BTS | HARDING PRIZE COMPETITION 2024


Due to the highly sensitive nature of SAA, it is


critical that they are installed securely and protected from accidental influence during adjacent works. For this reason, and in particular whilst baseline readings are established, it is important to have a separate distinct method of correlating and understanding the data being collected. Otherwise, works may need to be paused to confirm the excavation is stable. Clear and meaningful trigger levels were defined to


inform the DRMs of the behaviour of each structure. The TSC trigger levels are based on percentages of predicted movement (75%, 100%, and 150%) determined from the finite element modelling (FEM) and consideration of the convergence confinement method.9


Liner installation, stability & removal At the centre of each head location, holes for the steel liners were excavated,10


using a drill mounted on an


offshore jack-up vessel (JUV). The liners were then lowered into position using a crane. Once placed into the ground, the extrados of each


liner is surrounded by high strength low shrinkage grouts with a shear key formed from a local enlargement into the rock. The shear key is crucial to the design. Therefore, to be conservative, any stability contributions from skin friction were ignored for the ultimate limit state design cases. When a liner is reached by pilot tunnel, driven


from the segmental tunnel, stability checks are then undertaken using multiple distinct monitoring techniques. Fresh water was added into the liners to resist buoyancy forces that would otherwise be generated by an empty shaft during initial offshore installation. Once it is determined through the DRMs that a liner is stable, it is then confirmed as acceptable through a RESS to drain the water, in a controlled manner using pre-installed valves. Using hydro-cutting, the bottom third of the 35mm-


thick liner has to be removed to make room for the SCL enlargement for the PW envelope. Once the initial horizontal cuts are undertaken in each


liner, it is critical to understand if the shear keys are functioning as designed and that no significant vertical movement is occurring. In conjunction with SAA and precise levelling, this can be achieved using calibrated rulers, vernier callipers or preferably using instruments such as tell-tale precision gauges, which can be attached either side of the cut lines (see Figure 8). To break the liner into manageable sizes, further


partial horizontal and vertical cuts are made. The final short cuts are made immediately prior to individual panels being removed with overhead lifting equipment.


Tunnel enlargement & back mining Following the sequential removal of the steel panels, the remainder of the adit must be enlarged to the required final profile. As this paper is written, the plan is for this to be done for the first two TSC locations in the first quarter of 2024, using a staged back mining process. Once this is complete, the PW installation can begin.


Change management Due to the complex nature of the TSC (including nuclear safety requirements), and changes in personnel within the site teams delivering it compared to when the design was initially developed, multiple different approaches were considered to build the TSC TW. As significant steps had to be overcome to achieve initial design approval (see above), it is important that any proposed post-design acceptance changes are only progressed where there is an overall construction benefit. To control the processing of potential changes, I


Above, figure 4: Grouting from the segmental tunnels PHOTO CREDIT: JACOBS 14 | May 2024


instigated and lead bi-weekly request for information (RFI) meetings with BB and BeMo to consider future planned works in a timely manner.


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41