Figure 5 : Cross Head Slabs Figure 6 : Anchorage Strands
Concerns regarding long term structural integrity of the pretensioning strands arose from the design and construction information , especially :
� Existence of voids in the grout within the ducts .
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Ability to fully grout the ducts and additional forces caused by bends in the ducts .
All four strands were socketed in a single steel casing , making it difficult to open the wires without misalignment .
During the socketing the hot metal used for galvanising may have become molten and any corrosion protection beyond the socket would be lost .
Water ingress reported during the construction may have caused corrosion to the wedges that hold the strands .
In 2005 previously unseen records and papers were acquired . They related to the construction of the anchorages and indicated that especially the southern anchorages might be in potentially worse condition than had been described before .
It was decided , as part of the feasibility study on the options for the replacement or augmentation of the main cables , that investigation into the condition and long term structural integrity of the main cable anchorages would be carried out . A Peer Review Panel was established to audit and review the work .
Three separate methods of investigation were proposed : �
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Excavation behind the anchorage chambers down to the top of the tunnel to expose , inspect and test the pre-tensioning strands .
The full scale load testing of a number of the sockets within the anchorage chamber ( which was not accepted ).
� Non-destructive testing – e . g . Acoustic monitoring , the use of radar , radiography and magnetostriction scanning .
The main task was to excavate down behind the south west and south east anchorage chambers to expose the top of both the west and east tunnels . The concrete was removed by hydro-demolition . All 18 ducts were opened , grout washed out and wires inspected .
Figure 7 : Excavation – overburden , rock and concrete
1 / 2017