DryStoneGarden

Failed Moss Rock Wall

In light of all today’s talk about tearing down that (Berlin) wall, here’s some photos of the biggest wall to come down in my neighborhood this past winter, when a section of hillside pushed a moss rock retaining wall into the road. Must have been a shock to the homeowner and whoever built it.

Though, without sounding like I’m piling on the unfortunate builder of the wall, it was pretty predictable with a wall that tall and skinny at the bottom of slope that big. A three foot high wall is supposed to be two feet thick at the base, and this wall isn’t even a foot thick. It also doesn’t have a proper foundation or gravel backfill. I see a lot of failed retaining walls around town, and the common denominator is that they are always backfilled with dirt instead of the drain rock or rubble that virtually every book or manual recommends.

Failed Moss Rock Wall

Intact Section of Wall

The wall is still intact where the bottom course has bigger rocks and the rocks are toed in against the curb.

Failing Section of Wall

Failing Section of Wall

Moss Rock

Two more waller nightmares are below. (more…)

mallorcan style wall

Posting about the angled stone wall in this year’s garden show seems like a good excuse to post about the Mallorcan style wall we did with Mariposa Gardening and Design in the show last year. It’s not as eye-catching as the angled stone, but the building style is unique in its own way. Personally, I didn’t lay a single stone on the wall–I was a bit skeptical about building a wall and then taking it apart five days later, part of the reason I like stone is that it is the longest-lasting building material on earth, so instead my contribution to the garden was some flagstone steps that I could afterwards re-install in a real garden–but it was a nice wall and it deserves to have some internet presence.  A lot of the stuff in the garden show is just facade work, but the crew built a real wall, thirteen tons worth, pretty cool and pretty crazy.

Mallorcan walls are also sometimes called polygonal walls because they use five-sided stones laid in an arch pattern; traditional walls use four-sided rectangular stones laid in linear courses. One of the sacred rules of traditional walls is to break every joint, but with a polygonal wall the joints zigzag enough that the rule doesn’t apply. Instead, the rule for a polygonal wall is to have every stone touched by five others. Instead of trying to create a flat surface for the next stone, you try to make a cradle for it, and instead of vertical and horizontal lines, you create arches. The idea is that the adjoining stones form an arch around any given stone, so if that stone falls out the other stones will still hold together and the wall won’t fail. In theory, if you pick a stone, you can see a little arch of other stones around it.

mallorcan wall arch detail

The walling style is really effective at making tall, strong, long-lasting walls out of irregular stone. Mallorca is full of walls hundreds of years old, and examples I’ve seen on the internet are often ten or twenty feet tall. To work on the walls, workers pound metal bars between the stones and then put boards across the bars to act as scaffolding. A lot of the walls are pretty rough looking, but at the same time really appealing because of their size and strength. The walls get capped with European-style vertical coping stones, which adds a nice touch of style and self-consciousness to the rather rough, naturalistic stonework. Our wall in the garden show was made with Napa basalt, the closest Bay Area equivalent to the limestone they have in Mallorca.

The Stone Foundation has a beautiful Mallorcan wall in their write-up for the 2007 workshop and another in their write-up for the upcoming 2009 one. DryStoneWalling has photos of two walls, one that’s retaining and one that’s freestanding. Below, I put one of my sets of flagstone steps with a section of Mallorcan cheek wall.

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Old Town Patio Stone

I wanted to post a few more photos of the freestanding wall from the garden show. I haven’t seen many walls with the courses running at an angle, and none quite like this one. To lay the stones at an angle goes against the “rules” I learned about building walls, but, apparently, stoneworkers have been doing it in Cornwall for centuries. The Cornish call their walls “hedges”, and do things like cover them with sod, and they have a whole tradition of stacking slate vertically or at an angle. Their slate doesn’t support weight well when stacked horizontally, so they turn it on its edge, which makes a certain amount of sense; I’ve worked with slate which would crumble from a single hammer blow across the flat, but could withstand repeated blows against the narrow edge. The Guild of Cornish Hedgers has a collection of photos including some walls built with a herringbone pattern. I particularly like this one with stiles for climbing over it. There’s a photo on a blog here and another photo in the Cornish collection in the Dry Stone Walling Association of Great Britain gallery.

the courses, with my camera rotated

Through the magic of turning the camera sideways, you can see that the wall is built with courses like a traditional wall, just that the courses run at an angle.

transition from traditional to angled

The horizontally laid stones of the arch set the angle for the slanted courses. A lot of the wall’s weight is going to be pushing against those horizontal courses and against that arch, but arches are strong and the wall could have stood for a lot longer than the five days of the garden show. Now it only exists in memories and photos.

The Dry Stone Walling Association of Canada has more photos of the wall on their site, and photos of another angled wall they built for a garden show in Canada last year.

WallsWithoutMortar has photos of another angled wall built in Danville, here and here.

I stuck a couple of detail photos of the arch below. (more…)

cabernet stone wall

This cabernet stone wall (not built by me) is a good example of how a wall looks when you build using a stone grinder. It has extremely regular courses and a very clean face, and every stone is rectangular, and I know, from working with cabernet myself, that the stone didn’t come like that in the pallet. The builder used a grinder wheel to straighten the sides and remove any unevenness.

cabernet stone

Personally, I find it unpleasant to use a grinding wheel so much and I like walls to look a little more handmade. I like to have some rough faces and some irregular joints. I think the different size and shape of the stones adds visual interest; that’s why you use stone instead of brick. But that said, this is a nice looking wall; the eye slides across it, very nice and very smooth. I know this wall because a potential client pointed me to this wall and told me that this is exactly what she wanted. I’m sure she liked it better than the cabernet walls I’ve built. 

cabernet stone with grinder scars

Up close you can see the white grinder scars along the tops of the stones, from the grinder wheel sanding away the stone. Old school dry stone wallers hate grinder scars, but most people don’t notice. If you can’t see the scars in the photo, you probably wouldn’t notice them in the wall either. A few more photos are below. (more…)

Cabernet Stone Corner

It seems like whenever clients call us about terracing a slope on their property, the slope is actually too steep to terrace with dry stone. The slope usually turns out to be steeper than 1:1, one foot of vertical for every foot of horizontal (a quick way I estimate is to stand on the slope and measure or eyeball the distance straight out from my shoulder, my shoulder is five feet high, so if the distance to the slope is five feet then the ratio would be 1:1, ten feet would be 1:2, fifteen feet would be 1:3, and so on; if the distance to my shoulder is less than five feet the slope is too steep), and that math just doesn’t lend itself well to dry stone retaining walls, which rely on their thickness and weight to hold back the weigh of the slopes they retain. For instance, a two foot high wall needs to be a foot thick, so if your wall rises two feet on a 1:1 slope, it only creates two horizontal feet and one of those feet will be taken up by the wall; your net gain is only one foot of flat planting space. It’s rarely worth the money or effort, so we usually end up building a wall at the base of the slope and then planting the rest of it with plants that thrive on slopes.

This little planting in San Francisco is the first time we’ve actually terraced a slope, though, in reality, it barely qualifies as terracing; it’s more like one wall split into two shorter walls. We could have built it as a single two and half foot high wall. But because the whole planting is at eye level on top of a thick concrete retaining wall, we didn’t want to be adding another giant wall to further loom over people. So we split the wall into two separate walls and then further softened the impact of the stone by setting the lower wall back from the concrete to create space for plants.

For the plants, we chose ones that are soft textured, drought-tolerant and mostly native to coastal California. A few of them are considered rock garden plants, a somewhat subjective term, but typically rock garden plants like sandy or gravelly soil, tolerate or enjoy reflected heat from stone, have a smaller size, and are best appreciated up close and at eye level, all elements of this planting. And then a few of the plants like the Myrica and the Phormium are standard landscaping plants for San Francisco. A photo of the whole little planting and the plantlist is below. (more…)

I like to use brick walls to explain dry stone concepts, and I’ll probably never find a better demonstration photo than this one on website of the Dry Stone Walling Association of Canada. Brick wall concepts are straightforward: The vertical line between each brick is a joint. Each horizontal layer is called a course. Each course should break the joints of the course below it, a brick laid across the top of each individual joint. That way, if a joint starts to crack open, the opening can only run a single course before it bumps into something solid in the next one; it’ll have to staircase it’s way up instead of splitting along a straight line.

Stone walls built with rectangular stone work the same as a brick wall. A dry-stacked wall uses the heavier mass of the stone, greater thickness of the wall, and batter (a slight lean into the hillside or towards the center of the wall) to make up for the absence of mortar. Like with bricks, the stones should be laid flat, and the joints should be broken. Every stone should sit on two stones instead of just one, and in turn should have two stones sitting on it. It’s a good idea to occasionally break the horizontal joints with a double-high stone, but it’s not really necessary. The builder of the wall in the photo did a good job absorbing the irregularities of the stone into nice horizontal courses with sufficiently broken joints.

The wall also demonstrates two ways of capping a wall–with a large flat capstone, or with coping stones stacked vertical like books. Personally, I usually find vertical coping stones a bit self-conscious for my taste, but in this case the stepped coping works perfectly for this already self-conscious wall. (more…)