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Downland chalk and post concrete: fixing posts on Wingham chalk substrate

Chalk sub-strate on the Wingham downland edge changes how post concrete cures and how post holes hold up. Notes from working the CT3 chalk belt.

Wingham sits between two chalk ridges - the North Downs edge coming down towards Ash and the Sandwich flats. Once you get past the village and up towards the downland behind, the topsoil thins fast and you hit chalk within 400mm to 600mm of the surface. That changes the post-fixing job.

What chalk does to a post hole

• Water-soluble. Chalk under sustained rain slumps. A hole dug in dry June and left overnight in wet weather can lose 100mm of side-wall before the concrete arrives.
• Non-cohesive when broken. Once you punch the auger through the chalk cap, the sides do not want to hold. Fast-set concrete needs to arrive with the auger still in the hole on chalk work.
• Excellent bearing when set. Once the post-mix has cured, chalk gives an excellent compression bearing. A properly-installed post on chalk substrate stands longer than the same post on London Clay - the trade-off is the awkward install day.

What we actually do

On confirmed-chalk sites (typically past the Preston Road ridge, past the Staple ridge, and on the Guilton side going up to Ash) we adjust:

• Dig each hole on the day of the install, not the day before.
• Use post-mix concrete (fast-set 20-30 minute cure) rather than general-purpose. Time-in-hole is the risk.
• Post depth 750mm minimum. On chalk we do not save time by going shallower - the chalk cap holds much better than the topsoil above it.
• Dry-mix hole-fill in the winter months. Wet slurry pooling at the base of a chalk hole draws the water sideways and softens the cap around the post.

How you can tell if you're on chalk

Dig a 400mm test hole with a garden fork. If the fork bounces and rings against a white-grey substrate before you get to full depth, that is the chalk cap. Once you know your rear boundary sits on it, tell the fencer at quote stage - the material spec changes.