Further studies are needed to evaluate the field performance of MLCS and the appropriateness of methods for erosion estimation. 106 Figure 5.17 Variation in soil loss depth due to erosion over a period of 12 months 107 Figure 5.18 Groove profile of a section describing influence of vegetation and boundary 108 Figure 5.19 Observed versus estimated soil loss for surface layer of submitted pilot MLCS 1.1. program for evaluation of translational slope stability of MLCS 114 Figure 6.2 Schematic overview of multi-layered cover system (MLCS) 115 Figure 6.3 Simplified MLCS for stability analysis along the nth interface 116 Figure 6.4 Interactions of MLCS at the bottom of the passive wedge 1.157 Typical configuration of RCRA subtitle C cover system 118 Figure 6.6 Design curves for FoS of MLCS with change in slope 120 Figure 6.7 Design curves for FoS of MLCS with change in slope length 121 Figure 6.8 Design curves for FoS of MLCS with change in interface shear strength 122 Figure 6.9 Stability improvement of MLCS using of toe berm installation 123 Figure 6.10 Design curves for FoS of MLCS using toe berm of unit width at different verticals.
Introduction
- General
- Significance of the study
- Objective and scope of the study
- Organization of thesis
The main objective of the study is to evaluate the surface layer performance and overall stability of MLCS in controlled laboratory experiments and field studies. Determination of long-term hydraulic performance of soil-geotextile filters and drainage layers of MLCS.
Literature review
General
Multi-layer cover configuration
- Surface layer
- Biotic barrier layer
- Vegetation layer
- Geosynthetic filter
- Drainage layer
- Toe drain
- Geomembrane /geosynthetic clay liner (GCL) barrier
- Compacted clay barrier
- Foundation layer
During moments of high erosion, a layer of vegetation develops above the surface protection layer to protect the surface layer from rain and runoff induced erosion activity. A thick compacted low-permeability clay layer with a minimum thickness of 0.6 m, made of soil with a permeability of less than 1 x 10-9 m/s, was provided as a final protection against infiltrating water.
Different cover systems
The study indicated that the desiccation can be completely eradicated if the compacted mulch is covered with a geomembrane filled with natural soil. 2001), discussed the probability of deviation of coating performance in a long-term scenario due to increased environmental disturbances and degraded materials in coating system. The barrier layer is made of compacted clay soil rolled through geomembrane for better performance. 2012) examined the Ohio landfill problem prior to the establishment of a capping system.
Review of cover failures
The failure of the drainage layer, due to erosion caused by sudden rainfall, can be prevented by limiting the construction area of the drainage layer at a certain stage and sequential planning of the further area. The water pressure was created due to the failure of the drainage layer and the gas pressure due to the lack of a proper gas collection system.
Review on surface erosion
Increasing surface roughness resulted in increased leaching density and it was observed that leaching density has an inverse effect on soil loss. 2011) have attempted to establish the relationship between the amount of soil loss versus the length and width of the agricultural plot.
Review on slope stability analysis
Bouazza and Michel (2000) have explained the complexity in assessing the slope stability of household waste in Poland where uncertainty was due to the drastic change in waste characteristics, high void ratio resulting in high compressibility, change in properties with degradation , sudden changes in runoff or seasonally changing groundwater table. The graphs clearly explain the effects of slope angle, slope height and waste material properties on landfill lateral displacement.
Critical appraisal of the reviewed literature
Analytical methods proposed by some of the researchers were found to be relatively superior, which, however, require some modifications for assessing the durability of multilayer roofing systems. To summarize, the integrity and sustainable performance of multilayer roofing systems can only be determined by having appropriate individual components of the roofing systems and having proper coordination at their interfaces.
Evaluation of surface soil and its compaction state for MLCS
General
- Pin hole method
- Pin hole characteristics .1 Effect of soil type
- Effect of soil type
- Effect of compaction state
- Infiltration characteristics .1 Effect of soil type
- Effect of compaction state
The erosion rate versus shear stress variation of different soil types compacted at optimum standard Proctor compaction for jet erosion test (JET) is depicted in Figure 3.18. The shear stress versus erosion rate variation for different compaction states (in Figure 3.2) of soil S3 is shown in Figure 3.20.
Volumetric shrinkage test
- Effect of soil type
- Effect of compaction state
Klepee and Olson (1985) noted that increase in laboratory volumetric shrinkage will lead to increased field desiccation. The results showed an increase in shrinkage behavior with increase in water content for any given density.
Unconfined compressive strength test
- Unconfined compressive strength characteristics
- Effect of compaction state
The strength results of the nine densification states are presented next to the compaction curve as shown in Figure 3.30. It is observed that the soil strength decreases with both decreasing density and increasing water content, while the failure strain is the opposite, as shown in Figure 3.31.
Summary
The acceptable zone here is limited by the erosion index on the dry side of optimum water content, while the drying-induced shrinkage zone on the wet side of optimum. Taken together, the final acceptance zone suggests constructing the surface layer of the tire system at moderate water content near the OMC to higher densities to achieve better performance.
Hydraulic performance of filter and drainage layers
General
Hydraulic performance of geotextile filter layer
- Geotechnical centrifuge modelling of long-term permeability test
- Derivation of scaling laws
The main factor affecting the compatibility of the combination of soil and geotextile is the movement and clogging of soil particles in the pores of the geotextile. Six long-term permeability settings were constructed in-house, according to standard specifications available in the literature (Almeida et al. 1995).
Long-term flow behaviour at 1-g condition .1 Effect of soil type
- Effect of geotextile type
The final equilibrium permeability (keq) was found to be marginally higher than the initial permeability (kinitial) for all soil-geotextile combinations, as reported in Table 4.3. The Ipf is observed to vary from 0.5 to 5.9 for the soil-geotextile combinations considered in this study.
Centrifuge modeling of long-term flow test .1 Long-term flow results corresponding to 72-g
- Scale factors for permeability at equilibrium
Therefore, the centrifugal design scale factors for the equivalent permeability (SFk) of different soil-geotextile combinations were respectively estimated using Eq. From the table it can be understood that the scale factor for the equivalent permeability varied from 0.95 to 1.21 for different soil-geotextiles.
Hydraulic-mechanical performance of drainage layer
- Effect of displacement rate on shear characteristics
- Effect of particle size and relative density on shear characteristics
- Combined variation of shear strength and seepage characteristics
Therefore, this study attempts to understand the effect of particle size and relative density of selected aggregates on the shear strength and seepage properties. It can be observed that the permeability of aggregates decreased and the frictional drag angle increased as the relative density increased.
Summary
A'', compacted with a relative density of about 60%, can be used as a drainage layer in MLCS of low-level radioactive waste NSDF.
Field performance of surface layer of multi-layered cover system
General
- Construction of MLCS
- Vegetation measurements
- Infiltration monitoring
- Erosion monitoring
- Rainfall–runoff erosivity factor
- Soil erodibility factor
- Topographic factor ‘LS’
- Vegetation cover management factor ‘C’
- Support practice factor P
Mixing the soil in certain proportions with the desired water content is done as shown in Figure 5.4 (a) and Figure 5.4 (b). 2014). The 'C' factor within RUSLE is used to reflect the effect of planting and management practices on soil erosion rates and is the factor most often used to compare the relative impacts of management options in conservation plans.
Results and discussion .1 Vegetation characteristics
- Infiltration characteristics
- Erosion characteristics .1 Erosion rate behaviour
- Surface erosion profile
Increased vegetation also helped in the reduction of desiccation cracks (visual observation), which further led to decrease in infiltration as observed in Figure 5.15. The influence of vegetation on erosion rate (as described in Figure 5.16) can be demonstrated from reduced variation in erosion profile in Figure 5.17, for the months January to April 2017.
Soil loss estimation using RUSLE
Observed vs. estimated soil loss for the surface layer of the pilot MLCS deposited During the natural rainfall period from May-16 to Sep-16, the RUSLE model shows a higher rate of soil erosion compared to the observed rate of soil erosion. During the simulated period of rainfall intensity of the order of 100 mm/h from November to January, the RUSLE model shows a lower rate of soil erosion than the observed rate of soil loss.
Summary
In summary, RUSLE is observed to overestimate the rain-induced soil loss by approx. 30% in moments of high vegetation and underestimate by approx. 20% in high intensity rain. However, RUSLE fails to estimate temporal variation in erosion behavior, and can easily be used to evaluate annual soil loss with errors within acceptable range.
Translational slope stability analysis of MLCS
Preface
Suggestions for modifications in existing materials, and suitable materials to strengthen different layers, of MLCS are also reported in this study. The performance of MLCS under seepage caused by heavy rainfall and earthquake situations is further described.
Analytical model
In the comprehensive analysis, the factor of safety (FoS) is determined at all interfaces of MLCS by considering the changes in dimensions of both active and passive wedges that contribute to stability. There is no relative motion at the different parent or child interfaces during slippage, that is, the motion occurs only at the predefined fault interface. iii).
Analysis of critical FoS for cover system
Coarse sand can also be used as a drainage layer, but taking into account significantly heavy rainfall in the tropical region; aggregate is used for the drainage layer. The aggregates in the drainage layer can pierce the geomembrane placed underneath. Therefore, to avoid such a scenario, a thin layer of river sand is placed between them.
Effect of slope inclination
- Effect of interface friction angle (Use of improved materials)
- Influence of veneer reinforcement on stability of MLCS
Therefore, to improve the stability of the MLCS accordingly, it is proposed to build a finger shaft of uniform width inside the passive zone. A simple and most superior method of various improvement techniques is to provide additional veneer reinforcement such as geogrid, geocell, high-performance geotextile, or geomembrane to increase stability (Carroll and Curtis 1991; Bouazza and Gassner 2005; Yu and Bathurst 2016). An important factor in this method is the choice of the type of reinforcement according to the covering layer to be improved; improper selection would increase MLCS destabilization (Christopher 1991; Koerner 2013).
Factors leading to the reduction in slope stability of MLCS
- Upward movement of compacting vehicle
- Downward movement of compacting vehicle
- Seepage forces (horizontal submergence)
- Seismic forces
After calculating the amount of forces generated in different directions, the FoS of the MLCS is determined as the vehicle moves down the slope. As expected, it can be understood that as the earthquake severity increases, the FoS of different MLCS interlayers decreases.
Summary
Conclusions and future scope
- General
- Conclusions
- Major contributions
- Limitations of present study
- Future scope
Process and physical based soil loss assessment methods should be studied for effective assessment. The impact of climate change should be considered keeping in mind the design life of the MLCS.
R EFERENCES
Post-construction changes in the hydraulic properties of the water balance covers soil.” Journal of Geotechnical and Geoenvironmental engineering. Effect of Antecedent Rainfall Patterns on Rainfall-Induced Slope Failure.” Journal of Geotechnical and Geoenvironmental Engineering.
