Risk management and the design of foundations on expansive clay soil

Most structural engineers never experience a single significant failure of a load-bearing structural element on one of their own projects. Conversely, most structural engineers practicing in areas with expansive clay soil have dealt with multiple projects where the foundation experiences excessive movement in service.Those are considered a failure by the owner.

Many of these incidents eventually involve litigation and result in significant financial losses to the structural engineer.

However, despite the large amount of litigation and property damage resulting from expansive soil, there has been little discussion within the structural engineering community regarding why problems with foundations on expansive clay are relatively common.

My experience during the past 15 years investigating numerous foundation failures of commercial structures constructed on expansive clay soil suggests that most foundation failures result from the failure of the project team to do the following: • recognize the differences between the design philosophy for foundations on expansive clay and other structural elements; • properly coordinate the design effort; and • educate the project team and building owner about special requirements of foundations on expansive clay.

This article discusses the differences in design philosophy that result in increased risk for the structural engineer and areas where education of, and coordination within, the project team can reduce project risk.

Philosophy of foundation design

Foundations on expansive clay soil require a significantly different design approach relative to the design of other structural elements. Foundations on expansive clay soil present unique challenges in five areas that the design team must consider and understand to achieve a successful project: shared design responsibility, variable probability of failure, unclear definition of failure (limit states), unknown loads and deflections, and lack of code guidance.

Design responsibility

For a foundation to exhibit successful performance during the life of the structure, the design must meld together the work of the structural engineer, geotechnical engineer, civil engineer, architect, and landscape architect. Fragmentation of design responsibility results in difficulties for the structural engineer if foundation problems do arise.When a foundation failure is investigated, investigators rarely discover a single, obvious "smoking gun" cause for the failure. As a result, all of the design professionals are typically brought into any resulting litigation.

Probability of failure

Most structural elements are designed effectively for a zero probability of failure, and no failures are expected in the structure’s lifetime. While the importance factor used in calculating wind and seismic loads does affect the probability of failure, its use typically is dictated by code, and failure is not anticipated in the structure’s lifetime regardless of the importance factor.

With foundations on expansive clay, however, the chances of failure are relatively high.While the probability of failure cannot be expressed quantitatively, structural engineers recognize that different structural solutions offer different risks of failure in service. For example, an engineer might consider the structural systems listed below for a moderate-sized one- or two-story commercial building on expansive clay. All of the structural systems in Table 1 are considered an acceptable method of construction, but all have different levels of performance—the possibility of failure during the structure’s lifetime increases from elevated structural slab (lowest possibility of failure) to slab-on-grade (highest).

Limit states

During the design process, the engineer of a foundation on expansive clay must consider the definition of failure. For most structural elements, failure is based on welldefined limit states such as yield and rupture; however, there is no generally accepted, quantitative definition of failure for foundations on expansive clay.

Although failure is identified generally by excessive differential movement, all foundations on expansive clay experience some differential movement.

Therefore, the point at which normal movement transitions into unacceptable movement is at issue.

The lack of defined limit states creates difficulties if the performance of the foundation does not meet the owner’s expectations. This is compounded by the fact that even movement within generally acceptable limits can result in cosmetic distress to the structure.

Calculation of loads and deflection

At its most basic level, structural engineering involves the calculation of loads and the design of members to resist the calculated loads—considering strength and deflection limit states. In general, the calculation of loads and deflections for structural elements are well understood and related codes are mandated. However, the deflections and associated forces imposed by expansive clay cannot be accurately predicted using currently available methods.

The most basic load parameters for foundations on expansive clay are the potential vertical rise (PVR) of the underlying soil and the profile of the soil mound. Neither of these parameters can be predicted accurately using the best available techniques. PVR calculations commonly used in actual practice, such as the TxDOT TEX-124-E methods, are even less precise, with unknown precision and bias. One of the most comprehensive treatments of the science of expansive clay is Expansive Soils by Nelson and Miller.

Regarding prediction of PVR, the authors state, "Heave prediction can be conducted in ways to imply various degrees of accuracy.

Earlier methods predicted heave in terms of ‘low,’ ‘medium,’ ‘high,’ and ‘very high.’ Perhaps this should be retained because it does not imply accuracies that are impossible to achieve." Design codes—While engineering judgment is required in the design of any structure, industry codes provide guidance for the design of most structural elements.

For example, the American Concrete Institute’s Building Code for Reinforced Concrete Structures (ACI 318) and the American Institute of Steel Construction’s (AISC) Manual of Steel Construction provide detailed design guidance, and the American Society of Civil Engineers’ Minimum Design Loads for Buildings and Other Structures provides detailed loading criteria. These documents are supplemented by authoritative publications—such as the Portland Cement Association’s Notes on ACI 318 and the AISC Design Guides—but little guidance of similar depth exists for foundations on expansive clay.

The International Code Council’s International Building Code (IBC) does provide instruction for foundations on expansive clay in Section 1805.8; however, guidance is meager and many critical details are not covered. The IBC provides explicit design guidance for stiffened slabs-on-grade only, incorporating, by reference, the Wire Reinforcement Institute Design of Slabon- Ground Foundations (WRI Manual) and the Post-Tensioning Institute Design and Construction of Post-Tensioned Slabs-on-Ground (PTI Manual). Both of these referenced methods have significant limitations.

Solving the problem

The differences between the design of foundations on expansive soil and of more typical structural elements result in significantly increased risk for the structural engineer. The most effective way to control this risk is to ensure proper education of, and communication with, the project team. For these purposes, the project team should include the owner, the structural engineer, the geotechnical engineer, the civil engineer, the architect, the landscape architect, and the general contractor, when possible.

Informed consent and selection of foundation system—Businesses usually are rewarded financially for accepting risk. Most engineers run their business on this principle, making an allowance for project risk when determining fees. However, when it comes to the design of foundations on expansive clay, many structural engineers turn this principle on its head.

On most projects, selection of any foundation system other than an elevated structural slab supported on deep piers increases the risk of foundation failure. However, it is common for the engineer to use a higher-risk, lowercost system such as a stiffened slab-ongrade.

If the owner is not involved in the decision to use a higher-risk foundation system, this foundation choice effectively increases the risk to the engineer while saving the owner money.

If the foundation fails, the owners often claim they did not realize that there was any risk associated with the selected system, claim they were unaware of any maintenance requirements or landscaping limitations, or indicate they would have selected a more robust and expensive system if they were aware of the risk involved. This may, in fact, even be true in some cases. Additionally, owners commonly file litigation claiming that the system does not perform to their expectations—even though the movement is within the design tolerance intended by the structural engineer.

One technique that reduces the structural engineer’s risk is informed consent, a procedure in which the owner is involved directly in the selection of the foundation system. With informed consent, the structural engineer and geotechnical engineer hold a meeting to educate the owner on the advantages and disadvantages of various foundation systems and document the proceedings and results of the meeting. For informed consent to be valid, the various systems must be explained carefully in plain, understandable language, and some significant discussion should occur. Topics covered should include the following:

• cost of construction;
• qualitative estimations of probability of failure;
• performance limitations (range of normal movement);
• maintenance requirements; and
• landscaping limitations.

Only the owner can determine his tolerance for risk and make decisions regarding the cost-benefit ratio for various foundation systems, since his idea of acceptable risk may not match that of the structural and geotechnical engineers. It is telling that the architect consults with the owner regarding the color of the interior paint, but there is frequently no discussion between the structural engineer and the owner regarding the foundation system. My experience suggests that most owners are receptive to being involved in the design decisions, since it is their building. In fact, after the risk associated with the various slab-on-grade and slab-on-fill systems are documented on paper, many institutional owners are extremely averse to any system except an elevated structural slab.

Coordination of design responsibility

Although standard industry practice makes design coordination difficult, the geotechnical, structural, and civil engineers and the landscape architect all have direct design input into foundation performance. The geotechnical engineer normally is retained directly by the owner, and the structural engineer normally is retained through the architect; the geotechnical report is frequently their only communication. Landscape design commonly is conducted under separate contract after the building is complete. It is rare that the civil engineer and landscape architect consider themselves part of the foundation design team or accept any responsibility for foundation performance.

To achieve a successful design, the design professionals must coordinate their actions. The importance of this coordination must be stressed to the owner and architect, and the consequences of improper coordination should be documented.

Drainage

One of the most common breakdowns in coordination among the project team is design of the local drainage. Adequate drainage within 10 to 25 feet of the building is critical for proper performance of the building foundation. However, the design of the drainage at the building perimeter frequently is dropped in the project, with no party taking responsibility.

The design team should assign responsibility for local drainage design to a specific team member who is familiar with the drainage requirements for foundations on expansive clay.

Landscaping

Improper landscaping is one of the most common causes of foundation distress. Landscaping problems are common for three reasons.

First, the landscape architect, the landscaper, or both do not realize the importance of landscaping in regard to foundation performance. Second, landscaping commonly is performed under separate contract with the owner and is not part of the main construction contract. Third, landscaping is changed frequently, and changes made after the original construction do not have input from the design professionals.

Ensuring that landscaping does not compromise the performance of the foundation normally requires that the structural engineer coordinate with, and educate, the landscape architect and owner regarding requirements for foundations on expansive clay.

Maintenance

Building maintenance plays an important role in the longterm performance of foundations on expansive clay. Important maintenance factors include proper drainage maintenance, including gutters and downspouts; vegetation (planting or removing trees); irrigation; and flower and shrub bed construction and maintenance.

Because maintenance plays an important role in foundation performance, the owner must be informed of maintenance requirements. No one would consider selling a $20,000 car without clearly communicating maintenance requirements to the owner, but buildings worth hundreds of times that amount are sold to owners who have no idea that improper maintenance can result in foundation failure.

The design team should communicate clearly the maintenance requirements—and the consequences of ignoring them—to the owner.

Role of the structural engineer

Some may argue that the suggestions contained in this article expand the role of, and hence the risk born by, the structural engineer. Certainly, some of the tasks fall outside the role currently performed by many structural engineers on foundation design projects. However, taking on these roles does not increase—but rather decreases—the engineer’s risk. Better design practices will result in a better performing project, happier owners, fewer lawsuits, and more profitable projects for engineers.

Eric C. Green, P.E., is a principal in the Structural Diagnostics Services Group at Walter P. Moore. He specializes in forensic investigations of distressed structures and repair design, and has investigated the performance of more than 100 foundations on expansive clay. He can be reached at egreen@walterpmoore.com.

Sidebar: Expansive clays

What are expansive clays? Expansive clays are soils that expand when they gain water and shrink when they lose water. Only certain types of clay are expansive—mainly those containing montmorillonite.

Vertical movement of more than 12 inches has been observed in buildings constructed directly on expansive clay.

These clays are found mainly in the Great Plains, the southeastern United States, and in California. Because movement is caused by changes in moisture content, areas with a semi-arid climate are most susceptible to damage. In 1987, it was estimated that expansive clay soils caused $9.0 billion annually in damage to structures in the United States ($15.1 billion in 2005 dollars).

Other researchers have determined that the average annual cost of damage from expansive clay soils exceeds the combined cost of earthquakes, hurricanes, and floods.