Excessive rainfall blamed for destructive landslide
>> Excessive rainfall last winter from a string of atmospheric river storms caused a landslide in Rolling Hills Estates in July that destroyed eight homes and rendered several others unsafe, according to a city report released this week.
The report, prepared by an outside geotechnical engineering firm, found “the primary factor that caused the Peartree Lane slope failure was the unusually heavy rainfall that occurred during the winter of 20222023.” The 277-page analysis, conducted by Irvinebased GeoKinetics and reviewed Friday by the Los Angeles Times, is considered a preliminary assessment.
The report also identified secondary factors that “impacted the timing of the slope failure,” including “long term changes in the hillside vegetation” and “failures along the relatively steep slope.” The findings were based on a review of historical geologic and topographic information, engineering analyses, soil testing and rainfall data.
But a different analysis prepared by a separate company for the neighborhood's homeowners association, as well as an ongoing examination requested by attorneys, has left some residents skeptical of the city's findings.
“We are not satisfied with the city's preliminary assessment,” said Steve Blum, an attorney specializing in landslide litigation who is representing a 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 principal engineer on the GeoKinetics report prepared for the city, stood by his findings Friday.
“The conclusions presented in our report are objective and based on the available facts and information,” Tofani said in an email. “Our conclusions are not affected by who we are working for.”
Tofani also pointed out that he worked in close contact with GMU Engineers and Geologists, an Orange County firm hired by the homeowners association.
However, the city's report also made clear that it remains “distinct and separate” from the homeowners association's private investigation from 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 city spokesperson 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 hilltop street started to sink. Within hours, many homes had fallen down the canyon slope, their roofs caved in and 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 more are still yellow-tagged because of utility damage from the land movement. Most of the redtagged homes have been ruled structurally unsound and all have been deemed unsafe for reentry. The yellow-tagged homes which do not sit atop the steep canyon wall have had their water shut off from a damaged sewer line, making them accessible by residents, but not livable.
Blum hasn't filed a lawsuit on residents' behalf yet. He said his team is waiting to complete its own analysis of what prompted the landslide. He questioned how this year's rains could be the primary cause, given prior rainy seasons that have been more extreme, something GeoKinetics noted in the city's report.
“Since the site was developed in the late 1970s and early 1980s, there have been three prior winter seasons with winter rainfall totals higher than that which occurred during the winter of 2022-2023,” the report said.
Blum didn't question that rain was likely a factor in the slide one of many but said that “over the years, there have been bigger (rainfalls) than this rainfall.”
The preliminary investigation from the HOA's geotechnical firm which homeowners officials said is ongoing pointed to “progressive failure” involving erosion from stormwater running through the bottom of the canyon, which “exposed weak bedding of the sedimentary rock, allowing local downslope movement to initiate,” according to a presentation shared with residents and reviewed by the Times. That movement caused surface cracking, which was “likely infiltrated” by groundwater and rainwater, resulting in the slope failure, the presentation said.
In contrast, the city's report from GeoKinetics did not focus on erosion or call it a factor in the slide.
“Erosional cutting along the canyon invert into the slope ... does not appear to have occurred,” the report said.