a) What the Code Says
The International Building Code (IBC), again, the code required to be used in San Antonio, Austin, San Marcos, Houston, Dallas, etc and many other cities in Texas, says the following about foundations on expansive soils (i.e. soils which expand and contract due to changes in moisture content… which is pretty much all Clays that cause most of the foundation problems and foundation repair problems in Texas). Below is a portion of the code which we’ll discuss in this Part 1.
1808.6 Design for expansive soils. Foundations for buildings and structures founded on expansive soils shall be designed in accordance with Section 1808.6.1 or 1808.6.2.
Exception: Foundation design need not comply with Section 1808.6.1 or 1808.6.2 where one of the following conditions is satisfied:
1. The soil is removed in accordance with Section 1808.6.3; or
2. The building official approves stabilization of the soil in accordance with Section 1808.6.4.
We won’t go into the exceptions from above quoted paragraph from the IBC until later (The exceptions talk about how to improve the soil, not on the foundation installed itself… more on that later, or just go to Foundation Repair Non-compliance Part 2), but for now what needs to be understood is that the code has only two options for designing a foundation on expansive soils (If you live in Austin and especially in San Antonio and Dallas, many if not the majority of residential foundations are supported on expansive soils). The two options for foundation designs are either through following the code section 1808.6.1 or 1808.6.2. Section 1808.6.2 talks specifically about design methods for a slab on grade. In our case, we want to know about what to do if you have a pile or pier Foundation Repair system, so the first paragraph of the code that let us know what to consider is 1808.6.1
1808.6.1 Foundations. Foundations placed on or within the active zone of expansive soils shall be designed to resist differential volume changes and to prevent structural damage to the supported structure. Deflection and racking of the supported structure shall be limited to that which will not interfere with the usability and serviceability of the structure.
Foundations placed below where volume change occurs or below expansive soil shall comply with the following provisions:
1. Foundations extending into or penetrating expansive soils shall be designed to prevent uplift of the supported structure.
2. Foundations penetrating expansive soils shall be designed to resist forces exerted on the foundation due to soil volume changes or shall be isolated from the expansive soil.
So, the number one requirement for a foundation (and foundation repair system) on expansive soils is that the foundation needs to prevent uplift of the supported structure. Uplift on piers is due to swelling soils or rising water table through skin friction on the piers. In other words, if the water table changes or if the soils swell, then the piers (or any foundation) will be pushed up. So the first requirement of the building code is that this pushing up (heaving or uplift) of the foundation needs to be prevented.
The second requirement for a foundation (and foundation repair system) on expansive soils is similar to the first requirement, except it covers movement in all directions (including settlement due to soil volume changes).
In other words, The first paragraph regarding foundations, and hence a foundation repair (a foundation repair implies installing a new foundation or making the old foundation meet the code), is that if you install a pier or pile foundation repair system, then those piers or piles cannot move up or down (uplift or settle) due to the swelling or shrinking of the soils from changes in moisture changes.