Clay soil can have a higher corrosion rate than sandy soils. And, soil contact can accelerate corrosion of metals where soil moisture is present. The relative acidity of the soil moisture is an important factor in the corrosion rate. For example with galvanized steel, in relatively lower pH soil the evolution of hydrogen can eliminate the possibility of protective film formation.

Generally, building codes (including plumbing codes) require that corrosive metals be protected in a manner approved by the building official. Protection that may be acceptable in an environment where the metals are not in contact with soil may not be acceptable if they are in contact with soil.

2021 IBC Section 1604.1 (Previous editions are similar):

“Building, structures and parts thereof shall be designed and construction in accordance with strength design, load and resistance factor design, allowable stress design, empirical design or conventional construction methods, as permitted by the applicable material chapters and referenced standards.”

2021 IBC Chapter 26 PLASTIC (Previous editions are similar) has no provisions permitting plastic to be used as a structural material.

2021 IBC Section 104 (previous versions of IBC are similar)

“An alternative material, design or method of construction shall be approved where the building official finds that the proposed alternative meets all of the following:

1.The alternative material, design or method of construction is satisfactory and complies with the intent of the provisions of this code,

2.The material, method or work offered is, for the purpose intended, not less than the equivalent of that prescribed in this code as it pertains to the following:

2.1 Quality.

2.2 Strength.

2.3 Effectiveness.

2.4 Fire resistance.

2.5 Durability.

2.6 Safety.”

Historical Example: Soil retainers adjacent to a void under a gradebeam, specified by the Structural Engineer of Record

As these soil retainers are in a simply supported condition, they are statically determinate, which allows significantly less testing. Material properties back-calculated from one test can be applied to non-tested conditions. (The example of a non-isolated void system with plastic side panels and crossbars, on the other hand, is statically indeterminate if tested as a three-dimensional assemblage with partial fixity at connections, requiring significantly more testing of load cases.)

If these soil retainers were to fail, the soil uplift would be resisted by a reinforced concrete gradebeam that has a significant amount of strength and stiffness, as well as typically the dead load of a suspended slab and the dead load of an exterior wall, which is a significant amount of resistance. (If a non-isolated void system with plastic side panels and crossbars fails, on the other hand, the impacted plumbing system often has little relative strength or stiffness.)

Some examples of alternative design considerations for plastic:

What design method is used to determine the appropriate Phi resistance factor for LRFD or allowable stress for ASD for plastic?

How will these values account for effects of temperature on plastic?

How will these values account for effects of ultra-violet light exposure before installation?

How will these values account for effects of cyclical loading fatigue?

How will these values account for variation of material properties?

What are the specified construction tolerances and how will these values account for specified construction tolerances?

Designers need to also carefully consider the effects of swell/shrink cycles on the deformed shape of non-isolated void systems.

As an example for plastic side panels and crossbars, in a shrinkage condition, any crossbars that bend upon contact with clevises or pipes (causing crossbar deflection) will have an initial mid-span deflection that can significantly reduce the axial buckling capacity of the crossbar in a swell condition, especially if there is “plastic” deformation (in this case referring to “plastic” as the opposite of elastic in material behavior).

Furthermore, cyclical stress reversals typically lead to failures due to material fatigue at significantly lower stresses than at initial conditions.

Non-Isolated Void Systems will shift horizontally under net horizontal forces (where horizontal forces are not balanced by equal horizontal forces on an opposing side of a Non-Isolated Void System). These shifts can damage plumbing (cracks and backdraining) by causing pipes to not only shift horizontally but also swing upward.

Soil swelling on one side of a Non-Isolated Void System can create net horizontal forces that push Non-Isolated Void Systems and damage plumbing. As an example, this could occur where plumbing runs near and parallel to an exterior grade beam and then exterior soil swells after being exposed to moisture.

Non-Isolated Void Systems are typically not labelled and listed products that have been deemed to comply with construction codes by a certification body for the purposes of protecting plumbing in expansive soil. If they were, conditions of the listing would define limitations.

Where soil retaining components and bracing for soil retaining components are expected to retain soil, the specifying professional needs to verify that they will be sufficient to resist axial compression buckling, out-of-plane flexural plastic hinge failure, failure due to combined compression and flexure, account for P-delta effects of bracing under beam column axial displacement of braces, and more.

It is uncommon for Mechanical Engineers to have the education and experience necessary to perform or review the structural engineering tasks necessary to design a non-isolated void system so that it complies with all applicable construction codes. If a Mechanical Engineer specifies a non-isolated void system, they are not allowed to rely on a manufacturer’s claims unless the manufacturer provides an engineered design with the seal of an engineer that is accepting this responsibility.

A design professional specifying a non-isolated void system should not attempt to specify that the design of the system be delegated during construction (e.g. with shop drawings bearing the seal of a Professional Engineer). It is the responsibility of the specifying professional to sufficiently define the scope of the Delegated Design Engineer. Simply specifying that the shop drawings be sealed does not define any responsibility for the Delegated Design Engineer. Furthermore, it is not appropriate for design professionals to try to divest their responsibility with vaguely defined and broad specification language for this complex multi-disciplinary issue where data in the industry is sorely lacking and other design professionals on a project need to coordinate scopes with this task. Therefore, for all of the reasons stated in this presentation and more, it is not feasible for a Mechanical Engineer to properly specify all of the multi-disciplinary criteria and collaboration required for any specific non-isolated void system.

RECOMMENDATION

We recommend Mechanical Engineers immediately cease and desist the practice of permitting Non-Isolated Plumbing (Buried Plumbing or Non-Isolated Void Systems) under Slab-on-Voidwork foundations, 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 demands to reduce the initial cost of construction and claims they will accept the risks of damage that would requiring extensive and repeated maintenance.

We recommend Mechanical Engineers immediately begin complying with the provisions already approved by ICC for the 2024 IPC (provided in this presentation) for protection of plumbing from expansive soil, which is also recommended by the Geoprofessional Business Association:

Non-Isolated Plumbing is allowed under Non-Isolated Slabs

Isolated Plumbing is required under Isolated Slabs