Three-Dimensional Finite Element Modeling of Pressure Injected Footing Foundations

Pressure injected footings (PIFs), or Franki piles, were a popular foundation option 50 years ago and are similar to rammed aggregate piers of today. Installation methods varied, but PIF creation generally involved ramming of a bulb of compacted concrete inside of a pipe to a certain depth. Once the desired depth was achieved, a “bulb” was often created by expelling the concrete plug and ramming additional concrete through the pipe. Besides creating a larger bearing area, the compaction effort involved in bulb creation densified the soil below. After bulb creation, the pipe was withdrawn as concrete was compacted in successive lifts to the surface. In some instances, the casing remained and in others, a rebar cage was installed, and normal slump concrete was then poured to the surface. Though these types of foundation elements are generally no longer used, they are often found in older buildings whose foundations are being reused for building repurposing and/or renovation. However, a methodology to account for their contribution to foundation capacity is lacking. Neglecting their support is often too conservative and not warranted; thus, a proper understanding of their behavior and how they interact with new foundation elements, such as micropiles, is critical for a safe, economic, and effective design. In this paper, a methodology is presented for the three-dimensional finite element (FE) modeling of PIFs and the compacted soil below the bulb. A model is presented and calibrated to a full-scale load test. Engineers can use the methodology presented in this paper to derive an understanding of the contribution of PIFs to foundation support and their interaction with new foundation elements during design.