Expansive soils are the costliest natural hazard in the United States, causing more damage than all other natural hazards combined, including earthquakes, floods, tornadoes and hurricanes, notes a 1982 Federal Emergency Management Agency report (https://bit.ly/3m1HLL4). This damage occurs more in areas with a combination of highly expansive clay and cycles from very dry to very wet climates, which cause the soil volume to change.

While structural engineers are better at protecting structures from expansive soil damage today than in the past, many plumbing engineers today do not protect plumbing from expansive soil under isolated slabs (see Figure 1). And perhaps more importantly, there is some debate on whether a plumbing engineer is even expected to protect plumbing from expansive soil movement. 

Current Plumbing Codes

Every published edition of the International Plumbing Code (IPC) since the 2000 edition states in Section 305, titled “Protection of Pipes and Plumbing System Components”:

“Stress and strain. Piping in a plumbing system shall be installed so as to prevent strains and stresses that exceed the structural strength of the pipe. Where necessary, provisions shall be made to protect piping from damage resulting from expansion, contraction and structural settlement.”

Expansive soil is not explicitly mentioned in this provision. And expansive soil movement will not occur if the soil moisture does not change. Therefore, this is a geotechnical engineering matter of probability. However, that probability typically depends on numerous complex factors, often beyond the control of any one party or even all parties. An important example is the weather itself. 

Today, some say that IPC Section 305 requires protection of plumbing from expansive soil and some say it does not; it is a matter of interpretation. All international codes (I-Codes) give authority to the building official to interpret the codes. However, the I-Codes clearly state that a building permit shall not be considered approval of any code violation. The codes even state that if a building official purports to waive a code provision in writing, such a waiver is invalid. 

It is important to note that some believe that IPC Section 305 is sufficiently clear and does not require an interpretation in this matter. If this issue is not a question of interpretation, plumbing engineers are required to protect plumbing regardless of what a building official says. 

Therefore, it may ultimately be the responsibility of judges and juries in future lawsuits to determine if a plumbing engineer should have considered Section 305 to be up for interpretation or not. Plumbing engineers should evaluate their practices and carefully consider how juries may sympathize with an owner. 

All published editions of the Uniform Plumbing Code include language similar to IPC Section 305.

The 1955 edition of what was then called the National Plumbing Code (A40.8-1955) enumerated 22 basic principles (https://bit.ly/3GCr4zl). If a building official were to interpret IPC Section 305 to say that protection of plumbing from expansive soil is not required, it would contradict Principles 9, 18, 19 and 22.

Common Plumbing Design Approaches

1. Slabs-on-ground. Where the slab bears on the ground, under-slab plumbing is typically buried. Expansive soil is typically removed or modified before pouring the slab to reduce the potential vertical movement (PVM) to 1 inch or less. The slab and plumbing underneath typically both move together as the soil swells/shrinks. 

When plumbing becomes damaged by expansive soil movement, the slab can typically be sawcut above the damage and the plumbing can be repaired as feasible
maintenance.

2. Slabs-on-crawlspace. Where a crawlspace is under the slab, the accessible under-floor space typically isolates most of the plumbing from expansive soil damage. The plumbing often becomes nonisolated at the perimeter, where the plumbing transitions to being buried outside of the building. Damage can occur, but the crawlspace access makes repairs feasible.

3. Slabs-on-voidform. The building code only requires under-floor access if an under-floor space is a crawlspace. Under-floor voidform spaces without access are not crawlspaces. The voidforms typically degrade or collapse over time to protect the slab from expansive soil damage. 

Unless the plumbing is isolated, it has to bridge between a moving system (the soil) and a nonmoving system (the slab), so reoccurring damage is common (see Figure 2, showing an example in failure mode). Any damage to the plumbing under the slab becomes extremely expensive to repair because there is no access. The slabs typically cannot be sawcut immediately over the damaged plumbing because it could (and has in some cases) cause a structural collapse of the slab. 

Small, temporary access openings are typically sawcut in the slab, in between pier lines in both plan directions. To reach the damaged plumbing, soil is hand-excavated and wheelbarrowed out of the building. And, after all this time and expense, the repairs still do not address how the damage occurred in the first place, so it reoccurs.

One can categorize approaches to plumbing design under slabs-on-voidform in two primary groups: nonisolated plumbing and isolated plumbing. 

Examples of nonisolated plumbing are buried plumbing (see Figure 2) and plumbing supported by nonisolated void systems (many of them patented) before the slab is poured (see Figure 3, showing an example in failure mode). Nonisolated void systems often appear to isolate plumbing systems at first glance. 

However, in any system supporting the plumbing before the slab is poured, where that support system remains in place under the slab and in contact with hangar rods after the slab is poured, the support system can impart soil-induced loads onto pipe, clevises, hangers, drains and slabs. They must be accounted for because they are not isolated from the expansive soil. Mechanical engineers specifying such systems should make sure they will actually protect the plumbing if it is the intent. 

So that engineers do not practice in areas outside of their education and experience, this would likely require a specialty geotechnical, structural and mechanical collaboration, warranting additional fees (for testing and design) as an additional service. 

Among other things, this specialty design team would need to ensure that vertical and horizontal soil-retaining elements resist factored soil loads from International Building Code Section 1610 (including at-rest soil pressures, equipment loading pressures, undrained hydrostatic pressures and horizontal soil swell pressures) without deflecting into the plumbing between clevis hangers. 

They also would need to ensure that hanger rods do not buckle upon soil-swelling, slabs accommodate the factored force required to punch through buried support elements and skin friction around the rods, alternative materials such as plastic are approved by the building official based upon sufficient test data (e.g., variations in material properties, temperature effects, cyclical fatigue effects, moment-axial load interaction, P-delta effects, etc.), and the piping can resist any loads on them without stresses and strains exceeding the structural capacity of the specified pipe. 

Specialty design teams should be careful when accepting field test data or laboratory test data that only load products in one direction. Soil properties can vary from site to site, most of these products are statically indeterminate systems, and soil swelling occurs vertically and horizontally. 

It is so challenging for a specialty design like this to be done successfully and be properly reviewed by a plumbing inspector, that it’s a fool’s errand. Consequently, the plumbing code is changing to prohibit these products.

Isolated plumbing under slabs-on-voidform, similar to when plumbing is isolated under slabs-on-crawlspace, can be post-installed or pre-installed. Post-installed plumbing isolated from expansive soil is typically only installed where an existing slab-on-voidform with nonisolated plumbing has a lot of damage from expansive soil. Instead of only repairing broken plumbing, extensive quantities of soil under the slab are removed and new plumbing is installed. 

Hangers are attached to the slab in a manner that the plumbing, supports and hangers are not in contact with the soil or any assemblage of materials in contact with the soil. 

However, a recent invention (see Figure 4) now allows pre-installed plumbing to be an option around the country where isolated plumbing under slabs-on-voidform is desired. In the pre-installed approach, strut channel framing at a mid-slab elevation is temporarily supported by vertical rods until the framing is wire-tied to slab reinforcing bars. The vertical rods are removed from above before the slab is poured.

Plumbing Damage

Claims where plumbing is buried under slabs-on-ground and the potential vertical movement is underestimated or the subgrade is not prepared properly are common. However, repairs are generally feasible. The intent of the plumbing engineer is to protect the plumbing by burying it in low PVM soil conditions where the slab should be expected to generally move with the plumbing. 

Much larger claims have been made when nonisolated plumbing under slabs-on-voidform is not designed to accommodate the estimated potential vertical movement of the subgrade. In a San Antonio lawsuit, one federal court ruled that it was negligent for the design professionals not to protect the plumbing in such a case (https://bit.ly/3wTMEuG). 

Case Study 108 by the Geoprofessional Business Association describes a $25 million lawsuit related to a building with an approximately 100,000-square-foot footprint where it cost more than $8 million to post-install the plumbing in such as case (www.gbapodcast.com/ch108). 

Nonisolated PVC sanitary sewer piping has cracked where the subgrade moved only 1/2 inch under a slab-on-voidform (see Figure 5).

New Language in the 2024 IPC

New language for the 2024 International Plumbing Code was proposed by the Structural Engineers Association of Texas during the International Code Council 2021-2022 Group A code action hearings, with a 14-page rationale statement asking for clarification of IPC Section 305. 

The proposed change and rationale statement were based on a review by a collaboration of Texas structural, mechanical and geotechnical engineers of the practice, recent litigation, a publication from the Foundation Performance Association, and various products on the market. The SEAoT document states:

“In these instances [referring to nonisolated void systems], the mechanical engineers are relying on the manufacturer’s claims as a proprietary product and not designing a system that includes these products. However, these products retain soil as they move in response to swelling and shrinking, and they impart loads onto the structure and hangers that must be accounted for.”

The code change was supported by the American Institute of Architects, the Geoprofessional Business Association, the American Council of Engineering Companies – Texas, and various mechanical, geotechnical and structural engineers. The item was approved, with some modifications, by a vote of building officials around the country. 

The new language explicitly requires the protection of plumbing from expansive soil. It allows buried pipe under slabs-on-ground, but it requires isolation of plumbing under crawlspace slabs and slabs-on-voidform: 

“It shall not be permitted for the plumbing, hangers and supports below the slab or below the framing to be in contact with soil or any assemblage of materials that is in contact with soil within the active zone.” 

Exception No. 1 allows buried pipe for drainage of crawlspaces. Exception No. 2 allows attachment to piers if the plumbing, hangers and supports are otherwise isolated. And the language requires flexible expansion joints in the plumbing at transitions to buried conditions outside of the building, where one would start with an initial vertical offset allowing positive drainage, even if the site heaves the potential vertical movement.

I recommend that plumbing engineers protect plumbing from expansive soil by complying with the language in the 2024 IPC, as described above and as recommended by the Geoprofessional Business Association (https://bit.ly/3MZbOyR). 

This is recommended regardless of whether the IPC or UPC applies or whatever edition applies, whatever the geotechnical engineer estimates to be the probability of movement occurring, whatever the building official will allow, and however hard the owner presses to reduce the initial cost of construction.

Don Penn, PE, is the owner of Image Engineering Group and Don Penn Consulting Engineers. He is a professional mechanical/electrical/plumbing engineer licensed in 49 states. He has experience designing various project types and serves as an expert witness nationally in litigation related to the design of geothermal, HVAC, plumbing and electrical systems.