Cary Triassic Basin Clay Soil and Foundation Guide

Cracks in a slab, sticking doors or a stair-step crack in a crawl space wall often lead Cary homeowners to the same question: is it the soil? In western Wake County the answer can involve the Triassic Basin, a band of old sedimentary rock that runs along the county's western edge. This guide explains what the state's own soils study says about those soils, what the North Carolina Residential Code requires of footings and drainage, and why North Carolina routes foundation diagnosis to a licensed professional engineer.

What the Wake County soils report found

The North Carolina Geological Survey's Open-File Report 73-01, a 1973 preliminary study of Wake County soils, maps the county's rock units and tests the soils above them (NCGS OFR 73-01, Wake County Soils). It describes the Triassic rocks as located on the western edge of the county: sedimentary rocks deposited in a basin formed by movement along the Jonesboro fault, including fanglomerates, arkoses, siltstones and mudstones, some of them red.

The report measures "activity," an index of how much active clay a soil contains, calculated by dividing the plasticity index by the clay fraction. It cites the State Highway Department's rule of thumb that soils with activity above about 0.40 are possible problem soils and above about 0.70 possibly undesirable. For the Triassic soils it reports a mean activity of 0.50, with a range of 0.13 to 1.21, and states that the Triassic soils "contain critical and undesirable material." Clay was the largest grain-size fraction in the Triassic samples, at a mean of 32.4%.

Two other findings matter for builders. First, depth is unpredictable: the bedded rock means a thin hard layer can sit over a thick, poorly consolidated bed of undesirable material that an auger will not detect. Second, diabase dikes cut through the Triassic and other rocks across Wake County, and the report says soil developed from diabase is very undesirable material, often very fine grained and very slick and sticky when moist. Across the county, the report notes, rock may outcrop at one spot while the soil is 70 to 80 feet deep a few hundred feet away.

For context, the same report describes Wake soils as extremely variable in depth because of steeply dipping metamorphic rock, jointed granite and the varied Triassic beds. The report is old and preliminary, and a county-scale map cannot tell you what is under one house. It is a reason to ask for site-specific information, not a diagnosis.

How active clay moves a foundation

Clays with a high activity take on water and swell, then shrink as they dry. Under a house that cycle is uneven: soil near downspouts, leaky gutters or a low corner stays wet, while soil under a large tree or along a sunny wall dries out in summer. The result is differential movement, one part of the foundation rising or settling relative to another, which shows up as diagonal cracks at window corners, gaps at trim and sloping floors. Drainage and moisture control are usually the first things an engineer looks at for that reason.

What the North Carolina Residential Code requires

North Carolina adopts its own edition of the residential code (NC OSFM, current codes). In the 2018 NC Residential Code, Chapter 4 (2018 NCRC Chapter 4, Foundations):

• R401.3 Drainage: lots must be graded to drain away from foundations, with a fall of at least 6 inches within the first 10 feet, and impervious surfaces within 10 feet of the building sloped at least 2% away.
• R401.4 Soil tests: where data indicate expansive, compressible, shifting or other questionable soil, the building official determines whether to require a soil test by an approved agency.
• R403.1.4 Minimum depth: footings must extend below the frost line and never less than 12 inches below finished grade.
• Footing thickness: the code's footing figure notes minimum thicknesses of 6 inches for one story, 8 inches for two and 10 inches for three, based on 2,000 psf soil.
• R403.1.8 Expansive soils: the model-code section on foundations on expansive soils is deleted in North Carolina's edition, so expansive-soil design is handled through the soil-test and engineering provisions instead.
• R403.1.9 Excavations near footings: undercutting within 10 feet of a footing at a 45-degree line requires evaluation by a registered design professional.

Engineer letters and why North Carolina requires a PE

The North Carolina Board of Examiners for Engineers and Surveyors publishes Foundation Repair Guidelines, revised October 22, 2024 (NCBELS Foundation Repair Guidelines). They say every company that evaluates a foundation and recommends whether repair is needed must have a licensed professional engineer and be a licensed engineering firm, or have a licensed engineering firm do the evaluation. A general contractor may recommend restoring prior conditions, such as repointing mortar or replacing damaged joists, but analyzing why a foundation failed, choosing a support system and designing the repair must be done by a PE. Manufacturers' product specifications do not replace that site-specific design, and home inspectors who suspect a structural issue should refer the client to a PE.

A useful engineer letter for a Cary home typically identifies the observed distress, its likely cause (for example moisture variation in clay soils, poor drainage or inadequate bearing), whether repair is needed, and the repair design or scope. If a sale is coming, the letter becomes part of what you disclose; see our guide to selling a Cary home with foundation cracks.

Practical steps for owners

• Fix drainage first: downspout extensions, regrading to the R401.3 slope, and repairing leaks near the foundation.
• Photograph and date cracks, and mark crack ends with a pencil to see if they grow over a season.
• Ask whether a geotechnical soil report is warranted before an expensive pier design.
• Get the PE's evaluation in writing before signing a repair contract.

Sources: Angi, foundation repair cost; Angi, structural engineer cost; Angi, French drain cost. Ranges are attributed to the publisher named; they are national or regional estimates compiled by that publisher, not quotes and not prices from any provider on this site.

Local rules and code references

• NC Geological Survey, OFR 73-01 Wake County Soils (1973, preliminary): Triassic rocks along the Jonesboro fault; activity thresholds; Triassic mean activity 0.50 (0.13 to 1.21); diabase soils.
• 2018 North Carolina Residential Code, Chapter 4 (ICC): R401.3 drainage, R401.4 soil tests, R403.1.4 footing depth, R403.1.8 deleted, R403.1.9 excavations near footings.
• NC OSFM, Codes current and past: North Carolina code editions in effect.
• NCBELS, Foundation Repair Guidelines (rev. 10/22/2024): PE evaluation and design required for foundation repair recommendations.
• Town of Cary, Residential permits FAQ: Town permit questions.

Code text, policy forms and department procedures change. Confirm current requirements with the jurisdiction, your insurer or the utility before work starts.