My structure is small; does it require a Geotechnical Investigation?
Unlike its residential counterpart (IRC) the IBC makes no such distinction based upon the relative geometry or use of a structure, to whether it requires a geotechnical investigation (§ 1803.2). If a structure is deemed exempt from a geotechnical investigation, it is predicated upon a decision by the building official based upon satisfactory data, rather than the engineer’s preference (§ 1803.2, exception).
The satisfactory data referenced above is conditions-based, in that it must substantiate no concern for the conditions outlined in sections 1803.5.1 – 1803.5.6, 1803.5.10 and 1803.5.11. One example of a situation where the building official could potentially waive the geotechnical investigation would be a new structure on a site where there are previous project(s), whose geotechnical reports provide adequate information to understand the substrata prior to design.
The decision to design a structure without a geotechnical investigation is not permitted to be made by the design professional alone, apart from the residing building official. There must be sufficient data to demonstrate that the site conditions are adequately understood. Engineering experience alone is not a substitute for the information needed to determine whether the structure can be safely designed and its behavior reasonably predicted.
If the geotechnical investigation is waived by the building official, the IBC still provides limited presumptive soil parameters that may be used where appropriate. Section 1806 includes presumptive load-bearing values and other foundation design provisions intended for sites where sufficient geotechnical information is not otherwise available. These values are conservative, have specific limitations, and do not eliminate the requirement that the foundation be appropriate for the actual site conditions. They are not a substitute for a geotechnical investigation where one is required or where site conditions are uncertain.