When the slab is poured, the slab reinforcement and concrete are supported by a layer of “voidwork”. However, the voidwork is designed to degrade, collapse or otherwise create a space under the slab. Deep foundations support the slab permanently, on a grid about 20 ft each way.

The original 3 hour presentation has many more slides on failure
modes of non-isolated plumbing under isolated slabs.

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.

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.

We recommend Mechanical Engineers immediately cease and desist the practice of permitting Non-Isolated Plumbing (Buried Plumbing or Non-Isolated Void Systems) under Slab-onVoidwork 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

Explicitly requires protection of plumbing from expansive soil and defines when expansive soil is present.

Allows buried plumbing under Slabs-on-Ground.

Requires complete isolation of plumbing under Slab-on-Crawlspace and Slab-on-Voidwork: “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 1 allows buried pipe for drainage of crawl spaces

Exception 2 allows attachment to piers if the plumbing, hangers and supports are otherwise isolated.

305.8 Expansive Soil. Where expansive soil is identified under buildings in accordance with Section 1803.5.3 of the International Building Code, but not removed in accordance with Section 1808.6.3 of the International Building Code, plumbing shall be protected in accordance with Section 305.8.1 or 305.8.2.

305.8.1 Non-Isolated Foundations. Under foundations with slabs that are structurally supported by a subgrade, it shall be permitted for plumbing to be buried.

305.8.2 Isolated Foundations. Under foundations with a slab or framing that structurally spans over an under-floor space which isolates the slab or framing from the effects of expansive soil swelling and shrinking in accordance with Section 1808.6.1 of the International Building Code, the plumbing shall be suspended so that plumbing, hangers and supports are isolated, by a voidspace, from the effects of expansive soil swelling and shrinking.

Exception: It shall be permitted for plumbing to be buried if the plumbing provides drainage of an under-floor space.

To protect the voidspace, soil shall be sloped, benched or retained in accordance with an approved design methodology.

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. It shall not be permitted for a slab and plumbing to be liftedas an assembly to create the voidspace unless the under-floor space is a crawlspace with access to allow inspection of plumbing after lifting.

Exception: It shall be permitted for the piping, fittings, hangers, and supports below the slab or below the framing to be in contact with structural elements of the foundation that are designed to resist the effects of expansive soil swelling and shrinking in accordance with Section 1808.6.1 of the International Building Code.

Organic materials subject to decay shall not be used for hangers, supports and soil retention systems. Materials subject to corrosion shall not be used for hangers, supports and soil retention systems unless protected in an approved manner.

Where plumbing transitions to a buried condition beyond the perimeter of the foundation, an adequately flexible expansion joint shall be provided in the plumbing system to accommodate the effects of expansive soil swelling and shrinking.

+ Plumbing can be easily inspected and adjusted before a slab pour (which is not true with non-isolated void systems around plumbing)

+ Plumbing can be easily inspected, repaired and replaced over time (which is not true with non-isolated void systems around plumbing) once utility trenches are accessed through an exterior gradebeam.

+ Cluster under-slab plumbing, having multiple points of connection to site utiltities, so as to reduce thedepth of excavation and reduce the total linear feet of plumbing under the building. While this may increase the total linear feet of utilities outside of the building, plumbing outside of the building will not be fighting the resistance of the isolated slab and it is often much more economical to repair utilities outside of the building if routine maintenance is required.

+ As part of the foundation system on deep foundation elements, provide vaults, outside of the building, to house plumbing flexible expansion joints to coordinate the connection between site utilities and under-slab utilities without interfering with the pour schedule for the building and while allowing routine inspection and maintenance of flexible expansion joints as the soil-supported end rises and falls with the soil elevation.

+ Provide an initial offset in the plumbing flexible expansion joint that is the sum of the offset necessary for minimum code slope and the offset necessary to overcome the potential vertical movement for the soil at that depth. Where possible, provide pvc flexible expansion joints instead of epoxy-coated cast iron, lowering the elevation of the plumbing flexible expansion joint to reduce the pvm expected so that pvc flexible expansion joints can be used. Coordinate the elevations with structural and civil engineers.

+ Discuss preliminary layouts of plumbing with a pre-installed isolation system detailer for discussion of the most economical layout before 100% Design Development, so that the other engineering disciplines can accommodate for their design responsibilities and cost estimators can account for all expected costs early.

Where Isolated Slabs are designed by Structural Engineers:

+ Require the plumbing, hangers and supports below the slab not be in contact with the subgrade and not be in contact with any assemblage of materials that is in contact with the subgrade, specifically prohibiting buried plumbing and non-isolated void systems (e.g. specifically identifying any common non-isolated void products that are prohibited, to avoid confusion during bidding). Regardless of the applicable building code, plumbing designers can specify protection of plumbing comply with 2024 IPC Section 305.8.2 as a project requirement, because this language was added to make enforcement of basic protection provisions (such as IPC Section 305.2 in all editions) easier for plumbing inspectors.

+ Permit both post-installed isolated plumbing (e.g. any of the various, non-proprietary methods of installing plumbing in an underfloor space such as under a slab-on-crawlspace even under a slab-on-voidwork) and pre-installed isolated plumbing (explicitly prohibiting the contractor infringe on any intellectual property rights related to any construction methods used). Allow the Contractor to determine which method (postinstalled vs. pre-installed) is most cost-effective given the project schedule. This specification approach is not
limited to a sole-source supplier and should therefore suitable on both public and private projects.

At the perimeter of Isolated Slabs designed by Structural Engineers:

+ To connect to site utilities, specify a “horizontal” flexible expansion joint with an initial vertical offset between the ends that is sufficient to allow upward PVM of the soil-supported site utiliites and still have sufficient slope to drain over the length of the flexible expansion joint as required by the applicable plumbing code for new plumbing, with the flexible expansion joint installed in a structurally engineered vault that is part of the foundation system but outside of the slab perimeter, with a pipe clamp mounted to the foundation at the slab perimeter and a reinforcing pipe collar where the utilities penetrate a sliding soil retainer that allows the plumbing to slide up and down a vertically slotted hole in the vault. Explicitly prohibit the contractor from infringing on any intellectual property rights related to any construction methods used so
as to avoid a claim by an inventor that you were inducing the Contractor to infringe.

1. If you suspect that expansive soil is present because of local history, ask the
Registered Design Professional in Responsible Charge or “RDPiRC” (e.g. Architect) which foundation areas, if any, are likely to have isolated slabs. Preliminary layouts of plumbing should provide economical isolated plumbing under those areas.

2. Ask the RDPiRC if a Geotechnical Investigation has been or will be performed for the project. If so, obtain a copy and read the report to determine if expansive soil is present, before a set of construction documents is issued. If additional information is needed, such as recommended Design Vertical Movement or maximum vertical to horizontal ratio for subgrade cut clearances, request the information.

3. Ask the Structural Engineer to identify in plan view any and all foundation areas which will have isolated slabs, before a set of construction documents is issued.

4. Under isolated slabs, specify isolated plumbing and provide flow line elevations at critical locations for bidding coordination between trades (e.g. earthwork under the building, voidwork, plumbing, site utilities and concrete subcontractors). At the perimeter of isolated slabs, specify isolated plumbing transitions. Under nonisolated slabs, specify buried plumbing with utility trench backfill in accordance with Geotechnical Engineering recommendations. Coordinate specifications and details with the Architect, Structural Engineer, Civil Engineer and Geotechnical Engineer.