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Constructing Terraces

From "Weekend Landscaping"
episode WKL-202

Before beginning terrace construction, check local building codes for construction requirements in your area. It will also be necessary to check for hidden utility or water lines. Areas with surface tree roots may prevent terrace construction unless the tree is removed. Remove rocks and vegetation from the area.

Slope ratio, also known as angle of slope, must be determined before construction can begin. The relationship between horizontal length and vertical height, this is important in terms of runoff, slope stability and the ability to plant and maintain planting materials. A slope ratio of greater than 2 to 1 requires a retaining wall.

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Figure A

Figure B

Figure C

Figure D

Figure E

To determine ratio, take a piece of string and attach it at the top of the slope (figure A). Making sure that it is level, measure out 4', and at the 4' mark let it drop to the soil line, making sure that it is plumb or vertical. Measure the vertical drop from the 4' mark. The drop here is 2', exactly within range of the 2 to 1 ratio.

Premeasure the space for the terraces and outline the ground with spray paint. Burns is building two terraces, each approximately 1-1/2' in height.
Dig out a 12" foundation trench. This trench will hold the first row of retaining blocks.
Hammer in stakes at each end of the trench and attach a leveled string to the stakes: this ensures a straight line and a level fit for the retaining blocks. Straight and level is crucial at this point in construction: if this row is not correct, it will create an uneven surface for upper terraces.
Place about 10" of sand in the trench and level using a board to drag (this is called screeting) across the sand (figure B)). Use a level as needed to check surfaces.
Cut a 6" piece of board to use as a spacer. Align blocks with string line and begin installation. Use the spacer as a guide between each block. Corner blocks are installed 90 degrees perpendicular to facing blocks (figure C). To ensure the backs of the blocks are level, place a board along the back of several blocks. The board should be flush against the row of blocks: if not refit the blocks to ensure a straight line.
When the first row of blocks are in place (figure D), backfill with sand and soil.
Three stacked rows of blocks make up each terrace. Blocks on the two upper rows straddle two of the lower blocks (figure E) -- the block lips act as a resister to insure that the blocks don't slide forward. Continue layering the blocks and filling in with soil and sand until the terrace is the desired height.