ENGINEERING PROPERTIES OF MIMA MOUND SOILS FROM TURNBULL NATIONAL WILDLIFE REFUGE, EASTERN WASHINGTON

Mima mounds are enigmatic, low-lying hills that occur in fields in eastern Washington and other locations around the world. The goal of this study was to (1) determine engineering properties of Mima mound soils using ASTM standard testing methods and (2) determine whether these properties were consistent with any of the extant origin hypotheses for Mima mounds. The Mima mounds from which samples were collected for testing lie in Turnbull National Wildlife Refuge near Cheney, Washington. Results show that Mima mound soils are predominantly fine sand, silt, and weathered basalt gravel. Gravel is absent from local Palouse soil, which consists of sand, silt, and clay. Calcium carbonate accumulation in Palouse soil suggests that it is significantly older than Mima mound soil. Plasticity indices indicate that both soils have low quantities of inactive clays. Optimal water content for compressive strength occurred for both soils at 10% water content. Consolidation testing demonstrated that Mima mound soil exhibits a much looser structure than Palouse soil. These characteristics of Mima mound soils may be most consistent with the seismic hypothesis for Mima mound formation.