Calculating a block retaining wall involves determining the number of blocks needed, which depends on the wall's desired length and height, as well as the block dimensions. You also need to consider factors like the base course being partially buried and proper drainage.
Calculating Block Requirements
Here's a breakdown of how to calculate the block requirements for your retaining wall:
-
Determine the Wall Length: Measure the total length of the area you want the retaining wall to cover. This is your total wall length.
-
Determine the Wall Height: Decide on the desired height of the finished retaining wall.
-
Determine Block Dimensions: Measure the length and height of the specific blocks you plan to use. These dimensions are crucial for accurate calculations.
-
Calculate Blocks per Row: Divide the total wall length by the block length. This gives you the approximate number of blocks required for each row. Consider adding a small percentage (5-10%) for cuts and breakage.
- Example: If your wall length is 20 feet (240 inches) and each block is 12 inches long, you'll need approximately 240 / 12 = 20 blocks per row.
-
Calculate Number of Rows: Divide the desired wall height by the block height. This will give you the number of rows needed. Remember that the first row is usually partially buried, so account for that when calculating the exposed height.
- Example: If your desired wall height is 3 feet (36 inches) and each block is 6 inches high, you'll need approximately 36 / 6 = 6 rows. If the first row is buried halfway (3 inches), you will need to subtract that amount to calculate the proper amount of blocks that would be visible. 36-3 = 33 inches to calculate how many more blocks you need for remaining visible height 33/6 = 5.5 so about 6 block rows for the visible height.
-
Total Blocks Needed: Multiply the number of blocks per row by the number of rows. This provides an estimate of the total number of blocks needed for your retaining wall.
- Example: If you need 20 blocks per row and have 6 rows, you'll need 20 * 6 = 120 blocks.
Additional Considerations:
-
Base Course: Retaining wall construction typically involves burying the first course of blocks partially. Account for this reduced height when calculating the total number of blocks.
-
Cuts and Waste: Order extra blocks (5-10%) to account for cuts needed at the ends of rows and any potential breakage during installation.
-
Caps: Many retaining walls have a cap course made of blocks specifically designed to fit along the top edge for aesthetics. Calculate the number of cap blocks needed based on the wall length and cap block dimensions.
-
Geogrid: Taller retaining walls may require reinforcement with geogrid to prevent soil movement. Consult with an engineer or experienced contractor to determine if geogrid is necessary and how much you'll need.
-
Drainage: Proper drainage is crucial for the longevity of a retaining wall. Install a drainage system behind the wall to prevent water buildup. This often involves gravel and a perforated pipe.
-
Local Codes: Check local building codes and regulations for specific requirements regarding retaining wall construction in your area.
Example Calculation:
Let's say you want to build a retaining wall that is 15 feet long and 4 feet high, using blocks that are 1 foot long and 8 inches high.
- Wall Length: 15 feet (180 inches)
- Wall Height: 4 feet (48 inches)
- Block Length: 1 foot (12 inches)
- Block Height: 8 inches
-
Blocks per Row: 180 inches / 12 inches/block = 15 blocks/row
-
Number of Rows: 48 inches / 8 inches/row = 6 rows
- Assume first row is buried 4 inches. Visible wall height is now 48-4 = 44 inches. 44/8 = 5.5 or approximately 6 rows.
-
Total Blocks: 15 blocks/row * 6 rows = 90 blocks. Add 10% for waste: 90 + (0.1 * 90) = 99 blocks (round up to 100).
Therefore, you would need approximately 100 blocks. You would also need to calculate the amount of gravel you will need for the base.
