© 2008 by Raymond Alexander Kukkee
"A retaining wall is much like a dam, and must withstand the forces of direct soil pressure"
Cinder Block Wall or Concrete Block?For purposes of retaining wall construction, building a cinder block wall may be considered equivalent of building a retaining wall with 'concrete' block. Cinder blocks are much lighter in weight and as such may be preferred, reducing the level of hard work required. The blocks may be the same standard dimensions, and in some instances may even be used interchangeably. Regardless, let's go with Retaining Walls: Build a Cinder Block Retaining Wall-for homeowners and DIY'ers fortunate enough to have access to high-quality cinder blocks. The photo below shows a retaining wall built of concrete blocks simply because cinder blocks are no longer available in the author's geographic location. "I like cinder block better, they are lighter to work with than concrete blocks, and won't rot like timbers do".
"Cinder block?" Why would you want to build your retaining wall out of cinder block?" What's the difference? Cinder block is cast using coal cinder. Concrete blocks are cast using stone aggregate and cement."But for a retaining wall? You want heavy block for a retaining wall, don't you?" When considering building a retaining wall with cinder block, that may be a reasonable and logical question. The purpose of any retaining wall is to prevent downhill or ground movement and soil erosion, so to some extent, weight is a relevant factor. A retaining wall is much like a dam, and must withstand the forces of direct soil pressure, but it must also withstand settling, and the destructive heaving and displacement caused by frost. All hollow-cavity blocks can be filled with rock or gravel ballast as the wall is constructed to provide for additional weight. A cinder block wall properly designed can respond to all of these challenges. Essential structural considerations include adequate design and provision for drainage, footings, stability, and mechanical reinforcement of the wall itself. Always remember that design safety criteria becomes more critical with increased height. Here is how to build a cinder block retaining wall, keeping these critical aspects in mind.
Design the Cinder Block Wall Carefully
- Make a drawing to ensure details are not overlooked.
- Are you building the retaining wall in an area that has deep, severe frost penetration? If so, your footings must be deeper than the frost penetrates to provide stability through seasonal freeze/thaw cycles. Why? Wet soil expands when frozen, 'heaving' or raising erratically any structure sitting on it, which can damage the structure. When it subsequently thaws, the soil level subsides in a friable condition, which is unstable. Clay soils are particularly susceptible to severe freezing.
- What are soil conditions like, is the ground wet continuously? Plan for control of surface water and groundwater disposal if necessary. Saturated soil is easily deformed, can slump at any time, and creeps downhill.
- How high will the wall be? Structural integrity and safety issues start to become a matter concern where retaining walls are over 6' in height. Consider building successive walls 2' high instead, and take advantage of the usable level spaces between them for landscaping and features.
- Will the wall be constructed with mortar? If so, you require a solid concrete footing. Shifting, settling and heaving will soon destroy mortar joints, at best leaving unsightly cracks requiring annual repairs.
- Do you need a building permit? Do obtain any necessary permits in your jurisdiction before commencing any construction.
- Locate all gas, water and electrical services prior to beginning any digging. Your local utility will gladly flag any services endangered by your construction project.
Tools and Supplies RequiredThe following common tools and supplies are essential.
- Carpenter's line and chalk line
- Carpenter's 4' level, line level (use laser level or transit if available)
- Measuring tape, hammer, wire-cutting pliers
- Cement trowels
- Hacksaw or grinder to cut steel rebar
- Shovel, Picks, hammer, sledge hammer
- Pickets for staking footing forms
- Dimensioned lumber to build footing forms (2x6, 2x8 etc. or as required)
- Cinder blocks 8x8x16 or 10x8x16
- Steel reinforcing bar, galvanized deadmen anchor cables
- A mortar mixer rental will come in handy for larger projects.
- Concrete (ready-mix delivery) and cement mortar for block-laying.
Let's Build a Cinder Block Retaining Wall1. Stake out with pickets and excavate organic, soft soil from footing area to level. If fill is required, consolidate and pack with a plate packer. If you are in a geographical zone where severe frost penetration occurs, the footing must be below the depth of frost penetration to avoid damage from freezing. 2. Install forms for the footing slab, setting the top of the forms to level. For a typical 3' high wall, footings should be 24" wide and 8" deep as a minimum. If your design specifies the wall "leans in" toward the hill, set the footing forms accordingly. 3. Install two rows of 1/2" steel reinforcing bar, each being 6" from the outside edge. Support the steel bar with small pieces of concrete or commercially available standoffs. The steel bar should be in the bottom 1/3 of the concrete slab, but not touching the ground. 4. Pour the concrete, ensuring the reinforcing rod is in the correct position. Fill to the top of the form. Tamp the cement and hammer the outside of the forms to avoid "honeycomb" on the outside of the slab which can weaken the slab. Finish the slab with a rough finish; screed the cement level across the form with a 2x4. 5. Optional: Establish a carpenter's line and install a row of "J" steel anchor bolts or anchors where the row of blocks will be installed. Ensure the position of the bolts will match the cavities of the blocks! Alternatively, you may wish to drill holes into the concrete later to insert vertical reinforcing bars. *Note: your design may position the wall on the outside 1/3rd of the footing to allow more soil weight on the inside of the footing for additional stability. 6. Cover the slab and allow the cement to cure for three days. Keep the slab wet for the best result. Remove the forms and proceed with building the cinder block wall. 7. Snap a chalk line to position the first row of cinder blocks. Lay a 1/2" thick mortar bed and apply mortar to the blocks, tap the blocks into position and level as required. 8. Install vertical reinforcing bars 4' apart or less. 9. Fill the first course of blocks with concrete . 10. For the second course, alternate the joints as is standard practice in masonry for maximum strength. Install a row of reinforcing steel on top of the second course of blocks, notching out the blocks as required. Wire the horizontal steel bar to the vertical bars securely using steel wire ties. Fill the block cavities with concrete or mortar. 11. If the retaining wall is to be over 3' and the slope is steep, install deadmen anchors perpendicular to the wall every 4' as a minimum. Use galvanized deadmen cables attached to treated timbers buried parallel to the wall uphill in undisturbed soil. Alternatively build deadmen blocks into the course or install a geogrid web between courses for additional stability. 12. Install a perforated weeping tile or pipe on a layer of crushed rock at the base of the footing on the uphill side. Install the pipe with the drainage holes facing down. Cover the pipe with crushed rock or very coarse, free-draining gravel. Ensure the ends of the pipe direct any water away from the wall. Camouflage the ends of the pipe as desired. 13. Back fill the first two courses adjacent to the wall with free-draining gravel and pack with a plate packer if available to minimize future settling. 14. Install all subsequent courses, filling ALL courses with cement and installing additional reinforcing bar and deadmen supports as required. 15. Install facing flagstone, brick, or alternatively apply mortar to the face (as stucco) if desired to finish the visible wall face as chosen. 16. Cap the final course as desired and complete the back filling. Compact the fill and install topsoil, and seed or sod and landscape. Your cinder block wall, as any other masonry structure, if carefully built upon a solid foundation adequately constructed, will provide many years of service. You're free! On to the next project!
**Is that Incoming I hear? © Photos by R.A.Kukkee +