Intermediate Foundations

February 29, 2024

We all know about shallow and deep foundations, but how about intermediate foundations? Given poor soil conditions and faced with excessive calculated settlements of planned structures, geotechnical engineers are forced to consider deep foundations or significant overexcavation and recompaction of on-site soil. Both of these options are expensive and are riddled with challenging logistical, environmental, and political issues.

Intermediate foundations can be an effective way of dispelling many of these issues. If your site requires an overexcavation and recompaction of on-site soil more than about 6 feet, or your site requires deep foundations bearing in bedrock or competent soil, there is good chance that intermediate foundations will be the best option. If so, you will reduce your grading and foundation preparation costs by using intermediate foundations.

Intermediate foundations consist of rammed aggregate piers that vary in depth depending upon the situation. They are often 10 to 15 feet long and 30 inches in diameter. The piers are installed by either drilling a hole and ramming it full with aggregate, or by “injecting” the aggregate into the ground and ramming it. The piers are installed in a grid pattern below individual foundation elements.

This process effectively increases the stiffness of the soil right where you need it and efficiently reduces the potential settlement of the foundation, including during an earthquake.

The need to overexcavate the entire building footprint or install driven pile foundations is eliminated if intermediate foundations work for your project. This means that the number of trucks coming to and from your site, the need to dispose of excavated poor soil and import good soil, the noise and vibrations associated with foundation installation, and the overall disruption of site activities are all minimized. These are not only cost benefits, they are also logistical, environmental and political benefits.

Of course intermediate foundations will not be the most efficient option in all situations. Here are some situations when intermediate foundations should definitely be considered:

Soft, compressible soils are present within the upper 6 or more feet;
Trucking and/or vibration annoyances during foundation construction are sensitive issues;
Overexcavation and shoring/underpinning are required adjacent to existing structures;
High ground water table conditions make even shallow overexcavation and foundation construction difficult;
Variable depths to bedrock or competent soil that cause high differential settlement of structures are present; and
Soils prone to earthquake-induced settlement are present within the upper 20 feet.

Intermediate foundations are not new in geotechnical engineering. The system as we know it today was developed by Geopier® in the early 1990’s and has been used in many infrastructure, commercial and educational facilities throughout the world. Twining has successfully teamed with Geopier® on several projects in the Southern California area, including projects at California State University Long Beach, University of California Irvine, and commercial projects within the city of Irvine.

If you believe your site may be appropriate for the use of intermediate foundation systems, feel free to contact us to discuss. If you already have geotechnical subsurface data available, with the cooperation of Geopier®, we will review your site at no cost and help you determine if intermediate foundations will work for your planned structures.