Many of the fill slopes exist in Hong Kong are found to be sub-standard. Failure of these slope cause huge loss in both human lives and monetary values. Soil nails is proposed as an alternative method used for speeding up the upgrading program. However, concerns have been raised on the effects of strain softening behavior exhibited by granular materials on the effectiveness of the soil nails in stabilizing loose fill slopes. Most of the research in strain softening behavior is based on experimental results on clean sand and artificially recompacted silty sand. Although there are some experimental results on decomposed granite, which is one of the most commonly used fill materials, the soil behavior of it is still not fully understood....[ Read more ]

Many of the fill slopes exist in Hong Kong are found to be sub-standard. Failure of these slope cause huge loss in both human lives and monetary values. Soil nails is proposed as an alternative method used for speeding up the upgrading program. However, concerns have been raised on the effects of strain softening behavior exhibited by granular materials on the effectiveness of the soil nails in stabilizing loose fill slopes. Most of the research in strain softening behavior is based on experimental results on clean sand and artificially recompacted silty sand. Although there are some experimental results on decomposed granite, which is one of the most commonly used fill materials, the soil behavior of it is still not fully understood.

The objective of this study is to investigate the fundamental mechanical behavior of the loosely compacted decomposed granite and understanding the behavior of loose fill slope in centrifuge modeling. The influence of various factors on static liquefaction behavior was studied by conducting eight series of triaxial tests. This includes the water content at preparation, clays content, initial stress ratios and stress paths. A series of preliminary centrifuge tests were also conducted to study the behavior of unsaturated loose fill slopes at different water contents.

Static liquefaction was observed in isotropically consolidated undrained tests on loosely compacted decomposed granite. Liquefaction resistance was found to increase with the water content at preparation away from the optimum. This may be caused by the changes in the location and slope of the isotropic compression line and initial state line in e-log p' plane. Soils with similar fines content but with different amounts of clays exhibited large difference in liquefaction potential. This means that the fines content is not an adequate quantifier for describing the liquefaction potential of soils.

The ductility of the post peak soil behavior decreases as the initial stress ratio increased in the anisotropically consolidated undrained tests. Collapse behavior was observed in constant deviator stress drained tests, which mimic the field stress path of slopes under heavy rainfall. The mobilized friction angles at collapse are higher than that of the instability line determined from isotropically consolidated undrained tests but smaller than the critical state internal frication angle. The variation of the mobilized friction angle at collapse was found to be a function of initial stress ratio.

Based on the experimental data, it is hypothesized that a linear relationship may exist between the state parameter and the liquefaction potential of soils in isotropically consolidated undrained tests. A postulated method to estimate the liquefaction potential is proposed.

Two types of failure mechanisms were identified in the unsaturated centrifuge tests with different initial water contents. They are shallow failure and excessive settlement. The stability of the model slopes and the observed failure mechanism were influenced by soil suction and initial void ratio, which were affected by the initial water content. Based on the back analyses of the centrifuge model tests using the limit equilibrium method, the occurrence of static liquefaction in the loose fill slopes could not be ruled out.