BTS MEETING/NORTH BRISTOL RELIEF SEWER | TECHNICAL
of one of the remaining 30m-deep shafts to account for the changes in the tunnel alignment. These four changes can be seen in Figure 2. The vertical alignment was also optimised by Murphy
during this stage with the main change being to convert the route into a single TBM drive instead of the 3 drives that the original design would have required. This would reduce the impact on third parties and provide programme and cost benefits to the project. In addition to the alignment development, there
was also a considerable amount of value engineering done around the deep shafts on the project. The minimum acceptable size for the two 30m deep shafts was selected to be 6.0m i.d.. This was due to the expected excavation equipment and the requirements for the TBM to pass through the bottom. Further value engineering was employed by Aecom who developed a bespoke solution for the internal weir walls – to install an overflow weir pipe, replacing the traditional weir walls that would have increased the project cost (see Figure 3). During refinement of the original design, the tender
team removed the need to build two additional deep shafts (MH08 and MH11), which also represented a carbon saving of approximately 118 tCO2
A key temporary works area in project design was
the TBM Launch pit, the main requirement for which was to be able to fully assemble the shield before the start of excavation. The solution was to construct a 6m-deep, 8m-wide and 65m-long sheet piled pit with surrounding steel waler beams and one level of steel props (see Figure 4). The launch pit was designed to have three main lifting zones, where the prop spacing was increased to allow for the TBM logistics. In addition, the front two props were designed to be removable for a short time to allow for the TBM shields to be lifted into position. During the design period, it was key to estimate
the expected ground conditions. From the ground investigation (GI). It was expected that most construction would be undertaken in weak Mercia Mudstone (0.6 MPa – 25MPa), with some evidence of small sections of weathered Mercia Mudstone. Following the contract award, Murphy undertook some additional GI and identified the potential for an approximately 600m-long section of strong basal conglomerate that was located within the tunnel horizon and also one of the remaining deep shafts, MH09. From the GI, the maximum groundwater head was expected to be around 33m (3 bar – 3.5 bar) at various points along the alignment. Figure 5 shows the face of the tunnel excavation during the anticipated ground conditions.
PROJECT CONSTRUCTION Site Establishment Construction work on the project started in August 2019, following three months of ecological mitigation works. The first stage of works was to establish the main TBM tunnelling compound and three satellite site compounds
e to the project.
(two deep shaft sinking sites (MH09 and MH10), and the TBM reception site at MH12. At this stage, heavy earthworks were undertaken at the main compound to alter the topography enough to allow the TBM compound to be constructed, as shown in Figure 6. Several bespoke temporary works designs were required, all undertaken by Murphy’s in-house design team, Murphy Applied Engineering (MAE).
TBM Refurbishment Whilst the site establishment stage of the project was being undertaken, the refurbishment of the TBM began at Murphy’s Ollerton facility between the summer of 2019 and the of spring 2020. All the refurbishment work for the TBM and ancillary equipment was undertaken by the in-house team of specialists supported by key suppliers. The TBM that had been selected to complete the
tunnel drive was a Murphy-owned Lovat EPB machine (called ‘Fionnuala’), originally manufactured in 2001. All the backup ancillary tunnelling equipment, rolling stock, grout silos, TBM services, etc., were also supplied from Murphy Plant. One of the primary refurbishment tasks was
modification of the TBM cutterhead. The EPBM had the option of two cutterheads – a soft rock option with ripper tools only or a hard rock option with cutting discs or ripper tools (via adaptor boxes). The anticipated ground conditions led to choosing the hard rock cutterhead with ripper tools and to have a contingency set of cutting discs available, if required, when tunnelling through the conglomerate section. However, with the hard rock cutterhead there were
concerns that the original opening ratio would cause clogging in the Mercia Mudstone, where smectite was anticipated. To mitigate this, Murphy developed a concept design to increase the opening ratio. This concept was then sent to Lovsuns in Canada, where a design and finite element analysis was undertaken to confirm that the structural integrity of the cutterhead would not be compromised with these modifications. The existing cutterhead was modified in line with the design applied to this machine (see Figure 7).
Tunnel Construction Following the completion of the site establishment and TBM refurbishment, tunnelling started on the NBRS project in the summer of 2020. Below are some key facts about the tunnelling. ● The tunnel ring was a 2.85m i.d. trapezoidal ring, consisting of six equal-sized segments, 1m long and 180mm thick. The ring design had a 40mm taper to allow for steering for the tunnel.
● Grouting of the tunnel lining was undertaken using a traditional two-component grout mix that was pumped through the cast-in port in the lining.
● Spoil was removed during tunnelling via rail-bound spoil wagons hauled by a battery-powered Schöma locomotive.
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