How-To Guy | Mid-State Equipment

Retaining Wall Construction

Now is the time to make some curb appealing changes. There are of course many things we can do both in and out of the house to increase property value, functionality, and appeal. I would like to cover how to build a basic landscape retaining wall. A modest size wall can usually be finished in a weekend, but try to line up some extra help. Retaining walls can transform a sloped unusable area into a nice level surface for playground equipment, decks, patios, and other recreational purposes. Retaining walls can be built in many ways, with many different materials. For now, I would like to cover walls 3 feet or less in height using interlocking pre-cast stones which are readily available, attractive, and simple to install. Consulting an experienced landscaping contractor or engineer would be wise for anything over 3 feet tall. If a wall is not built properly, it will not stand up over time and could even pose a safety risk.

Gravity and hydrostatic pressure (the pressure exerted on a fluid by gravity, water in this case) are important factors to consider before building a wall. Not only will gravity pull soil towards the core of the Earth, but it also causes lateral force as the soil spreads out under it’s own weight. When working on hills, as is the case with most retaining walls, this idea is critical. The wall will actually only be holding back a long wedge shaped section of material between the wall and the failure plane, not the entire slope. This “failure plane” in basic terms, is the steepest slope that resting soil can hold itself together without collapsing. Still, since one cubic yard of moist soil can weigh as much as 2,000 lbs, a retaining wall must be strong enough to withstand natural forces over time. Water can be a wall’s number one enemy in a couple of ways. For one, frost heave is an issue in any region with a frigid winter season. Stored moisture in the ground will freeze and when this happens the affected soil will expand in all directions causing a poorly constructed wall to fail. Proper drainage during non-winter months is also very important. A well-drained wall can prevent base washout, which if not accounted for can cause a wall to topple, sink, or and a number of problems.

Ok, let’s get started! Here are some recommended tools and supplies for the project:

¨ Digging Tools (shovel, pick, metal rake, etc…)

¨ Measuring Tape

¨ Rubber Mallet

¨ Mini-Sledge and 2-3” Stone Chisel

¨ Stakes and String, or Marking Paint, or a Garden Hose

¨ Levels

¨ Wheelbarrow

¨ Broom

¨ Work Gloves, Safety Glasses, Knee Pads

¨ Vibratory Plate Compactor and Hand Tamper

¨ Powered Masonry Saw

¨ Rounded Gravel (drainage fill)

¨ 4” Drain Tile (round corrugated tubing)

¨ Leveling Sand

¨ Crushed Gravel (base)

¨ Impermeable Soil (like clay)

¨ Interlocking Wall Stone

¨ Capstone

¨ Landscaping Fabric

¨ Masonry (stone) Adhesive

¨ Building permit? (Typically not needed if wall is less than 4 ft tall and set back from the road, but it is best to check with your local community planning and development department if in doubt.)

1) Prepare the Site:

Start out by laying out the shape of the soon to be wall with a garden hose, marking paint, or stakes and string.

2) Dig:

Starting at the lowest grade of your layout, dig around 10-12 inches deep and 24-32 inches wide along your mark. Create a slant toward the back trench wall as you dig. Try to dig as evenly as possible without getting too obsessive. (See trench profile)

Note: A trench floor of 32” wide would provide adequate room for use of a 20” wide plate compactor as opposed to a hand tamper. This is favorable because it’s easier on the back, and can provide a better quality compaction than what is achievable with a hand tamper.

3) Prepare the Base:

Once the digging is done, clear out any loose material and compact the trench floor with a vibratory plate compactor. Next, covering the entire trench floor, lay about 5 inches of crushed gravel to serve as the base. Level out the gravel (hint: use a level attached to a 2x4 to sweep across the area). After leveling, compact the area well. Walk across the gravel base, if you can feel your feet sinking in take another pass with the compactor. Finish off the base with 1” of leveling sand spread evenly.

4) Lay Base Course:

If the block type has interlocking tabs on the bottom, use a hammer and chisel to break this tab off for each block that will be used in the base course. Next, lay the base course of stone leaving no less than 12 inches (note: leave at least 22 inches if using a plate compactor) of space on the backside to allow for drainage fill material. Level each stone side-to-side and front-to-back. Use a rubber mallet to help set and level each stone, you can also add a bit of sand under the block to raise if needed. If this first course is done correctly the following rows will go much smoother. It’s much better to spend a little extra time leveling the first course then tear the wall down later to fix a base problem.

5) Drain Tile & Fill

Lay drain tile along the backside of the base course as shown below and add landscaping fabric against entire backside of the trench wall. Next, add round gravel fill material behind the wall level with top of the base course. To prevent kick-out of the base course add crushed gravel (same as base material) along the front side of the wall, leaving room for a couple inches of topsoil. Compact gravel on both sides of the wall and verify that base course is still sitting level. Finishing off this step, sweep the entire length of the row clean to get ready for the second course of block.

6) Lay 2^nd Course:

Using a running bond pattern, lay the next course so that each block overlaps a seam from the previous row by half the length of the block. You may have an extra step here if your layout does not fade the walls ends back into your slope or runs an end up against the foundation of your house. If so, use a brick (masonry) saw to cut the outermost block in half, you will need to do this on every-other course of block as you go. This will create a nice straight edge on the end(s) of your wall. Add round gravel fill level with the top of this course and compact. Make sure and sweep the row clean as in step 5.

(Below are side and front views of what you should have so far)

7) Continue To Desired Height:

Continue laying courses as was done in step 6 remembering to compact each layer of fill until desired height of 3 feet or less is achieved. Note, add round gravel fill material only as high as the bottom of your final course of wall block. (See final profile below)

8) Finish:

Time to finish off the wall! Add a layer of compacted impermeable soil (like clay) up to the top of the final course of wall block. This will further help direct water off the wall. If there are any cap blocks that will require a special cut, cut and test fit them prior to final installation. Once the blocks are ready, liberally apply adhesive across the top course of wall block and install cap row. After caps have had time to dry, add topsoil (do not compact), seed, clean up, and you have yourself a mighty fine retaining wall.

(See final profile)

Tips:

When calculating the amount of blocks and caps, buy a bit more than needed. Different batches of the same block can have slightly different shades. You can typically return what you do not use, but double check before you buy.

If the slope is too steep for a 3-foot wall to do the job, consider making multiple 3-foot walls spaced at least 6 feet apart. This can create a series of nice terraces.