Seismic Response of Peaty Organic Soils As a Levee Foundation Material

The dynamic response of peaty organic soils is not well understood but its poor competency as an engineering material is well known. Its use as a foundation material is generally avoided through careful selection of project location, or removal and replacement with more suitable material. In certain situations peaty organic soils are unavoidable because of prior land use and subsequent development of infrastructure. In a delta environment levees are often found to be overlying peaty organic deposits, the California Bay Delta and New Orleans levee systems are two important examples. The Bay Delta is in a seismically active area and dynamic response of peaty organics underlying these levees is a concern. This research develops a first-order estimate of the large strain dynamic response of peaty organic soils, with the goal of improving system reliability estimates of the Bay Delta levee network. Suites of cyclic triaxial tests on manufactured and sampled peaty organic soil specimens have been carried out, the results of which are used to inform finite element modeling of the levees subjected to seismic loading. We use a finite element program with existing constitutive models to provide deformation estimates of typical levee cross-sections in the Bay Delta. The deformation estimates are in turn used to improve the system reliability estimates of the Bay Delta levees. A full understanding of the dynamic response of peaty organics will require more testing and the development of more specific constitutive models; however for the goal of improving system reliability estimates this first-order approach is adequate within the bounds of all other contributing uncertainties.