A sand stockpile that added to the load on the soil beneath it and the operations of a rock crusher are among the factors that may have contributed to a large landslide in Westbrook last month.

Those findings are part of a report issued by the U.S. Army Corps of Engineers following a site visit Sept. 19 that also provided recommendations on how to prevent future landslides and stabilize the area to ensure damage is not done to public utilities.

The landslide occurred Sept. 16 on the properties of Les Wilson & Sons, an excavating company, and Delta Realty on Warren Avenue and prompted a flash flood warning and emergency order from the city after about 2 acres of land collapsed and slid into the river.

According to the new report, a sand stockpile in the area where the crown of the landslide was noted increased the load applied to the soil beneath it. If the stockpile had been placed in a short time frame, rapid loading could have resulted in increased pressure on the soft clay beneath it, the report said.

Sand was placed on the pile the day of the landslide, according to a Maine Geologic Survey referenced in the report, and about 6,000 cubic yards of the pile were lost in the landslide.

The Maine Geologic Survey also indicated a rock crusher was operating on site the day of the landslide and the vibrations may have resulted in reduced strength of the clay known to be underground in the area.

Chris Wilson, owner of Les Wilsons & Son, said he was uncertain whether the company was operating a rock crusher the day of the landslide and would have to go back and look at what was going on that day. He said there have been sand stockpiles on the property for decades.

“Nothing different happened on the land that’s ever happened there, no different operations, no different jobs, no different piles, no land expansion,” Wilson said.

Attempts to reach Delta Realty on Friday night were unsuccessful.

Several public utilities are near the landslide, but none appears to have been damaged, according to the Army Corps report. However, the steep slope of the landslide scarp and loss of root systems and vegetation could lead to future erosion and threaten water and gas lines.

Army Corps officials have met with Westbrook’s city engineer and suggested exploring programmatic assistance for riverbank stabilization, removing the remaining stockpiled sand from the crown of the landslide to reduce the load and filling tension cracks to prevent water intrusion into the failed land mass during future rainfall.

Additional recommendations in the report include implementing temporary setback requirements for storing material and operating heavy equipment on the properties of Delta Realty and Les Wilson, regrading the properties and providing temporary diversions to drain surface water away from the landslide and placing stakes at the landslide toe and river bank to monitor for continued movement.

The report also recommends placing rip rap at the toe of the landslide to prevent erosion during floods, inspecting areas up and downstream from the landslide to observe conditions that may contribute to similar soil movement and increasing public awareness and outreach for landslide hazards in the area.

Wilson said he is working with the city on the recommendations. “We’re working with the city on everything right now,” he said. “We’re still working through it.”

Michelle Clements, a spokeswoman for the Portland Water District, said crews removed debris and stabilized the district’s portion of the riverbank on Sept. 23 when pending rain and the possibility of a sudden surge of material flowing over the area threatened to disrupt Westbrook’s wastewater treatment system and jeopardize the public water supply.

Work is complete on the Warren Avenue side of the river and the district expects to begin work on the other side next week, Clements said. She said the district remains concerned about the stability of the area and future landslides as well as the threat of additional land movement on the slope.

Clements said she did not have an estimate Friday night on what the total cost of the emergency response will be, but there are no state and local funds available to assist and the cost will be borne by the water district and ratepayers. The district is exploring options for reimbursement and met Friday with Gov. Janet Mills’ office and others, including the Maine Emergency Management Agency, city of Westbrook and the Maine Department of Environmental Protection, which will take the lead on coordinating next steps.

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