▪ Abstract Field observations, laboratory experiments, and theoretical analyses indicate that landslides mobilize to form debris flows by three processes: (a) widespread Coulomb failure within a sloping soil, rock, or sediment mass, (b) partial or complete liquefaction of the mass by high pore-fluid pressures, and (c) conversion of landslide translational energy to internal vibrational energy (i.e. granular temperature). These processes can operate independently, but in many circumstances they appear to operate simultaneously and synergistically. Early work on debris-flow mobilization described a similar interplay of processes but relied on mechanical models in which debris behavior was assumed to be fixed and governed by a Bingham or Bagnold rheology. In contrast, this review emphasizes models in which debris behavior evolves in response to changing pore pressures and granular temperatures. One-dimensional infinite-slope models provide insight by quantifying how pore pressures and granular temperatures c...

Debris-flow mobilization from landslides

Published by the American Geophysical Union as part of the Water Resources Monograph Series, Volume 18. Landslides are a constant in shaping our landscape. Whether by large episodic, or smaller chronic, mass movements, our mountains, hills, valleys, rivers, and streams bear evidence of change from landslides. Combined with anthropogenic factors, especially the development and settlement of unstable terrain, landslides (as natural processes) have become natural disasters. This book charts our understanding of landslide processes, prediction methods, and related land use issues. How and where do landslides initiate? What are the human and economic consequences? What hazard assessment and prediction methods are available, and how well do they work? How does land use, from timber harvesting and road building to urban and industrial development, affect landslide distribution in time and space? And what is the effect of land use and climate change on landslides? [Book Synopsis]

Landslides: Processes, Prediction, and Land Use

Movements and failure of cuts and natural slopes constitute an important geotechnical problem that involves a variety of geomaterials in a variety of geological and climatic contexts, and which has a major socio-economic impact in many countries. The paper reviews the state of knowledge in this domain, examining the basic aspects of soil mechanics that are relevant to the context of slopes, the importance of water on slope behaviour, and then the different stages of slope movements: pre-failure, failure, post-failure and reactivation. Finally it is shown how the geotechnical characterisation of slope movements can be used, in particular for assessing risk associated with such movements. Emphasis is put on for the brittleness of soils and its practical implications for the progressive failure developing at the pre-failure stage and on the characteristics of post-failure movements. The influence of other factors such as creep, fatigue, destructuration, partial saturation and infiltration is also considered.

https://www.icevirtuallibrary.com/doi/pdf/10.1680/geot.2001.51.3.197

Natural slopes and cuts: movement and failure mechanisms

Rainfall-induced debris flows are flow failures in residual and colluvial soils initiated by a reduction of confining stress as a result of pore-water pressure rise during or following periods of intense rainfall. To establish appropriate stability analysis procedures, the behavior of soil from a debris flow source area is investigated. Constant-shear-drained triaxial tests, tests that mimic the field stress path, and anisotropically consolidated undrained triaxial tests performed at the in-situ stress level indicate that undrained loading is a prerequisite for flow failure. Mechanisms of stress transfer are proposed to explain how the initially drained deformation that occurs along the field stress path can lead to undrained mobilization. Because a debris flow involves both drained initiation and undrained mobilization, a complete stability analysis includes an effective stress analysis with constant-shear-drained parameters to determine the potential for failure initiation, and an undrained strength analysis using undrained residual strength to determine the potential for debris flow mobilization.

Analysis of Rainfall-Induced Debris Flows

Since the 1980s, several field studies of pipeflow hydrology have been conducted in forested, steep headwater catchments. However, adequate information is lacking with regard to questions as to how representative these previous studies are and how widespread the phenomena might be. Thus, the aim of this study is to review some studies of pipeflow hydrology on forested steep hillslopes. Several points were clarified. (1) The maximum discharge of pipeflow was mainly determined by the soil pipe diameter. When the condition of both the soil pipe diameter and the slope gradient in forest soil were similar to those in peaty podzol, the maximum discharge of pipeflow on forest slopes was slighter than that in peaty podzol. (2) Pipeflow was delivered from a variety of sources, and the contributing area of pipeflow extended as the soil layer became wetter. Therefore, the dominant contributor (new water and old water) was varied and the contribution of pipeflow to streamflow increased with the increase of rainfall magnitude. (3) The roles of pipeflow in the storm runoff generation processes demonstrated two roles: the concentration of water and the rapid drainage to downslopes. Thus, soil pipes can increase the peak discharge from the hillslope and decrease the peak lag time of the storm hydrograph, when compared to the unpiped hillslope. (4) The roles of pipeflow on hydrological process suggested that the soil pipes contribute to the slope stability by increasing the rate of soil drainage and limiting the development of perched groundwater conditions. However, if the rate of water concentration to the soil pipe network is in excess of the pipeflow transmission capacity, the cavity of the soil pipe could readily be filled with water during a rain event, increasing pore water pressure in the surrounding matrix. In this case, the soil pipe induced slope instability. (5) Moreover, pipe erosion has significant impact on the runoff characteristics of pipeflow, since pipe erosion contributed to a change in the limited drainage capacity of soil pipe. Thus, pipe erosion plays an important role in shallow landslide initiation. Copyright © 2001 John Wiley & Sons, Ltd.

Effects of pipeflow on hydrological process and its relation to landslide: a review of pipeflow studies in forested headwater catchments

Three experiments were performed on natural slopes to investigate variations of soil pore-water pressure during induced slope failure. Two sites in the Wasatch Range, Utah, and one site in the San Dimas Experimental Forest of southern California were forced to fail by artificial subsurface irrigation. The sites were instrumented with electronic piezometers and displacement meters to record induced pore pressures and movements of the slopes during failure. Piezometer records show a consistent trend of increasing pressure during the early stages of infiltration and abrupt decreases in pressure from 5 to 50 minutes before failure. Displacement meters failed to register the amount of movement, due to location and ineffectual coupling of meter pins to soil. Observations during the experiments indicate that fractures and macropores controlled the flow of water through the slope and that both water-flow paths and permeability within the slopes were not constant in space or time but changed continually during the course of the experiments.

Wade G. Wells

Papers

Pore pressure response during failure in soils