SLOPE STABILITY OF PEORIA LOESS ON THE MOLINE UPLANDS, QUAD CITIES

Augustana College is situated in a large, forested ravine system, which dissects the loess-covered Moline Upland areas of the Quad Cities. North-northeast drainage is towards the Mississippi River. Hillslopes are composed primarily of Peoria Loess, which experiences ongoing soil creep, characterized by pistol-grip tree trunks, leaning retaining walls, offset sidewalks, etc. Occasional slumping occurs as well. In an effort to characterize the slope stability on campus and to understand better the controlling characteristics, two sites were studied and compared: a northeast-facing and southwest-facing slope. These two slopes are located on Augustana College’s property and threaten several buildings and pathways within the campus. Slope angles range from seven to thirty degrees. Forty soil samples (twenty for each slope) were taken from the slopes and tested in the laboratory for wet and dry bulk densities, shear stress, and shear strain values. In-situ shear stress measurements were made in pits using a vane shear device. Measurements reveal that the northeast-facing slope has greater shear strength values than the southwest-facing slope. The southwest-facing slope revealed a shear strength value of 24.5 N/ cm² at the surface of the loess, while the northeast-facing slope revealed a shear strength value of 31.9 N/cm². A great deal of difference could be seen in shear strength values measured at depths of one foot. The southwest-facing slope had shear strength value of 17.2 N/cm² at the bottom of the pit, with side strengths of 12.3 N/cm² and 17.2 N/cm². The northeast-facing slope had shear strength values of 41.7 N/cm² at the bottom of the pit, with side strengths of 31.9 N/cm² and 29.4 N/cm². A factor-of-safety map was created, based on field measurements as well as lab measurements in order to depict areas prone to failure. The southwest-facing slope experiences warmer and wetter weather conditions during winter months, which probably results in greater weathering and reduced strength.