Learning Center · Texas Clay
What is the plasticity index of soil?
What is the plasticity index of soil? It is the number that says how much water a clay can take on before it stops behaving like a solid and starts behaving like a liquid, and you get it by subtracting the plastic limit from the liquid limit. Both of those limits are water contents, measured in a lab as a percentage of the soil's dry weight, so the plasticity index describes a width rather than a level. A wide index means a clay that holds a great deal of water and changes volume a great deal as that water arrives and leaves. That is why the number matters to a house here. The deep clays of the Blackland Prairie sit at the wide end of the scale, the thin soils over limestone west of the Balcones sit at the narrow end, and the gap between those two grounds is most of what separates a quiet foundation from a busy one.
THE ATTERBERG LIMITS ON A WATER CONTENT SCALE
What is the formula for the plasticity index of soil?
The formula is the plasticity index equals the liquid limit minus the plastic limit. The USDA engineering classification handbook puts it the same way, describing the plasticity index as the span of water content over which a soil behaves plastically and stating that it is numerically the difference between the two limits. Run a liquid limit of 70 and a plastic limit of 25 through that subtraction and the plasticity index is 45. Both inputs are percentages of dry weight, so the answer carries no unit of its own. It is a span of moisture and nothing more complicated than that, which is why it can be compared across soils from anywhere in the country.
What is the plastic limit for soil?
The plastic limit is the water content where a soil stops being mouldable and starts crumbling. Dry a clay past that point and it behaves like a solid lump. Wet it above that point and it takes a shape and holds it, the way modelling clay does in a child's hand. The field version of the test is a hand roll. You work a small piece of moist soil into a thread about an eighth of an inch across, fold it and roll it again, and the water content at which the thread finally breaks up instead of rolling out is the plastic limit. USDA field guidance notes that soils with no plasticity cannot be rolled into a thread at any water content at all, and that the tougher the thread feels near the plastic limit, the higher the plasticity index behind it.
How to do a soil plasticity test?
A real plasticity test is lab work, not yard work. The sample is dried, broken down and sieved, and the Atterberg limit tests are run on the fraction that passes the number 40 sieve, which is the fine sand and everything smaller. The liquid limit comes from a standard procedure that pins down the water content at which the wet soil starts to flow. The plastic limit comes from the thread roll described above. Each result is a water content, so the technician weighs the sample wet, oven dries it, weighs it again, and works the moisture out as a percentage of the dry weight. Subtract the second number from the first and the plasticity index falls out. Most homeowners never need to commission one, because the ground under this corridor has already been sampled and published, which is the subject of a later section.
What is considered a high plasticity index?
High starts in the double digits and gets serious in the forties. In the Unified Soil Classification System the letter H at the end of a soil group stands for high liquid limit, so CH is the group the trade calls fat clay. USDA geotechnical guidance singles those fat clays out as the problem material and puts the worst of them above a liquid limit of 60 and a plasticity index of 40, noting that the severity of the trouble tracks both numbers directly. The same guidance points out that these are the soils a survey flags for high shrink-swell potential and classifies as Vertisols. Blackland clay is that material, which is why a foundation conversation in this corridor starts with the ground and only then moves to the house.
Thresholds as published by the USDA Natural Resources Conservation Service in its engineering classification and geotechnical guidance. The last row describes the fat clays that dominate the Blackland Prairie.
What is considered a low plasticity index?
Single digits. The Unified system draws its lines at 4 and 7. Fines below 4 plot on the silt side of the chart, fines above 7 plot as clay, and anything caught between the two carries a dual CL-ML label. USDA guidance on soil strength treats a plasticity index above roughly 7 as the point where a fine soil has meaningful plasticity at all. A soil down at 3 or 5 packs firm and stays put through a wet winter, because there is very little water it can pull into its structure and very little volume change available to it. That is also the reasoning behind the select fill used against a foundation, covered in our guide to filling holes around a house foundation, where the right material is a low-plasticity clayey soil rather than sand.
What is likely to occur when soil has a high plasticity index?
The ground changes volume with the weather and the house rides it. Clay with a wide plasticity index takes on water through a wet season and swells. It gives that water up in a drought and shrinks. Only the upper layer answers the weather, the depth engineers call the moisture active zone, which on our clays can extend several feet down and reaches deeper where a big tree, poor drainage or an old plumbing leak is pulling moisture around. That is more than enough to lift or drop a slab edge, and the house reports it in a familiar order. A door starts binding at the top corner. A drywall crack opens diagonally off a door or window corner. Brick steps up through the mortar joints near the corner of the house. Tile grout splits in a straight line across a floor. On the driest ground the movement shows without a house on it at all, and the USDA description of the Houston Black series, the Blackland soil that runs from its Travis County type location down through the corridor, records surface cracks of half an inch to four inches wide at a depth of one foot during dry periods. A slab edge sitting over cracks like that has lost the support it was poured on. Which direction the house went matters for the repair, and that read is the subject of settlement versus heave.
Which soil type has the highest plasticity index?
Smectite clays, the group older references call montmorillonite. The USDA classification handbook explains the mechanism plainly. Kaolinite clay is built from two-layer sheets bonded firmly together, so water cannot get between them and the particle does not swell. Montmorillonite is built from three-layer sheets held together weakly, so water moves in between the sheets, the particle expands, and it contracts again when the water leaves. Illite has the same structure with stronger bonding and less movement. Texas A&M AgriLife describes Vertisols as the soils defined by smectite, points to the surface cracking they produce in long dry spells, and places them across the Central Texas Blackland Prairies. Houston Black, the Blackland series AgriLife calls the unofficial state soil of Texas, is classified as a fine, smectitic Vertisol, and its official description carries both the high shrink-swell rating and the slickensides that repeated swelling and shrinking leave polished into the profile. That is the top of the scale, and it is what most of this corridor is built on. Our full plain-English treatment of it lives on the Texas clay soil page.
How to find the plasticity index?
Three routes, and only one of them costs money. The first is the USDA soil survey. Web Soil Survey publishes an engineering properties report for every mapped soil in the country, and that report lists estimated liquid limit and plasticity index ranges next to the classification. Draw a boundary around your lot, run the report, and the numbers come back for the map units under it. USDA notes that those estimates are rounded to the nearest 5 percent, so read them as a range for an area rather than a measurement of your yard. The second route is a geotechnical report, if one was produced for your subdivision or pulled when the house was built. The third is paying a geotechnical lab to run Atterberg limits on a sample from your own lot, which is what a Texas-licensed engineer does when a design decision turns on the exact figure.
How the plasticity index relates to the ZIP soil scores on this site
Our ZIP dataset measures the same behaviour with a different instrument. We score foundation-movement risk for 331 ZIP codes across 35 Central Texas counties, and the input is linear extensibility, the USDA measure of how much a soil shrinks and swells with moisture, taken over the top 100 centimetres of the dominant soils. We use it instead of the plasticity index because it is mapped consistently for every ZIP in the survey, and because it reports volume change directly rather than predicting it from a pair of water contents. Plasticity index is the lab number underneath. Linear extensibility is the field consequence. Across the corridor that consequence runs from 0.5 percent to 17 percent, a 34 times spread, which is why two towns twenty minutes apart can carry completely different repair markets. 104 of the scored ZIPs, 31 percent of them, land in the High or Elevated bands, and inside San Antonio the share is half, 30 of the city's 59 scored ZIPs. The whole table, the method and the free CSV are on the soil risk dataset page, and you can find your own ZIP on the Central Texas foundation risk map. We do not publish a plasticity index by ZIP, and you should be wary of anyone who does, because one lab figure cannot honestly stand for every lot inside a postal code.
What to do if your lot sits on high plasticity clay
Two habits cover most of it, keeping the soil moisture steady and keeping a dated record of what the floor is doing. Clay only moves when its moisture changes, so the work is holding the moisture at the slab edge as level as a Central Texas year allows. Gutters should discharge well away from the foundation. Grade should fall away from the slab. Soaker hoses in a drought should run evenly on all four sides, not just the side that browns first. A big tree close to the house earns a root barrier. Where movement has already happened, piers work because they carry the load below the active zone and into ground whose moisture does not answer the weather, which is the reasoning laid out in drilled piers and the moisture active zone. None of that requires knowing your exact plasticity index. It requires knowing you are on clay, which the risk map answers in seconds, and knowing what your floor is doing today, which only a survey answers. We are ICC-certified and inspector-led, we work alongside independent Texas-licensed engineers when a job calls for a stamped design, and about a third of the time we find no repair needed.
