ARS | Publication request: Slope Failure Mechanisms Due to Seepage: Three-Dimensional Soil Block Experiments

Submitted to: Proceedings of the American Society of Agricultural and Biological Engineers International (ASABE)
Publication Type: Proceedings
Publication Acceptance Date: June 25, 2008
Publication Date: July 7, 2008
Citation: Chu-Agor, M., Cancienne, R., Fox, G.A., Wilson, G.V. 2008. Slope Failure Mechanisms Due to Seepage: Three-Dimensional Soil Block Experiments. Proceedings of the American Society of Agricultural and Biological Engineers International (ASABE). St. Joseph, MI. CD-ROM

Interpretive Summary: Erosion by seepage plays an important role in failure of steambanks and gully formation. However, although seepage erosion affects streambanks in all three directions, previous research used soil profiles that represented two directions to analyze for the hydraulic and soil property controls on loss of bank stability. In this research, a soil block (50 cm by 50 cm by 50 cm) was constructed to investigate the three-dimensional nature of seepage undercutting and to derive an improved sediment transport equation. Two soil types (sand and loamy sand) were utilized in these three-dimensional experiments with varying inflow water levels (15 cm, 25 cm, and 35 cm). Each soil type was packed at bulk densities of 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. The front face of the soil block was shaved back to bank angles of 90o, 75o, and 60o such that the horizontal centerline for each bank was 20 cm from the water inlet. As water flowed through the soil block, a laser scanner was used to obtain the volume of the eroded bank at approximately 15 to 30 s intervals. The hydraulic conditions producing seepage failure 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.

Technical 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.