How to Build a Keystone Block Retaining Wall

A keystone block retaining wall can be a great landscaping feature in your place. They’re a good way to level off an area in your yard, to create some useful space, or to make a raised garden. They can look a lot better than wood, and are pretty easy to build yourself.

Rules & Regulations

You can build a retaining wall up to 1.5m high without needing building consent. However the Keystone block retaining wall product we’re using in this guide can only be built up to 900mm. Other block retaining wall products may differ.
If you are building a retaining wall higher than 1.5m or retaining anything like a driveway, parking space or building above the wall, you will need an engineer to draw up your plans and submit them to council for a building consent.
For retaining walls 1m or higher you may be required to install a safety barrier. This is likely to be required where the wall is beside an access path to the house.
For block walls over 3 layers, or around 300mm high, you will require ground water drainage behind the wall.
If you are building your retaining wall on a boundary, the entire cut behind the wall, your fill, and drainage ALL need to be on you r side of the boundary. So make sure you take this into account when planning.
As a general rule, your wall will need to be set back from the boundary the same distance as, or greater than, the wall height.
For these reasons it’s always best to check with your local council before starting, to see if your retaining wall needs to comply with any unitary plan, or resource management act requirements.
It is also a good idea to check your plans for the location of any pipes before digging, and call your electricity provider for information on any underground wires, as these may not be shown on your plans.

Product Information

This wall will be built with Firth Keystone retaining blocks. They come in a variety of styles, and in this guide we’re using the Country Manor style. They’re pre-cast in small medium and large sizes, and sit on top of each other in a staggered join. They’re fixed together with fiberglass pins and finished with a cap on top.

This product is perfect for retaining walls up to 900mm high.

Instructions

Establish the position of the wall

You need to factor in the depth of the blocks, which are 250mm, and at least 250mm for the drainage coil and scoria backfill behind the wall – so allow for at least 500mm behind the face of the wall.
Set up a string line where the face of your wall will be positioned.
If you’re creating any curves use a conduit pipe and pegs to help you mark out smooth curves.
Dig out at least 500mm from the front face of the wall.

Dig out footing trench

Mark and dig a footing trench 500mm wide by 300mm deep. This needs to sit 50mm forward of the front face of the wall.
If your site is sloping, start at the lowest point and dig the trench so that it steps up in 150mm increments, which is the height of the blocks.

Fill with base course

Make sure the footing trench is free of any debris or loose soil.
Add 75mm of GAP20.
Compact evenly with a plate compactor.
Add another 75mm of GAP20 to achieve a 150mm base.
Compact evenly with a plate compactor.
Check the base is completely level.
You could hire a laser level for improved accuracy. You should now have 150mm for the first layer of blocks to sit in. The top of them will be flush with the ground, for rigidity.

Lay first course (layer) of blocks

Lay the blocks end-to-end 50mm from the front of the trench. The three pinholes should be facing upwards, and the long groove facing down.
For alignment of straight walls, set up a string line aligned to the pinholes for accuracy. To do this lay one block at the beginning and one block at the end, and then set up your string line, aligned to the pinholes.
Using a spirit level make sure each block is level side-to-side and back-to-back. Use a rubber mallet to knock any blocks into place. Levelling the first course is critical for best results.
Create any pattern you like with the big, medium, and small blocks. You could create a regular pattern, or a random pattern depending on what you like.

Install drainage coil

Cut drainage coil to size, leaving it longer on the end that will be connected to a silt trap, and storm water. A Drainlayer will install the silt trap, then connect to the stormwater drain.
Put the filter sock over the drainage coil and tie it up on the other end.
Place the coil behind the first layer.
Backfill with scoria.
While backfilling, also fill the open cavities of the large blocks with scoria.

Insert fiberglass pins

Insert the fiberglass pins into the pinholes on the blocks. These help the blocks lock together. Only one pin per grouping of three holes should be used.
If your wall is to be vertical, and is up to or under 600mm high, place the pins in the middle hole.
Place the pins in the rear hole if your wall is to be built up to 900mm high. This will give you an approximate 9.5° setback per course.
The front hole can be used randomly if you require a forward projection of a specific block. This is primarily for design purposes, for accent and variation in the wall appearance.
You may need to clear out the pinholes to allow the pegs to fit in.
Our wall will be over 600mm high, so we are placing the fiberglass pins into the rear hole, to give us a setback, for added strength.

Lay the second course

Brush the first layer clean, ready for the second.
Lay them down in your chosen pattern, making sure you have a staggered join between layers. You may need to adjust your pattern to suit.
Backfill with scoria, making sure you fill the open cavities of the large blocks.

Continue laying blocks

Lay your other courses using the same technique.
Insert pins, brush clean, lay blocks, and backfill making sure you fill the cavities in your blocks.
Continue until you reach your desired height. However, note that vertical walls made with these Keystone retaining blocks can only be up to 600mm in height, or up to 900mm if you are building it with a setback.
You can decide to stagger your layers to follow the contour of the land, or keep the wall a consistent height – that’s up to you.

Install caps

Set up a string line aligned to the back edge of your blocks, and use a rubber mallet to make sure your blocks are all straight. This will ensure your capping will be laid in a straight line.
Lay the caps on top and glue them in place with brick and concrete adhesive.
Caps can sit flush or overhanging according to your design.
If any need to be cut to size, use a bolster, or a block cutter from the hire shop.