HARDING PRIZE COMPETITION 2024 | BTS
Rock bolting When installed, the staggered arrangement of 2.4m-long rock bolts act to stitch together the horizontally layered bedrock to form an effective rock arch, which redistributes the stresses induced by the excavation formation. As shown in Figure 6, the smaller pilot tunnels used
in the TSC were designed to not require rock bolts until enlarged, later in the excavation process. The pilot tunnels did not require planned bolting as design modelling demonstrated they would remain stable without. If rock wedges or other instabilities are identified
during excavation, they are secured using additional spot-bolts or other agreed toolbox measures. This is to avoid localised ground failure that could have adverse impacts. When undertaking rock bolting in marine systems,
it is important to assess for potentially aggressive groundwater conditions that may be present during the planned life of the structure in which they are to be installed.7 Following the identification of potentially aggressive
groundwater conditions at the TSC, subject matter experts were engaged to review the sample data and agree mitigation measures with BB for coating bolts for certain locations. As remotely operated rigs are used to drill the probe
holes, PRHs and to install rock bolts, it was important to ensure all equipment was thoroughly space proofed for all activities before being procured.
Setting out & surveying Whilst software packages, such as Amberg Profile,8
can
provide reliable and traceable means for ensuring that design profiles are built correctly and SCL thicknesses are achieved, it is not always practical to have surveyors present when fresh SCL has been installed. To define boundaries whilst reducing the likelihood
of over-spraying material for the complicated TW geometries and encroaching into the PW envelope, it remains effective to utilise simple additional control measures. These measures can be such as the installation of setting out pins and reinforcement lacers, that can be embedded into the final SCL profile (see Figure 7). On future projects, it would be beneficial to align
software packages at the design development stage to reduce the amount of manual processing required between different programs. To reduce potential error in the transfer of data and processing time, there may be benefit from the development of automation tools using programs such as Autodesk Dynamo and Civil 3D.
Ground movement monitoring Due to the relative inaccessibility of the TSC and to allow for constant monitoring at sub-millimetre accuracy, Shape Acceleration Arrays (SAA) were installed at the interface of the steel liner and SCL to provide a means of understanding the real-time behaviour of structures interacting with the ground without requiring personnel to be at the works face. This was particularly helpful during an extended shutdown period over Christmas.
Above: Completed temporary opening between segmental and SCL tunnels PHOTO CREDIT: BALFOUR BEATTY May 2024 | 13
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