Dublin Waste to Energy Facility, Covanta

Finite Element Analysis

AGL Consulting designed the temporary cofferdam that facilitated construction of the pump station for the cooling water intake for the Dublin Waste to Energy incineration facility in Dublin Bay. AGL also designed the permanent sheet piled inlet channel.

The key features of the temporary cofferdam design were:

• The cofferdam was constructed partly within an existing tidal channel and partly on the quay side, which introduced significant out of balance forces for the required excavation depth of 12m. The construction stages were modelled using the PLAXIS 2D finite element program to account for the differential ground and groundwater levels and allow the benefits of interaction between the cofferdam walls to be considered.
• Support options considered included high level ground anchors combined with a low level propping frame or 2 No. levels of internal propping. The latter option gave greater ground movement but was more economical and had greater programme certainty. AGL were able to demonstrate that ground movements were acceptable so this option was implemented. Prop forces were very significant with the lower propping arrangement the biggest ever used by Groundforce in Ireland.
• The temporary cofferdam comprised a fully-interlocked sealed cofferdam with the sheet-piles penetrating below permeable gravels into the underlying Port Clay so that the excavation and construction works could be carried out in the dry.
• Dewatering/relief wells were designed to prevent an uplift failure mechanism occurring in the Port Clay due to the presence of permeable sand layers.
• AGL also carried out the design of the driving frame used to install the sheet piles.

The key features of the permanent inlet channel were:

• The inlet channel comprises a deep open trench supported by sheet piles installed in an existing channel bank on one side and in the seabed on the other side. The channel bed is up to 15m below the bank level.
• The inner row of sheet piles is supported by permanent ground anchors with double corrosion protection.
• The outer sheet pile wall is supported by a propping frame connected to the inner wall.
• A corrosion study was undertaken for design of the sheet piles by extracting old sheet piles in the existing ESB outfall channel, measuring the loss in steel thickness and using this to obtain site specific corrosion rates.