COMPACTIVE EFFORT AND UNCONFINED COMPRESSIVE STRENGTH OF MT. MAZAMA ASH AND LATAH CREEK FLOODPLAIN SOIL IN EASTERN WASHINGTON

The eruption of Mt. Mazama nearly 7,700 years ago blanketed the Pacific Northwest in volcanic ash and resulted in the formation of Crater Lake in southwest Oregon. Remnants of this volcanic ash outcrop in the Spokane, WA area as localized channel and basin deposits that are, in some locations, as much as one to two meters in thickness. We tested Mt. Mazama ash from the Latah Creek area of Spokane (adjacent to the Latah Fault) for its geotechnical engineering properties according to ASTM standards. We determined the specific gravity, Atterberg limits, particle size distribution, and the optimum water content for compaction. We conducted unconfined compressive strength tests on compacted Mt. Mazama ash and Latah Creek soil according to ASTM Standards D698 and D2166. We then varied compactive effort and tested soil strength to determine the sensitivity of strength to compactive effort. When compacted to the ASTM standard, Mazama ash was significantly stronger than Latah Creek soil. At 60% of ASTM standard compactive effort, Mazama ash exhibited a 44% decrease in strength and Latah Creek soil exhibited a 35% decrease in strength. At 140% of ASTM standard compactive effort, Mazama ash was 30% stronger and Latah Creek soil was 28% stronger. Based on these results, the Mazama ash is more sensitive to compactive effort than the surrounding Latah Creek soils. Ash beds underlay areas undergoing development in Spokane, so understanding their engineering properties is vital to predicting the long-term stability of surface infrastructure.