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Extreme rain caused landslide in Rolling Hills Estates, report says

Excessive rain last winter from a series of atmospheric river storms caused a landslide in Rolling Hills Estates in July that destroyed eight homes and deemed several others unsafe, according to a city report released this week.

The report, prepared by an external geotechnical engineering firm, reveals that “the main factor which caused the collapse of the Peartree Lane embankment was the unusually heavy rainfall that occurred during the winter of 2022-2023”. The 277-page analysis, conducted by Irvine-based GeoKinetics and reviewed Friday by The Times, is considered a preliminary assessment.

The report also identified secondary factors that “impacted the timing of slope failure,” including “long-term changes in hillside vegetation” and “along-slope failures.” relatively steep. The results were based on a review of historical geological and topographical information, engineering analyses, soil analyzes and rainfall data.

But a different analysis prepared by a separate firm for the neighborhood’s homeowners association, as well as an ongoing review requested by attorneys, have left some residents skeptical of the city’s findings.

“We are not satisfied with the city’s preliminary assessment,” said Steve Blum, an attorney who specializes in landslide litigation and represents the majority of residents affected by the landslide. “It would not be unusual for a report like this prepared for a client… to be self-serving. »

But Glenn Tofani, the lead engineer on the GeoKinetics report prepared for the city, stood by his findings Friday.

“The findings presented in our report are objective and based on available facts and information,” Tofani said in an email. “Our conclusions are not affected by who we work for.”

Tofani also emphasized that he works closely with GMU Engineers and Geologists, an Orange County company hired by the homeowners association.

“Based on my discussions with GMU representatives, I think our conclusions are generally very similar,” he said. “We continue to work together to gather additional information and identify actions that should be taken in the future.”

However, the city’s report also makes clear that it remains “distinct and separate” from the homeowners association’s private investigation of GMU.

The city’s report, compiled with support from the Los Angeles County Office of Emergency Management, was prepared as part of a disaster reimbursement from the Federal Emergency Management Agency, according to the spokesperson. word of the town, Alexa Davis. The report was submitted to FEMA last week, she said.

The devastating landslide on Peartree Lane in the Rolling Hills Park Villas community forced the evacuation of a dozen homes on July 8, when the street at the top of the hill began to sink. Within hours, many houses collapsed down the slope of the canyon, their roofs collapsed and their foundations destroyed.

The new report shows the landslide moved the existing slope about 45 feet down the canyon. Eight homes were destroyed, four others remain red-tagged and five others still carry a yellow tag due to damage to utilities caused by land movement. Most of the houses marked red were deemed structurally unsound and all were deemed unsafe for re-entry. Homes labeled yellow — which are not atop the steep canyon wall — had their water cut off from a damaged sewer line, making them accessible to residents, but uninhabitable.

Blum has not yet filed a lawsuit on behalf of residents. He said his team was waiting to complete its own analysis of the causes of the landslide. He questioned how this year’s rains could be the primary cause, given previous rainy seasons that have been more extreme, which GeoKinetics noted in the city’s report.

“Since the site was developed in the late 1970s and early 1980s, there have been three winter seasons with winter precipitation totals greater than winter 2022-2023,” the report states.

Blum did not question that rain was likely a factor in the slide — one of many — but said that “over the years there have been more (precipitations) important than these precipitations.

The HOA’s geotechnical company’s preliminary investigation — which homeowner officials say is ongoing — found a “progressive failure” involving erosion of stormwater flowing through the canyon floor, which ” exposed the weak stratification of the sedimentary rock, allowing local movement downslope to initiate’. ”, according to a presentation shared with residents and reviewed by the Times. This movement caused surface cracks, which were “probably infiltrated” by groundwater and rainwater and led to the collapse of the slope, the presentation said.

In contrast, the city’s GeoKinetics report did not focus on erosion or call it a factor in landslides.

“Erosive digging along the canyon into the slope … does not appear to have occurred,” the report said.

Instead, it pointed to the presence of surface breaks “along the relatively steep slope” that were not apparent in previous wet years but had developed last winter, the report said. These failures “probably resulted from saturation of the slope side during major storms.”

The report refers to previous ground movement problems in the area – including a 1986 incident at 21 Peartree Lane, just west of the recent slope break – which was caused by “poor compaction of the embankment and associated settlement/creep.” Consultants at the time found “no indication of gross slope instability,” the report said.

The owners’ management company, Scott Management, declined to comment or answer questions from The Times. GMU officials also did not respond to questions.

The city’s report also disputed some concerns from Rolling Hills Estates neighbors, saying its team found “no pre-existing slope failures.”

“The recent failure appears to represent a new feature as opposed to the reactivation of a pre-existing slope failure,” the report states. It also excluded leaks from utilities.

Residents, already worried about the safety and value of their homes after the disaster, say they now face a lengthy and costly process to try to mitigate the slope collapse, a process the HOA has announced to its members would cost $4.3 million. They recently presented a plan to split that cost among nearly 200 homeowners, to the tune of nearly $24,000 each.

The first step is to work on a city-mandated “winterization” of the slope, including a stormwater pipe through the canyon, sandbags, plastic sheeting and filling cracks with concrete, according to the presentation of the owners association.

Los Angeles Times

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