Abstract
The generation and development of excess pore water pressure directly affects the grain interaction in debris flow, which can significantly reduce the friction strength and promote the movement of debris flow. It has been found that coarse grains favor the increase in excess pore water pressure, but the effect due to grain configuration is missing in studies. In order to study the influence of grain configuration, field investigations and laboratory tests were carried out for two typical cases, i.e., flow with coarse grains evenly mixed (case I) and flow with coarse grains floating on the surface (case II). The results show that case II generates much higher excess pore water pressure than case I. The variation of relative excess pore water pressure (Ur) with time (t) satisfies the power function relationship: Ur = mt–n. Case II often has a smaller n value, meaning a low dissipation rate of excess pore water pressure. This study is helpful for a better understanding of granular effects in debris flow.
Highlights
The wide-graded grain composition plays a decisive role in the formation and movement of debris flow
The experiments indicate that the overlying coarse-grained layer facilitates the generation of high excess pore pressure in high-density debris flows, where the pressure can’t dissipate readily and the grains in the underlying layer shows a semi-fluid property, which helps promote the overall migration of grains and reduce the placement and deposition of debris along the flow track (Guthrie et al, 2010)
It has been widely recognized that fine grains can increase debrisflow mobility through a “ball-bearing-like effect” (Hsü, 1975; Iverson, 1997; Brewster, 2004; Roche et al, 2005; Phillips et al, 2006; Iverson et al, 2010; Roche, 2012), and our experiments reveal the acceleration effect due to coarse grains
Summary
The wide-graded grain composition (from 10−6 m clay to 1 m gravel) plays a decisive role in the formation and movement of debris flow. Under certain initial velocity conditions, the high-density, wide-gradation, and coarse-grain separation characteristics of debris flows will have acceleration / deceleration effects on it through changes in excess pore water pressure. This experiment shows that when the content of fine grains is large, the pore water pressure dissipates slowly, corresponding to a small n value. The experiments indicate that the overlying coarse-grained layer facilitates the generation of high excess pore pressure in high-density debris flows, where the pressure can’t dissipate readily and the grains in the underlying layer shows a semi-fluid property, which helps promote the overall migration of grains and reduce the placement and deposition of debris along the flow track (Guthrie et al, 2010)
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