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REPAIR ‘TEST BED’ — TATTYNURE BRIDGE
The repair works to the above bridge have been completed and as stated in my report to you dated 29 April 1988 1 herein submit a further report which includes a description of the works carried out and measures taken to accommodate bats and dippers within the structure.
Tattynure Bridge is located approximately 6 miles north of Omagh on the C614 route (grid ref: 836795). The Bridge is a 200 year old, 7.5 metre span masonry arch structure across the Cappagh Burn. The Bridge required repairs which included the removal of vegetation, underpinning of the east end of the south abutment, rebuilding several areas of loose masonry, stitching cracks in the arch barrel, pointing joints in the masonry and the injection of cement grout to the entire structure.
Repair work commenced in June with the Contractor initially concentrating his efforts on the foundations of the bridge, underpinning to arrest settlement of the east side of the south abutment and also forming a plinth along its entire length. The Contractor then drilled and grouted the foundations and spandrel walls but did not allow grout to rise into the arch of the bridge. When this work was completed the Contractor started to work on the arch.
Prior to the commencement of the works consideration was given to methods of preserving the habitat of the bats and dippers within the arch masonry. These methods included the use of inflatable bladders and polystyrene void formers, prefabricated dipper boxes and the fixing of slabs to the soffit of the arch. The following paragraphs show that during the Works much simpler and more practical ideas were adopted and proved to be effective.
Immediately before the Contractor started work on the arch of the bridge, Mrs. A Archdale, a local of the area and member of the Northern Ireland Bat Group, circled with paint the holes, cracks and crevices (hereafter known as voids) which she thought were used by the bats and dippers. The timing of the Works, early June to mid—August was fortunate for 2 reasons. Firstly, Tattynure Bridge is used by bats as a roost during Autumn through to Spring and not as a nursery during the summer months so that we could be almost certain that no bats were present. Secondly, the dipper breeding season had just ended and the dippers were using the bridge as a roost only.
After Mrs. Archdale had painted the voids we discussed with here which voids we thought could be preserved and which ones would have to be blocked up in order to protect the structural integrity of the bridge. It was also agreed to form additional voids at suitable locations.
The Contractor’s main concern was to prevent the inflow of grout to the voids, which were mainly located in a deep (450 mm maximum) diagonal crack, which extended along the arch barrel. He achieved this in 2 ways. Firstly, existing masonry stones adjacent to the voids were removed and set aside. The surfaces of the enlarged voids were then rendered by hand with cement mortar which acted as a sealant. The removed masonry was then rebuilt into the arch in its original position. Secondly, where it proved impractical to remove masonry the voids were filled with twigs, straw and other vegetable matter and this remained in place until the grouting operation was completed. The natural voids preserved were generally around 300 mm long by 25 mm wide by up to 300 mm deep.
The additional voids were formed by removing individual stones within the masonry, cutting the stone and replacing it so that an opening 50 mm x 100 mm x 100 mm leading to a larger void behind 150 mm x 100 mm x 150 mm deep was formed. These voids were protected from the inflow of grout as described above by rendering the inside surfaces. In all 7 natural voids were preserved and 2 additional voids formed.
The Contractor then inserted 24 stitch bars across the crack in the arch barrel and the entire arch was then pressure pointed and grouted. Approximately 25 tonnes of cement were used in the grouting operation.
In addition to the above measures a 600 mm x 600 mm x 12 mm thick cement plasterboard slab was fixed to the crown of the arch soffit and camouflaged with rough dash mortar. The segment shaped void formed by the arch soffit (circumference) and the slab (chord) was left open at one end.
The position of the slab relative to the profile of the arch was considered prior to fixing. It was felt that the horizontal orientation of the slab at the crown of the arch would not suit “hanging” bats and that the slab should be fixed in a more vertical position further down the arch profile. However, at Tattynure Bridge flood levels relative to the arch profile determined that the slab should be located at the crown of the arch. In other bridges, where the arch rests on tall abutments, and where flooding is not a problem, it would be practical to fix the slabs in a vertical position. This measure was experimental and intended to accommodate bats only.
Mrs. Archdale monitored the activities of the dippers and watched for bats throughout the duration of the Works. In September she reported that the bats had not yet returned to the Bridge but she was hopeful that they would do so next year. The dippers were much more tolerant and roosted in the bridge each night during the Works and have continued to do so. She also reported that they had used some of the new voids.
(Mrs Archdale, made a follow up reports for 1991 & 1992.)
The cost of the additional work necessary to accommodate the bats and dippers at this bridge was £220 and is equivalent to 4% of the total cost of the repairs.
In my earlier report I mentioned that I had intended carrying out repairs to Crew Bridge (1-1315845), which required pressure grouting. Originally, Tattynure Bridge was not to be pressure grouted but shortly before the works commenced it was decided to do so which for test purposes obviated the need to pressure grout Crew Bridge.
R J HARVEY
Principal Engineer (Works)
23 December 1988