Unconfined Compressive Strength of Some Streambank Soils with Herbaceous Roots

Streambank stability requires that bank strength exceed destabilizing forces, including compressive forces from sources such as trampling and foot traffic. This study was conducted to determine the unconfined compressive strength associated with herbaceously rooted streambanks, and whether greater compressive strength is developed from greater root‐length density (length of roots per volume of sample) or specific plant communities. The unconfined compressive strength, root‐length density, plant community, and engineering soil characteristics were measured on 122 samples from six meadow stream reaches. Sample compressive strength emulated an elastic condition in highly rooted samples, and developed negligible compressive strength in samples containing very small amounts of roots. Additionally, compressive strength was found to increase nonlinearly with increases in very fine root‐length density (herbaceous roots <0.5 mm in diam.), and reached an apparent asymptote at 50 kPa. The greatest compressive strength was derived from samples dominated by Nebraska sedge ( Carex nebrascensis Dewey). Correlations between unconfined compressive strength and engineering soil properties were mostly insignificant, primarily because samples were derived from unconsolidated alluvium. Stream morphology parameters showed little correlation to compressive strength, root‐length density, or other independent variables, suggesting other unmeasured factors may have an equal or greater influence on stream morphology. Herbaceous roots appear to supply most of the compressive strength and soil stability found in meadow streambanks, especially those dominated by Nebraska sedge.