Concrete block is ideal for building walls to hold back the soil after you dig into a slope for a pathway, patio, or other project. Block retaining walls are generally the same as freestanding block walls with a few important differences.
A retaining wall must provide a way to release the water that builds up in the slope behind it. Without a pressure-relief system, the weight of the water in the soil would crack, or even buckle, the wall. Weep holes—lengths of 3/4-inch pipe inserted along the top of the first course—escort some of the water out. The other part of the system is a plastic drainpipe covered with gravel. As water accumulates behind the wall, it percolates through the gravel into the drainpipe, which carries it off safely.
Retaining walls must be stronger than freestanding walls. Insert rebar in the footing when you pour it—every three blocks, or at intervals specified by your local codes. As a last step, fill the cores around the rebar with mortar from the bottom to top.
Expect to spend 20-36 hours building a 3x10 foot wall. Before you begin, design the layout and pour the footing.
Lay out the wall and excavate the slope and the footing trench. Build the footing and insert rebar into the wet concrete at intervals corresponding to the cores in the blocks. Prepare the footing, spread mortar, and build leads, sliding the block over the rebar as needed.
As you lay the second course of block, insert weep holes—3/4-inch pipe—into the mortar every third block. Chip off a small recess in the block and trowel a mortar bed for the pipe, sloping it slightly toward the front. Then chip out a recess on the next block, butter the ears, and set the block.
Stop occasionally to check the mortar. When it begins to set up slightly, scrape off the excess with an upswing of the edge of the trowel. Do not plug the weep holes with excess mortar as you remove it.
After you've laid at least two courses, spread landscape fabric on the soil behind the base of the wall, temporarily laying the excess over the grass on the slope. Lay rocks on the fabric to hold it while you work. Between the rear of the wall and the slope, backfill the wall with gravel, laying perforated drainpipe on the gravel bed level with the top of the first course. Face the holes down.
Shovel more gravel on top of the drainpipe. Backfill with additional gravel as you add courses to the wall. Fold the landscape fabric over the top of the gravel fill, and backfill with soil. Replace the sod.
When you reach the finished height of the wall, fill those cores containing rebar to the top with mortar. Mix the fill mortar slightly wetter than what you'd use for joints. Smooth the top with a trowel. Spread additional mortar, and lay cap block to finish the wall.
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