How to build a retaining wall: Step by Step
By David Gordon – Baywide Dingos Napier, Hawkes Bay, NZ
These projects usually have a generic way to do them but each job is different with different levels and slopes, steps & tricks. Here are a few guide lines and general steps to learn before starting:
- Excavate the wall line with a machine. Make sure you leave enough room usually 200-300mm from the wall position to the excavated bank so that you can nail the boards on to the posts. Run a string line along the bottom from either end and make sure you have an equal level all the way down. This is critical as when you lay your 1st board down it has to be exactly level otherwise you will be out when you get to the top board.
- Leave the string line up and use it to make out where your holes will go. As a general rule depending on the post size or engineering specifications 1m to 1.2m is the average post distance apart (centre to centre) for a wall up to 1.5m high. Spray the holes with a small dot rather than a cross
- Drill the holes to the required depth. The rule of thumb – the post should be 2/3 of the height of wall in the ground. So if your wall is 1.2m high – your post hole should be 800mm deep. The hole diameter should be large enough to allow for slight movement of the auger when drilling and enough concrete around the post to hold it. Typically if we use 125mm square posts the hole should be 300mm – 350mm diameter. Always put at least 10% rake (slight angle toward the wall). The rake adds a great amount of strength to the wall – too much though and it can look out bad. Clear out the holes and remove all loose soil
- Set up a post at either end of the wall with props. Fix both posts to the exact position they will be and either concrete these in or make sure the wood props holding them are strong
- Run two strings top and bottom from these two posts and place remaining posts in each of the holes. Check alignment with the strings to every post. Make sure your strings are very tight especially over a long distance. Check each post touches the string with enough clearance from the hole
- Concrete the posts in, checking the posts for level as you go. Make sure you use the correct MPa rating concrete for the specifications and don’t fill the holes up so they are overflowing with concrete – leave them about 30mm down (you’ll know why when you go to panel up)
- Leave to set for at least a few days to allow to build strength
- You now can panel up the wall. Remove strings and rake any dirt, concrete slag out of the way. Start with the bottom board and take your time to line it up dead level. Cut each board to length so it fits in the middle of a post and nail the start and end then check middle for sag – then nail. Carry on and lay the panels making sure the Tongue and Groove is seated properly.
- Saw the top of the post off to height and place top capping if required taking care to make sure you do a nice job as this is the bit you see – use decent sized nails! 5 inch at least because the capping will try to do its best to warp
- Place drainage pipe and metal behind wall & backfill, taking care you get a fall from one end to the other.
- You are now ready to backfill the wall with fill. Keep in mind the fill will compact over time no matter how much you compact the dirt. It pays to leave for as long as possible and soaking it before you do anything above it. NOTE: be very careful running a machine parallel with the wall whilst backfilling – it is very easy to bow a wall out – even if you have installed tie back supports.
Good luck – any questions I’m happy to help and guide you email