How do I dig a post hole in hard or stony ground?

Getting a post hole into clay, stone, rubble or compacted ground with hand tools, and the point at which the tool has to change.

Difficulty: intermediate
Time: 1 hr
Read: 4 min
Safety: warning

Short answer

Break the ground with a digging bar and lift the spoil with post hole diggers, wetting hard clay the night before. In flint, rubble or old concrete, switch to a bar and a bolster rather than fighting an auger — augers stall on the first large stone.

Post hole digging is easy in loam and miserable in everything else. The trick is matching the tool to what is actually in the ground, and softening it before you start rather than after you have exhausted yourself.

Safety

Check for buried services before you start. Electricity, gas, water and telecoms runs are often shallower than people expect near boundaries and outbuildings — a cable service plan or a CAT scanner is worth the trouble. Hand-dig the first 300mm with a spade. Never use a breaker or auger before you have proved the top layer by hand. Wear eye protection when breaking stone.

What you'll need

A digging bar or pinch bar
Post hole diggers (double-handled shuv-holers)
A narrow spade and a trowel
A bucket for spoil (optional)
A hose or watering can (optional)

Step by step

Prove the ground for services by hand.Spade out the top 300mm carefully across the whole hole footprint before any bar or power tool goes in. Cables and pipes near boundaries and outbuildings are often laid shallow.
Soften hard clay the day before.Flood the marked hole position with water and leave it overnight. Baked summer clay is close to impossible dry and workable wet. Do not do this in ground that is already saturated.
Use a digging bar to break, diggers to lift.The bar loosens; the post hole diggers pinch the spoil out without widening the hole. Trying to dig the whole thing with a spade produces a crater instead of a hole.
Keep the hole the same width all the way down.A 300mm square hole means 300mm at the bottom too. A cone-shaped hole gives the post nothing to resist against and wastes concrete.
Work stones loose rather than levering hard.Dig around a large stone with a trowel until it is free on most sides, then lift it out. Levering against an embedded stone bends bars and breaks handles.
Know when to move the hole.If you hit a slab of old concrete, a footing or a rock you cannot free in twenty minutes, move the post 100-200mm along the line. A slightly uneven bay is better than a shallow post.
Handle rubble and made ground carefully.Loose rubble collapses into the hole as fast as you dig it. A short length of drainpipe or a bottomless bucket as a temporary liner holds the sides while you work.
Clean the bottom before setting the post.Loose spoil at the bottom compresses under the post. Scrape it out, then add and compact 100mm of gravel for drainage.

Tips

A long digging bar does more than a heavy one. The leverage comes from the length, and a lighter bar is usable for longer.
Two shorter sessions dig a hard hole faster than one long one, because the water has time to work into the clay.
Hire a petrol auger only if you know the ground is stone-free. In flinty or rubble-filled ground it kicks back and stalls constantly.

Common mistakes

Making the hole shallower because the ground is hard — Depth is what stops a post levering out. A 400mm hole in hard ground is still a 400mm hole and the fence will lean. Move the hole rather than shorten it.
Using a breaker or auger before hand-digging the first spade depth — This is how buried cables and water pipes get hit. The top 300mm is where services are, and it is the one layer that is easy to dig by hand.

If it doesn't work

Hole keeps collapsing

Cause: Loose or made ground, or water ingress — Fix: Line the hole temporarily with a section of pipe or a bottomless bucket, or dig and concrete the same day so the sides do not have time to fall in.

Water filling the hole faster than you dig

Cause: High water table or a broken land drain nearby — Fix: Bail it, work fast and use postcrete, which sets in water. If it recurs along the run, there is a drainage problem to solve first.

Hit solid concrete at 300mm

Cause: An old footing or a previous post base — Fix: Move the hole 100-200mm along the line. Breaking out an old footing by hand is rarely worth the day it takes.

Questions people ask

Is it worth hiring a petrol auger?

Only in clean, stone-free soil with several holes to dig. In flint, rubble or clay with stones it jams on the first obstruction and a digging bar is faster and far less dangerous.

How do I dig a hole in solid chalk or rock?

A digging bar and a bolster chisel, breaking it down in layers, or a shallower hole with a much wider concrete collar to compensate. In genuine rock, a resin-anchored post shoe onto the rock face is a legitimate alternative.