Building a Retaining Wall
Build a retaining wall.
Base Preparation for Allan Block Retaining Walls
- To start your layout, place stakes to represent the location of the front of the wall. Using a string line or paint, mark out the entire length. A garden hose is an excellent tool to use when laying out curved walls.
- Excavate the area by removing all surface vegetation and organic materials from the area. These cannot be used as backfill material.
- If reinforcement is needed excavate behind the wall to accommodate the design length of the geogrid. Refer to your approved plans for exact length.
- Starting at the lowest point, dig a base trench the length of the wall. For walls where the base trench steps up a slope see stepping up a slope for retaining wall for more information.
- Dig a base trench 24 in. (600 mm) wide the length of the wall.**
- The depth of the trench will be 6 in. (150 mm) plus an additional 1 in. (25 mm) for each 1 ft. (300 mm) of wall height for the amount of buried block that is needed.**
- Compact the base trench making a minimum of two passes with a walk behind plate compactor.
- Foundation soils at the bottom of the base trench must be firm and solid. If the soils are made up of heavy clay or wet soils, or the areas have been previously excavated, remove this material and replace with a granular material, compacting in 8 in. (200 mm) lifts or less.
** For walls under 4 ft. (1.2 m) dig the base trench 18 in. wide (460 mm) and 4 in. deep (100 mm) plus additional to account for the amount of buried block needed.
Mark area
Excavate
Video: Build a Proper Base Trench 
Video: Install Base Course 
Video: Backfill and Compact 
Video: Build a Gravity Retaining Wall Base Material
- A drain pipe is required for any reinforced wall, gravity walls over 4 ft. (1.2 m) tall or sites with poor drainage. Place the drain pipe at the lowest possible point toward the back of the trench and vent to daylight every 50 ft. (15 m). See approved plans for location and specifications.
- Place a minimum of 6 in. (150 mm) of wall rock in the base trench and rake smooth.
- Compact the wall rock making a minimum of two passes with a plate compactor.
- Check the entire length for level, and adjust as needed.
Install Base Course
See here for base course installation when building with AB Fieldstone.
- Begin the base course at the lowest wall elevation. For more information on stepping up the base course see Stepping up a slope for retaining wall
- Place all blocks with the raised front lip facing up and forward on the base material in the center of the base trench.
- Check and adjust each block for level and alignment as it is installed. Check the blocks for level frequently from side-to-side and front-to-back. Verify the proper position of all the AB blocks by examining a string line across the back of the blocks or by sighting down the back of the raised front lip.
- Make minor adjustments by tapping the AB blocks with a dead blow hammer or by placing up to 0.5 in. (13 mm) of coarse sand under the blocks.
- Irregularities in the base course become larger as the wall stacks up. Careful attention to a straight and level base course will ensure a quality finished wall.
Backfilling and Compaction
- Fill in the area in front of the blocks with on-site soils. This will keep the base course blocks from shifting while filling and compacting.
- Fill the hollow cores of the base course and 12 in. (300 mm) behind the block with wall rock to the height of the block.
- Use infill or approved on-site soils to backfill behind the wall rock in lifts of no more than 8 in. (200 mm).
- Use a plate compactor to consolidate the wall rock directly behind the block then compact in a path parallel to the wall, working from the back of the block to the back of the excavated area with a minimum of 2 passes. More information on compacting your retaining wall.
- Check the base course for level and adjust as necessary.
- Every course after the first course requires compaction starting on the block.
Compact
Install drain pipe, wall rock and level
Install base course, level and align
Backfill wall rock and infill soils
Additional Courses
If installing a patterned block retaining wall, see installation steps for next course.
- Remove all excess material from the top surface of all blocks. This can be done when installing the next course of block, by sliding the blocks into place.
- If reinforcement is needed, go to reinforced walls to continue the installation process.
- Stack the next course of blocks so that the vertical seams are offset from the blocks below by at least 1/4 the length of the block.
- Check each block for level and alignment and make adjustments as needed.
- Fill the hollow cores and 12 in. (300 mm) behind the block with wall rock to the height of the block.
- Use infill or approved on site soils to backfill behind the wall rock in lifts of no more than 8 in. (200 mm).
- From the 2nd course and above use a plate compactor to compact directly on the blocks as well as the area behind the blocks. Compact in lifts of 8 in. (200 mm) or less.
- Repeating these steps, complete the wall to the desired height. On the last course, fill behind the blocks with organic soils in place of infill or approved on-site soils. This will assist in any plantings above the wall and also to direct water from running behind the blocks. More information on finishing your retaining wall.
Compact
Install next course, level and align
Backfill and compact