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Gravity-Driven Consolidation of Granular Slurries--Implications for Debris-Flow Deposition and Deposit Characteristics
Jon J. Major
Fresh debris-flow deposits consolidate under their own weight. How quickly they consolidate (dissipate excess pore-fluid pressure and compact) affects their resistance to remobilization as well as their sedimentologic and stratigraphic characteristics. Here, analysis of small-volume (0.05 m3) noncohesive debris-flow slurries and larger (10 m3) experimental debris-flow deposits reveals the nature, rate, and magnitude of consolidation of typical debris-flow deposits.
A simple, linear, one-dimensional model describing the diffusion of excess pore-fluid pressure satisfactorily approximates the overall timing and magnitude of consolidation of noncohesive debris-flow deposits. The model and measurements of pore-fluid pressure demonstrate that changes in fluid pressure and effective stress evolve upward from the base of a deposit, and show that hydraulic diffusivities of muddy slurries containing about 5 to 50 wt% mud are remarkably similar, about 10-6-10-7 m2/s. By comparison, sandy-gravel debris-flow deposits containing <2 wt% mud have higher hydraulic diffusivities, 10-4 m2/s. Pore-fluid seepage across a permeable basal boundary accelerates consolidation response time in the lower stratum compared to that over a no-flow boundary. However, changes in sediment fabric resulting from porosity changes alter hydraulic properties of basal debris and retard expected decay of fluid pressure immediately above the bed. This result suggests that fluid infiltration to the substrate does not contribute significantly toward debris-flow deposition.
Low hydraulic diffusivities promote high and persistent pore-fluid pressure in debris flows, key factors enhancing mobilization. Elsewhere, pore-fluid pressures nearly sufficient to liquefy debris have been shown to persist through transit and deposition. Here, I show that significant dissipation of such fluid pressure is restricted to postdepositional consolidation. Therefore, neither uniform decay of excess pore-fluid pressure nor intrinsic viscoplastic yield strength explain debris-flow deposition. Instead, debris-flow deposition results from friction concentrated along flow margins where high pore-fluid pressures are absent. Sustained high pore-fluid pressure following deposition fosters deposit remobilization, which can mute or obliterate stratigraphic evidence for multiple events. A thick deposit of homogeneous, poorly sorted debris can result from mingling of soft deposits and recurrent surges rather than from a single flow wave if deposit consolidation time greatly exceeds typical sediment emplacement times.
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