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Cut the angle of the embankment back 30 – 45 degrees from plumb to give you room to place the stones and the gravel for drainage behind the wall rock. Loose or sandy soils tend to collapse, so cut the angle more sharply.
In soil that drains well, excavate a 9-inch deep by 18-inch wide trench along the length of the wall. Remove all grass, sod, roots and large rocks. Put drainage fabric in the trench and drape up the excavated wall leaving enough to fold back over. The fabric will allow moisture and air to penetrate but will prevent the soil from settling into the aggregate bed.
Starting at one end of the wall, carefully fit each wall rock, seating it firmly in the aggregate bed. Use your largest stone for the first course, not only to create a good base, but also to avoid having to lift and adjust these heavy pieces at higher levels.
Dig out under the stone and fill in any void spaces with aggregate, if necessary, to get the stone to sit firmly without wobbling. Pack the spaces between stones in the first course to give the wall a stable base.
Set the next few courses of stone on top of the first, keeping in mind that you want the inside of the wall to slant back at least 2 inches for every foot of height. As mentioned, the front of the wall can be plumb or it can follow the slope.
Lay wall boulders in successive courses so that they overlap the stone above and below. Avoid creating continuous, straight vertical joints. The overlapping pattern will produce a stronger wall giving it the stability it needs to resist pressure from the soil. Install long stone that extend into the back fill about every 4 feet horizontally in each course.
Set the remaining courses of stone in the same manner. Remember to let each course of the inside wall jut in an inch or so farther toward the embankment so that the inside face will slant about 2 inches per foot.
When building up the wall, carefully select each stone for the best fit and check its fit as you lay each course. If a stone wobbles on a point or sharp corner, use a brick hammer and a point or pitching chisel to shape it so it will sit more securely. You also can use small pieces of stone as shims to make the stones fit more tightly together; although for the best looking wall, you will want to minimize shims. If you do use shims, insert them from the outside of the wall.
You have some design options here. One approach is to stop building 6 – 8” inches before you reach the top of the slope. Pack dirt between the top course of stones and then cover the wall with soil to bring it up to the tip grade. Then you can plant a ground cover above the wall. The plant’s root network will help prevent erosion and help hold the wall together.
This method of building a retaining wall looks very nice in a casual country garden. Another approach is to leave the top of the wall exposed as you would for any freestanding stonewall. You can mortar the stone of the top course in place or cap the top course with mortar