Building Pad Preparation Across Texas and Oklahoma
A slab is designed against a soil the engineer described in a report. Pad preparation is the work that makes the ground under the slab match that description: undercut to the depth the report sets, moisture brought into its window and held there, chemical treatment or a select fill cap where the report calls for one, and every lift compacted and tested before the next. IRONWILL Earthworks builds structural pads on commercial, industrial and public sites across Texas and Oklahoma, to the report and to the code.
Undercut, moisture, treatment and density from the engineer's numbers, not a borrowed detail
Conditioned full depth and held, because a clay compacted dry has its swell ahead of it
Lime, cement, fly ash, select fill or removal, chosen by the test results
Proof rolled, then density and moisture on every lift, then the slab
A building pad is a designed element with a number attached to it
Everything above a pad is calculated. The slab thickness, the reinforcement or the post-tensioning, the bearing pressure and the movement the structure can tolerate are all worked out against an assumption about the ground. The building code makes the assumption explicit: where a shallow foundation bears on compacted fill, that fill has to comply with an approved geotechnical report1, and where the soil is expansive, the foundation has to be designed for it1. The pad is where those two requirements are built.
The ground gives the assumption teeth. Clay like the Blackland Prairie's Houston Black runs 40 to 60 percent clay with high shrink-swell potential, and in a dry spell it opens cracks half an inch to four inches wide from the surface to a foot or more down4. How much a particular site will move is not in the soil's name; it is in the geotechnical report, measured in inches as potential vertical rise, Texas's own test for how far a soil profile can lift whatever rests on it when it takes on water2. A pad that passes density on a Friday and heaves the slab two winters later was built to the wrong number.
Below: what the code and the report require of a pad, how pad preparation differs from grading, what the PVR number means and how it is brought down, why moisture governs the work, which treatment applies to which soil, what is tested before the pad is released, and the order we work in. The code sections, test methods and specification items behind every number here are listed at the end.
What building pad preparation is
Building pad preparation turns the ground beneath a structure into a uniform, engineered subgrade that carries the design load without moving. The building code defines the target: an expansive soil is one with a plasticity index of 15 or more, more than 10 percent passing the No. 200 sieve, more than 10 percent finer than 5 micrometers, and an expansion index above 20, with the expansion index alone allowed to settle it1. Most native clay in the metroplex meets all four4. Pad preparation is everything done to that soil so the slab designed on top of it behaves as designed: removal of the soil that would move, moisture control of the soil that stays, chemical treatment or a select fill cap where the report specifies it, and compaction proven lift by lift.
| Criterion | Threshold | Test method |
|---|---|---|
| Plasticity index | 15 or greater | ASTM D4318 |
| Fines | More than 10 percent passing the No. 200 sieve | ASTM D422 |
| Clay-size fraction | More than 10 percent finer than 5 micrometers | ASTM D422 |
| Expansion index | Greater than 20 | ASTM D48293 |
The operative word is uniform. The code's own design requirement for foundations on expansive soil is that they resist differential volume change and keep deflection and racking within what the structure can use1. A pad fails not because it is weak everywhere but because one corner behaves differently from the rest: soft material left under part of the footprint, a lift placed dry on one side and wet on the other, a treated zone that stops short of the slab edge. Each of those produces differential movement, and differential movement is what cracks slabs and racks door frames.
That is why pad preparation is prescriptive. Depth of undercut, moisture window, treatment type and rate, lift thickness, required density and the number of tests per lift are written in the geotechnical report, and the code ties the pad's acceptance to that report rather than to appearance1. The pad is finished when the results say so.
Pad preparation is not grading, and the code treats them differently
Grading gets the pad to the right elevation. Pad preparation gets it to the right condition. Only one of those is what the slab bears on.
Mass grading
Moving bulk volume to bring the site to design elevations: cut, fill, haul and balance across the whole footprint. It establishes where the ground is, and it is measured in cubic yards.
Fine grading
Trimming the surface to line and grade within a tight tolerance, usually just ahead of concrete. It establishes how flat the ground is, and it is measured in hundredths of a foot.
Pad preparation
Changing what the ground is made of and how it behaves: undercut, moisture conditioning, treatment or replacement, and compaction to a required density. It establishes what the ground will do under load, and it is measured by the laboratory and accepted against the geotechnical report1.
All three happen on the same site, often in the same week, and the sequence matters: mass grading brings the site to subgrade, pad preparation builds the structural section, and fine grading trims the finished surface. Trouble starts when a scope carries pad preparation as a line inside grading. One is measured by the cubic yard; the other by the square foot at a stated depth, and it carries water trucks, mellowing days and a testing schedule the other does not.
What pad preparation includes
Every pad is different because every report is different. On private work the report is the document the pad is built to; where it borrows a TxDOT test method, as Texas reports commonly do, that method is the yardstick, not the specification the job is built under. Most reports call for one of three caps, select fill, flex base or a lime-stabilized cap, and we build all three. These are the operations that make up the work, in the combination the soil, the structure and the report call for.
Subgrade preparation and undercut
Topsoil, organics and existing fill come off, then the expansive soil is removed to the depth the report sets. The code's test for that depth is that the soil left behind holds a constant moisture content, with the depth reducible where confining pressure exceeds the swell pressure1. Anything that pumps under a loaded roller comes out before anything structural goes back.
Moisture conditioning
Water worked into the soil to full depth, by disc, water truck or aerator, until the moisture reads inside the window the laboratory curve sets, and held there through compaction. On expansive clay this is the operation that decides the pad, and the reason is in the specification: a soil above PI 35 is compacted at or above its optimum moisture5. Where a report offers the owner moisture conditioning or chemical or water injection, most of the owners we work for choose moisture conditioning, and it is the method we build.
Chemical treatment
Lime, cement or fly ash mixed into the clay full depth to change it. On TxDOT's tree, lime enters at a plasticity index of 15 and above, cement below it, with blends across the middle6; the code lists chemical treatment beside dewatering and presaturation as ways to stabilize the active zone1. The mix design, not habit, picks the additive and its rate.
Select fill placement
Imported low-plasticity material placed in controlled lifts where the report calls for a non-expansive cap of a set thickness. The TxDOT definition of a select granular embankment material is a useful yardstick: liquid limit 45 or less, plasticity index 15 or less, bar linear shrinkage 2 or more5. The cap gives the slab a section that does not shrink and swell with the seasons.
Aggregate and flexible base
Crushed limestone base or select aggregate hauled in, spread, brought within two percent of its optimum moisture and compacted to 100 percent of its laboratory density5: the working surface for slab construction and the drained layer under it.
Compaction and testing support
Lift-by-lift compaction with the right roller for the material, proof rolling ahead of the laboratory, and coordination with the testing firm so a failing area is reworked the same day rather than the day of the pour.
Moisture decides the pad, and it is the one thing you cannot see
A soil reaches its maximum density at one moisture content. Compact it drier and the particles do not lock; compact it wetter and the water carries the load. That relationship is established in the laboratory by a compaction test: ASTM D698 at standard effort, 12,400 foot-pounds per cubic foot from a 5.5-pound rammer dropped 12 inches7, or ASTM D1557 at modified effort, 56,000 foot-pounds from a 10-pound rammer dropped 18 inches8. On a TxDOT-referenced job the equivalent curve for subgrade soils is Tex-114-E9. The field target is written as a percentage of that maximum dry density inside a stated moisture range.
On expansive clay there is a second reason to care, and it is the reason the moisture window sits where it does. A clay compacted dry has somewhere to go: it takes on water later, swells, and lifts the slab. TxDOT's embankment item makes the rule explicit by plasticity: a soil with a PI of 15 or less needs only 98 percent of maximum density with no moisture floor; between 15 and 35 it must be at or above optimum and between 98 and 102 percent; above 35 it must be at or above optimum and between 95 and 100 percent5. The density ceiling on a fat clay is deliberate. Overcompacting it dry stores swell, so the specification trades a little density for moisture the clay has already taken on. A pad that hits density but misses the window has passed half a test.
The equipment reflects that. Water trucks, discs and reclaimers are the primary equipment on a pad job, not the support equipment, and a pad that comes in too wet is dried by working it rather than by waiting for weather that may not arrive. Once the window is met it is kept: TxDOT requires the moisture of a finished clay layer to be held no lower than four percent below optimum until the next layer covers it5.
Optimum moisture from the laboratory curve, not from feel
Expansive clay placed at or above optimum, by rule
Disced and mixed full depth, never surface-watered
Aerated and dried when the soil comes in too wet
Moisture read with the density on every lift
Sealed and drained until the slab goes down
Which treatment suits which soil
Treatment is not one product applied to every pad. The building code allows removal, chemical treatment, dewatering and presaturation as ways to handle an expansive soil1; TxDOT's guidelines add that the choice follows the soil's mineralogy, plasticity, gradation, sulfate and organic content, the goal of the treatment, the design life and the drainage6. Choosing wrong is expensive in both directions, whether that means treating a soil that did not need it or building on one that did.
| Treatment | What it changes | What governs it |
|---|---|---|
| Undercut and replace | Removes the swelling soil from the active zone so what remains holds a constant moisture content1 | IBC 1808.6.3 sets the depth by that test; the geotechnical report sets the number1 |
| Moisture conditioning | Brings the clay to or above its optimum before compaction so the swell is spent before the slab, not after | Item 132 holds a clay above PI 35 at or above optimum, 95 to 100 percent of maximum density5 |
| Chemical treatment | Lime alters the clay's particle structure; cement and fly ash bind particles6 | The TxDOT selection tree by PI and gradation, then a mix design; sulfate checked first612 |
| Select fill cap | Replaces the top of the section with a low-plasticity material that does not shrink and swell | Item 132 Type A: liquid limit 45 or less, PI 15 or less, bar linear shrinkage 2 or more5 |
| Flexible base or aggregate | A drained, load-spreading working surface under the slab | Item 247: graded material table, 100 percent of its Tex-113-E density5 |
| Presaturation or dewatering | Stabilizes the active zone's moisture rather than its chemistry | IBC 1808.6.4 permits it as an alternative to removal1 |
Lime treatment
One of the options for a high-plasticity clay, and the one TxDOT's tree lists first from a PI of 15 upward6. The reaction is with the clay's own silica and alumina, and it rearranges the particle structure, which is why lime is reached for to cut shrink and swell rather than to add strength6. The rate is the lowest lime content that holds a soil-lime pH of 12.4 by ASTM D627614, and on TxDOT work the mix is compacted to at least 95 percent of its maximum density after one to four days of mellowing5. Its reach has a limit the federal manual states plainly: two to three feet is generally the maximum depth lime can be mixed in13, so a deeper active zone needs removal or a cap above the treated layer.
Cement treatment
Suited to granular and low-plasticity soils, where cement binds particles and builds strength directly; the TxDOT tree offers it first below a PI of 156. Speed is the constraint as much as the benefit: compaction has to be complete within two hours of water reaching the mix5. On a pad it is most often the answer under a pavement-grade section, or where sulfate rules lime out.
Select fill
Where the native soil is not worth improving, replace the top of it. Imported low-plasticity fill placed over the prepared subgrade in controlled lifts, to the thickness the report sets, gives the structure a section that will not shrink and swell with the seasons. The federal pavement manual notes the same trade-off for roads: where the weak or expansive zone is thin, removal and replacement is usually the preferred alternative13.
Aggregate and flexible base
Crushed limestone base placed and compacted over the prepared subgrade: a stable working surface for slab construction, load distribution under the structure, and a drainage layer that keeps water from collecting at the subgrade. On public work TxDOT's flexible base item compacts it to 100 percent of its laboratory maximum dry density and checks its depth before acceptance5; on private work the report sets the density and the thickness.
One caution belongs ahead of any lime decision in this region. Soluble sulfate in the soil combines with calcium from the lime and alumina from the clay to form ettringite, a mineral more than 200 percent larger in volume than its constituents13, and the result is heave rather than its prevention. TxDOT measures soluble sulfate by colorimeter under Tex-145-E12, routes anything above 3,000 parts per million to its sulfate-rich soil guidelines6, and forbids dry quicklime above that level5. Where the number comes back high, the answer is a different treatment, a select fill cap or a deeper removal, and that decision belongs to the geotechnical engineer.
How a finished pad is verified
A pad is accepted on evidence. These are the four things that decide whether it is ready for the structure, and the reason a schedule that skips mellowing time or testing turnaround will slip anyway.
The geotechnical report
The document the pad is built to, and the one the building code names as the standard a compacted fill has to comply with1. Borings, classification, plasticity, PVR and the recommendations that follow: undercut depth, moisture window, treatment type and rate, lift thickness and required density. Everything else on this page is downstream of it.
Proof rolling
A loaded pneumatic roller, at least 25 tons on a TxDOT job and no more than 50, crossing the prepared subgrade at 2 to 6 miles an hour in at least two coverages5, watched for rutting, pumping and deflection. It finds the soft zones a density test would have walked past.
Density and moisture testing
Field density and moisture read lift by lift against the laboratory curve: by nuclear gauge under ASTM D6938, which reads density by gamma attenuation to about 300 millimeters in direct transmission11, or under Tex-115-E, which also prescribes how the test locations are chosen at random10. Both numbers matter. Passing density at the wrong moisture is not a passing lift on expansive clay.
Potential vertical rise
PVR estimates, in inches, how much the soil profile below a foundation can rise if it takes on water2. TxDOT's method samples the profile to 15 feet, and to 20 feet where highly expansive clays are present, and computes the rise layer by layer from each layer's liquid limit, moisture content, fine fraction and plasticity index2. The worksheet then plots rise against depth so the engineer can read off the depth of modification needed for an allowable PVR2. That plotted depth is what usually sets the undercut, the treatment depth or the cap thickness.
Our pad preparation sequence
Pad preparation is sequential work, and the order is not negotiable, because each step depends on the one before it being right. Below is that order, with the days each step takes.
Review the geotechnical report
We start with the report, not the plan sheet. The PVR, the undercut depth, the moisture window, the treatment and its rate, the lift thickness and the required density set the entire sequence and most of the schedule.
Strip, undercut and proof roll
Topsoil, organics and existing fill come off. Expansive soil is undercut to the specified depth and the exposed subgrade is proof rolled5 so soft areas are found and removed before anything structural is placed over them.
Bring the subgrade to its moisture window
The subgrade is disced, watered or aerated full depth until it sits inside the window of its curve, at or above optimum on a fat clay5, and held there through compaction rather than left to dry overnight.
Treat, or import fill
Lime or cement spread at the mix-design rate, mixed full depth, mellowed where the item requires it and remixed5; or select fill and aggregate base hauled in and placed where replacement is the specified solution.
Compact in lifts and test
Material placed in lifts of the specified thickness, never more than 16 inches loose or 12 inches compacted under density control on a TxDOT-referenced job5, and compacted with the right roller for the material. Density and moisture are tested on each lift; anything that fails is reworked and retested before the next lift goes on.
Finish, protect and hand over
The pad is trimmed to grade and tolerance, sealed against rain, and drained. The code requires the ground next to a foundation to fall away from it at no less than one vertical in twenty horizontal1, and a finished pad that ponds water for a week is not a finished pad, so it is protected until the slab goes down.
What a correctly built pad prevents
Pad failures rarely show up during construction. They show up in the second and third year as cracks running diagonally from slab corners, doors that stop latching, masonry that opens at the joints and floors that read out of level on a laser. By then the fix is not earthwork; it is piers and litigation.
Almost all of it traces back to differential movement, one part of the pad moving relative to another, which is exactly the behavior the code requires a foundation on expansive soil to resist1. Uniform moisture, uniform material and uniform compaction are the whole defense. This is the least visible part of the site work and the one most regretted when it is cut.
There is a schedule argument too. A pad that passes testing the first time releases the slab on the date the pour was scheduled, and everything stacked behind that pour, framing, envelope and inspections, keeps its sequence.
Uniform bearing across the whole slab footprint
Swell taken out of the active zone before the structure goes up
Density and moisture on record for every lift
Slab pours that are not waiting on a failed test
Fewer differential-movement cracks years later
A drained working surface crews can build on in any weather
Have a pad to build?
Send the geotechnical report and the pad footprint. We will tell you what the PVR and the undercut mean for the section, which treatment the soil points to, and how many days the treatment and its testing will take off the front of the schedule.
Pads we build
From a single equipment foundation to a warehouse floor covering several acres, the question is the same: what does the report require, what is the soil doing, and what has to be true before the concrete arrives.
Commercial buildings and retail
Slab-on-grade pads for offices, retail shells, restaurants and medical buildings, where finish tolerances are tight and the slab is the floor everyone sees.
Industrial and warehouse floor slabs
Large-footprint pads under racking loads and forklift traffic, where flatness and uniform bearing across hundreds of thousands of square feet is the entire specification.
Multifamily and mixed-use
Building pads for apartment and mixed-use construction on expansive clay, where a post-tensioned slab designed to PTI DC 10.5 still depends on what the subgrade does1.
Tilt-wall and structural pads
Pads that serve twice, as a casting surface for tilt-wall panels and as the finished floor section afterward.
Equipment and tank foundations
Smaller, heavily loaded pads for equipment bases, tanks and process structures, where settlement tolerance is measured in fractions of an inch.
Municipal and institutional
Pads for public buildings, schools and civic facilities, built and documented to the testing regime the owner's program requires, often under our construction management scope.
How we approach pad preparation
Built to the report
The geotechnical recommendations drive the work, not a detail borrowed from the last job.
Moisture taken seriously
Water trucks, discs and mellowing days carried as real scope, not as an afterthought.
Clay experience
Expansive soil is what we work in every day, including the sites where lime is the wrong answer.
Testing coordinated, not dreaded
Proof rolled before the laboratory arrives, failures reworked the same day, results documented.
Pad preparation across Texas and Oklahoma, from Dallas-Fort Worth
We build structural building pads on commercial, industrial, multifamily and municipal sites across Texas and Oklahoma, to the report and to the code. Crews, water trucks and compaction equipment are based in Dallas.
Pads are what our crews build most weeks, so ask about a site anywhere in the metroplex or the counties around it.
Site address and pad footprint
Geotechnical report, if you have one
Required compaction and moisture range
Structure type - slab, tilt-wall, equipment
Target date for the slab pour
Pad preparation FAQ
What does building pad preparation include?
Removal of the soil that would move, moisture control of the soil that stays, chemical treatment or a select fill cap where the report calls for one, an aggregate working surface where specified, and compaction proven lift by lift. The building code ties acceptance of the fill to an approved geotechnical report1.
How is pad preparation different from grading?
Grading changes where the ground is; pad preparation changes what it is made of and how it behaves. Neither mass grading nor fine grading alters the soil's moisture, plasticity or bearing. The code's fill and expansive-soil provisions apply to the pad, not to the grade1.
What does the PVR number in my report mean?
Potential vertical rise is the height, in inches, the soil profile under the foundation could lift if it took on water2. TxDOT's Tex-124-E computes it layer by layer to 15 feet, or 20 where the clay is highly expansive, and plots it against depth so the engineer can read the depth of modification for the rise the structure can tolerate2. That depth becomes the undercut, the treatment depth or the cap.
What compaction is required for a building pad?
Whatever the geotechnical report sets, expressed as a percentage of a laboratory maximum inside a moisture range. The code's own floor, for a fill of 12 inches or less placed without a full report, is 90 percent of the ASTM D1557 maximum verified by special inspection1. On expansive clay the moisture side of the requirement is the one that gets missed.
When is lime used instead of cement, or neither?
The state's guidelines put the dividing line at a plasticity index of 15, with lime at or above it, cement below it and blends across the middle6. The code also allows removal, presaturation and dewatering as ways to stabilize the active zone1, and where sulfate exceeds 3,000 parts per million the calcium-based additives are referred to the sulfate guidelines first6.
How is the lime rate determined?
By a pH series on the project soil, ASTM D6276: increasing percentages of lime are mixed with the soil and the design rate is the lowest percentage that holds the mixture at a pH of about 12.414. Cement and fly ash rates come from strength testing at trial percentages.
Can lime make a pad worse?
What is select fill and when is it used?
Imported low-plasticity soil placed in controlled lifts where the report calls for a non-expansive cap. A useful benchmark for what qualifies is TxDOT's Type A embankment material: liquid limit of 45 or less, plasticity index of 15 or less, bar linear shrinkage of 2 or more5.
What is proof rolling and why is it done?
A pneumatic roller loaded to between 25 and 50 tons, walked over the finished surface twice at no more than 6 miles an hour while somebody watches for a rut or a pump5. It finds the soft spots between density test locations, which is exactly where a test would otherwise miss them.
How long does pad preparation take?
The schedule is set by chemistry and testing more than by earthmoving. Lime needs one to four days of mellowing before final compaction, seven or more where sulfate is between 3,000 and 7,000 parts per million5; density and moisture results have to come back before the next lift; and rain can put a clay pad outside its moisture window for days. Those items belong on the schedule from the start.
Sources
Every number and every rule on this page is cited here. Reviewed September 17, 2026.
- 2021 International Building Code, Chapter 18: Soils and Foundations (as adopted in Texas)International Code Council, via UpCodes, 2021Section 1803.5.3 (the four criteria that define an expansive soil), 1804.4 (ground sloped away from the foundation at 1 in 20), 1804.6 (compacted fill under shallow foundations complies with the geotechnical report; the 12-inch, 90 percent D1557 exception), 1808.6 (design for expansive soils; slab-on-ground to WRI/CRSI or PTI DC 10.5), 1808.6.3 (removal to a depth that holds constant moisture) and 1808.6.4 (chemical, dewatering or presaturation stabilization).
- Tex-124-E, Potential Vertical Rise of Natural Subgrade SoilsTexas Department of Transportation, Construction Division, 2017PVR defined in inches; sampling to 15 feet, and 20 feet with highly expansive clays; the rise computed by layer from liquid limit, moisture, fines and plasticity index; the plot of rise against depth used to set a depth of modification.
- ASTM D4829-21, Standard Test Method for Expansion Index of SoilsASTM International, 2021The expansion index the building code uses as its single sufficient test for expansive soil.
- Official Series Description: Houston BlackUSDA Natural Resources Conservation ServiceClay content of 40 to 60 percent, high shrink-swell potential, and cracks 0.5 to 4 inches wide to 12 inches or more deep when dry.
- Standard Specifications for Construction and Maintenance of Highways, Streets, and Bridges, adopted September 2024Texas Department of Transportation, 2024Item 132 Embankment (Type A select material limits; density and moisture by plasticity index; lift thickness; moisture maintenance), Item 216 Proof Rolling, Item 247 Flexible Base, Item 260 Lime Treatment (mellowing and sulfate rules), Item 275 Cement Treatment (two-hour compaction).
- Guidelines for Modification and Stabilization of Soils and Base for Use in Pavement StructuresTexas Department of Transportation, Construction Division, 2005The factors behind additive selection, the mechanisms of lime, cement and fly ash, the selection tree by plasticity index, and the 3,000 ppm sulfate referral.
- ASTM D698-12(2021), Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Standard Effort (12,400 ft-lbf/ft3)ASTM International, 2021The standard-effort compaction curve: 5.5-pound rammer, 12-inch drop, maximum dry unit weight and optimum water content.
- ASTM D1557-12(2021), Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Modified Effort (56,000 ft-lbf/ft3)ASTM International, 2021The modified-effort curve: 10-pound rammer, 18-inch drop.
- Tex-114-E, Laboratory Compaction Characteristics and Moisture-Density Relationship of Subgrade, Embankment Soils, and Backfill MaterialTexas Department of Transportation, Construction Division, 2011The laboratory curve for subgrade soils on TxDOT-referenced work.
- Tex-115-E, Measuring In-Place Density and Moisture Content of Soils and Base MaterialsTexas Department of Transportation, Materials and Tests Division, 2025Field density and moisture, and the random selection of test locations.
- ASTM D6938-23, Standard Test Methods for In-Place Density and Water Content of Soil and Soil-Aggregate by Nuclear Methods (Shallow Depth)ASTM International, 2023The nuclear gauge: gamma attenuation for density, direct transmission to about 300 millimeters, thermal neutrons for water content.
- Tex-145-E, Sulfate Content of Materials Using a ColorimeterTexas Department of Transportation, Materials and Tests Division, 2024The soluble sulfate test run before any calcium-based treatment.
- Geotechnical Aspects of Pavements Reference Manual, Section 7.6: Subgrade Improvement and Strengthening (FHWA NHI-05-037)Federal Highway Administration, 2006The two-to-three-foot practical limit on lime mixing depth, removal and replacement where the poor zone is thin, and ettringite growth from sulfates.
- ASTM D6276-19, Standard Test Method for Using pH to Estimate the Soil-Lime Proportion Requirement for Soil StabilizationASTM International, 2019The pH method that sets the lime rate at a soil-lime pH of about 12.4.