Use a post hold borer with a 300 mm diameter to dig the holes for your posts and concrete. You will need to angle the holes so that the post is on a lean towards the bank. The lean should be 1:10 - in other words, for every 100 mm you go up, the post should angle towards the wall 10mm.
Hole diameter must be at least 450mm for a one metre high retaining wall. For a one metre wall, the post holes must be dug 1100mm deep.
Pour concrete into holes and level. The post holes into which the retaining wall posts will be placed shall be 450mm diameter with minimum 100mm concrete cover below the post. Footing depth is typically equal to the height of the retaining wall.
Dig post hole so diameter of the hole is 3 times the width of the post (i.e., the hole for a 4” wood post should be about 12 inches wide). The depth of the hole should be 1/3-1/2 the post height above ground (i.e., a 6-foot tall fence would require a hole depth of at least 2 feet).
A general rule of thumb is to dig the diameter of the hole at least 3-4 times the diameter of the post. For example, if setting 4 x 4 wood posts, dig holes 9-12" in diameter.
The theory I work on is – half the height of the wall plus 100mm. For example if your wall is going to be 800mm high, the holes for your posts should be 500mm deep. Time now to concrete your posts into position. Sleeper retaining wall posts come in two varieties – steel galvanised H Beams or a vertical sleepers.
Lay out where you are going to build your retaining wall by running a string line between two stakes at either end. Make sure you set your string line to the back of the retaining wall. This will help you keep your posts in line and your retaining wall straight.
Timber Retaining Walls
With the continuous exposure to moisture and external elements, wood will break down and deteriorate. Generally, the expected lifespan of timber walls ranges from 25 to 35 years.
Auger Bit Diameter
Match the bit diameter to the hole size you need for your project. If you need to dig holes to set 4-by-4 posts for a mail box or fence, you'll need an auger that can bore a 10- to 12-inch diameter hole. This allows for the post and well as the fill dirt or concrete to hold the post in place.
If you have been planning an ambitious project around the garden which shall require many post holes, then a power auger shall be your saving grace. A Post Hole Auger will help you put in precise holes quickly. Also, a Post Hole Auger will help prevent you from breaking your back through tough terrain.
Each hole is to be dug to the recommended depth (dependant on the wall height – rule of thumb half in the ground half out ie. 600mm high wall will require 600mm of post to be cemented into the ground) * Add 100mm to the depth of each hole for 20mm drainage gravel.
The length of the wall will determine how many 4×4 posts will be required. Note that unlike a fence with posts eight feet apart, we like to space the retaining wall posts five feet apart since they are not only supporting the wall structure, but the pressure of the slope to be retained.
While you can use smaller augers for planting bulbs and annuals, they won't work for post holes. A 4- to 7-inch auger is about as small as you can use for fences. Even better, look for augers in the 8- to 9-inch category and above.
Galvanized steel is an ideal material for retaining wall posts due to its excellent corrosion resistance and strength. Similarly, fence posts made from high-quality materials like steel are essential for ensuring the stability and longevity of fencing structures.
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.
The footing should be 16in wide (or more depending on you block depth) and 6-8in of compacted crushed gravel. The depth of the trench should also include the height of one full row of blocks which will be buried below the finished grade. So if your blocks are 6in tall, your footing trench should be 12-14″ deep.
The generally recommended depth for any fence post hole is one-third the total length of your posts. For example, pressure-treated pine 4×4 posts are typically sold in 8-foot lengths. One-third of 8 feet is 32 inches. So for a 6-foot fence with 8-foot posts, you would dig the holes 32 inches deep.
The base of a retaining wall should be set below ground level. The taller a wall is, the further below ground level it should be set. Crucial for supporting the rest of the wall, a good base is made of compacted soil and at least a six inch layer of compacted sand and gravel.
Rules of thumb commonly used by designers to establish the geometry of the wall include (refer to diagram): Base width = 1/2 to 1/3 of the height of the wall. Base thickness = 1/8 of the height of the wall but not less than 12 inches. Stem thickness = 6 inches + ¼ inch for each foot of wall height.
Auger drill sizes
The size of the drill bit determines the diameter of the hole. 2- to 3-inch bits: Useful for planting bulbs or installing small poles. 6-inch bit: Often used to dig holes for small plants and steel fence posts. 8- to 10-inch bit: Can be used to dig holes for larger plants or 4 x 4 wooden fence posts.
The depth of the post should be a minimum of ⅓ of the height of what's above ground. This means that if you want a 6-foot tall fence, you need at least 2 feet of the post to go below ground. If this was the case, you would need at least 2 bags of concrete per fence post, with each individual bag being a 50lb bag.
If you're wondering how deep can a post-hole digger dig, the answer depends on the application and the tool. Most augers dig about 3 feet deep, but an extension rod can be used to gain another 12 inches of depth.