Hydraulic Conductivity and Compressibility of Organic Clays in New Orleans

Navid H. Jafari, Civil and Environmental Engineering, Louisiana State University, Baton Rouge, LA, 70803, njafari@lsu.edu

Evaluating the field-scale hydraulic conductivity and compressibility of soft organic clays that dominate fluvial-deltaic deposits is important for determining migration of contaminants and magnitude of subsidence. For example, the large concentration of chemical industry and population along the U.S. Gulf Coast results in these clay formations hosting a large number of chemical spills, superfund sites, and waste disposal facilities. Because the organic clays also overlie coastal aquifer systems, long-term groundwater withdrawals can result in a significant consolidation of aquitards and ultimately manifest as land subsidence at the ground surface. Urban and agricultural regions affected by groundwater induced subsidence of thick normally consolidated organic layers include the San Jacinto Basin and San Joaquin Valley, California; Houston-Galveston, Texas; and coastal Louisiana (Baton Rouge and New Orleans).

Hydraulic conductivity can be scale-dependent., i.e., there is an overall increase in hydraulic conductivity as one moves from laboratory scale (tested volume is 10-5 to 10-3 m3) to borehole test scale (10-3 to 101 m3) and large pumping test scale (101 m3 or more). Neuzil (1986) describes the many challenges faced to obtain estimates of hydraulic conductivity in low hydraulic conductivity formations, e.g., extrapolating small-scale and short-time period tests to larger dimension and long periods of time. As a result, limited data is available on the field-scale hydraulic conductivity and compressibility of organic clay layers. To quantify the scale effects of hydraulic conductivity, this presentation uses the comprehensive field and laboratory testing program performed at the Inner Harbor Navigational Canal (IHNC) floodwalls in New Orleans, Louisiana. Because field pump tests are rarely performed in fine-grained strata, this presentation also describes the equipment and analysis procedures of four field pump tests, provides a comparison between field and laboratory hydraulic conductivity of organic clays, and discusses the spatial variability of hydraulic conductivity along the IHNC.