ALLEN

Allen went from a small town to a city of a hundred thousand in about twenty five years, and nearly all of that was built on Blackland Prairie clay. A post-tension slab poured on a builder’s schedule over ground that was scraped and filled behaves well for a decade or so, and then finds its level.

The calls we take in Allen are consistent: a house between ten and twenty five years old, one corner low, and an owner who reasonably assumed a newer home would not have this problem. On this soil, age is not the protection people expect it to be.

What the ground under Allen is actually doing

Allen sits mostly on Austin soil — around 45% clay but only 4.7 for linear extensibility, because Austin is a shallow, chalky soil formed over marl rather than a deep expansive clay. That combination surprises people: high clay content, moderate movement.

The part that does move is Houston Black, under roughly 15% of the city, at an extensibility of 13.7 — one of the most expansive soils in Texas. So Allen is genuinely two cities underground, and which one your house is on matters far more than which subdivision it is in.