We’ve been planning a retaining wall project in the garden for some time. Last week, after numerous distractions, we finally managed to focus all of our attention and energy on constructing this 37 foot long wall to control erosion in the garden just below our apiary.
Although hardly any great feat of engineering or construction compared to say the Pyramids, or the actual Great Wall, it was a key project that needed to be completed before we can finish installing the rest of the vegetable garden.
Forward of the wall is where the new greenhouse will be installed. The greenhouse arrived some time ago, but because the wall wasn’t done, we couldn’t install it.
Due to the slight slope behind where the greenhouse will be, and the nature of our friable soils, we knew it would be critical to contain the erosion from the slope, or face problems in the future with soil building up behind the greenhouse wall later.
We’d seen how our soils behave when the previous owners of the property had installed a 6ftx10ft plastic kit shed in front of a similar slope, and by the time we tore that shed down this spring, more than 18 inches of soil had walled up against the exterior wall, and had caused the shed wall to fail where it met the floor.
We knew the wall wouldn’t need to be very high, 3 feet would suffice, but first we had to decide what sort of material to construct it out of. As we planned to do the work ourselves, the lighter the material the better, but it would also need to be sturdy enough to get the job done. A number of walls in this area are simply constructed from large format pressure treated timbers or redwood, often with steel posts. However, we didn’t want to use pressure treated lumber, and even rot resistant timbers do eventually rot, especially when in direct soil contact. We didn’t want to build the wall, and then have to replace it in 10 or 15 years. There’s enough to do around here, so we wanted to build it, and forget about it.
After walking around some local supply yards it seemed that our best, homeowner-friendly retaining wall system option, was using some sort of interlocking, mortarless block. There are a number of manufacturers of such products, but we eventually chose a wall system where the blocks were readily obtainable, locally, and easy to install.
Rather than stare at a perfectly aligned run of parallel running bond blocks, we preferred the less rigid pattern using assorted block sizes, to help add some visual interest to an otherwise potentially boring stretch of wall. We also chose a block color that was as close to the color of our native soils as possible, in the hopes the wall would blend into the landscape, rather than detract from it.
The manufacturer provided clear instructions for determining the number of blocks needed for the project, based on the pattern used when constructing the wall. After double checking our math, we ordered the various blocks, the base rock, 3/4 inch drain rock, drain pipe, and filter fabric, and while waiting for the delivery, set to work on the foundation.
I will be the first to admit that with our terrain here, the tractor was completely indispensable, both for moving materials around the job site due to our elevation changes here, and for prepping the wall base. Our soils are comprised mostly of Santa Margarita Sandstone, and Santa Cruz Mudstone in this part of the garden. Although these soils, once exposed, weather to sand, the surface soil in some areas can be almost as hard as concrete.
As you can see, it’s not really soil at all, which is why we plant in raised beds in this part of the garden. The backhoe made excavating the foundation trench a snap though. However, if we’d been blessed with loamy soils, a shovel would have worked just fine. Mudstone isn’t all bad though, as it does help to provide a solid base, providing it’s not disturbed too much.
By the time the trench was dug, four pallets of concrete blocks had been delivered along with the other materials we needed for the project. With the trench excavated and level, the next step was to add a few inches of base rock (usually a 3/4 inch gravel mixed with fines so it compacts).
The base rock is tamped and compacted to provide a solid base, and then using string level lines as a guide the first foundation row of large blocks was set into place.
That may sound easy, but this part of the project took a day and a half to complete. We’re sticklers for getting any project’s prep done right, so we didn’t rush this step.
We knew if the foundation wasn’t perfect, any leveling errors would easily be compounded up the wall as it was built.
It’s easier to build a straight wall, but our hillsides aren’t straight. Rather than disturb the fragile soils at the toe of the slope too much, we chose instead to curve the wall around the terrain. We especially liked that this hollow-block system has ‘tabs’ at the rear of the block, so when building outside (convex) curves, the tabs can simply be snapped off with a quick hammer blow to allow for a tighter inside radius.
It saved a tremendous amount of time not having to use a masonry saw to make these adjustments.
Once the foundation blocks were in position, the insides of the blocks were filled with clean 3/4 inch drain rock. This helps to prevent the blocks from shifting, and helps to lock the wall together.
Of course, it’s always important during these projects to have some expert on-site supervision.
This Coast Range Fence Lizard oversaw part of our project on Tuesday morning, and seemed to approve of the wall’s location. I hadn’t really thought about the fact that this wall will probably provide some much welcome lizard habitat as they love to bask on rocks and logs in the garden, so why not a 37 foot long concrete wall?
Before installing more courses of block, we needed to install a toe drain behind the wall. Our soils drain very well, but it can also rain very hard here during the winter months. To prevent future wall failure, it’s important to install the toe drain so that any excess water that accumulates behind the wall, especially once our soils are saturated in late winter, is carried away, at a steady pitch down slope.
We used a triple wall perforated drain pipe, being careful to install the pipe with the holes facing DOWNWARD.
A retaining wall constructed by the previous owners had the pipe installed incorrectly, and that wall has now failed as the drain pipe has clogged. Now water pools at the top of the wall during periods of heavy rain.
The drain pipe was wrapped in a filter fabric to prevent clogging, and installed with a downward grade out toward a gravel dry well. The pipe behind the wall was laid on a bed of drain rock, and then subsequently covered in the same 3/4 inch drain rock.
Some wall manufacturers recommend lining immediately behind the wall with a weed-blocking fabric, before filling the space behind the wall with gravel. This is supposed to prevent fine soil particles from marring the face of the wall during winter weather. However, this system specifically recommended against using such fabric, especially in areas with fine soils, because if the weed fabric becomes clogged, it can lead to increased hydrostatic pressure behind the wall, and lead to early wall failure. We omitted the fabric, and simply filled the space behind the wall with more 3/4 inch drain rock.
With the foundation in place, the drain pipe installed and secured, it was time for the fun to begin. Although it took a day and a half to complete the preparation of the foundation and base row of block, we managed to install 75% of the entire rest of the wall, with just two of us, in less than 8 hours (my biceps are much stronger now!).
Working from the suggested pattern from the manufacturer, it was just a matter of putting together a rather large, and very heavy, jigsaw puzzle (the larger stones in this pattern weigh 75lbs each — even the capstones weigh 55lbs). As a row of blocks was installed, we continued to fill the blocks with the drain rock, all the way up the wall to secure it.
As the rows of blocks are installed, the offset on the face of the blocks causes the wall to naturally step back with a 6 degree slope.
This makes the wall stronger, as it’s less prone to being pushed forward over time by the pressure of the soil behind it. As our wall was only being built to 3 feet in height, and the slope immediately behind it is relatively shallow, we didn’t need to install geotextile fabric to anchor this wall. We do have another wall to build on site that will be taller, with a much more severe slope behind it, so that wall’s installation will require multiple rows of geo-grid to anchor the wall into the hillside, but that’s another project!
Once the last course was complete, and we’d finished filling in behind the wall with gravel, we could add the capstones to finish off the wall. As you can see above, we tapered the ends of the walls. We did this for two reasons. Most importantly, as this wall has curves in it, it minimized the cutting of capstones. The capstones are solid, and rectangular, so not inherently suited to following a curve.
Tapering the ends also meant we needed less block, but it also makes the wall more visually interesting, and for its location, less formal in appearance with the staggered ends.
By dropping the height of the wall at the ends, the capstones help to conceal the side of the blocks and yield a more finished appearance. However, at the end of the wall that is most visible as you enter the garden, we used special end blocks that were finished on two sides.
It feels great to finally strike this project off our list of things to do. In all it only took four days for the two of us to construct, much faster than either of us anticipated, and by doing it ourselves, we saved a lot on labor costs, which is a large part of the expense with wall projects such as these.
Personally I think it turned out great, and it’s certainly tidied up the appearance of the slope below the bee hives.
So now we’ve built the ‘great garden wall’ we can FINALLY turn our attention toward the construction of our greenhouse…
If all goes well, I’m hoping to show off a greenhouse where this patch of bare gravel is within the next couple of weeks. Stay tuned!