The effect of inÆltration on the stability of the slopes of a dry ash dump

Abstract

The effects of prolonged periods of infiltration on the stability of the slopes of a dry ash dump are examined. The ash, which is placed by stackers, comes to rest at slope angles that are steeper than those predicted by conventional slope stability analyses using effective stress strength parameters. It is shown that the additional strength required to ensure stability of the ash dump slopes is provided by matric suction developed in the unsaturated ash. Infiltration of rainfall into the slopes can reduce the suction sufficiently to trigger shallow slope failures. Two methods for analysing this problem, an approximate method based on statistical rainfall records together with a simplified approach to simulating the rate of infiltration into a slope, and a more rigorous finite element approach, are discussed. Although the approximate method is more conservative than the finite element method, the results are similar and illustrate the importance of accurately quantifying the contribution of matric suction to the shear strength of an unsaturated soil when considering slope stability problems. The relevance of antecedent infiltration on the susceptibility of a slope to rainfallinduced instabilities is highlighted, and a strategy to take account of this factor is suggested. Nous examinons l'effet de pluies prolongées sur la stabilité des pentes d'un terril de cendre sèche. La cendre, qui est amenée par les engins emplileurs, se dépose en formant des déclivités plus abruptes que celles prévues dans les analyses de stabilité de pente conventionnelles utilisant des paramètres effectifs de resistance à la contrainte. Nous montrons que la résistance additionnelle nécessaire pour assurer la stabilité des pentes d'un terril de cendre est donnée par la succion matrique développée dans la cendre partiellement saturée. L'infiltration des pluies dans les pentes peut suffisamment réduire la succion pour déclencher des ruptures dans les pentes peu profondes. Nous examinons deux méthodes pour analyser ce probl&graveme: une méthode approximative basée sur les statistiques de pluviosité, employée avec une simulation simple du taux d'infiltration dans une pente; et une méthode d'éléments finis, plus rigoureuse. Bien que la méthode approximative soit plus conservatrice que la méthode d'élément finis, les résultats sont similaires et illustrent l'importance de bien quantifier l'influence de la succion matrique sur la résistance au cisaillement d'un sol partiellement saturé en ce qui concerne les problèmes de stabilité de pente. Nous montrons que les taux de pluviosité passés ont une grande influence sur la propension des pentes à se déstabiliser et nous suggérons une stratégie permettant de prendre en compte ce facteur.

Effect of Rainfall Intensity on Infiltration into Partly Saturated Slopes

“…It is used extensively by hydrologists and geotechnical engineers, and has been extended to predict the infiltration response due to steady rainfall Mein and Larson (1973), and unsteady Chu (1978) rainfall events, and has been applied to the assessment of infiltration in slopes by Pradel and Raad (1993), Fourie et al (1999) and Cho and Lee (2002). The two layer model assumes that a wetting front of downward percolating water results in the formation of a perched water table beneath the ponded water (see Fig.…”

“…Pradel and Raad (1993) and Fourie et al (1999) show that the stability of unsaturated slopes is critically influenced by the depth of wetting front and the suction values at this depth. The wetting front depth and suction are influenced by the total infiltration of water (I c ) into an unsaturated soil.…”

“…For this reason, the proportion of the total rainfall that results in infiltration and run-off changes continuously during a rainfall event. The process of the reduction of in-situ suction, results in a decrease in the effective stress (and therefore strength) in the near surface soils, and may result in slope failure (Fourie et al 1999). Whilst geotechnical engineers are therefore exclusively concerned with predicting the amount of infiltration that will result under a given rainfall intensity, hydrologists are primarily interested in the amount of run-off for catchment flooding studies.…”

Consideration of Minimum Safety Factors for Cut-slope Infiltration Analysis

“…and Rahardjo, 1995;Park et al, 1995;Spierenburg and van Esch, 1995;Ng and Shi, 1998;Fourier et al, 1999). …”

Effect of fine grain content on unsaturated shear strength of embankment material

“…Generally, decrement of the effective stress, that is, the decrement of shear strength on the potential failure surface is caused by the increase of the pore water pressure and the seepage induced by rainfall (Sidle and Swanston 1982), and it triggers the collapse of the embankment (Rahardjo and Leong 1997). In particular, because the embankment material over water level in river is under unsaturated condition, it should be considered that the decrease of the suction due to the seepage or the rise of the water level at rainfall affects the shear strength of embank material (Fourie et al 1999). In addition, the embankment material is comprised of gravel, sand, silt, and clay, the fine grain content (F c , under 75μm through sieve #200) has a very important role for the shear strength of embankment material.…”