Avalanche of support for Edgehill Mountain
Dangerous monster home plan unites locals in environmental preservation effort
by Glenn Gullmes
San Francisco Times
Waves of emotion, ranging from fear to sadness to outrage, swept over the quiet residential neighborhoods encircling Edgehill Mountain when word spread that a developer had unexpectedly applied for three – of an anticipated five – permits to build a monster-home development on the steep, rocky bluffs along Kensington Way and Vasquez Avenue.
In response, neighborhood representatives from Community Action to Preserve Edgehill (CARE) called a Nov. 13 community meeting with Board of Supervisors President Norman Yee, who represents District 7, at New Life Church of the Nazarene on Knockash Hill, site of numerous landslides dating back to the mid-twentieth century.
With a full house of extremely concerned residents in attendance, the developer, geotechnical engineer and property owner calmly described their plans to remove an acre of mature forest, excavate a massive portion of Edgehill Mountain and build five 5-story 5000+ square-foot homes along Kensington Way.
The audience was stunned by the scale of the project and remained thoroughly unconvinced by the developer's reassurances that their proposal was entirely safe.
CARE responded with a slideshow presentation detailing the long history of catastrophic landslides in the immediate vicinity, inevitable dangers associated with destabilizing the fragile hillside during the construction of multi-home developments on Edgehill Mountain, and the environmental devastation to the urban greenbelt and watershed that would result from such an undertaking.
The neighborhood group presented compelling personal testimony, as well as impressive historical and scientific evidence relating to the various life-threatening scenarios the proposed project posed to residents in the area – and pointed to recent engineering failures such as Millennium Tower (which employed high-priced, well established firms), as one reason for not blindly relying on "expert" safety assurances.
After expressing strong opposition to the project in no uncertain terms, former District 7 Supervisor Tony Hall (who attended the meeting on crutches just a few days after knee surgery), explained how he had previously helped prevent a dangerous building proposal by arranging a land swap to expand Edgehill Mountain's Open Space parkland.
The meeting was then opened up to the audience, which raised a host of questions on issues ranging from neighborhood safety, congestion and emergency vehicle access to the scale and architectural appropriateness of the buildings.
The owner was encouraged to walk away from the project before wasting any additional money on an expensive plan that was unlikely to succeed. A lengthy, contentious neighborhood battle would only serve to increase community resentment as the planning process dragged on.
In a city where land use issues rank among the most contentious topics of the day, rarely is there general consensus, much less unanimity in the community, about a development proposal.
This is different. The neighborhood stands strongly united to oppose the massive building project and support local efforts to save existing open space on Edgehill Mountain. Longtime residents are already heavily invested in saving this cherished local treasure. They remember the years of persistence needed to create and expand the Edgehill Mountain Open Space parkland as a way to both protect the idyllic urban wilderness and prevent risky development.
Given its decades-long history as a champion of Edgehill Mountain, the Greater West Portal Neighborhood Association immediately came out against the project for safety reasons.
Six weeks earlier, representatives from SIA Consulting (the project developer) and City Planner Jeffrey Horn were taken aback by the overflow crowd of about 100 residents who came downtown to a Sept. 26 pre-application meeting at the Planning Department to voice their opposition to the proposed development.
Typical pre-application meetings attract three or four concerned neighbors, at most.
Families, Homes in Danger
History clearly shows that extensive excavation associated with multi-home construction projects on Edgehill Mountain inevitably leads to extremely dangerous and costly landslides. Always.
Locals vividly recall the terrifying, catastrophic landslides of the late 1990s as if they happened just yesterday. They have no wish to re-live a recurring nightmare.
Frank Rollo, the City’s pre-eminent geotechnical consultant when it comes to urban landslide emergencies, knows full well the dangers of the area’s steep slopes. Highly respected in his field, Rollo also respects Edgehill Mountain. And, as an author of the Edgehill Mountain Slope Protection Area section of the SF Planning Code, he understands the dangers in a way few ever could.
He narrowly escaped serious injury when the hillside gave way while he was surveying damage from a 1997 rockslide at Knockash Hill. Moments earlier he had been standing with his son right below where the entire slope suddenly collapsed. Had the excavated Franciscan Chert cliffs not been disturbed in the first place, the situation could have been avoided.
“Left to itself, chert remains in place for a long time,” said Rollo after the Knockash Hill disaster, noting that, in all probability, the hillside would have remained stable for thousands of years had it not been for excavations undertaken on the rocky slope. “Most of the hills with unstable conditions, it’s because of factors introduced by man,” he explained.
Rollo’s subsequent engineering efforts to make the treacherous cliff as safe as possible for those living at its base were, in a word, heroic. But the drastic measures that were required came at a very steep cost.
As part of the $2 million mitigation efforts, Edgehill Way’s iconic cantilevered clifftop octagonal home had to be demolished before the residents forced to evacuate the thirteen Knockash Hill Court homes could safely return. The slope angle above Knockash was also reduced, sealed with shotcrete, covered with metal netting, and then anchored and bolted deep into the hillside. The lengthy procedure was dangerous, but necessary under the circumstances. Such drastic and costly measures aren’t typically undertaken in non-emergency situations.
Despite input from top geotechnical firms prior to the massive 1997 landslide, many of Knockash Hill’s pre-construction mitigation prescriptions proved either monumentally inadequate or too dangerous and prohibitively expensive. The firms strongly disagreed on mitigation strategies, with estimates ranging from about $600,000 to almost $3 million.
The Knockash Hill builders and geotechnical engineers weren’t caught entirely unawares by the 1997 disaster, however.
They knew full well the dangers of undertaking large-scale construction in an area known for landslides and riddled with fragile, crumbling Franciscan Chert. After all, the long-abandoned quarries in the area date back to the 18th century. And, since 1935, aerial photos have dramatically depicted the hillside retreating up to six feet per year, depositing ever-growing piles of boulders, soil and debris at the base of the cliff.
Recipe for Disaster: Just Add Water
All significant instances of rockfalls, landslides and ongoing subsurface drainage problems on Edgehill Mountain were precipitated by rainstorms or seismic events – and were located in the immediate vicinity of areas that had been excavated and where the watershed was disrupted.
At any point during the planned excavation, as digging undermines Kensington’s rocky cliffs, a single rainstorm could instantly trigger a catastrophic landslide event. And, predictably – although landslide intervals could range from hours to years – heavy rains would continue to saturate and destabilize the existing equilibrium on the steep, excavated hillsides, eventually resulting in slope failure – over and over again (see page 3).
Problems still occur on neighboring San Miguel Hills with similar topography, as winter rockfalls continue to rumble down the slopes of Mt. Davidson and O’Shaughnessy Boulevard’s roadcut above Glen Canyon.
Over the past few years, big projects have been shelved due to community opposition, delays and high costs associated with the dangers posed to neighbors on steep slopes near Forest Hill (250 Laguna Honda Blvd.) and on Forest Knolls’ side of Mt. Sutro.
Most dramatic was the 2016 emergency demolition of a Sherwood Forest home at 256 Casitas Ave. that threatened to undermine adjacent homes and tumble towards Miraloma Way after subsurface leaking caused it to slip 14 inches away from the curb – and 12 inches downhill – within four days.
Edgehill Mountain has been enjoyed by generations of San Franciscans. A photo taken in 1910 before the West Portal of the Twin Peaks Tunnel opened the wild west “Outside Lands” to development – clearly shows a grove of native Californian Monterey Cypress trees and Edgehill’s prominent geological feature – a signature stretching hundreds of feet across, halfway up the fifth highest hill in the City.
Faced with the prospect of a monstrous, lengthy construction project disrupting this rare urban wilderness area, some residents have experienced profound sadness at the potential loss of an acre of historic forest – including a mature stand of Monterey Cypress – and the destruction of a geological wonder comprised of elaborate fossilized layers of Franciscan Chert that have withstood eons of upheaval.
Some likened the destruction of the long stretch of stacked, swirling ribbons of vertical chert to defacing a geological work of art. Millennia in the making, the weathered cliffs are like a mural in stone – a Mesozoic mosaic, if you will.
Naturalists point out that the greenbelt’s habitat would also be under attack. In addition to the existence of native ferns and succulents, surveys of the area’s flora and fauna have noted a number of threatened bird and plant species on Edgehill Mountain.
With a tree canopy of just 14 percent, San Francisco currently ranks near the bottom of the list of major US cities in terms of urban tree cover, according to the SF Planning Department. The concrete jungles of New York, Chicago and Los Angeles all have much larger percentages of tree canopy. And while the City acknowledges the importance of open space, scientists worldwide consider environmental preservation in the face of the existential threat posed by climate change to be a top priority. City forests are essential.
History shows that Edgehill Mountain excavations associated with multi-home construction projects have resulted in disastrous consequences for both the developer and those living in the immediate vicinity.
Over the past three decades, every single time a multi-home development was undertaken on Edgehill Mountain (as evidenced by projects on Kensington Way and at Knockash Hill), construction was eventually halted by rockfalls and landslides that required emergency action. Every single time.
Experienced contractors agree that the proposed project is an incredibly massive undertaking, using heavy equipment ranging from chainsaws and jackhammers to backhoes and bulldozers and generating pollution in the form of relentless noise, exhaust fumes and particulate matter.
The project would require countless truckloads over the course of many months to remove an acre of forest with well-established root systems, along with the debris from 200 feet of vertical Franciscan Chert cliffs (from excavations at least 30 feet into the hill), buried boulders and untold tons of crumbled rock, sandy soil and clay.
Disrupted watershed runoff and groundwater drainage, and erosion due to the destruction of the forest canopy and root system, would multiply the lasting environmental damage. The unique urban wilderness, once destroyed, could never be replaced.
Other potential problems lie deep below the surface. What can’t be easily quantified are the immense, often invisible, underground geological and hydrological pressures, compounded by environmental factors predicted by the latest climate models, such as the increasingly extreme cycles of extended drought and torrential rainstorms.
At Knockash Hill, a previously undetected presence of a “slope parallel shear zone 30 to 40 feet below the ground” was among the primary factors for the 1997 rockslide failure within the ribbon chert, according to Cotton, Shires & Associates Consulting Engineers and Geologists. They noted that intense rainfall caused rapid swelling of exposed sandy shale and accelerated weathering effects that fractured and dilated the rock mass on a hill previously destabilized by excavation.
And if that isn’t cause enough for alarm, the Edgehill Mountain Slope Protection Area falls within an Earthquake-Induced Landslide Zone – in a city with a 98 percent chance of a 6.0 magnitude earthquake within the next 25 years.
Giant Walls, Monster Homes
The scale of the proposed monster-home development on Edgehill Mountain boggles the mind. In fact, the staggering amount of mountain that would be excavated and removed from the five lots is hard to comprehend.
In addition to removing an acre of forest, plans call for street level excavation along Kensington towards Vasquez over 30 feet into the base of the hill – which currently includes a rock cliff spanning hundreds of feet. In some respects, the vertical chert wall currently in place acts as a natural retaining wall supporting the hillside above it.
One rock formation near the rear of the properties in question features a mammoth, multi-ton overhanging boulder the size of a Smart car. Left undisturbed, the overhang is one of the most impressive geological formations in San Francisco.
However, any excavation or construction vibration in the vicinity would disturb a precarious balance achieved over the centuries and immediately introduce a life threatening situation to surrounding homes. It would be no exaggeration to say that if construction begins, neighbors would live in constant fear for their lives.
“I’ve seen what can happen with Edgehill landslides – more than once – and I’ll fight day and night to protect my family from anything that threatens them,” said one neighbor who requested anonymity.
On average, the five lots are at least 70 feet deep or more, with a total of over 300 feet fronting Kensington Way and Vasquez Ave. – an area spanning an entire football field. With a stepped foundation and retaining walls at the back and sides of the property reaching more than 40 feet above street level, the foundation would be incredibly massive, stretching over 200 feet.
The homes themselves are out of character and scale for Kensington Way. Modern buildings that size are better suited for Edgehill Way, the privately-maintained road up the hill that loops around the exclusive enclave atop Edgehill Mountain.
For example, 166 Kensington Way, which would be located on the extremely narrow curve at Vasquez Avenue, would total approximately 5900 square feet of floor space (including a two-car garage), which is more than twice as large as the closest homes on Kensington.
From street level, the Kensington neighbors’ one- and two-story homes would be dwarfed by the five-story behemoths towering above them across the narrow street. The neighbors would have to crane their necks in order to be able to see the roofline of the giant houses. In some ways, it’s a scene reminiscent of David and Goliath.
As the story goes, one small stone brought a threatening giant tumbling to the ground. In this situation, however, the neighbors don’t want to see anything or anyone come crashing down. They just want to live in peace and preserve Edgehill Mountain for generations to come.