ASTM D4318 is not just a lab procedure here in San Antonio — it is the first line of defense against foundation distress in a city built on expansive clays. The Atterberg limits test measures the liquid limit, plastic limit, and plasticity index of fine-grained soils, directly quantifying how much water a soil can absorb before it shifts from a plastic state to a liquid one. In a region where summer drought cycles alternate with intense rainstorms, the shrink-swell potential derived from these numbers often dictates whether a slab-on-grade foundation needs post-tensioning or deep piers. Our lab processes samples from across Bexar County weekly, and the pattern is consistent: soils with a plasticity index above 25 require special engineering consideration for any structure larger than a single-story wood-frame home.
A plasticity index above 25 in San Antonio clays almost always signals a high shrink-swell potential that must be addressed in the foundation design.
Methodology and scope
Local ground factors
The Casagrande cup sits on a hard rubber base in our San Antonio lab, its brass cup rising and falling at two drops per second while the technician counts the number of blows needed to close a 13-millimeter groove in the soil paste. This is not a test you can rush. The difference between 24 and 26 blows at a given moisture content can shift the liquid limit enough to mis-classify a soil as CL instead of CH, which in turn alters the allowable bearing pressure assumptions in the geotechnical report. In San Antonio, where shallow foundations dominate residential construction, a mis-classified high-plasticity clay can lead to slab edge lift, drywall cracks, and floor heave within the first two years. We run every sample in duplicate because the cost of getting it wrong is a foundation repair that can exceed the original structural budget.
Regulatory framework
ASTM D4318 – Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils, ASTM D2487 – Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System), IBC 2021 – Section 1803 Geotechnical Investigations, ASCE 7-22 – Minimum Design Loads and Associated Criteria for Buildings and Other Structures
Associated technical services
Standard Atterberg Limits (ASTM D4318)
Liquid limit by Casagrande cup method, plastic limit by hand-rolling, and calculated plasticity index. Turnaround in 3 business days for standard-volume projects.
Atterberg + Full Classification Package
Combines Atterberg limits with sieve and hydrometer analysis to produce a complete USCS classification and soil description for the geotechnical report.
Shrink-Swell Potential Evaluation
Using the plasticity index and correlation methods from the Texas Department of Transportation, we provide a quantitative shrink-swell rating for expansive clay mitigation design.
Typical parameters
Frequently asked questions
What do Atterberg limits actually measure?
The test measures the moisture contents at which a fine-grained soil transitions between four consistency states: solid, semisolid, plastic, and liquid. The liquid limit is the moisture content at the boundary between plastic and liquid behavior. The plastic limit is the moisture content at the boundary between semisolid and plastic behavior. The numerical difference between them is the plasticity index, which engineers use to estimate shrink-swell potential and soil classification under the Unified Soil Classification System.
How much does Atterberg limits testing cost in San Antonio?
A standard Atterberg limits test on a single sample typically costs between US$70 and US$100 depending on the number of samples and whether it is part of a larger geotechnical investigation package. Volume discounts apply for projects requiring more than ten samples.
Why are Atterberg limits so important for foundations in San Antonio?
San Antonio sits on the Eagle Ford Shale and associated weathered clays that exhibit significant shrink-swell behavior with seasonal moisture changes. Knowing the plasticity index allows the geotechnical engineer to predict how much a foundation will move during wet and dry cycles and to specify the appropriate foundation type — whether a stiffened slab, post-tensioned slab, or deep pier system — to manage that movement.
How long does the test take from sample to report?
The laboratory procedure itself takes about 24 hours including drying, sieving, and the Casagrande cup and hand-rolling steps. With sample preparation, duplicate runs for quality control, and report writing, our standard turnaround is three business days. Expedited two-day service is available for time-sensitive projects. More info.