Home & Garden Columns
In many ways my job is quite a joy. I get paid to do what many do on a free Sunday as their idea of fun. Looking at houses is both complex and deeply satisfying. Houses, especially those built in the early part of the 20th century (if not earlier) have so many pleasing features that it often feels more like art appreciation during my work day than data gathering. Then there’s the complex part.
Yesterday, the complex part had less to do with the house than with where it was located. While some aspects of the geology of the site were alluded to during my introduction to the house, it was not until many hours later at a computer screen that I, my client and his realtor got a close look at precisely where the property was situated with respect to a local slide-zone.
Berkeley has a bunch of these and they dribble across the hillside as though part of a Jackson Pollack commission. An array of the smaller ones near the crest of the hillside often strike me as a cartoon of tear drops (I’m really not trying to be funny here). But more than anything, they appear, on the map as tiny pools that formed in the lowlands or gullies of these hummocky, rolling hills. That word, hummock first came to me from the estimable geotechnical engineer Alan Kropp. A local luminary in the world of rocks and dirt, Kropp explained to me and a few colleagues some years ago that hummocks were lumps or humps in the soils formed through the crunchy world of geological formation. Berkeley is a hummocky place and explains a lot about why one house will have a wet basement while a nearby neighbor will be literally “high and dry.” Are you down in the hummocks today or riding the crest of the wavy earth?
Another feature of these hills and many communities like ours is that, over time, earth will break away, mostly from steeper terrain and slide down into the ravines below to be captured by the friction created by this lumpy landscape. Obviously, the lumpier things are, the more they hold this newly deposited turf and the higher the hummocks, the deeper some of these deposits will be.
With all this in mind, it’s interesting to go back to Mr. Kropp’s famous slide map and look again. What becomes clear is that the slide zones he demarks in red and yellow (active or potentially active) are nothing more than pools of soil held in place by friction and the shape of the landscape. Since the ancient landscape below this newer fill (and by newer, I mean hundreds of years old) can be deeply variegated, the depth of the landslide at any given point can vary enormously.
All this came to bear yesterday in a rather intense conversation about where my client’s house was located. Studying the map against other maps, we attempted to determine if the house was located in one of these active or potentially active zones and, if so, were we on the edge or the middle. This last portion is apparently of some special concern. When landslides move (the deposited soils across the older consolidated soils and other substrates such as rock) they take houses, streets and power poles with them. If structures are located on the edges of these pools of matter, the effect may be more dramatic, or at least more noticeable.
If you are located in the middle of a very large landslide area, it is more likely that the relative positions of your house and adjacent houses will appear unchanged. The same house located on the edge of a landslide may seem, after a dramatic shift in this unconsolidated matter, to have moved some distance from nearby landmarks but more importantly may have moved in ways that would not happen, were it adrift, as it were, in the middle of the slide. If one side of your house is resting on ancient firm soils or rock and the other side is resting on part of a 300-year-old slide, it is possible that over time or (more dramatically) during an earthquake that one side of the house will remain stationary while the other pulls or turns resulting in damage to the house. Earthquakes are good at creating slides and probably created the ones many of us (myself included) are living in. If you’re in a slide zone near an active fault line, the likelihood of a sudden change of address is more likely still.
Locally, we’re all near active faults and the difference between locations may not be all that significant since epicenters don’t consistently fall where we would expect them to be. Tectonic plates aren’t smooth or well mapped, they vary like the mountains and the spot that does the big slam may be deep in the earth and far from the apparent surface-identified fault. So it makes sense to not place too much focus on location and more on soils-mobility and the structure of the home. That said, having a home that is literally ON the fault is troubling and homes can be badly traumatized when they bridge or rest directly upon the fault when large quakes occur since faults can slip by up to several feet.
Foundation type and shear resistant framing methods can make a world of difference in slide zones. Mat or “raft” foundation by their names suggest something that can tolerate sliding better than most other types and my very uneducated eye has a great preference for these types where slides are likely (in either the slow continual creep or the quick visit to your neighbor’s property). These are essentially fat slabs of concrete, often specially shaped or possessing thick edges, and can move about as a whole preventing distortions in the shape of the house above. They seem to float on the surface of the ground and will tend to maintain an erect posture as they buffer the varying soils heights and densities changing below them. Even if they do end up out of plumb, the house as a whole will be uniform in level thus preventing doors and windows from sticking along with the other features we like to refer to as Berkeley Charm.
The pier and grade beam systems of the past (something akin to a table and its legs set into the earth) may tolerate soils subsidence (flowing away from the site) fairly well but extremely deep or powerful slides can bend or break those table legs in some cases. Engineers are all over that red and yellow map about these things and our concepts of how to live on moving earth continue to evolve. Methods I’d seen eschewed 20 years ago are more common today and, naturally, everything usually comes down to money and who can do something for a few thousand less.
My client and his wife had some hard thinking to do last night. A truly beautiful home on a lovely street on the arcadian moors of Berkeley, vested with Himalayan food, polarity massage and the worlds best coffee; and the possibility that for well over a million bucks they might be buying a mobile home.
Yea, I know, that joke’s getting old.