Coring

14 August 2017

With all the construction going on around town, you’ll see lots of drill rigs taking geotechnical cores. This one was at work at 2330 Webster, where the Webster Alexan development will go.

Just a few days earlier, a rig was collecting cores in the parking lot at 20th and Telegraph, slated to become one of two residential towers.

Crews like this are testing the ground to firm up the construction plans. The weight of these buildings requires a foundation that won’t sink, buckle or deform under the expected loads over the building’s life, including earthquake loads. At the Telegraph site two holes were bored, at opposite ends of the lot.

It takes a couple of workers to run the rig and a geologist to log the hole. They look the same — vests, boots, hardhats — except the geologist carries a clipboard and isn’t quite as muddy. The geologist on this job was a young guy, crouched in the sun and processing sediment plugs that looked like this.

It’s nice, clean marine clay from the lower part of the hole. I refrained from nibbling on a piece to gauge its silt content. It was real firm, not sticky. I’d put a house on it, no problem.

The geologist was poking at the plugs with a pocket tool and keeping them properly organized. He told me the hole was around 90 feet deep, with this stuff at the bottom. The top 20 feet was sand and gravel, then about 30 feet of clay, then some more sand and gravel and finally this clay. It’s a common pattern around the Bay, reflecting the changes in sea level over the last few hundred thousand years.

The crew was finished in less than a day, and they tidied up nicely afterward.

The U.S. Geological Survey’s Open-File Report 2014-1127, “Geologic Logs of Geotechnical Cores from the Subsurface Sacramento-San Joaquin Delta, California,” will give an idea of what core logging involves when it’s done right. What seems like painstaking drudgery is essential for building safely, and geologists can get called into court to vouch for the accuracy of their core logs.

The smaller creeks of Leona Heights

7 August 2017

Last week I lingered over Leona Creek, in the Leona Heights area, but there are three other streams in the land between Horseshoe Creek and Chimes Creek. I’ll label them on the watershed map from 1 to 3, putting the numerals where they enter culverts.

Creek 1 passes through the old Crusher Quarry grounds, now occupied by houses, so I’ll call it Crusher Creek. This is the best view I can find of its catchment, looking across Mountain Boulevard up Bermuda Avenue to the quarry’s old working face. The headwaters are to the right, and I haven’t explored there yet.

Behind me, the creek emerges in a steep gulch that passes around the old Chabot Observatory (now a school), then disappears under route 13.

At the moment it’s all dry, so I can’t tell what shape Crusher Creek’s water is in. Probably not so good, considering the amount of sulfuric acid being weathered out of the old quarry leaving all that red iron oxide behind.

Under the freeway Crusher Creek joins the orange water from the Leona Sulfur Mine, seen here where Leona Creek enters its culvert at Twitter Court.

Creek 2 comes down next to Rusting Avenue, so I’ll call it Rusting Creek. This is not to prejudice its water quality, which is good where I’ve seen it. This view shows the entrance to its culvert at Mountain Boulevard, looking up its catchment.

Higher up, just above Mountain View Avenue, the streambed displays the variety of rock that comes from the Leona volcanics, in a spectrum from nearly white and greenish-gray to dark red.

Here are two hunks of the pristine rock with a piece between them of the ocher crust that forms on top of it after centuries of weathering.

Creek 3 has a fire road up its valley, behind a gate at the end of Leona Street, that used to be called Russell Avenue, so this is Russell Creek. It’s worth an outing, as long as you park down the road and walk in quietly. However, at the top the fire road ends at a locked gate on Ridgemont Drive, and you’ll have to walk back down. I can attest that trying anything else is foolhardy.

Here’s the view down from near the top. At the lower left you can see the fire road, which ends next to the house at the far left. (Russell Avenue appears on Apple Maps, but not Google Maps. Trust Google on this one.)

The rocks exposed along the fire road show the full range of the Leona volcanics.

Close up, they’re quite colorful with their oxide skins, and there are many fine oaks and vistas along the way. This may be Oakland’s best trail you never heard of.

Finally, from Leona Road you can take a fire road to the former Leona Quarry, now called “Monte Vista Villas at Leona Quarry” and filling up with townhomes. I can’t easily describe how to access it except to recommend you take it first from the other end, at the entrance to the development next to this fine wall made of quarry boulders.

It leads along the freeway and ends at a nameless spur off of Leona Road.

The stroll is nice, if you ignore the freeway noise, and you’ll see that the hillsides here, which look so tempting from a distance or from the seat of a car stuck in traffic, are quite inaccessible. Stay on the path.

The mine drainage of Leona Creek revisited

31 July 2017

Over the years I’ve done a lot of poking around Leona Heights, the large hill looming over the south end of the Warren Freeway. You’d think I have a nice photo after all this time, but instead here’s a vertical view from Google Maps, terrain view. It shows the area between Horseshoe Creek, at the top, and the former Leona Quarry at the lower right.

The hillside is deeply eroded by several steep gullies, all of which still have running water in them at this time of year. Those fascinate me. And the satellite view of the same area shows how much of it is forest, which also fascinates me.

Most of this land is inaccessible. There’s only two fire roads, few trails, and very steep slopes well guarded by brush and poison oak. Apparently the city owns much of it.

And here’s the stream map to help with the creek names, because they’re confusing. Each of the Leona Heights gullies is interesting, and I’ll be showing them to you in coming weeks, but this week I’ll focus on the one labeled Leona Creek.

The streams in these hills all feed Lion Creek, originally named Arroyo del Leon. The rules of river names say that the name of a stream applies to the most vigorous branch, and if the stream splits into branches of the same vigor the name can be arbitrarily assigned to one branch, or none. Thus the upstream end of Sausal Creek is where Shepard Creek and Palo Seco Creek join. So on this map I’m extending Lion Creek up its northernmost branch (although it may well be that by this criterion Horseshoe Creek should be Lion Creek). People also talk about “Leona Creek,” sometimes applying the name to all of Lion Creek and sometimes applying it to the nameless creek that has the former Leona Mine on it. Because the mine site is so important, I’m giving that particular stream the name Leona Creek.

This creek once had potential. It has a nice catchment, seen here from the Merritt College parking lot toward the north end of Ridgemont Avenue. The woods are impenetrable.

But then the creek reaches the old mine.

By early last year, the mine site had been fixed up so it looked clean.

And down below the mine the stream looked pretty good.

But as of a couple weeks ago, it was back to its old trick: acid mine drainage.

What we’re looking at is yellow and orange iron oxides, precipitated out of the acidic water as it’s neutralized. They aren’t poisonous in themselves, and the water won’t eat the flesh off your fingers. But other metals are dissolved in drainage water besides iron, which are more toxic. I don’t have any chemical data from the water, so I can’t address the true hazard. But this stuff is harmful in other ways, specifically by blanketing the streambed so that living things can’t live on and in the gravel like they’re used to — insects and insect larvae, which feed other insects and birds and so on.

Acid drainage is natural in the Leona Heights, to a certain degree. The rocks hold a lot of pyrite, which oxidizes to yield sulfuric acid, so there’s always a little acid around. The mine, however, opened up the richest part of the hill and gave it access to oxygen.

The raw chemistry of pyrite oxidation is not that fast. But sulfur-oxidizing bacteria make their living by eating pyrite and pissing out acid, and the old mine is like a giant party condo for them. They won’t stop for anything short of encasing the whole hillside in concrete. And we won’t do that.