Concrete block retaining walls are effective structural features for keeping slopes in place. These walls are flexible enough to be constructed in just about any situation, and can be relatively easy to build compared to other structures. The cost of retaining walls varies greatly depending on their size; because of this, using an estimation formula is the best way to forecast construction costs.
Calculate the total surface area of the proposed retaining wall by multiplying the wall length by its height. Use your tape measure to obtain these measurements.
Divide this number by 1.125. This is the amount of common 8x8x16 concrete block that will fill a square foot of wall surface. The resulting product will be the number of blocks needed to build the retaining wall. Multiply this number by 1.06 to add a 6 per cent waste to your calculation.
Multiply the total surface area of the wall by 3.2 per cent to get the amount of mortar needed for 8x8x16 block per square foot. Then multiply this product by 1.4 to add a 40 per cent waste factor.
Divide the length of the wall by 2 to find the amount of vertical rebar needed. The spacing of rebar is commonly set at 24 inches on centre. Add 1 for the end. Multiply this number by the height of the wall; this will give you a total length of rebar needed for the wall.
Multiply the surface area of the wall by its thickness, 8 inches in this case, to find the volume of the wall. This will be the amount of concrete you will need to fill the wall. Filling is the final step in creating a solid retaining wall.
Call a home improvement store to quote current unit pricing for block and mortar, then multiply these costs by the amount calculations found in steps 2 through 5 to find the sub-total cost of materials; add these together to find the total cost of materials.
Multiply the length of the wall by the size of the footing (this is the cross-sectional dimension, and is usually 8 by 16 inches, but can vary) to find the total footing volume in cubic feet and thus the total amount of concrete needed for the footing.
Multiply the length of the wall by 2 to calculate the total length of wood needed for concrete forms (if the footing is 8 inches high, use a common 2 by 10 for the forms, or cut plywood of the same dimension). For every length of wood, consider 3 to 4 stakes (wood or metal) needed to hold the forms in place.
Multiply the total amount of concrete needed by the current unit price of concrete (this will commonly be a cubic foot unit), to find the cost of concrete.
Multiply the amount of wood form work and stakes (calculated in step 2) by their current unit price to find their total cost.
Add the cost from steps 3 and 4 to calculate the total cost of footings.
You can also estimate the cost of a retaining wall if you have the building plans. Use the method above, but instead of measuring using a tape measure, use an architect or engineering scale to measure and calculate the wall size. You may also need to add any cost of equipment rental or purchase if you do not already posses them.