CORRELATION OF MINERALOGY AND INDEX PROPERTIES WITH FULLY-SOFTENED SHEAR STRENGTH OF THE DALLAS FLOODWAY SYSTEM

Laboratory testing was performed on high plasticity clays and clay shales from the Dallas Floodway System’s Trinity River levees within Dallas County, Texas. Index tests such as the liquid limit, plasticity index, and clay fraction were determined while mineralogical analyses were also performed via x-ray diffraction (XRD). The mineralogy for these samples is primarily quartz, calcite, micas, and kaolinites. These results were compared to the index tests for two soil types: alluvial clays and the Eagle Ford Shale. A mineral’s resistance to weathering was taken into account and the data compared in terms of stability.

Previous research has shown that weathering can reduce the strength of a soil. The drained strength available in weathered soils is referred to as the fully-softened shear strength (FSS). This strength was replicated in the lab through blenderizing and is lower than the undrained peak strength of the soil and is higher than the residual strength. The soils tested for the Dallas levees have experienced significant weathering which may yield a high consequence of failure. The procedures developed by the Engineer Research and Development Center (ERDC) to determine FSS of these soils were compared to the clay mineral content. An attempt was also made to correlate the clay mineral content to the design strength and ultimately predict the level of weathering.