Stack blocks just might be the answer for that sloped garden area

By Lloyd Thompson
WSU Chelan/Douglas County Master Gardener
August 17, 2022

Terraced garden behind house.
WSU Master Gardener Lloyd Thompson used stack blocks to replace old railroad ties in the terrace garden at his home. – Provided photo/Lloyd Thompson

Lloyd Thompson – WSU Extension Chelan/Douglas County Master Gardener – Provided photo

Do you have a troublesome slope or need a level area to build something?

I remember the days when railroad ties were a hot commodity for building retaining walls. There weren’t a lot of alternatives then. The advantages for using ties included a fairly long life, uniform size and they were readily available. Of course, they were covered in creosote, which on hot days caused it to melt and transfer to anything that touched it (not a good feature during a “heat dome event”). In addition, the splinters, cracks and the occasional twist made them difficult to stack.

If you didn’t want to deal with those issues, you could also set forms and pour concrete walls, but that included equipment that most people lack for excavating and backfilling. Dry stack rock and mortared rock were also available choices, but required some skill and effort to install.

We had one of these situations at Eastmont High School outside the ag shop. A bank covered with roses, while great for capturing errant tennis balls from the nearby tennis courts, was a rather steep slope. The roses bordered the access driveway behind the school and, at best, left room for parallel parking and a single-lane access for freight deliveries. Around spring break one year, our horticulture students were wanting to be outside rather than in the classroom and wanting to try and fix the problem. After a conversation, our CTE director agreed to rent a front loader for excavation and buy the block for the wall. The kids agreed to rip out the old roses and create the block wall. My teaching partner and I agreed it would be a meaningful educational experience for everyone.

We lacked experience in installing stack blocks (it would be years before YouTube would be able to help with an instructional video) so we had to rely on the advice of the sales person. With a few restarts, we realized to make sure you have a level base of compacted gravel to start with, and it may need to have multiple tiers depending on the slope of your site. The first error we made was not sloping the bank more as we cut it back with the front end loader. We had failed to take into account the spacing lip, which makes each level sit back a half inch or so farther than the one it is sitting on. Since we had nearly 30 kids with shovels to make adjustments on that job, it wasn’t a big deal.

The success of your project really depends on good preparation for that first tier of blocks you install. If you don’t have a compacted bed of gravel under the block it can settle and cause issues. The gravel also helps with too much water which can cause a wall to bulge out and fail depending on the soil type and rainfall. If your area is prone to water from runoff, installing a perforated drainage pipe is a good idea. I also like to use some landscape cloth behind the wall to prevent dirt from seeping through.

If the first tier of blocks are not level or straight, then everything after that will get worse, and at some point you will decide to redo that first level. The moral of the story: it is less work to do it right the first time.

I like to use a 4- or 6-foot level for the length of the wall and a 6-inch torpedo-style level for checking the front to back of the block direction. I have found a hand tamper for packing down the gravel will work, but renting a power tamper on a larger project saves time and lots of effort. If possible, I start on the low side of the slope so I can stair step up when the block is level with the ground. The straighter and closer together you keep the blocks on each level makes the next one easier to do.

A well-constructed block wall not only solves problems, but it can really enhance the use of the space.

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