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Begin by laying the foundation. Start at the most visible edge of the wall. Cut off the top tongue of the base stones, if necessary. Use coarse sand and a rubber mallet to level off the first layer of blocks. Cut individual blocks to complete the first layer, if necessary. Use crushed stone or gravel for backfill on your first layer of blocks. Place a filter fabric over the backfill. Sweep the first layer with a broom.
These are the most critical blocks in your wall. If they aren't level or adequately support the top half of your retaining wall, the whole project is going to look less than professional. Make sure that the foundational blocks are leveled off, sturdy, and tightly packed together. Add gravel or crushed rock to level the stone, if necessary. Add the first block to the trench, using a cornerstone. Make sure it is level from front to back and side to side.  Alternately, if there's no edge to the wall that is more visible than another, start at the edge that will be closest to another structure (usually a house). If you're building a straight or rectangular retaining wall, make sure that the backs of the blocks line up with one another perfectly; if you're building a curved retaining wall, make sure that the fronts of the blocks line up with one another perfectly. Some contractors prefer cutting off the top tongue or groove from the base stones before laying them down. Check for sturdiness yourself and knock the tongue off the block with a hammer and chisel, if necessary. Understand that curved retaining walls with tongues may not benefit from the interlocking grooves. These grooves will need to be cut off with a hammer and chisel if the layout of the pattern does not fit the direction of the grooves. This will complete the entire foundation. If you took the time to level off the bed, laying the first row should be easy. Use coarse sand where necessary to get a level finish on your foundation. Hammer the blocks down with your rubber mallet. Simply mark them at the appropriate length and cut with a mason's saw. Always use proper protection when cutting. This will provide excellent support, keeping your bottom layer from slipping back with time and erosion. This will prevent frost heave and keeps the soil from mixing with the backfill. Depending on how tall your retaining wall is, you may want to drape the filler along the back side of the trench or house, fill the trench with backfill until it's anchored the filter fabric down, and then drape the fabric flat out, on top of the backfill. This will free up any dirt or dust.