TECHNICAL | SHAFTS, CAVERNS - BTS HARDING PRIZE COMPETITION


Photos (L to R):


Some images from the top 5m of the shaft construction Left:


Void formers installed to create the opening for ventilation pipes to transport foul air


brackets fabricated on site. The process from initial concept to fabrication was accelerated to avoid delays to the slipforming operation. Fabrication was carried out during the night shift following advice from the slipform team, with the temporary works design finalised the following day. Once the design was confirmed, an additional gusset plate was welded onto the brackets, after which they were ready for installation. They were then fixed to the cast secondary lining. Relocating the climbing tubes to reduce the cantilever


length, combined with limiting the load placed on the widened deck section, resulted in the slipform rig moving back towards its intended design position. An alternative solution could have involved adding supplementary support beneath the cantilever to stiffen the structure and reduce deflection; however, this would have increased the overall rig weight. The specified tolerance for the finished concrete


lining was +25mm to 0mm, and I set out the lining at +10mm to remain within this range. I submitted the as-built survey data to the designer to confirm whether the lining could be accepted as constructed. Although the change in diameter exceeded the absolute tolerance of ±50mm stated in CESWI clause 11.8, when expressed as a percentage of the design diameter (80mm / 25,000mm = 0.32%), it remained below the permissible 1% threshold. On this basis, the designer accepted the secondary lining diameter as built. This is based on the worst-case scenario at level 75.4m ATD whereby the shutter had pulled-in by 32mm on one side, and 48mm on the other, totalling an 80mm reduction in diameter for the 25m-wide shaft. A schematic of this is shown on Figure 12.


3.3. Rig alterations As the rig approached the final 5m of the slipform, planned stoppages were introduced to accommodate the increased lining thickness, which rose from


26 | April 2026


600mm to 1200mm and required additional steel reinforcement. Rig alterations were also undertaken to facilitate the installation of structural components for the cover slab and ventilation systems, as illustrated in Figure 13 and Figure 14. This stage represented an exceptionally busy


period and coincided with a time when I was the sole engineer on site. To manage the workload effectively, I set up two EDMs on the rig, enabling me to move between assisting the steel fixers and carpenters, ensuring that no gang was left without work. By anticipating requirements and marking out points during operative breaks, I successfully oversaw the correct installation of five box outs, four void formers, and multiple sections of couplers as well as alterations to the shutter pans. At the conclusion of my shift pattern, I produced a


clear diagram identifying the type and location of each cast-in item and marked these positions directly on the slipform deck. This ensured continuity across shifts and provided clarity for subsequent teams during this critical phase of construction. Following completion of the slipform, the rig was


disassembled, and the concrete lining was scanned to assess the dimensional accuracy. The scan results, presented in Figure 15, illustrate the variation in lining thickness across the shaft. Areas highlighted in red indicate sections where the lining was 50mm thicker than the design specified, while blue areas denote sections 50mm thinner. Green areas correspond to the correct design thickness of 600mm. In addition to thickness variation, the scan


captured the locations of the various structures installed during slipforming which were created in preparation for the subsequent phase of works to complete the shaft internals. This survey provided a comprehensive record of the as-built condition and confirmed the lining was acceptable to use as constructed.


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