DECLARATION 1: PLAGIARISM
2. CHAPTER 2: LITERATURE REVIEW
2.9 Case Studies
2.9.3 Case Study: FEM analysis and dimensioning of a sinkhole overbridging system
The case study identified that the design guides and analytical procedures currently present were inaccurate and only applicable under certain circumstances. Due to this numerical procedures were employed in order to investigate the behaviour of a cement stabilised railway bed with a layer of geosynthetic reinforcement present on the LeipZig-Halle line. The high- speed rail link LeipZig-Halle travels at up to 300km/h, facilitating the transport of both passengers and heavy cargo. The 800m long and 120m wide railway section was constructed over a post-mining area in the Groebers region of Germany. The whole area was prone to subsidence with sinkhole diameters of less than 4m occurring. The collapse in the area was
attributed to “sub-ground excavations” in karstic regions. In order to reinforce the soil embankment against collapse various methods were considered (e.g. deep foundation on piles, stiff reinforced concrete plate, and dynamic compaction after deep excavation). The methods generally employed in this region are spanning the void with concrete slabs or backfilling.
These methods of reinforcement were not sufficiently cost-effective, due to the vast area that the rail-line covers. Geosynthetic reinforcement was a more economical method and was used for the project. High strength basal reinforcement was used extensively in Great Britain for the similar purpose of bridging voids. The design was based on two fundamental concepts, ensuring serviceability for a given period of time after a void forms and a sinkhole detection warning system. In the event of a sinkhole taking place, the speed of the trains of the line were limited until the void can be filled and the fill rehabilitated so that the stability of the line will not be compromised.
Figure 2.13 above shows the embankment considered, with the basal reinforcement placed above the potential sinkhole
2.9.3.1 Soil reinforcement:
The investigation used a combination of cemented soils, high strength geosynthetic reinforcement and a warning system. A cement stabilized bearing layer made from cohesive soil, a mixture of cement and soil and a thin layer of gravel was used above the geosynthetic Figure 2.13 Cross section of the embankment including the stabilized layer, the placement of the geosynthetic reinforcement and sinkhole detection system.
reinforcement layer. An electronic warning system was integrated into the geosynthetic reinforcement; this consisted of insulated and embedded wires. Rupture of the geosynthetic results in a change in the electrical resistance of the wires. The systems design life spans 60 years, with a serviceability limit of a month to rehabilitate the fill after the void forms.
The specialized method developed within the project is the use of the CSBL (Cement stabilized bearing layer). When collapse occurs, a stable arch must form in the CSBL layer to ensure serviceability until permanent reinforcement can be placed. In order to ensure serviceability, the train speed was decreased from 300km/h to 100km/h (and the sinkhole must then be backfilled).
The design limitations identified were as follows:
• Due to the high speed of the trains on the line, the specified serviceability life of 30 days was too high
• Analytical methods provide a guide for design when non-cohesive soils are present, even then the guides are not sufficiently accurate. No guidance at all exists for a bridging system designed in cohesive soils
• The position of the sinkhole cannot be predefined in this 120m wide track (spans about 8 tracks)
Subsequent to these limitations, the design cannot be performed without a numerical analysis, the FEM technique.
2.9.3.2 Experimental procedure:
The two design conditions considered were:
a) Low coverage, where the overburden height of soil was small hence the geosynthetic deflection was equal to the soil surface deflection.
b) In the second case the maximum allowable tension of the geotextile was reached.
The analysis of the design factors, stress, strain, deflections, arching in the cement stabilised layer and the diameter of the resulting surface depression were all estimated by a combination of the finite element method and analytical methods. The design calculations for the ultimate
and serviceability deflection level of the geotextile was performed using the German codes for geotechnical structures.
2.9.3.3 Finite element method:
FEM was performed on the PLAXIS software, the FEM process was found to be inefficient in the post-failure analysis. In order to gain a better understanding of post-failure stress and strains, analytical procedures were used. The model used was a linear elastic perfect-plastic Mohr- Coulomb model.
The finite element method was performed successfully but it was found that the FEM- calculations would be insufficient for the complexity of the problem. This problem was also experienced in Jaros et al. (2009) and Munian (2010) where soil collapse caused convergence problems within the finite element model.
2.9.3.4 Geosynthetic:
The analysis of the geosynthetic reinforcement was simplified in that it took into account the membrane effect of the reinforcement alone (any occurrence of soil arching was localized to the cement stabilised layer). The membrane effect is generally used when considering geotextiles and only takes into account the vertical loading/surcharge forces and not the upward reaction of the soil. A sufficient cover distance was required in the geotextile so that the pull- out resistance of the soil can be calculated. The geogrid was modelled as a linear elastic liner.
The ultimate tensile strength of the geosynthetic was between 1200 – 1400 kN/m with low creep capacity. One of the limitations in the geosynthetic modelling procedure was the post failure analysis of the geosynthetic. The tensile modulus J was measured in kN/m (J =force, kN/m / strain). Alexiew et al. 2002 stated that “theoretically the reinforcement can strain infinitely mobilizing a never ending force and can never fail, which is not correct”. According to Alexiew et al. 2002 this makes it difficult to test the reinforcement to failure, because the tensile force within the reinforcement must be monitored after each simulation to determine whether it exceeds the ultimate tensile strength of the reinforcement.
2.9.3.5 Case study conclusions:
The propagation of the sub-grade void can be successfully modelled using finite element analysis PLAXIS software, until the point where collapse occurs. In order to obtain more
accurate results for the post failure analysis, analytical methods were used. The analytical solution solved for the critical tension in the geotextile. The analysis of the cement stabilised layer is a relatively new concept and arching was expected in the highly cohesive soil block.
2.9.4 Case Study: Stability charts for predicting sinkholes in weakly cemented sand over