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Project Difficulty:
Difficult
Estimated
Project Time: 5 to 7 days
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Shovel
Wheelbarrow
Level
Measuring tape
Interlocking concrete blocks
4-lb. sledgehammer
Brickset
Safety glasses
Landscape fabric
Sand
3/8" to 3/4" gravel or crushed stone
Cap blocks
Concrete adhesive and caulking gun
Soil as backfill for cap course
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This project discusses how to build a 36-inch-high semicircular tree well, but you can use the same general principles when building a circular version.
Because the design of interlocking concrete blocks differs among manufacturers, the block you buy may have different construction criteria. Every brand of interlocking block will come with manufacturer's specifications regarding height limitations, loading strength, and base requirements. Read these specifications carefully before building.
For any tree well, you'll need to set the wall far enough away from the tree so that you won't damage the root system when digging the footing. For most trees, the diameter of the root system equals the spread of the limbs. (Fig. 1)
To determine the number of blocks you need for the semicircular tree well, first calculate half the circumference of the circle defined by the tree's limbs. To do this, multiply the diameter of the circle by pi (3.14), and divide by two. Then divide that number by the length of a single block to determine how many blocks are needed for each course. To determine the number of courses, divide the planned wall height by the individual block height. Multiply that number by the number of blocks in each course to determine the total number of blocks needed. You'll also need special solid capping blocks for the top course, and an extra course of blocks set below grade. It is also a good idea to buy about 10 to 15 percent extra material to compensate for wastage and future damage.
Because blocks differ among manufacturers, the block you buy may have different construction criteria. Every brand will come with manufacturer's specifications regarding height limitations, loading strength, and base requirements. Read these specifications carefully before building.
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1.
Excavate the Embankment
Excavate into the hill until you reach your planned vertical rise, and continue excavating until the embankment falls away 30 to 45 degrees from vertical to allow for any soil that may fall forward. Make sure you cut the embankment to make a flat base on which the units can sit and to allow for the gravel backfill. (Fig. 2)
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2.
Excavate the Trench
In well-drained areas, excavate a trench along the length of the embankment 6 inches deep (4-inch-thick block plus 2 inches of sand) and 24 inches wide (12-inch-wide block plus 12 inches of gravel backfill). In areas with poor drainage, excavate to 10 inches instead of 6 to allow for 4 inches of 3/8- to 3/4-inch gravel or crushed stone to act as a drainage bed. If the drainage bed is required, add the gravel and cover with landscape fabric. If not, merely place the landscape fabric in the bottom of the trench, overlapping sections by at least 6 inches. Cover the fabric with a 2-inch-thick sand bed, making sure the surface is flat. (Figs. 3 and 4)
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3.
Split Units for Corner
Some systems may come with specially shaped blocks, making this step unnecessary. If not, split half- and three-quarter units for the corners using a 4-pound sledgehammer and brickset. First, score the unit all the way around, and then break it with a few sharp blows. To avoid eye injury, wear safety glasses. (Fig. 5)
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4.
Set the First Course Corner
Starting at the 90-degree corner between the straight and curved wall sections, lay the base course corner, butting each unit snugly against the next. To form the corner, alternate the half- and three-quarter units as shown in Figure 6 (or follow the directions for the manufacturer's prefabricated corner units).
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5.
Lay the First Course
Lay the first course of blocks so that their tops are level with the grade. Level each block individually and to adjacent blocks. It is essential that this course be level, or the wall will not be stable. Do not align the blocks according to their rough faces; instead, align their pin holes or their machine-smooth backs. (Fig. 7)
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6.
Fill Any Voids and Backfill
Shovel 3/8- to 3/4-inch gravel or crushed stone behind the first course and between the blocks in the curved section to secure them in place and to provide drainage (Fig. 8). Also fill any voids in the blocks themselves. (Repeat this step after each course is laid).
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7.
Set the Second Course
Clean off any backfill material from the top of the first course, and then set the second course on top of the first, offsetting each unit one-half the length of the block below to form a running-bond pattern. To secure the units to one another, insert fiberglass pins in the holes on the top of the first course and lower a block onto the pins of two adjacent blocks. There are usually two or more pin holes on each side of the block. By placing the pins in the back holes (as suggested by the manufacturer), the wall will slope backward at an attractive angle rather than running near-vertical. Other kinds of retaining wall systems align the blocks using a lip on the back of the unit or by other means. (Fig. 9)
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8.
Set the Remaining Courses
Set the remaining courses in the same manner. Alternate the half- and three-quarter units in each course at the corner, insert the locking pins, and place the gravel backfill inside the wall-up to the height desired. (Fig. 10)
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9.
Cap the Wall
To hide drainage voids and pin holes, cap your retaining wall with a solid unit. Manufacturers make cap units specific to their wall blocks and either make or recommend concrete adhesive that is applied with a caulking gun to secure the units. To place the cap course, clean off the top course of blocks, and then apply the adhesive. Lay the cap blocks next to one another, backfilling them with soil. (Fig. 11)
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