Soil Education

How to Build on Expansive Soil: Foundation Solutions That Work

Expansive soil is one of the most damaging geologic conditions in construction. It exists across North America, damaging structures, roadways, and infrastructure as it swells and shrinks through cycles of high and low environmental moisture. 

For contractors working in affected regions, understanding how to build on sites with expansive soil protects structures over the long term and protects your reputation from preventable concrete failure. Ultimately, concrete projects and foundations on expansive soil are an engineer’s problem, but with proven solutions for mitigating risk.

What Is Expansive Soil?

Expansive soil is a clay-rich geologic formation that undergoes dramatic shrink-swell cycles in response to moisture changes in the surrounding environment. Because of the soil’s structure, it readily absorbs water and expands. Then, as it dries out, it contracts, dramatically sinking and cracking. 

According to the U.S. Army Corps of Engineers, soils can expand in volume by up to 1,500%. That movement generates upward and lateral pressures on anything built on top. Depending on the site, expansive soil pressures can reach upwards of 30,000 pounds per square foot — more than enough to crack slabs, shift grade beams, buckle walls, and reduce the building’s functional lifespan.

Expansive soils are present in all 50 states and across Canada. In the U.S., the condition causes an estimated $13 billion in damage annually. For perspective, this is more than the combined damage caused by floods, hurricanes, tornadoes, and earthquakes. 

The American Society of Civil Engineers has previously estimated that 25% of American homes have some degree of damage from expansive soil. Today, as development moves into rural areas, the risk of expansive soil to structures is growing.

Many rich agricultural soils, like Texas’s famous Blackland Prairie soil, are prime examples of this ongoing issue. These soils were valuable for farming because of their clay content, mineral deposits, and water retention, but they are problematic in construction for the same reasons. As development moves into more affordable rural regions, the expansive nature of the soil must be taken into consideration.

Common signs of expansive soil damage include diagonal wall cracks, sticking doors and windows, uneven or heaving floors, and visible foundation displacement. By the time these symptoms appear, the damage is already done.

Site Preparation for Expansive Soil: Start with Testing

Sound construction on expansive soil starts long before the design phase. You can’t build around problems that haven’t been identified, which is why geotechnical testing is the very first step.

Soil Testing and Soil Strata Mapping

Engage a geotechnical engineer to conduct a full site investigation. Critical data points include the Plasticity Index (PI), the expansion index, depth of the active zone, and seasonal moisture variation. 

The higher PI indicates greater swell potential. Standard lab tests include Atterberg Limits, Swell Index, and Expansion Pressure tests. The geotechnical report will specify the Potential Vertical Rise (PVR) or Potential Vertical Movement (PVM), which directly determines how much swell could be expected, which is crucial data for determining the void depth required beneath structural concrete elements.

Image credit: BSK Associates

For projects specifying deep foundations, such as drilled piers where pier depth must be determined by the location of the stable bearing layer, the geotechnical testing may include Borehole logs. This work identifies the stable bearing layer below the active zone. For deep foundation systems, this data determines pier depth. 

The geotechnical report will inform the greater site design, specifically if additional preparations are needed to mitigate on-site conditions.

Despite its challenges, Blackland Prairie Clay has historically supported productive farmland and remnant prairie ecosystems. But for engineers and builders across the South, its expansive nature remains one of the main geological risks that must be addressed well before construction begins.

Image credit: constructionbusinessowner.com

Grading and Drainage

To manage the water brought by seasonal weather, as well as issues linked to on-site conditions like high water tables, grading, and drainage preparations are non-negotiable. 

Grading recommendations will vary by landscape and project, but as an example, the recommendations may be to grade the site to slope away from the foundation at a minimum of 2% for several meters. At the basic level, grading and drainage prevent water from pooling or running near the structure. Saturation events near the structure are among the primary triggers for catastrophic swell events.

Moisture Management for Expansive Soils

Expansive soil responds to moisture. Managing water around the foundation is as structurally important as the foundation design itself.

Subsurface Drainage

French drains and perforated perimeter pipes divert groundwater away from the foundation zone, reducing moisture fluctuations that trigger swell cycles. In high-risk areas, like those with high groundwater levels, sump systems may be warranted.

Controlled Landscaping

Large trees and deep-rooted shrubs extract moisture unevenly from the soil, creating differential drying zones that cause asymmetric shrink-swell activity. Keep significant vegetation away from the foundation perimeter. Alternatively, in arid regions, drip irrigation along the foundation line can maintain more consistent soil moisture and reduce the amplitude of seasonal shrink-swell movement.

Surface Grading

Maintain positive drainage away from the structure throughout the life of the building. Grade degradation over time is a common cause of long-term foundation problems in otherwise well-built structures.

Foundation and Structural Solutions

for Expansive Soil

Expansive clay soils demand foundation systems that will withstand or prevent damage to the structure during its functional lifespan.

What is the best foundation for expansive soil? The right solution depends on soil expansive risk and behavior, as determined by the geotechnical report as well as the final structure design and the related financial impact. Many successful projects combine more than one approach.

Here are a few ways to protect and stabilize the foundation on clay soil:

Void Form Systems

Void forms are the most direct structural solution for managing expansive soil pressure. A void form creates a designated space between the soil surface and the foundation, grade beam, wall, or pier. When the soil swells, it expands into that predetermined void space rather than exerting pressure on the concrete above. The structure is protected because the force is directed into the void, rather than into the structure.

Importantly, the void depth is not arbitrary. It is specified by the geotechnical report, specifically the PVR or PVM value, and must be precisely maintained throughout concrete placement. This is why material selection matters.

Degradable Cardboard (Carton) Void Forms

VoidForm’s SureVoid® system is the most specified solution for expansive soil conditions. Made from corrugated paper with a wax-coated exterior, SureVoid is installed in dry conditions and designed to support the weight of freshly poured concrete and the construction loads. Once the concrete cures, the form gradually absorbs moisture from the surrounding soil and environment and degrades, leaving behind the exact void space specified in the geotechnical report. This managed breakdown is an intentional process, proven under millions of square feet of structures in the US and Canada.

SureVoid covers the full range of structural applications, including slabs, grade beams, walls, piers, and more.

Non-Degradable Void Forms

As noted above, corrugated paper or carton void forms are suitable for installation on dry sites. For locations with wet on-site conditions or facing seasonal or periodic inclement weather, VoidForm’s StormVoid® system is designed for wet conditions during placement. 

StormVoid uses a waterproof, fluted copolymer polypropylene (PPC) interior structure wrapped in moisture-resistant corrugated paper. The flexible plastic internal structure maintains the specified void dimension even under heavy loads in saturated conditions, yet collapses under soil uplift pressures. 

Like SureVoid, StormVoid is pre-manufactured to project-specific sizes and strengths and can be configured for a broad range of applications.

EPS Foam

Expanded polystyrene (EPS) foam has historically been used as a void form material. Cut on site and placed beneath foundations and other concrete structures, it’s a familiar product in Canada. 

While technically compressible, it eventually compresses into a rigid brick-like structure under sustained soil pressure. Unlike corrugated paper or flexible plastic void forms, EPS does not degrade or properly yield, meaning it could, in theory, transfer soil pressure to the structure.

Styrofoam. Sheets of Factory manufacturing

Deep Foundations: Drilled Piers and Grade Beams

For soils with moderate to high swell potential, drilled piers extending below the active expansive soil zone into stable soil or bedrock are another proven structural foundation option. The pier bypasses the shrink-swell layer entirely, transferring structural loads to solid load-bearing material. Grade beams span pier heads, elevating the structure above the most active soil zone.

Void forms are often deployed alongside drilled piers and grade beams because, despite the depth of the drilled piers, the pier caps and beams remain subject to the shrink-swell forces of the surrounding expansive soil. 

In this regard, void forms are a critical element in pier-and-grade-beam construction. Without void forms at these locations, even a well-designed deep foundation system can be damaged as soil swells up and into the caps and grade beams themselves.

Reinforced Concrete Slabs and Grade Beams

Heavy reinforcement in slabs and grade beams allows structural elements to resist bending and distortion caused by differential soil pressure. Post-tensioned slabs, which use steel tendons tensioned after the concrete cures, are an alternative used in expansive soil regions because the slab can behave as a single rigid unit, redistributing stress rather than concentrating it at a point of weakness. 

They’re less effective where predicted movement is severe or where one area of the site is significantly more active than another. In these cases, the slab’s rigidity works against it when the ground beneath it isn’t moving consistently.

Regardless of slab type, reinforcement alone is not a substitute for a properly designed void system. A heavily reinforced slab without a void form beneath it still transmits all soil swell pressure directly to the structure.

Image credit: ucdavis.edu

Chemical Stabilization

Chemical stabilization is an expansive soil treatment that involves mixing additives like lime or cement into the soil to reduce its plasticity and swell potential. It’s commonly used on large-scale roadway, airfield subgrades, embankments, and similar infrastructure projects.

However, its feasibility is limited by site conditions. Soils with moderate to low plasticity, high organic content, or high sand and gravel content often lack sufficient clay for chemical additives to react effectively. 

Cold or saturated sites can further hinder the chemical bonding process, and projects near sensitive ecosystems may face environmental restrictions on stabilizer use altogether. Where it does apply, chemical stabilization reduces swell potential, but the site is still at risk of residual heave. A secondary strategy, like void form system and/or drilled piers, will be required to protect structures built above the treated soil.

Over-Excavation and Replacement

Over-excavation involves removing expansive clay to a specified depth and replacing it with compacted, non-expansive engineered fill. It’s straightforward in concept but costly and logistically complex in practice, particularly where the active zone runs deep or the project footprint is large. For example, in Northern Mississippi, there are areas of expansive soil type called Porter’s Creek Clay that can reach depths of 200 feet, making excavation unworkable.

Protection for Below Grade Utilities

It’s not just a foundation on expansive soil that puts a contractor’s reputation on the line. Plumbing, conduit, and mechanical lines installed below the slab are vulnerable to the same forces that damage concrete. Soil heave can shear rigid connections, break pipe joints, and cause costly failures that often go undetected until water damage appears. Flexible utility connections and/or dedicated protective enclosures allow for soil movement without utility failure.

VoidForm’s PlumbingVoid® System is a modular system made with lightweight,corrosion-proof materials, protecting below-slab plumbing from expansive and corrosive soil. It’s installed in conjunction with conventional plumbing designs. The system assembles quickly from lightweight plastic components, allows slope adjustment in the field, and is designed with inspection-friendly removable caps.

Construction Practices To Protect the Void System

The best-specified void form system on paper can fail if installation practices are not followed.

  • Protect installed void forms before concrete is placed.
    Protect void forms from point-load damage during rebar placement and concrete delivery by installing Cover Boards before the pour. This will also offer protection against accidental foot traffic and equipment damage or displacement. Place BackFill Retainer around the installed voids under grade beams and slabs to prevent damage or displacement from backfilled soil.
  • Maintain compaction standards during fill placement.
    Differential compaction in engineered fill is a source of settlement that compounds swell-related movement. Fill placed in inconsistent lifts or without adequate compaction testing creates zones of varying density that settle unevenly under load — producing the same cracking and distortion patterns that expansive soil causes, but from a problem that was entirely preventable.
  • Use low-permeability backfill near foundations.
    High-permeability material adjacent to the foundation creates a pathway for water infiltration and accelerates moisture cycling in the active zone.
  • Install void forms in appropriate conditions.
    SureVoid is designed for dry or normal soil conditions and should be protected from pre-installation moisture exposure. Where wet conditions are anticipated or unavoidable, StormVoid is the correct specification.

How To Build on Expansive Soils: Protect Your Next Project with VoidForm

You can build a foundation on clay, but working on sites on expansive clay soil requires specialized strategies to mitigate the risk. VoidForm has been manufacturing void form systems in the USA for over 45 years, protecting more than 178 million square feet of structures.

If voids have been specified on the project plans, use the Quick Start Guide or get in touch to request a quote. Free take-off services are available for most projects, and our team coordinates directly with contractors to keep delivery aligned with project timelines.

With five manufacturing plants and coast-to-coast distribution ensuring products arrive on time, pre-manufactured to the exact void depth, strength, pier diameter, and structural configuration specified in the geotechnical report.

Expansive soil is one of the most damaging geological conditions in the country. A successful project built on expansive soil relies on geotechnical data, smart foundation design, and a proven void form system beneath it all. Protect your next project and protect your reputation with VoidForm.

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