Identifying Rigid Inclusion Design and Construction Risks for Support of Shallow Foundations in New England

Rigid inclusions (RI), a form of ground improvement, has become accepted in the United States as a cost-effective alternative to support low- to mid-rise buildings, parking garages, tanks, retaining walls, and embankments, especially in areas with marginal, soft, and compressible ground. RI systems are commonly implemented as a cost-effective alternate to deep foundations for the growing warehouse, distribution, and data center markets. Originally developed by specialty contractors in Europe, this system of support was imported to the US in the early 2000s. For the purposes of this paper RI are defined as unreinforced grout or concrete columns. These systems are typically procured through a design build process and though similar in appearance, the drilling equipment, installation methods, inclusion material, and design methodology, are often proprietary and specific to each specialty contractor. As with other methods of ground improvement, the lack of Building Code regulation, misunderstanding of the technology and poor-quality project specifications can lead to variability in performance, project team expectations, quality control, and verification testing. This paper addresses the authors’ local industry approach of RI systems as support for buildings in New England, specifically in the presence of soft compressible soils such as soft organic or clay deposits. This paper will discuss sites where RI can be considered and will highlight areas of design risk including load transfer layers (i.e., aggregate pads that are used to separate the inclusions from the supported structural foundations), bearing capacity, element structural capacity, settlement of the system, and interactions with building foundations and slabs. Construction risks may be mitigated with thoughtful pre-mobilization planning between Owners, Consultants and Specialty Subcontractors, and robust quality control procedures to address project specific risks (including load testing of elements). This paper will distinctly identify design and construction considerations that should be considered by the engineer and contractor as the use of this technology continues to grow and evolve.