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.COME RAIN COME SHINE

GEOTECHNICS OF TRANSPORT

The Conwy Valley railway, which lost parts of its embankment to floods in February, is due to reopen this month. Max Soudain reports

The rain in North Wales falls mainly on the Conwy River valley, it seems. It certainly did on 3 and 4 February, causing the river to overtop flood defences north and south of the town of Llanrwst. Parts of the railway embankment between Llandudno Junction and Blaenau Ffestiniog were washed away, leaving the line suspended in mid-air in places. There was minor damage all along the 45km route, which had to be closed.

The single-track passenger line, opened in the 1860s and used mainly for tourist traffic, is about to reopen after emergency embankment repairs. Edmund Nuttall was main contractor for the £5M project, working for client Network Rail North West Region under the North West Structures Alliance agreement.

Nuttall geotechnical engineer Derek Arnott says the five-year contract agreement allowed repairs to start almost immediately. Nuttall brought in its consultant Scott Wilson to advise on the scope of the problem and design of repairs.

'We were on site inspecting damage within a week, after flood water had subsided, ' Arnott says. 'On the following day, a helicopter survey helped identify damage not found during the walkover survey.'

The first flooding was south of Llanrwst, over a 1.5km stretch running south from the A470 bridge over the River Conwy.

Overtopping of Environment Agency flood banks upstream led to flooding east of the railway, according to Scott Wilson Geotechnics project manager Ian Shelley.'This meant water was higher on the east side [not on the river side], with overtopping of the railway embankment from east to west.' Here the railway runs between 3m and 3.5m above the flood plain, 20m to 50m east of the river.

Shelley says the west side of the railway embankment here had already been damaged by river scour in 1994 (during a 1 in 20 year flood event) and was reinforced with rock fill protection and gabions. But in February the situation was far worse.'The small number of culverts had insufficient capacity to carry the flood waters and the embankment breached in six places, over lengths of up to 40m, over a 1km stretch, ' he says.

Breaching did have one positive effect, in that it appears to have averted flooding of the Parc Conwy housing estate on the edge of Llanrwst. Some gardens were deluged but no homes were damaged.

On the second day of heavy rainfall the situation worsened, when the river's tidal section flooded about 4km north of the town. Here the railway embankment is lower, typically 2m to 2.5m above the flood plain, but it is further from the river, between 250m and 500m to the east.

'Flooding overtopped the railway, with water flowing from west to east, causing severe damage and in places complete loss of the embankment over about 1km, ' Shelley says.

Following the walkover and helicopter surveys, 20 window sample boreholes were put down to 5m on the north site and six cable percussion boreholes between 10m and 15m deep were sunk on the south site. These revealed between 2m and 4m of soft to firm alluvial clay lay beneath the embankments, overlying river gravels and sands.

Arnott says the latter showed some evidence of sub-artesian water, with blowing sand encountered, making it difficult to form boreholes in the gravels and sands.

'The embankments are made of poor quality, initially cemented platy river gravels with a reworked -alluvial 'spongy' clay core in places, ' Shelley says. Damage was caused by turbulence as the flood water flowed over the embankment, scouring out ballast and eating back into the earth structure.

Scott Wilson set about designing replacement structures. About 330m and 575m of embankment was removed in the north in two discrete sections, and 260m removed in the south. 'We wanted to take the opportunity to better mitigate the risk of future problems, ' Shelley says.

Design also had to consider future use, explains Network Rail earthworks asset steward Gerry Manley. 'The National Assembly of Wales is exploring the viability of reintroducing freight services on the line, carrying slate from the quarry at Bleanau Ffestiniog, so design had to take this into account, ' he says.

Nuttall and Network Rail agreed the design without going through a time-consuming, formal approval process. The Environment Agency also had a say because of possible effects on river discharges, but this process was also fasttracked, with approval in three weeks rather than the normal ten, Arnott says.

Repairs were under way in earnest by the end of February, with subcontractors WA Developments working on the north site and J Murphy and Sons on the south. Vegetation clearance and minor ballast repairs on other sections were carried out by contractor Jarvis, working directly for Network Rail.

Failed sections were excavated out to typically 500mm below ground level, before placing a locally sourced limestone rock fill starter layer.

'This was punched into the alluvial clay to provide a stiff foundation, ' Shelley says.

The embankment was then built up in 200mm layers of compacted limestone granular fill sitting on a separation geotextile.

Construction was tested to ensure it met Network Rail's earthworks specification. Tests included insitu sand replacement and plate load tests.

Importation of about 100,000t of this good quality fill was dictated by the need to carry out emergency works quickly during poor weather under the risk of further flooding, Shelley explains. 'Because of the emergency nature of the work, design had to consider what was possible.' The core was wrapped in the geotextile to improve protection.

Embankment slopes in both areas are protected by riprap scour protection of 17,500m 3of imported Penmaenmawr Microdiorite rock fill (up to 600mm size) to prevent scour damage from future floods.

'Inevitably there will be some short-term settlement, ' Shelley says.'But this will be minor and should be compensated by compaction and extra track ballast. The ground beneath the embankment has been surcharged for 150 years and essentially the new structure creates the same loading.' The new sections are benched into the embankment in 10m to 15m long transition zones which should minimise any chance of differential settlement.

More than 46,000m 3of material was excavated from the damaged embankments.'At the design stage there was no allowance for reusing this material but we found there was enough space [particularly in the northern site] to use it for extra protection, ' Shelley says.

Material not used on these embankment shoulders will be used elsewhere on site.The EA is considering using some of the alluvial spoil to rebuild its defences.

Final additional flood relief culverts were installed to reduce the risk of future overtopping. Two 1,050mm diameter triple pipe concrete culverts and an 1,800mm diameter concrete culvert were placed at the south site and six 1,050mm diameter concrete and gabion culverts at the north site.

Since the flooding, Network Rail and the Environment Agency have worked closely to understand the failure of both organisations' infrastructure, Manley says.

'This has proved invaluable in enabling us to implement remedial works allowing for potential repeat events.'He believes the development of a very close working and efficient team of consultants and contractors over a very short period ensured the scheme's success.

Arnott says: 'The framework agreement allows us to undertake a number of value management processes. We were able to react during work to obtain a cost-effective solution. If it happened again, the design of the new sections means there won't be catastrophic failure, but local loss of ballast - any damage will be repaired simply using local ballast replacement.'

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