In-situ strength tests of coarse, cohesionless debris on scree slopes

Debris strength is a key factor in the initiation of debris flows. Therefore debris strength must be measured to assess the initiation conditions of debris flows. It is difficult to test the strength of coarse debris in the laboratory, as large quantities of material are needed to eliminate single-particle effects. Therefore in-situ strength tests have been conducted on scree slopes in the southern French Alps to measure the strength of dry, coarse, matrixless debris. The test method consisted of bringing a debris mass into movement parallel to the slope surface on slopes at or near the critical slope angle. , coarse, matrixless debris is essentially cohesionless, its strength can be characterized by its internal friction angle. Mean kinetic internal friction angles vary from 360° to 387° for five debris types with mean stone sizes ranging from 33–50 mm. Stone size sorting is the most important cause of variations in kinetic internal friction angle. Stone shape also influences the kinetic internal friction angle, but it is less important. Stone size, stone shape sorting and rock type have no influence. However, rock type may indirectly influence kinetic internal friction angle through stone size sorting and stone shape.