how to extreme

Build a Retaining Wall

Construction How-To, Landscaping January 2, 2008 Sonia


I love building retaining walls. It’s fun to be out in warm weather cutting big timbers and doing a project that sees quick, upward progress. However, the standard stack of horizontal 6-by-6′s wasn’t a design that worked for me, so when it came time to design a wall for my own yard, I went looking for something with a little more texture and style but with the same mass and power as a standard style wall. I found it, and all it took was standing those 6-by-6′s on their head.

This post-and-plank system is based on the way seawalls (or bulkheads) work. You set posts into the ground, vertically, then plank behind them. This creates a wall with texture and shadow lines with nice hollows between the posts for plantings or grass. And, because there’s so much post buried in the earth, a wall like this is solid state and resists moving, even if the soil on the hill you’re holding back is mobile.

Save yourself some headaches and have the material delivered. Lumber yards with forklifts, can help you stage the material right where you want it.

Save yourself some headaches and have the material delivered. Lumber yards with forklifts, can help you stage the material right where you want it.

 

Prep Work

Preparation is key to getting a project off on the right foot. For this one, there were two critical steps to a good kick-off; obtaining a building permit and sourcing materials and equipment. Because my build time is limited (weekends), it’s important to me to source all materials and equipment in one place. This saves phone calls and leg work. For me, this was my local Home Depot. I could purchase all the landscaping supplies including lumber and fasteners, and even rent the excavation equipment, all under one roof.

Permit. The easiest way to know if you need a permit (and what’s required if you do) is to call your building inspector’s office and have them walk you through the process. Mine was very helpful and easy to work with. Although building inspectors are busy and shouldn’t be used as a design source, mine offered me some good advice.

Materials. The lumber for this wall will be ACQ pressure-treated 6-by-6 and 2-by-8′s with double hot-dipped galvanized fasteners. There’s no beating pressure-treated lumber in ground-contact situations. The reason for double hot-dipped galvanized fasteners, as opposed to electro-galvanized or coated, is that the new formula for pressure treating lumber has one serious drawback: It corrodes unprotected steel.

The last tip on materials is this: Get it delivered, even if the yard charges for delivery. Lots of DIY-ers like to go to the lumber yard and max out their trucks. With a load as heavy and long as this one, that’s a waste of your time and your truck’s shocks.

Equipment. My Home Depot store is also a full-dress rental center, and I was able to rent an AC Prowler, a mini skid-steer. The beauty of this package is that it comes with a bucket and several augers, so I can use it to drill post holes (auger attachment) and move earth (bucket attachment) both for construction prep and final backfill.

Hack away a couple of feet of hill with a mattock to loosen it so the loader can cart it away. I took out about 2 feet of hill for this project, but wish I had taken away another 6 inches.

Hack away a couple of feet of hill with a mattock to loosen it so the loader can cart it away. I took out about 2 feet of hill for this project, but wish I had taken away another 6 inches.

 

Excavation and Layout

So I can have access to the back of the posts later to install the plank, I notch out the hill with the Prowler’s bucket.

Next, because I’m building against a sidewalk, I sweep up and layout my post locations, which is 3-feet on center. I mark them in chalk on the sidewalk. If I were doing this elsewhere (like on grass) I’d set up a string between 2-foot rebar rods set 1-foot into the ground to mark where I want the front of the posts to land. Next, I’d set 40-penny spikes in the center of each hole location, and then remove the string so I could work (Don’t remove the rebar; you’ll need it later).

We dug 30 4-foot deep holes in one long day with this machine, including all the bucket work.

We dug 30 4-foot deep holes in one long day with this machine, including all the bucket work.

 

Drill the holes. It’s smart to go down at least 3 feet. This provides the posts the muscle they need to hold back that hill.

Cut the posts to length, drop them in the hole, set them to the string height, plumb in both directions, then brace with 2-by at the base.

Cut the posts to length, drop them in the hole, set them to the string height, plumb in both directions, then brace with 2-by at the base.

 

Post Placement and Concrete

Level the Wall Top. Set your layout string up again. It’s important to set the post faces straight along your layout line. In my case, I lucked out with a sidewalk to follow, though I had to use a chipping hammer to carefully remove some concrete below grade to get the posts flush to the edge. To get the wall top level (as opposed to following the grade of the earth) set up a second layout string at the exact height you want your wall to finish out. Securely braced 2-by-4′s or rebar will work well to hold the string. A line-level will help you get a level string-line. You can also use a laser level. Keep your 2-by-4 string holders at least 4 feet from the wall location on each end so you have room to work.

There's always that one hole you have to dig by hand. In our case, there was a pile of subterranean concrete impeding a post. A 14-pound chipping hammer does the trick.

There’s always that one hole you have to dig by hand. In our case, there was a pile of subterranean concrete impeding a post. A 14-pound chipping hammer does the trick.

 

We held the posts in position with temporary 2-by bracing to make sure they stayed level and plumb.

We held the posts in position with temporary 2-by bracing to make sure they stayed level and plumb.