COMHAIRLE CONTAE ÁTHA CLIATH THEAS
SOUTH DUBLIN COUNTY COUNCIL

MEETING OF RATHFARNHAM AREA COMMITTEE

Tuesday, February 14, 2012

HEADED ITEM NO. 17

UPDATE OF THE CLOSURE OF THE GLENBROOK FOOTBRIDGE

Glenbrook Bridge carries a pedestrian access off Willbrook Road over the Owendoher River, in Rathfarnham, Dublin 14. This is a relatively lightly trafficked pedestrian bridge leading to Glenbrook Park housing estate.

There is a recent history of scour and undermining at the bridge which has resulted in significant temporary repairs being undertaken in February 2011 and more recently the closure of the bridge following the major flood events on 24th October 2011. The findings of the most recent inspection are summarized as follows;

• The damage to the bridge is far more extensive this time than that which was previously experienced. The abutment  has collapsed from the springing level of the abutment.
• There is no sign of the reno mattresses / rock armour that was placed across the river bed to protect the structure from scour.
• In terms of the arch footbridge, there is no evidence of distress on the superstructure at the moment (no cracking, distortion, rotation).
• The mechanism of collapse appears to be very similar to the previous deterioration whereby the gravels under the riverbed have scoured out immediately below the weir which has created a large hole and undermined the abutment and the new gabion basket wall downstream of the bridge.
• The general condition of the arch barrel appears to be good with no signs of structural distress in the form of cracking, lateral separation, arch deformation, or loss of bricks.

An inspection was carried out on the Glenbrook Footbridge  on the 8th November 2011 by the Consulting Engineer from Roughan and O' Donovan. It was recommended that, in the short term, the footbridge be closed. In the longer term, it was recommended that the location of the weir in the middle of the bridge be considered and an engineered solution to the problem designed to ensure the stability of the bridge.

3  Options to strenghten the superstructure were considered and are briefly described below:

Option 1   Relocate the hydrometric station to a point approximately 50m downstream of the existing bridge (adjacent to the next weir) and construct an in-situ concrete trough slab for a distance of at least 5m upstream and 25m downstream. The appearance of the bridge would only be marginally affected by the works.

Option 2   Rebuild the existing weir, to preserve the hydrometric data, with an in-situ concrete trough slab for a distance of at least 5m upstream and 15m downstream. The appearance of the bridge would not be changed.

Option 3   Demolish the existing arch and overspan the bridge with precast prestressed standard inverted double T footbridge beam with integral piled abutments set well back from the existing in structure.

Option 1 was discounted as it would mean relocating the hydrometric station and consequent loss of flow data.

Option 3 was discounted as it would entail demolition of the existing bridge.

Following a meeting with the consultant on the 7th February, it is proposed to proceed with Option 2 and get feedback from the Fisheries Board prior to completing contract documents. No work can commence on site until the spawning season closes on the 1st May. However the intervening time will be used for documents preparation and tender procurement. Once work begins, it will take 10 to 12 weeks on site to complete the works.The preliminary estimate for the works is €70,000 and there is no budget allocation for this repair work.