TRENCHLESS | SMALL DIAMETER TBM/PIPEJACKING
AboveAbove left, figure 20: Proposed infilling of existing structure Above right, figure 21: Proposed structure during construction
which ensured the construction works during the
possession were achievable in confined working environments. To construct the pits and the compound, a temporary purchase of land was required. A large number of trees were removed to facilitate the construction of the compounds. To meet Network Rail’s net biodiversity increase objectives, all the trees will be replanted and including an additional 10%. As part of the compound construction, track matting
was used to allow for vehicular access. This option negated the need for 3000 tonnes of imported stone. Additionally hybrid plant, solar panel site cabins and tower lights were used.
CRANE AND EXCAVATOR TEMPORARY WORKS CHECK Close collaboration was required between Systra (Principal Designer) and Active Tunnelling (Subcontractor to Principal Contractor) to achieve the construction works. One of the checks completed was to consider the effects of a 42 tonne excavator and a 50 tonne crane in close proximity to the pits. The crane was required to lower the tunnel boring machines into place. The excavator was required for withdrawal of the temporary sheet piles as well as other duties. Refer to Figures 17-19 for load diagrams and analysis results. The crane and excavator specifications were provided
by the Principal Contractor (J Murphy and Sons). The machines respective weights were converted to calculate lateral and vertical dispersion of finite line loads for calculating horizontal surcharge pressures. Loads were checked using finite element (FE) analysis software, MIDAS. The calculations determined that the loads generated from the self-weight of the machines were less than the maximum bending capacity and shear force capacity of the walls/base slab. A minimum offset from the centreline of the machines’ first axle to the pit was provided to the Contractor as a safe working limit.
18 | September 2024
INFILL OF EXISTING STRUCTURE Figure 20 shows the proposed infilling of the existing structure. These works would remove the future maintenance liability of the structure. Foam concrete was initially proposed to undertake
the infill of the structure in line with standard Network Rail details. Foamed concrete is a lightweight fill suitable for infilling stable structures as a void filler but has a compressive strength that lowers as it gets lighter. With settlement likely to be ongoing, a material was required that would aid both the long term stability and structural integrity of the existing culvert. Through design development it was identified that expandable polystyrene (EPS) blocks contained within a high performance hydrocarbon and chemical resistant liner was the best option. This provides a material with a high compressible strength and low density to reduce long term settlements from the infill material. A combination of dowel bars and flexible woven geogrids allow the infill blocks and the embankment to work together globally.
CONCLUSION This paper demonstrates that it is possible to install three culverts using tunnel boring machines, under an operational railway, during a 54 hour possession. A total of 615 tonnes of earth was removed during the possession. This was achieved by integrating temporary works and permanent works designs. Weekly meetings with key stakeholders and the construction team allowed me as the designer to understand key risks and construction best practice. Weekly discussions with the geotechnical team also played a huge role in the successful construction and delivery of the scheme. Extensive physical and non-physical ground investigation, discussions with the tunnelling contractor and operating as an Alliance made this scheme possible. Whilst three culverts have been sequentially installed adjacent to each other, we believe this is the first time that three culverts have been jacked simultaneously.
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