Steel–Concrete–Steel Sandwich Immersed Tunnels For Large Spans

Steel–Concrete–Steel Sandwich Immersed Tunnels For Large Spans

Bekarlar, K.Z.

Jonkman, S.N. (mentor)
Bakker, K.J. (mentor)
Braam, C.R. (mentor)
't Hart, C.M.P. (mentor)

Civil Engineering and Geosciences

Hydraulic Engineering

Hydraulic Structures

2016-08-30

Traditional reinforced concrete tunnels show limits regarding large spans in the transverse direction. There was a lack of knowledge whether the steel-concrete-steel (SCS) sandwich tunnel can be a solution for tunnels with extreme large spans. Research was needed to understand the structural response of a large span SCS tunnel to the load applied. For the detailed analysis of the distribution of internal forces a finite element program was used. In order to compare a SCS sandwich tunnel for a large span with a reinforced tunnel, two base case tunnels were designed. From this comparison the critical span for each type was determined. It was seen that the reinforced tunnel critical span is 18 / 19 m, whereas the SCS tunnel could be designed for a span of 27 m (boundary condition reference project). In this thesis two 2-D models have been analysed in DIANA: one simplified model and a detailed model. Both models use linear-elastic material behaviour. The results of the hand calculations are first compared with that from the simplified model, to verify the simplified model. The results of both FEM models are compared and the differences are investigated. From the stress / strain analysis of the SCS tunnel cross section for a large span, it was seen that the tensile strength of the concrete was reached. This would result in the formation of tensile cracks. The allowable compressive stresses / strains were only locally exceeded. Concrete cracking and plasticity however may have impact on the degree of connection between the steel and concrete. Due to the cracks the shear stiffness of the steel and concrete connection can decrease. This may have impact on the overall stiffness of the structure. From the durability point of view these cracks have no impact on the durability of the structure since the concrete is situated in a confined space. Although the other side the exceedance of the stress is only locally, it might result in a redistribution of forces. By using a detailed FEM analysis, detailed insight was obtained in the distribution of internal design forces. This resulted in a significant reduction of the amount of steel applied. Namely 21 %. In absolute values, this is a reduction of 2,51 m3 of steel per meter in the axial direction. Since the reinforced concrete tunnel was not able to have a span up to 27 m, it was investigated, whether prestressing (post tensioning) the tunnel could be a solution. From the new design it was concluded that a span of 27 m is not a feasible solution when using prestressing. This is due to the large size of the prestress tendon anchors and the large axial forces which the concrete cross section could not resist. Further there was observed that a steel shell tunnel is a feasible solution for tunnels with large spans up to 28 m. From the costs analysis it was seen that steel shell tunnel variant 1 (regular steel shell) would cost 315 000 euros per meter length. Steel shell tunnel variant 2 (with steel cover plates on the inner side) is slightly more expensive than variant 1. The costs for this tunnel per meter length is 351 000 euros. The same analysis was performed for the SCS tunnel. This variant is with 421 000 euros per meter length, more expensive than the other two steel shell tunnel variants. It can be stated that in terms of costs, a reinforced concrete tunnel is the preferred solution for tunnels with a span up to 18 / 19 m. This is also the limiting span for a reinforced concrete tunnel. For a span from 19 till 28 m, the steel shell is a more cost efficient solution than the SCS tunnel. However, applying the SCS for spans shorter than 29 m, has some advantages as well, since the shear force and bending moment capacity of a SCS tunnel are larger than in case of a steel shell tunnel. This advantage can be important for changing boundary conditions or accidental loading on the tunnel structure, e.g. an earthquake loading, explosion, sunken ship on top of the tunnel, extra loading due to sedimentation on top of the tunnel, erosion below the tunnel floor or more ductile behaviour. When the construction area is in a region where the risks for earthquakes are significant, the SCS sandwich is the preferred solution for a span between 19-28 m. This is the case for the reference project used. Finally it can be stated that the SCS sandwich tunnel is the only solution available for spans larger than 28 m.

Immersed tunnels
Steel – concrete – steel sandwich immersed tunnels
Steel concrete composite
Large span
FEM analysis DIANA
Structural analysis

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Student theses

master thesis

(c) 2016 Bekarlar, K.Z.