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Section 1802.2 of the 2006 IBC specifies where a foundation and soils investigation (hereinafter referred to as a soils report) is required to be submitted to the building official. There are some specific cases set forth in the IBC where soils reports are required, such as in the presence of questionable soil, expansive soil, and a high groundwater table. These triggers are irrespective of the seismic design category (SDC). What may come as a surprise to the code user is that even when these specific situations do not exist, the IBC requires a soils report for a structure assigned to SDC C, D, E, or F. And there is a laundry list of issues that need to be addressed in the soils report, including slope stability, liquefaction, and surface rupture. In SDC D and higher, the lateral pressure on basement and retaining walls due to earthquake motions needs to be determined. This requirement for a soils report can very much work to the engineer’s and owner’s advantage, because the soil type is conclusively determined, which may mean a more favorable SDC assigned to the structure.

What may come as even more of a surprise is that the exception for a soils report found in Section 1802.2, which reads as follows, is not applicable to structures assigned to SDCs D through F:

Exception: The building official need not require a foundation or soils investigation where satisfactory data from adjacent areas is available that demonstrates an investigation is not necessary for any of the conditions in Sections 1802.2.1 through 1802.2.6.

A code user may not initially recognize that the exception does not apply to structures assigned to SDCs D through F. However, upon close reading of the exception, one notices that it only references Sections 1802.2.1 through 1802.2.6, and it is Section 1802.2.7 that requires a soils report for any structure assigned to SDC D, E, or F. This mandatory requirement of the IBC for a soils report for any structure assigned to SDC D, E, or F will result in a major change in California practice where the UBC has not automatically required a soils report based on seismic zone.

Answers to FAQs:

Q: My structure has been assigned to SDC D. Section 1802.2.7 mandates quite an extensive list of requirements for the soils report. Are there any applicable exceptions? The exception for an investigation in Section 1802.2 does not apply to Section 1802.2.7. How does the exception at the end of Section 1802.2.7 work?

A: There are no exceptions to the requirement for a soils report for an SDC D, E, or F structure. The exception at the end of Section 1802.2.7 applies only to the site-specific study requirement for the purpose of determining the peak ground acceleration. The peak ground acceleration needs to be known by the geotechnical engineer to evaluate the potential for liquefaction and soil strength loss. The exception allows an estimate of SDS/2.5 to be used to represent the peak ground acceleration.

Q: Can you cite any references on the seismic design of retaining walls?

A: Following is a list of references for the seismic design analysis of "yielding" retaining walls (walls that can move sufficiently to develop minimum active earth pressures), such as that illustrated in Figure 1806. These references (and others) and their application are discussed in Section 7.5 of the 2003 NEHRP Commentary.

Nadim, F., and R.V. Whitman. 1984. "Coupled Sliding and Tilting of Gravity Retaining Walls During Earthquakes." Proceedings of the Eighth World Conference on Earthquake Engineering, pp. 477-484.

Seed, H.B., and R.V. Whitman. 1970. "Design of Earth Retaining Structures for Dynamic Loads," Proceedings, ASCE Specialty Conference on Lateral Stresses in the Ground and Design of Earth-Retaining Structures, pp. 103-147. Ithaca, New York: Cornell University.

Siddarthan, R.S. Ara, and G. Norris. 1992. "Simple Rigid Plastic Model for Seismic Tilting of Rigid Walls," Journal of Structural Engineering, ASCE, pp. 469-487.

Steedman, R.S., and X. Zeng. 1996. "Rotation of Large Gravity Retaining Walls on Rigid Foundations Under Seismic Loading." Analysis and Design of Retaining Walls Against Earthquakes, ASCE Geotechnical Special Publication 60, edited by S. Prakash, pp. 38-56. New York: ASCE.

Following is a reference for the seismic design analysis of "nonyielding" retaining walls (walls that cannot move sufficiently to develop minimum active earth pressures), such as a basement retaining wall.

Wood, J.H. 1973. Earthquake-Induced Soil Pressures on Structures, Report EERL 73-05. Pasadena: California Institute of Technology.

S.K. Ghosh Associates Inc., is a structural, seismic, and code consulting firm located in Palatine, Ill., and Laguna Niguel, Calif. President S.K. Ghosh, Ph.D., and Susan Dowty, S.E., are active in the development and interpretation of national structural code provisions. They can be contacted at skghosh@aol.com and dowtyskga@cox.net, respectively, or at www.skghoshassociates.com.