Slope Failure Mechanisms due to Seepage: Three-Dimensional Soil Block Experiments

Abstract

Seepage erosion has been suggested to potentially play an important role in streambank failure and gully formation. However, although seepage erosion has three-dimensional characteristics, two-dimensional lysimeters were used in previous research to analyze for the hydraulic and geotechnical controls on this mechanism of instability. In this research, a three-dimensional soil block (50 cm by 50 cm by 50 cm with an inflow reservoir capable of generating water heads of 50 cm) was constructed to investigate the three-dimensional nature of seepage undercutting and to derive an improved sediment transport function. Two different soil types (sand and loamy sand) were utilized in these three-dimensional experiments with varying water inflow reservoir heads (15 cm, 25 cm, and 35 cm). Each soil type was packed at various bulk densities (1300, 1450 and 1600 kg/m3 for the sand and 1300, 1450, 1600 and 1700 kg/m3 for the loamy sand) to create a 25 cm tall bank. The bottom of the soil block was lined with densely packed clay to a height of 2.5 cm to serve as a restrictive layer and the rest of the block was packed with soil to the desired bulk density in 2.5 cm lifts. The soil was then cut to simulate various bank angles (90o, 75o, and 60o) such that the horizontal centerline for each bank remained 20 cm away from the water inlet. As water flowed through the column, a three-dimensional laser scanner was utilized to obtain the volume of the eroded bank at approximately 15 to 30 s intervals.The hydraulic conditions producing different seepage failure mechanisms were evaluated and the three-dimensional nature of seepage erosion was investigated. Information from these three-dimensional experiments can be used in a stability model for incorporating seepage undercutting as a failure mechanism.