

When a barber drove over the spot in his 2007 Dodge Ram 2500 diesel truck Saturday morning, the parking lot collapsed.

Miller said the collapse was not anticipated because there was no evidence of erosion when the main drain was inspected about two years ago.

City Engineer Jerry Fuccillo said Thursday the main drain system dates to approximately 1870 through 1880 and it carries storm runoff from the surface down a low elevation line toward Sonora Creek.

The drainage tunnel has slate rock walls and in some places, iron doors along the top.

“It’s a pretty good drain and it’s lasted all these years,” Fuccillo said. “We occasionally get these collapses and we got one there in the Bank of America parking lot. That drain was there before the parking lot was there.”

Fuccillo estimated that the parking lot was built in 1968, just under a century after the construction of the drainage tunnel.

The main drain begins at a point behind Gold Country Gas and the slope of South Washington Street toward Green Street. The main drain goes under the Sonora Armory, the parking lot and Trado building, Fuccillo said.

By Tuesday, enough debris was removed to restore storm water flow in the drain for rains that fell on Tuesday and Wednesday.

Shotcrete, a sprayed concrete of mortar material, was used to fill in the holes along the drainage system and reform the walls of the drain. The shotcrete was applied by a pump and will also be used to create a roof structure. A void will also be left in the top of reconstructed drainage to allow access from a vertical manhole cover.

Miller described it as a “concrete tunnel in lieu of the drain that was there.”

Multiple cylindrical concrete tubes will be stacked on top of one another below the manhole providing access to the drainage. The hole will reach down approximately eight to nine feet from the surface to the top of the drain.

Miller said ongoing work in the drain may be inhibited by forecasts of rain later in the week and over the weekend.

“They can’t work in the drain if it’s filling with water and they can’t apply the shotcrete if the rain is too intense,” he said.

Crews will also be unable to set the manhole structure or backfill the excavation site if it is too wet, he said.

Hannah Chandler-Cooley, a meteorologist with the National Weather Service, said between a half inch and three quarters of an inch of rain was expected to drop in Sonora from Friday through Saturday.

The rain will be low intensity with most rain falling Friday night before isolated showers on Saturday.

Fuccillo said a high intensity and short duration rainfall made the drain most susceptible to damage. Access to a manhole could all city crews to more readily remove debris which could cause more blockages and urban infrastructure damage, he said.

“You need to leave it open so you can get it open and just in case there’s another breakage you can get in there and take rocks out,” Fuccillo said.

Years ago, a full wheel from a truck was in the drain and ended up in Sonora Creek, he said. There have been several major collapses in the drain, including during a flood in May 1996 that caused water to back up into several businesses on the 200 block of South Washington Street.

Miller and Fuccillo said the blockage did not cause damage to the Trado building.

“Any soil erosion under the building has been replaced by the shotcrete that will replace and stabilizes the sub-structure,” Miller said.

The city also has access to another manhole a few hundred feet away, located inside of the Sonora Armory, a not-yet-open entertainment venue and beer garden owned by Doug Kennedy, a developer who also owns Trado.

As part of an agreement with Kennedy, the city has access to the manhole and the main drain inside the Armory, Miller said.

Originally, Kennedy was tapped to construct a new manhole and make nearly $100,000 worth of improvements to the city-owned parking lot in exchange for the city granting an easement needed to provide water, sewer, propane and electricity to his project.

Fuccillo said another manhole access to the main drain outside the Armory structure would not have avoided the collapse.

“The failure was downstream of it and you can never tell where the heck it’s going to fail,” he said.

The Sonora City Council has declared a local state of emergency due to damage from the Mar. 6 storm. A failed storm drain in the Sunrise Hills area north of Morning Star Drive is expected to cost about $200,000 and is separate from the main drain cost estimate.

Contact Giuseppe Ricapito at (209) 588-4526 or . Follow him on Twitter @gsepinsonora.