Blackland clay and foundations in Allen
The soil under much of Collin County cracks wide open in dry weather. Here are the numbers.
The soil under Collin County
According to the Handbook of Texas, the central and western parts of Collin County are covered by deep clay soils over marl and chalk, and the county lies in the Blackland Prairie. Allen, Lucas and Parker are all in that part of the county.
The Texas Almanac, using USDA soil data, describes Blackland Prairie soils as deep, dark-gray to black alkaline clays. It calls them "cracking clays" because deep cracks open in dry weather, and says this shrink-swell property can seriously damage foundations, highways and other structures.
Houston Black, measured
Houston Black clay is the best-known Blackland soil. The USDA's official description of the series gives numbers:
- In dry periods, cracks 0.5 to 4 inches wide extend 12 inches or more down, and stay open 90 to 150 days in most years.
- The surface can form gilgai, a pattern of small mounds and dips repeating every 10 to 24 feet, with 3 to 18 inches of relief.
- Its shrink-swell potential comes from smectite clay.
Not every lot sits on Houston Black. The USDA Web Soil Survey shows the soil mapped at your address.
What this means for a house
When clay under part of a slab dries and shrinks while another part stays wet, the slab is no longer evenly supported. That uneven movement is what shows up as cracked drywall and sticking doors. The Texas Section ASCE guidelines stress keeping soil moisture near uniform on all sides, while warning against overwatering.
Our page on watering your foundation has the practical side.
Sources
- Texas State Historical Association, Handbook of Texas: Collin County
- Texas Almanac: Soils of Texas (source: USDA Natural Resources Conservation Service)
- USDA NRCS Official Soil Series Description: Houston Black series (revised 08/2017)
- USDA NRCS Web Soil Survey
- Texas Section, American Society of Civil Engineers: Guidelines for the Evaluation and Repair of Residential Foundations, Version 3 (adopted April 1, 2022)
Last checked October 7, 2026. Rules, costs and contact details change, so check the source before you act on them.
Good questions
What soil is Allen, TX built on?
Much of central and western Collin County, including Allen, has deep clay soils over marl and chalk in the Blackland Prairie, according to the Handbook of Texas. Your own lot can be checked in the USDA Web Soil Survey.
How big do the cracks in Houston Black clay get?
The USDA's official series description says cracks 0.5 to 4 inches wide extend 12 inches or more down in dry periods, staying open 90 to 150 days in most years.
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