TECHNICAL | SOFT GROUND TUNNELLING
Young’s modulus. There was no reason to invest
more time in a complex model of the concrete as the load on the tunnels was in any case rather small due to the low cover.
Q4: What were the considerations regarding fixings onto the final lining? A4: Fixings were installed into the final linings and were handed over to the next contracts to complete the installation of the catenaries.
Q5: Was any part of the I&M (instrumentation and monitoring) retained operational after the completion of the construction? A5: There was a period of one year between substantial completion and final completion when monitoring was operational but then all was decommissioned.
Q6: How were the pre-support pipes monitored? Also, the pipes where shown bent downwards. Were there cases where the pipes bent upwards and affected the highway? A6: The installation of the pipes was monitored through the centre of the auger. In the longer drives, a navigating rod was drilled-in first and then the auger boring followed. There were, luckily, no cases where the pipes bent upwards.
Q7: What was the condition of the existing tunnel prior to the start of construction? In particular, was there any deterioration due to the extreme temperature changes? A7: The structure was from the 1960s and there were a lot of cracks, which were monitored. A very detailed monitoring system was installed to assess the impact of the tunnelling and there was no impact.
Q8: What was the actual staging of the two tunnels? Which tunnel to go first? A8: The selection of which tunnel will start first was purely done so that the tunnel that is farther away from the existing tunnel would start first, in particular due to the weak backfill on the sides of
the existing cut & cover tunnel. The only requirement from the designer was for 20m distance between the headings of the two tunnels to allow the shotcrete in the first tunnel to gain full strength. There was no requirement to install the final lining in one tunnel first. The first tunnel went through and it was almost by completion of the second when the installation of the final lining started in the first tunnel. The tunnels were 180m long each and advanced at up to about 1m/day with a long average of 50-70cm per day due to the removal of obstructions.
Q9: What was the biggest risk as encountered during construction? A9: The pipe jacking operation was done with three machines in order to accelerate the schedule. The biggest risk were the obstructions (boulders,
wooden pieces, steel pieces) where the auger could not go through but there was a risk-budget allocated. The second was the drive through the wing-walls of the existing tunnel which was a tedious operation due to the very soft backfill.
Q10: What was the process of convincing the client? A10: It started with the contractor’s outreach where the client realised that the concept design would not work. The client believed in the mined solution and introduced modifications to allow this solution in the tender. During the RFP process there were a lot of
people on the stakeholder side involved and it was a tedious process trying to convince and explain the particularities of the project as there were no tunnelling experts on the other side of the table who would understand the proposed method.
Q11: How were obstacles removed through a 800mm pipe? The UK standard is 1.2m to allow personnel entering a pipe. A11: The diameter of the pipe was chosen (as opposed to 500mm) to allow removal of the auger and allowing a person to enter the pipe providing necessary light, ventilation and quick escape means and this person was manually removing the obstacles.
16 | November 2024
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