Archive for the ‘Oakland hazards’ Category

Oakland builders, what are you thinking?

12 June 2017

Californians have always known we’re prone to earthquakes. The first Californians didn’t have our worries about it, though, because their structures were small and limber, no larger than a temescal sweathouse. Things changed when the missionaries of New Spain came into the country starting in the late 1700s. When the earthquake of 8 December 1812 took down the six-year-old stone church at San Juan Capistrano during the day’s first service, the forty natives who died were probably the first Californians ever, in thousands of years, to be killed by a structural collapse from an earthquake.

To the Americans who succeeded the Spanish and Mexicans in the Bay area, earthquakes were well known. By my count, after Oakland incorporated in 1852 its inhabitants experienced thirteen notable earthquakes in the 54 years before 1906. As the American cities grew up around the Bay, builders sought to guard against quakes with thick walls of ever-stronger materials, culminating in concrete and steel. In the century since the 1906 San Francisco quake, engineers and architects have repeatedly improved the building codes.

Today, buildings of almost any shape and size can be designed to withstand the largest earthquakes. That doesn’t mean we’ll trust them. Just as some of us get vertigo looking at photos of confident rock climbers, appearances can outweigh reason.

In recent months, two large buildings have been proposed in downtown Oakland that actually included overhangs. The one originally submitted for 325 22nd Street, facing the Ordway Building, looked like this, with a cantilevered soffit (as seen on SocketSite).

The Planning Department didn’t like its bulkiness and awkward fit with its neighboring buildings, so that design was replaced with a more traditional set of boxes.

The other building site is at 1100 Broadway, the lot next to the old Key System Building, both of which have been vacant since the 1989 earthquake. The latest project, from new owners Ellis Partners, is supposed to renovate and integrate with the Key Building, and this overbearing design is what they came up with last month.

After getting feedback from planners and the Oakland Heritage Alliance, they submitted a revised design last week. They just love that looming cantilever — in fact they added more on other sides.

Builders and planners are pros, so the overhangs are designed to hold. I understand that very well in my head. But how many of us will feel secure beneath — or inside — a cantilevered structure?

The National Earthquake Hazard Reduction Program (NEHRP) has funded and published research into building standards since the 1970s. Its publication Earthquake-Resistant Design Concepts spells out seven characteristics that are necessary for buildings in areas prone to large earthquakes. Two of those are especially pertinent for cantilevered designs: continuous load paths and regularity. The first means that the forces a quake imposes on a structure need to be guided down to the ground. The second means that irregular buildings must be extra strong, as “the damage can be concentrated in one or a few locations, resulting in extreme local damage and a loss of the structure’s ability to survive the shaking.”

I think these are obvious to most people, and that’s why, say, the new Kaiser hospital is reassuring in its continuity and regularity. The Transamerica Pyramid, even the new Salesforce Tower — reassuring.

The design for 1100 Broadway is a textbook example of discontinuous and irregular, especially in its integration with the hundred-year-old, damaged Key System Building. The NEHRP Concepts classify downtown Oakland office buildings in Seismic Design Category E, just short of the most stringent category used for hospitals, police and fire stations and other critical structures.

After the East Bay’s next big earthquake, our perceptions will change. It’s important to think about that. Old-timers who were here for the Loma Prieta quake in 1989 can tell you. For years afterward, this fine old town will feel like Doctor Caligari’s city, angular and foreboding.

When large quakes strike, buildings can sustain damage in an earthquake and then collapse in aftershocks, of which we will have plenty after a magnitude 6.7 event. I use that magnitude because the official odds are based on it, and they give the Hayward fault a one-in-three chance of producing one before the year 2043, within the useful life of these proposed buildings.

No one alive has experienced such a quake in Oakland. Once we do, buildings with overhangs, even if they perform superbly, will no longer look vibrant or stylish — they’ll look deadly to our newly cautious eyes. And with that they’ll be effectively worthless, except maybe for low-income housing (which would be a good thing). And the city that thought such a building was a cool statement will be judged for that statement.

I submit that builders and the city should be very conservative in not just their designs, but the appearance of their designs.

Shepherd Canyon landslides

10 April 2017

Last week I went to visit a landslide that had been in the news. As it happened, I saw three.

Shepherd Canyon always gets a lot of landslides, like its neighboring canyons in the high hills. The main reason is that Shephard Creek has a lot of cutting power, thanks to its relatively large watershed and the low base level provided by Dimond Canyon. That creates steep slopes and V-shaped valley profiles. A secondary reason is the relatively soft mudstone underlying those slopes.

My destination was the landslide that came down on the south side of Banning Drive. But along the way my path was blocked by two more mass movements. They’re marked by white asterisks on the geologic map below.

The Montclair Railroad Trail, my usual route, offers walkers good access to the canyon. On the inner side of the sharp curve and cut leading into the canyon, this slope failure exposed the rears of two houses. I classify it as a debris fall.

The majority of the material is broken rock, hence the term debris, and it tumbled in a heap rather than traveling any distance, hence the term fall. Only a little mud was present.

The area is mapped as the Oakland Conglomerate (Ko), although the debris appeared to consist mostly of fine sandstone and a little shale, like this. The rainwashed stone is well displayed.

Picking my way past that was no problem. Farther up the trail, though, was a complete blockage.

Like the lower slope failure, this one involved debris, but unlike it the material slid, so I classify it as a debris slide. Several large trees that came down with the rock didn’t appear to be to blame. However, this time of year is the most dangerous for trees because the ground is sopping wet and the limbs are heavy with young leaves, making them prone to catch the wind. Maybe they triggered the slide. Maybe the other way around.

Fortunately no houses appeared to be threatened above the headscarp, but now the slope is highly vulnerable.

A sewer line runs beneath the trail, so the city may have to clear the slide once the ground is no longer saturated. Meanwhile this is too dangerous to approach. It could fall with no warning.

The debris is made of fine-grained sandstone of the Redwood Canyon Formation (Kr).

Finally I got up to Banning Drive. It’s situated in one of the major side valleys in Shepherd Canyon, and the walls are exceptionally steep.

I classify this slope failure as a debris flow, what the news media often calls a mudslide. It traveled downhill a good hundred meters in a thick semifluid mass. The mud content was greater than the other two slides, and muck spilled into and around several homes on Banning. There’s plenty of footage of the scene online, so I don’t need to show you that. It was hard to watch the residents clearing out their red-tagged homes while the news vans gathered round.

I didn’t need to be there once I’d seen it. Presently I went uphill to Aitken Drive, where the slide originated.

Note a couple of things. Right beyond the gap in the road, a telephone pole was snapped off and the wires were hanging low. (The power was off.) The extra load caused the pole at the left edge of the photo to lean inward. The scar in the road reveals a wall of sandbags (I assume they were filled with concrete) that must have been put there after a previous slide.

Landslides occur where previous slides did. And sure enough, looking uphill I could see the young scar of a small rockslide, nestled in turn within a concavity in the hillside that looked like the scar of a much older slide.

There is another street higher up, Chelton Drive, but no houses up there appeared to be endangered. Meanwhile East Bay MUD had the road blocked while they were making sure the water lines underneath wouldn’t break and make more trouble.

Who’s responsible? Perhaps no one. The problem is above my pay grade, as I’m not a licensed geologist. But I can see the signs and warnings of landslides, and so can you if you pay attention to the landscape. The U.S. Geological Survey has resources, and so does the California Geological Survey.

Old fill and made land

6 March 2017

Between the 1840s and roughly 1960, the Bay area made colossal amounts of dry land through “reclamation,” a euphemism for filling in marshland with whatever was handy. Oakland was no exception. Here’s a portion of the geologic map centered on Jack London Square, Oakland’s original harbor. Reclaimed land, or artificial fill, is shown in pink.

jlsgeomap

We were lucky in Oakland that the waterfront was largely developed by the railroads. They had the money and foresight to do the job right, by mid-1800s standards, and the fill is pretty good. An exception is the building that houses Heinold’s First and Last Chance Saloon.

heinolds

The structure was built in 1880 of ship’s timbers, and in accordance with good practice it was placed on piles, driven into the fill and the mud beneath. But it has settled, especially after the 1906 earthquake, and the land has been built up around it. You won’t notice from the outside until you approach the door, which sits below the surrounding ground.

heinoldsdoor

Go inside (as every Oaklander should at least once), and you’re in a funhouse of ancient furniture and creaky walls covered with old papers, all set off by a crazily tilted floor. Fortunately it’s merely a bar and not some place people have to live in.

For real landfill folly, you’ll want to visit San Francisco. The land south of Market Street was originally a mixed bag — the bedrock of Rincon Hill and a ridge of sand dunes to its west offered firm ground, and Sullivan Marsh to their south surrounded Mission Bay with poor ground. On the geologic map, the old marsh is mapped as “Qaf,” or artificial fill. It extends to the Civic Center.

sfmarshgeomap

The area south of Market began to be developed around 1870, and the marsh was filled rapidly and willy-nilly. Within a few decades the good ground was occupied by grand hotels, office buildings and luxury residences while the bad ground — the made land — was full of working-class boarding houses, small shops and warehouses.

The marsh contained several meters of peat on the surface and saturated sand and mud beneath. Loading this land with landfill rubbish, then placing buildings on it, left it highly vulnerable to settling and, during earthquakes, liquefaction — complete loss of strength.

When the 1906 quake struck, the buildings on firm ground did well while the former marsh area was largely ruined. But everything south of Market, rich and poor, burned down within a day. All of it was rebuilt, just like before, as rapidly as possible. And for the next century, the made land has kept on subsiding.

Main streets, like 7th Street, were regularly built up to maintain their grade. Side streets like Natoma were not.

sfslump-4

Many homes sank at the same time the street were built up. This one is typical.

sfslump-3

At the same time, many buildings suffered differential or uneven settling, like Heinold’s did. Residents, then and now, just put up with it.

sfslump-2

Today redevelopment is replacing these tired survivers. The new blocks have a hip and trendy look now, if you don’t happen to notice the occasional straggler.

sfslump-1

Soon enough, the past will vanish from the South of Market. But just wait another century, or less if they have a major quake before then, and some of these new buildings will slowly turn into antique funhouses like Heinold’s.

Oakland has been pretty good about avoiding our own versions of Sullivan Marsh, but we do have strips of fill to watch out for.