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SULIS - Sustainable Urban Landscape Information Series.
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Installing Block Retaining Walls

Description/Purpose:

There are several different brands of prefabricated concrete blocks on the market in a variety of shapes, textures, and colors. These blocks can be used to create sturdy, long-lasting retaining walls. Each brand has its own way of connecting the blocks together, but the general method of building modular walls is the same.

Image of a Retaining Wall

Building long or tall walls can be difficult. Often there are many site-specific questions to consider. Always read the manufacturer's specifications provided with your block. Don't build beyond your capabilities and experience.

Amounts, Specifications & Supplies:

  • Prefabricated concrete blocks come in a variety of sizes. The size you choose will depend on the size of the wall being built. Short walls (under 3 feet) can be built with smaller blocks, often called half-unit blocks, which measure approximately 6" x 8" x 12" and weigh 40 pounds.

  • Short walls may not need granular fill for drainage. Taller walls need larger, heavier blocks, which usually measure approximately 6" x 18" x 12" and weigh 80 to 90 pounds.

    Taller Wall

  • Smaller and larger blocks are also available.

  • Many manufacturers offer specialty blocks for corners and steps. These blocks are square or rectangular. Tapered blocks are used for walks and curves.

  • Some brands also have cap units, which are smaller blocks that go along the top of the wall.

    Smaller Blocks on the Top of the Wall

  • Some brands connect by use of pins.

  • A soil retention grid mat (geogrid) is needed for larger walls.

  • Crushed rock or class 5 fill--enough for a base 4-5" deep the length of the wall.

  • Drainage aggregate ¼-¾" pea gravel or angular clear stone, - enough to backfill 6-8" against wall.

  • Drain tile - 4" perforated, may include a filter fabric sock.

  • Filter fabric is sometimes specified to separate drainage gravel or stone from soil.

  • Coarse sand enough for 1" layer to cover the base along the length of the wall.

Tools and Equipment:

  • Always read information or operation manuals for instructions and safety considerations.

  • Safety goggles

  • Spade for digging trench and backfilling or backhoe for large jobs

  • String with stakes for setting straight lines

  • Tape measure

  • Tamping bar for compacting soil. A power tamper is recommended for most jobs.

  • Skid loader (optional) for moving blocks and backfilling

  • Carpenters level(s)

  • 3-4" brick chisel for splitting blocks if necessary (or circular saw with concrete blade)

  • 1 lb hammer

  • Rubber mallet for leveling block

Site Considerations:

The size of the wall will depend on the site. It is usually better to build two shorter, terraced walls rather than one tall wall. The size of the wall will determine the size of the blocks.

Bobcat Digging for the Wall

Larger walls require a soil retention grid mat (geogrid) to hold the wall in place. A drainage tile is often specified behind the wall.

Geogrid

Step-By-Step Process:

1. Measure and lay out the area where the modular wall is to be built. Use stakes and string to create a straight line. Garden hose, flags, or paint can be used to lay out curves. Curve radius is very important. Read the manufacturer's specifications carefully to determine the minimum radius for the block you are using.

Measuring the Wall

Minimum radius must be determined at the top of the wall. Radius becomes smaller as each row of block is installed.

Diagram of the Radius

2. Dig a trench deep enough to accommodate the depth of one block plus 5-7". Some manufacturers specify two blocks under the soil surface. The trench should be wide enough to allow 2-3" of compacted base on the front and 6-8" on the back side of each block. The first row of blocks will be below grade to provide stability and strength for the wall.

Building the Wall

3. Fill the trench with approximately 4-6" of crushed rock or class 5 fill. Be sure the layer is level along the length of the wall and tamp thoroughly. A power tamper will make this job easier and do a better job. The appearance and strength of your wall depends on a firm and level base. A string line will aid leveling.

4. Over the crushed rock, spread a 1" layer of coarse sand, and tamp thoroughly. The sand makes the leveling process much easier.

5. Set the first course of blocks into the trench. Blocks should abut each other with no gaps. Each block must be perfectly level in all directions. Use a 4' level for the length of the wall and a shorter 12" level for individual blocks. Check level with a string line every few blocks. Remember, this first course will be below grade level. This process is very time consuming for do-it-yourselfers.

Inserting the Lower Blocks Using a Level

6. Drain tile should be installed when walls exceed 3' in height, are tiered, or installed in soils that retain water. A 4" perforated drain tile should be installed on top of the compacted base, behind the wall and covered on all sides by the drainage aggregate.

Diagram of the Lower Portion of a Retaining Wall

7. An alternate process uses 1" x 2-1/2" x 15" grade stakes, 6" wide 1/4" masonite strips and 1" sheetrock screws to create a form. The masonite is set to level and the base gravel or rock screeded (leveled) between the masonite.

Stakes

8. For drainage backfill ¼-¾" pea gravel, angular clear stone or another easily permeable aggregate against the back of the first course and tamp thoroughly. Soaking backfill with water is an effective way to settle the soil for do-it-yourselfers with minimal equipment.

9. The second and subsequent courses should be set back into the hill to provide strength for the wall. Each block system has its own way of providing for this setback. If the blocks you are using require connecting pins, insert them into the first course and set the second course over the pins.

10. Continue adding each course, being sure to backfill with drainage aggregate against the wall. Tamp all backfill soil thoroughly. Add geogrid per manufacturer's specifications as courses are added.

Diagram of the Entire Retaining Wall

11. The top course of the wall will be cap units, if available. After all blocks are in place, be sure backfilling is complete and well tamped. The cap blocks can be attached with cement adhesive for added security.

Backfill

12. Plants will soften the appearance of the wall and blend it into the landscape. Planting between blocks is only possible in very low walls where drainage aggregate is not necessary.

Plants

13. Use poly or aluminum edging to create mow strips to eliminate trimming on herbicide applications.

Mow Strips

This implementation report is based on landscape projects completed by University of Minnesota students enrolled in landscape design and implementation courses, Department of Horticultural Science.

 
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