House Foundation Repair Systems
If you are considering having foundation repair or stabilization work done to your house or small commercial building because of damage caused by expansive clay soil, be wary of the different methods advertised and promoted. Many are potentially inadequate for resisting any remaining uplift forces and swell potential in the clay soil or future cycles of shrinkage and swelling. Here are some comments concerning the different foundation repair options. Note that "stabilization" is not the same as "re-leveling" or "repair".
Note that no information on this website is intended to provide legal advice. We are providing information solely for educational purposes for a homeowner, Builder, commercial property owner, Structural Engineer, or Architect.
Methods of foundation stabilization or repair include:
- Pressed concrete piles (a more accurate term would be pushed concrete cylinders)
- Pressed concrete piles with steel cable tie
- Drilled concrete piers (straight shaft or belled depending on geology)
- Pushed steel pipe piles
- Helical soil anchors
Warning: An important piece of fine print common in the foundation repair company contract is that their system is intended only to resist future settlement, NOT HEAVE. Which means if there is any remaining swell potential in the clay soil under the structure, and you did not have the slab turned into a structurally supported element over a void space (via jacking and/or hand excavation) subsequent damage can still occur and you will not have any recourse of warranty work with the repair contractor. The contractor is not trying to fool anyone, this is simply the nature of the soil conditions and foundation behavior.
It is possible to design and construct a foundation repair system that will also resist localized heave, but such as system needs to be "engineered" (i.e. by a Geotechnical Engineer and a Structural Engineer). You can contact us for assistance in this matter. We can refer you to several local Structural Engineers who provide design services in our local market.
Pressed Concrete Cylinders (aka "concrete pilings", "pressed pilings", or "pressed piles"):
Pressed piles have been a common method of attempted foundation repair because of their relatively low cost. This system is essentially the hydraulic pushing of concrete cylinders into the ground. The cylinders at best will be tied together with a cable running through a hollow cored hole up and down the center alignment of the cylinders, or spliced reinforcing steel bars will be installed to help prevent lateral displacement.
In his 2006 PhD thesis at the University of Texas at Arlington, W. Thomas Witherspoon noted the following in his research of different foundation repair elements:
"[...] depth will vary significantly at a single house. [...] at the corners of a structure the available driving pressure will be much less [than along an edge or in the center]. This reduced pressure is found because the house mass and soil contact is half that of a location along a long wall. Therefore, driving depths will normally be much less at a corner than along a wall length."
It is not unusual for pressed piles to encounter push refusal at a more shallow depth at the corners, therefore pressed piles commonly have inconsistent foundation embedment and this translated to inconsistent uplift resistance and inconsistent foundation performance. During his research, Mr. Witherspoon also noted half of the pressed piles tested broke during installation or load testing, possibly due to inconsistent soil shrinkage away from the pressed pile cylinders that creates a loss of lateral support along part of the cylinder pile system and possible shear force and bending moment that exceed capacity and cause failure. Imperfections in a concrete cylinder can also lead to weakness and failure of that particular element and the loss of load transfer between cylinders.
Pressed concrete cylinders are typically not installed deep enough and will not provide uplift resistance if the surrounding clay swells, potentially pushing up the cylinder column and slab along with the clay mass. Even a deep cylinder column may come apart from swelling clay soils if the cable lock system is inadequate. This type of design (or lack thereof) is a violation of the IBC (code) Section 1808.6.1 Pressed cylinders are pushed to "refusal", but unfortunately here in Central Texas a hard and dry clay, when it is at its maximum swell potential, can cause refusal.
A typical pressed pile detail is provided below.
This image is of a drilled shaft for foundation repair. Note the concrete cylinders adjacent to the shaft, these are "pressed piles" or what we prefer to call "pushed cylinders". That system failed so another foundation repair company that does not use pressed pilings was called to improve the foundation system using drilled concrete piers. There are countless examples of homeowners that spent money on pressed piles with poor results, we hope you do not make the same mistake. Another image of the top of a pressed cylinder system is shown below, this is not a foundation element as defined by the building code.
Drilled Concrete Piers:
This is our preferred foundation improvement element. Properly designed and constructed drilled piers will prevent future settlement AND provide some uplift resistance to swelling clay soil for the pier element itself (i.e. using adequately deep straight shaft piers or using a belled pier), reducing the risk of the slab element being pushed up by new foundation elements. If the slab we re-leveled (jacked upwards), a void space was likely created under the slab and beam near the pier, providing some accommodation for future swell that would otherwise affect the nearby slab. An example of a belled pier is presented below.
There is currently only one foundation repair company (contractor) in the Austin region that we can recommend based on having performed significant quality control inspections (pre-pour inspections) of their field crews' work and that is Capital Foundations of Austin, Texas (no relation to Capital Geotechnical Services PLLC). Their website link is below.
Capital Foundations of Austin, Texas
There might be other repair contractors that might provide suitable installations but we are not aware of the quality of their work until we perform a series of inspections of their work product.
Segmental Steel Pipe Piles:
Another common foundation repair method uses hydraulically pushed steel pipe segments. There are a wide variety of differences in systems used by different companies, from the brackets to the pipes to the installation method. A very simplified conceptual detail is shown below. The pipe connections do no have notable pull-out strength therefore if clay expands around one pipe segment relative to an underlying segment, this system could come apart if the spicing is not long enough and lead to lateral displacement, vertical heave, or vertical shrinkage settlement. The pipe can typically be pushed deeper than concrete cylinders, but refusal can still be encountered at too shallow a depth on very dry and hard clay or a rock cobble or gravelly layer within clay. A shallow element is useless for preventing future shrinkage settlement or swelling heave. This system can be considered but its limitations must be clearly expressed to the homeowner. From a geotechnical engineering perspective, we do not recommend this type of system for long term stability unless drilled piers are not feasible (i.e. significant groundwater presence and/or collapsible soil presence).
Helical Soil Anchors:
Helical soil anchors are steel elements that are screwed into the soil and provide downward and upward force capacity through the interaction between the helical plates and soil. Helical soil anchors can provide adequate downward capacity and uplift capacity if properly installed and embedded (i.e. long anchors, not short ones), but they can also suffer the same limitations as other methods if installed too shallow due to refusal. This type of system can be considered but must be properly engineered (i.e. input from a Structural Engineer and from a Geotechnical Engineer for your site specific conditions).
Micropiles are common in limited access areas and although a variety of designs exist they basically consist of a rotary drilled hole (6 to 10 inches in diameter) that is either pressure grouted or gravity grouted with a large steel reinforcing bar to provide tensile strength. The element will provide most of its capacity from skin friction. This type of system can be considered but must be property engineered (i.e. input from a Structural Engineer and from a Geotechnical Engineer).
BOTTOM LINE: Foundation repair or improvement systems should be designed for the unique subsurface conditions of a site. A system intended to be a stabilizing system (and not just an improvement system) must be designed by a Structural Engineer and a Geotechnical Engineer who will characterize the subsurface conditions. The engineers can describe the limitations of a particular system selected so that there are no misunderstandings with regards to long term performance expectations. A system intended to simply improve the foundation system and limit settlement can be installed without structural engineering design and oversight, but the homeowner or commercial building owner must recognize and accept the difference in expectations.