Stability analysis of layered slopes in unsaturated soils

This study presents stability analyses of layered soil slopes in unsaturated conditions and uses a limit equilibrium method to determine the factor of safety involving suction stress of unsaturated soil.One-dimensional steady infiltration and evaporation conditions are considered in the stability analyses.An example of a two-layered slope in clay and silt is selected to verify the used method by comparing with the results of other methods.Parametric analyses are conducted to explore the influences of the matric suction on the stability of layered soil slopes.The obtained results show that larger suction stress provided in unsaturated clay dominates the stability of the layered slopes.Therefore,the location and thickness of the clay layer have significant influences on slope stability.As the water level decreases,the factor of safety reduces and then increases gradually in most cases.Infiltration/evaporation can obviously affect the stability of unsaturated layered slopes,but their influences depend on the soil property and thickness of the lower soil layer.

Integrated simulation and monitoring to analyze failure mechanism of the anti-dip layered slope with soft and hard rock interbedding

The significant difference between the mechanical properties of soft rock and hard rock results in the complexity of the failure mode of the anti-dip layered slope with soft and hard rock interbedding.In order to reveal the landslide mechanism,taking the north slope of Fushun West Open-pit Mine as an example,this paper analyzed the failure mechanism of different landslides with monitoring and field surveys,and simulated the evolution of landslides.The study indicated that when the green mudstone(hard rock)of the anti-dip slope contains siltized intercalations(soft rock),the existence of weak layers not only aggravates the toppling deformation of anti-dip layered slope with high dip,but also causes the shear failure of anti-dip layered slope with stable low dip.The shear failure including subsidence induced sliding and wedge failure mainly exists in the unloading zone of the slope.Its failure depth and failure time were far less than that of toppling failure.In terms of the development characteristics of deformation,toppling deformation has the long-term and progressive characteristics,but shear failure deformation has the abrupt and transient characteristics.This study has deepened the understanding of such slope landslide mechanism,and can provide reference for similar engineering.

Characteristics of pressure gradient force errors in a terrain-following coordinate

A terrain-following coordinate （a-coordinate） in which the computational form of pressure gradient force （PGF） is two-term （the so-called classic method） has significant PGF errors near steep terrain. Using the covariant equations of the a-coordinate to create a one-term PGF （the covariant method） can reduce the PGF errors. This study investigates the factors inducing the PGF errors of these two methods, through geometric analysis and idealized experiments. The geometric analysis first demonstrates that the terrain slope and the vertical pressure gradient can induce the PGF errors of the classic method, and then generalize the effect of the terrain slope to the effect of the slope of each vertical layer （φ）. More importantly, a new factor, the direction of PGF （a）, is proposed by the geometric analysis, and the effects of φ and a are quantified by tan φ.tan a. When tan φ.tan a is greater than 1/9 or smaller than -10/9, the two terms of PGF of the classic method are of the same order but opposite in sign, and then the PGF errors of the classic method are large. Finally, the effects of three factors on inducing the PGF errors of the classic method are validated by a series of idealized experiments using various terrain types and pressure fields. The experimental results also demonstrate that the PGF errors of the covariant method are affected little by the three factors.

Effect of rangeland rehabilitation on the herbaceous species composition and diversity in Suswa catchment,Narok County,Kenya

Introduction:Land degradation is a serious environmental problem of our time.In Kenya,it is estimated that 30%of the total land mass is severely degraded.Suswa catchment within Narok County is a good example with gullies of over 25 m deep and 30 m wide.In response to the increasing land degradation in the area,the Sustainable Land Management(SLM)project rehabilitated the catchment through establishment of soil and water conservation structures such as cutoff drains,semicircular bunds,and water retention ditches.Despite the various rehabilitation approaches carried out,little research has been done to ascertain their effect on successful vegetation recovery.This coupled with the fact that many restoration approaches have failed in East Africa and Kenya in particular gave drive to the study.The objective of this study was to determine the effects of rangeland rehabilitation on herbaceous species composition and diversity in a severely degraded rangeland.Methods:To assess the diversity of aboveground herbaceous layer in the rehabilitated and degraded areas along a slope(upper,middle,and lower),line transect and quadrat count methods were used.Within each slope position,three 100-m-long transects were placed across the hill parallel to one another 30 m apart using a tape.The species hit,the closest species to the hit,and hits on bare ground were recorded.Along the same transects,1-m2 quadrats were placed 25 m apart and aboveground biomass determined by the use of the destructive method.In the determination of species richness,diversity,relative abundance,percentage cover,and species composition,the Shannon-Wiener diversity index was used.Data collected on vegetation attributes was subjected to analysis of variance(ANOVA)using Genstat and Tukey’s HSD post hoc used in means separation where F values were significant.Results:The results showed that percent cover(74.67%),aboveground biomass(1459 kg/ha),relative abundance,richness,composition,and diversity of perennial grasses significantly(P≤0.05)increased downslope and were higher in the rehabilitated area than in the degraded area.On the contrary,forbs and annual grasses were significantly(P≤0.05)higher within the degraded area compared to the rehabilitated area and increased upslope.Conclusions:In general,herbaceous species diversity,species richness,relative abundance,percentcomposition,biomass production,and percent cover of perennial grasses significantly increased downslopeand were higher in the rehabilitated area compared to the degraded area.On the contrary,the sameattributes for forbs and annual grasses were higher in the degraded area and increased upslope.The studyconcluded that effective rangeland rehabilitation has the potential to enhance vegetation regeneration andhence forage productivity.Conclusions:ln general,herbaceous species diversity,species richness,relative abundance,percent composition,biomass production,and percent cover of perennial grasses significantly increased downslopeand were higher in the rehabilitated area compared to the degraded area On the contrary,the same attributes for forbs and annual grasses were higher in the degraded area and increased upslope.The studyconduded that effective rangeland rehabilitation has the potential to enhance vegetation regeneration andhence forage productivity.