BTS | HARDING PRIZE COMPETITION 2024
Hydrology, geology & pressure relief
At the location of the TSC, the geological formation is primarily horizontally bedded limestone and mudstone of the Lias Group.5
Due to the presence of discontinuities
and fractures and potential pathways linked to the Bristol Channel, significant water pressure could act upon the lining. To account for this potential pressure and to optimise
the overall TW design of the TSC, arrays of pressure relief holes (PRH) were prescribed to be installed throughout the structures to a depth of 3m beyond the extrados of the SCL (see Figure 5). This means that
the SCL is not required to resist the full groundwater pressure that would otherwise act upon it (up to circa 5 bar). Theory of stress increases in horizontally bedded rock6
water pressure build-up and must therefore be relieved to ensure continued stability. The required length of the PRHs is, however, harder to determine and should be based upon engineering judgment for the specific construction case being considered. Whilst it can seem counter-intuitive, a PRH that
remains dry is still functioning as required in relation to the design requirements. If significant flow occurs within a PRH it would need to be piped and pumped away until the completion of the PW, at which point it can be backfilled for the PW lining is designed to withstand the water pressures. For the correct formation of the PRHs, which must
reach a specific target zone, and undertaking rock bolting (see below), the space proofing of plant is critical in relation to required drilling and installation angles. To resist the uplift of the shaft liner due to buoyancy
resulting from the displaced water (see below), additional PRHs are prescribed to be installed below the shaft location.
Tunnel excavation & SCL For each of TSC, approx. 800m3
of rock needs to be
excavated, in 50 No. approx. planned advances and enlargements. As part of each advance, the geology is assessed (see
Figure 6) and a Rock Mass Rating (RMR) determined. This is then compared to the design assumptions and used to estimate rock stand-up times. In the TSC, the SCL is applied in two separate passes
of 100mm minimum thickness. The primary function of the first layer is to immediately stabilise the exposed ground prior to the installation of rock bolts; the second layer provides the required design thickness. As the TSC SCL does not need to resist the full groundwater pressure, it can be relatively thin at nominally 200mm thickness, which reduces material use and provides a significant carbon saving to the project. The primary SCL used in the TSC is a pre-blended mix
that must be stored distant from the works face, in silos, at the entrance of the segmental tunnels. To maintain mix consistency, it is important to not allow the silos to fully empty as the contents could potentially segregate by particle size within the silo. As an emergency measure, should there be an issue with supply of the primary mix, bagged shotcrete is available at the active works locations. Onsite meetings between the Jacobs and BB materials
teams were facilitated to review the requirements of the mix and to ensure alignment between the teams. During trialling, consideration was given to the
Above, figure 3: Probing from the segmental tunnels PHOTO CREDIT: JACOBS 12 | May 2024
development of summer and winter mixes to allow for more consistent and predictable performance throughout the year. The quantity of water used when batching must also be carefully controlled to ensure early age strength gain is achieved.
demonstrates that it can become significant due to
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