Internal instability is a form of internal erosion in broadly-graded cohesionless soils in which fine particles can be eroded at lower hydraulic gradients than predicted by classical theory for piping or heave. A key mechanism enabling internal instability is the formation of a stress-transmitting matrix dominated by the coarse particles that leaves the finer particles under lower effective stress. In this study discrete element modeling is used to analyze the fabric and effective stress distribution within idealized gap-graded samples with varying potential for internal stability. The reduction in stress within the finer fraction of the materials is directly quantified from grain-scale data. The particle size distribution, percentage finer fraction and relative density are found to influence the stress distribution. In particular, effective stress transfer within a critical finer fraction between 24% and 35% is shown to be highly sensitive to relative density.

Internal instability describes the loss of soil particles as a result of seepage. Two criteria are commonly used to determine the potential for such instability within a granular soil that is subject to seepage flow. The criteria are similar, in that they both require an evaluation of the slope of the grain-size distribution curve. However, the manner in which the evaluation is made yields a subtle difference between the respective methods. Comparison of the methods reveals that the difference can have implications for evaluation of broadly graded soils, and guidance is suggested for their use with greater confidence in engineering practice.

Abstract. The extreme hydrologic response of gravelly, sandy soils in the Carnation Creek watershed is examined from observations at 12 standpipe piezometers. The nearly continuous piezometric data are reported as a time series of monthly maximum readings. Ten locations of measurement appear to exhibit an upper limit to the pore water pressure head that is independent of rainfall intensity and duration. Two locations exhibit artesian pressures that appear directly influenced by rainfall characteristics and may last for several hours. We found the impact of individual storms to be highly variable. The spatial variation in hydrologic response is attributed to the influence of preferential flow paths in the soil matrix.

An empirical model is presented for analysis of debris flows based on field observations of landslides from clear-cuts in the Queen Charlotte Islands, British Columbia. Given an initial failure volume, changes in event magnitude arising from entrainment and deposition along the path of movement are used to establish the point at which the cumulative flow volume diminishes to zero, and therefore the total travel distance. Hillslope morphology is used to assign three types of flow behaviour: unconfined, confined, and transition flow. Flow behaviour and slope angle determine the occurrence of entrainment or deposition in the model, for every reach of the event path. Volume change in a reach is calculated from regression analysis of the Queen Charlotte Islands field observations. Predictor variables are reach length, width (of entrainment and (or) deposition), and slope angle, together with a bend-angle function and the incoming flow volume.Key words: slope stability, debris flow, travel distance, runout, risk, clear-cut.