“Piping in connection with a plumbing system shall be so installed that piping or connections will not be subject to undue strains or stresses, and provisions shall be made for expansion, contraction, and structural settlement.”

Some of the relevant “Basic Principles” in this document indicate that protection of plumbing from expansive soil is a health, safety and welfare issue.

Principle No. 18: Where a plumbing drainage system may be subjected to backflow of sewage, suitable provision shall be made to prevent its overflow in the building.

Principle No. 22: Sewage or other waste from a plumbing system which may be deleterious to surface or subsurface waters shall not be discharged into the ground or into any waterway unless it has first been rendered innocuous through subjection to some acceptable form of treatment.

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.

IAPMO rejected the proposed change because they believe it is clear that UPC already requires that plumbing engineers design/specify plumbing so as to protect plumbing from expansive soil, and those engineers would be at fault even if someone were to argue that the code is not sufficiently clear.

“I’m going to reject this proposal on a few reasons. Number one, I think the code is already pretty clear in requirements to support and protect piping. And, if the soil is no good to support the piping, they have to provide alternative methods of supporting the piping, not just the soil. I’ve personally installed many piping systems underground that are hanging from the slab on grade because of conditions similar to this. Secondly, the engineering of a system, the design of a system, is based in part on the plumbing code, not in whole on the plumbing code.”

“I’m really aware of this problem. I have first-hand knowledge of it in areas up in the Mission District of San Francisco. They have these issues. It is expensive to repair. But, here’s what my problem is. As a plumber, I rely on the soils engineers or I rely on the mechanical plumbing engineers to design these systems and it kind of bothers me when it’s said, “Well, the owner doesn’t want to do it because it’s too expensive so we’ll throw it on the plumber.” When I make a recommendation in plumbing that an owner do something and I think there’s a significant liability risk and they turn me down, I walk away from the project.”

Cracking pipes in walls at wall penetrations (e.g. wall cleanouts).
Cracking walls at wall penetrations.
Lifting walls and ceilings up at wall penetrations.
Reversing slope in lateral ss pipes, in walls and/or above ceilings.
Lifting toilets.
Cracking pipes below slab if the sleeve does not allow enough slip.

At floor drains, upward movement can lift floor drains up above the floor so they no longer are able to drain the floor, this upward movement can cause damage to flooring and, if a P-Trap cracks due to the stresses, the P-Trap can drain out and allow noxious odors to enter the occupied space, while the plumbing crack allows untreated sewage to enter the subgrade which can also cause additional heave that can progressively worsen problems.

Design of non-isolated void systems to protect plumbing often requires a specialized understanding of geotechnical, structural and mechanical engineering and their interactions. The specifying professional (typically the Mechanical Engineer of Record) should ensure that they have necessary education and experience or that they hire those that do to assist in the design.

Furthermore, the following illustrations show how many Non-Isolated Void Systems can impart loads onto non-plumbing systems, such as foundations, flooring, walls, ceilings, etc… When these systems are specified, regardless of whether or not the intent is to actually protect the plumbing or not, the Specifying Professional (typically the Mechanical EOR) should provide the locations and magnitudes of any such loads to other relevant design professionals (Structural EOR, Architect of Record, Civil EOR, etc…) so that they can design for said loads.

If a design professional specifies a product that can be used to protect plumbing but does not specify what is necessary for the product to protect the plumbing, the design professional should not expect that the product will protect the plumbing. There are design professionals who specify products like this simply because their understanding is that an Owner believes a product may perform better than other approaches, without any expectation that the system will protect the plumbing.

If, instead, a design professional actually intends to protect plumbing from expansive soil by using a non-isolated void system, there are often many features that must be specified. This can be a performance specification (e.g. design for 20,000 psf swell pressure, 9” design vertical movement up and down, etc…) and/or hard specification (e.g. three layers of 3/4” thick plastic side boards, ¼” thick washer, 9” clearance, etc…)

Some product manufacturers do not engineer their products for each project. And, product engineers often limit their responsibility.

Vertical sheets of expanded metal lath mesh in compression act similar to a classic spring when in compression. As in physics, F = kx. The compressive force equals the spring coefficient times the distance compressed. This does not change what was discussed in debunking the knife edge theory. It simply means that the resistance capacity of a mesh system is going to be a function of the distance compressed.

The bearing area of vertical sheets of expanded metal lath can increase as the mesh is compressed.

The bearing area of vertical sheets of expanded metal lath can also increase as soil swells in between mesh elements.

The effective bearing area of vertical fluted plastic retainer boards, with plastic webs that connect two planes of plastic sheets, can be as large as a solid sheet with a thickness equal to the gap between the two planes if soil bridges across the planes by “arching action”, where an upside-down compression arch forms in the soil.

A similar phenomenon is seen in much larger foundations: open-end driven piles. In some cases a soil “plug” forms in the bottom of the pile, significantly increasing resistance to pile driving.

In classic geotechnical engineering, three specific types of lateral soil pressures are often addressed:

Active (sliding at a downward angle into a cantilevered retaining wall)
At-Rest (sliding at a downward angle into a “basement” wall)
Passive (soil resisting the sliding of a retaining wall)

In addition, hydrostatic pore water pressures can occur where the soil is not drained (e.g. weep holes in a retaining wall).

Vertical surcharge loading nearby such as compaction equipment loads can create horizontal pressures too.

With expansive soil, there is an additional type of lateral pressure caused by soil swelling, which is a three-dimensional phenomenon. It is often difficult for geotechnical engineers to be able to estimate these pressures. The potential horizontal expansive soil swelling movement can be greater than the potential vertical movement and the horizontal expansive soil swelling swell pressure can be greater than the vertical swell pressure.

2021 IBC Section 1610.1 excerpt (Previous editions similar):

“Foundation walls and retaining walls shall be designed to resist lateral soil loads from adjacent soil. Soil loads specified in Table 1610.1 shall be used as the minimum design lateral soil loads unless determined otherwise by a geotechnical investigation in accordance with Section 1803. Foundation walls and other walls in which horizontal movement is restricted at the top shall be designed for at-rest pressure. Retaining walls free to move and rotate at the top shall be permitted to be designed for active pressure.

Lateral pressure from surcharge loads shall be added to the lateral soil load. Lateral pressure shall be increased if expansive soils are present at the site. Foundation walls shall be designed to support the weight of the full hydrostatic pressure of undrained backfill unless a drainage system is installed in accordance with Sections 1805.4.2 and 1805.4.3.”

2021 IBC Section 1610.1 excerpt (previous editions similar):

It is important to note that Table 1610.1 Lateral Soil Load indicates that inorganic clays of high plasticity are “unsuitable as backfill material”. These types of soils and other soil types also “unsuitable as backfill” are common in many areas where slab-on-voidwork foundations are used.

2021 IBC Chapter 16 STRUCTRUAL DESIGN (Previous editions are similar):

H = “Load due to lateral earth pressures, ground water pressure or pressure of bulk materials.”

Load Resistance Factor Design (LRFD) Load Combinations:

Load Factor of 1.6 for lateral earth pressure

Allowable Stress Design (ASD) Load Combinations:

Load Factor of 1.0 for lateral earth pressure

If an engineer were to assume a soil arch forms and reduces the lateral pressure on a soil-retainer that prevents soil from damaging plumbing, it would be important for the engineer to verify that the arch will function under gravity. If the arched shape is compromised, the lateral arch can fail and cause all lateral loads to push on the soil retaining system. There needs to be sufficient vertical resistance at the top and bottom of the lateral arch to prevent the lateral arch from spreading open; where soil retainers are near the subgrade, this may be problematic. There needs to be sufficient horizontal resistance; therefore, horizontal components that keep soil retainers apart must
be able to act as horizontal columns resisting
this load without buckling. As the arch itself
consists of soil and lateral swelling pressures
can be significant, the modulus of elasticity of
the soil (where it compresses under load) may
not be sufficient to maintain the arch shape if wet.
And, as the arch itself consists of material that
is expanding as the lateral swelling occurs, the
very expansion of the arch itself may push itself
inward, causing the arch shape to collapse.

In addition to overcoming the skin-friction resistance of any soil in contact with the threaded rod, for a threaded support rod to not buckle and allow a soil-retention system (that also functions as the support system for the nut during installation to support the plumbing before the slab is poured) to slide upward during soil swelling, the nut on top of the support system will need to either push through the support system in a punching-shear failure or need to deform the support system so that the nut can pass through the support system.

As an example, the IBC requires that sheet metal washers comply with AISI S100 “North American Specification for the Design of Cold-formed Steel Structural Members”. AISI S100 has provisions that provide the DESIGN strength permitted; however, the applicable value for the design of a threaded rod would be the ULTIMATE capacity, multiplying the design strength by an appropriate increase factor to account for the coefficient of variation in the statistical data used in determining the ultimate capacity.