INFLUENCE OF MAZAMA ASH ON THE GEOTECHNICAL PROPERTIES OF SOIL FROM A RESIDENTIAL DEVELOPMENT IN CHENEY, WA

We tested soil from the Harvest Bluff residential development in Cheney, WA. Harvest Bluff sits at the boundary between loess hills to the north and the Four Lakes Ice Age flood path to the south. We also tested Mt. Mazama volcanic ash, which is locally abundant in the Spokane-Cheney region, to assess the impact of volcanic ash on soil properties. We determined optimal water content for compaction (ASTM D698), California Bearing Ratio (ASTM D1883-14), unconfined compressive strength (ASTM D2166-85), particle size distribution, and Atterberg limits for Harvest Bluff (HB) soil and Mazama ash. According to the Unified Soil Classification System (USCS), Harvest Bluff soil is classified as a low liquid limit silt, and Mazama ash is classified as a non-plastic silt. Optimal water content for compaction of Harvest Bluff soil is 16% with a max dry unit weight of 101 pcf. The CBR test reveals penetrative resistance of soil as well as suitability for use beneath roads, runways, and landing pads. The CBR value for HB soil is 28%, which indicates suitability for excellent subgrade, good base, and good subbase. Unconfined compressive strength testing reveals the behavior of soil under loads and its failure point, defined as ultimate strength. Compacted HB soil has an ultimate strength of 5486 psf. Pure Mazama ash has an optimal water content of 41%, CBR value of 30.4%, and maximum strength of 5347 psf at 35% water content. A mixture of 50% Harvest Bluff and 50% Mazama ash by weight has an optimal water content of 31%, CBR value of 20.8%, and maximum strength of 4692 psf at 25% water content. Addition of Mazama ash to Harvest Bluff soil increased the optimal water content for compaction, decreased the maximum dry unit weight and decreased the CBR value while maintaining suitability standards. Addition of Mazama ash decreased ultimate strength at lower water contents while increasing strengths at higher water contents. We hypothesize that Mazama ash, where present locally, will have similar impacts on other soils undergoing development.