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Chalk substrate and post concrete on Thanet ground: the honest gotchas

Thanet sits on chalk. That changes how a post hole behaves, how post concrete cures, and why some Margate runs need a wider dig than the specifier expects.

Why Thanet ground is different

Most of the Isle of Thanet sits on the Upper Chalk of the Kent Chalk Group, overlaid with variable thicknesses of clay-with-flints and topsoil. In practice for a fencer, that means the ground behaves in three broad ways depending on where you are digging:

• Deep topsoil over chalk (Garlinge, Birchington): straightforward dig to spec-sheet depth, standard post-concrete performance.
• Shallow topsoil, chalk close to surface (Palm Bay, cliff-adjacent Cliftonville): chalk hits at 400-500mm. Dig hits solid resistance before spec depth is reached.
• Reclaimed or made-up ground (some Old Town harbour-adjacent plots, Westbrook seafront): variable fill over chalk. Post bearing is unpredictable across a single garden.

Post depth on shallow-chalk ground

The rule of thumb for post depth is 25-33% of the total post length below ground. For a 6ft finished-height fence on a 2.4m post, that is 0.6-0.8m in the ground. When you hit chalk at 400mm, you cannot get spec-sheet 800mm depth without genuinely drilling into the chalk (which is possible but takes time and specialist bits).

The pragmatic fix is a wider concrete collar. A standard post hole is 300mm x 300mm at 800mm depth. On a shallow-chalk plot, dig to whatever firm bearing you can reach - typically 500-600mm on chalk - and expand the concrete collar out to 400mm x 400mm. The collar acts as a wider bearing plate against the chalk, and the load-carrying capacity of the post is preserved even at reduced depth. This is genuinely what the physics wants; it is not a shortcut.

Post concrete: mix on site, not dry-thrown

The construction-industry standard for post concrete is dry-thrown post-mix from a bag: pour the dry mix into the hole, add water, wait. That works on domestic-sized posts in low-load fencing and it is the reason DIY fence installs use it. For load-bearing fencing on the Thanet coast where the wind load on a 6ft fence is real, dry-thrown post-mix is not the right answer.

The honest spec is site-mixed ballast:sand:cement at 4:2:1, mixed wet in a barrow before it goes in the hole. Site-mixed concrete flows to fill the hole geometry and cures to full-strength standard. Dry-thrown post-mix cures faster (24 hours vs 72 for site-mix to full-strength) but the compressive strength is lower and it can develop internal voids where the water did not reach.

On urgent same-day storm-damage repair I will use rapid-set post-mix to get the run back up before the client's Friday changeover. On any planned build, site-mixed concrete is the standard.

Cure time and loading

Even on rapid-set, do not load a fresh post the same day. Site-mix wants 72 hours to full-strength; rapid-set wants 24 hours. Loading a post the day the concrete is poured stresses the collar-to-post interface before the collar has cured, and results in slow post lean over the following months even if the run looks fine on completion.

The practical scheduling for a fence job on any decent-length run: day 1 strip-out and post-setting, posts left overnight to cure, day 2 board or panel installation onto cured posts. Rushing this saves half a day at the cost of the fence sagging in six months.

Chalk pack for the collar

On the shallowest-chalk plots (worst-case Palm Bay), some fencers pack the concrete collar with chalk rubble excavated from the hole. This is fine - it does not weaken the concrete meaningfully at pack ratios under 20% - and it uses spoil that would otherwise need removing. On any plot where the excavated chalk is clean enough to reuse, pack away.

Concrete gravel boards on chalk substrate

The other half of the story is the gravel board. On Thanet chalk-clay ground the water table can sit high in winter, and the ground damp works up through the concrete gravel board into any timber sitting on it. A gravel board with a proper drip-edge detail (bottom edge angled outward to shed water off the face rather than run down onto the timber) is worth specifying. Standard flat-bottom concrete gravel boards are fine on well-drained plots; drip-edge boards are worth the small upcharge on the wet plots.