Slow-motion slides

[torquill]

I spent some time today staring at the hillside above the lower driveway.

I've studied the ground when hiking to the water box, as the trail is above that, so I know that the driveway runs along the bottom end of a long ridge. Over time, the last bit of that ridge has crumbled, despite the huge trees anchoring it, and slightly raised one rut of the driveway for about 20 feet. I've been mulling over the best way to stabilize that situation ever since I came here nearly a year ago. The nearest part of the slope is in excess of its angle of repose, given the sandy loam it's made of, so it's just going to continue to intrude on the driveway without some intervention.

There are various ferns and small plants there, doing their best, but the small cliff at the top argues that simply planting the slope beneath it won't help. I can try to get some more thimbleberry in there anyway, and perhaps add something like Mahonia, but odds are good that the next big storm will just slide them down to the foot before they really take hold. The cliff edge is about ten feet up, with a slope above it, and maybe five or six feet of run to the driveway -- far too steep for a decent retaining wall or terracing without professional engineering. We have better things to spend that kind of money on.

I decided that I can shovel out the latest set of slides, which I would need to do anyway, and level the driveway, and mix that soil with compost to fill raised beds in the garden; it's poor soil by itself, but it drains well, and it would solve the problem of getting enough bulk to actually fill the beds. With proper amendment I suspect it would be very good planting mix. So, with that quantity out of the way, what to do about the rest of the hill?

I can't make a retaining wall that will withstand the mass of soil there in the winter.... but I can put up a containment wall, like they put on slide-prone roads. A low wooden palisade which will act as a stop for small slides and tumbling rocks, to keep the driveway clear. Every year or two it could be cleared out on the back side in the dry season. I could easily make it out of pressure-treated 4x4s and 1x6s, and it wouldn't require the gravel and drainage pipe needed for actual retaining walls.

It won't stop the hill from sliding, but I suspect that the soil will erode to a point, then become more or less stable again. If we can wait that out, planting shrubs and ground cover on the result should make it stable under normal circumstances. It may take twenty years, but that's about how long a good stout wall will last, and we can take another look then.

Comments

[klrmn]

have you considered gabion boxes? i'm not sure if they would totally eliminate the needs-engineering part, and i don't know if you've got the rocks to fill them on-site, but they are supposedly both good for allowing drainage, and becoming more stable as they age rather than less.

[torquill]

From what I can see, gabion boxes are simply a class of retaining wall... while they probably deal with the drainage problem, they have less structural integrity than a solid wall, so they would be even less suited to this situation than something like wood or reinforced concrete.

I suspect I have not conveyed the scale of the problem well -- even if the soil were firm, I could not climb this slope without a ladder. Retaining walls are for far gentler rises.

[klrmn]

gabion boxes would not hold in sand, that much is true, so if it's that texture when dry.

but given that the place you most often see them is roadside holding up cliffs, i don't think steepness is the problem.

you could even build a short one like your short fence, and instead of digging it out when the back side of it fills up, you put in the next row, slightly behind the first.

hey, it's an academic reference https://web.mst.edu/~rogersda/umrcourses/ge441/online_lectures/retention_structures/GE441-Lecture6-3.pdf

but it is in fact also highly likely that i'm not picturing things well, or misunderstanding some crux of the issue. i keep flashing to a particular hairpin turn encountered on a motorcycle ride behind lake chabot. now that i've provided a pointer to 'it's not just for aesthetics', i'll assume you're probably more capable of interpreting it than me.

[torquill]

huh. That is not what I found when searching for gabions.

Yes, it looks like that might work... but on a scale where I would not feel comfortable, or even capable, doing it myself. These structures look like they require a front-loader at the very least, and some knowledge of how to anchor the toe so that it doesn't push out under load. So while you're probably right, and gabions might be appropriate for this hillside, I think they would take a soil engineer to implement.

[klrmn]

the cages themselves can be ordered in pretty much any size and guage, but if the scale is still too big, then the scale is still too big.

[torquill]

It's mostly that I don't want to half-ass it because I'm only doing it at the scale I can physically manage myself, and then have it fail and cause a bigger mess later.

Here's the most slide-prone spot: https://photos.app.goo.gl/YAeqGVw9ov3pfmQD8