Dredging Method Statement

Dublin Port Company Ltd. Berth 50A Development – Method Statement for Dredging and Retention of Silt _____________________________________________________________________________

1.

Scope Dublin Port Company Ltd. is in the process of developing a new berth to be known as Berth 50A at the southern end of the existing Dublin Ferryport Terminals Lo-Lo facility at Breakwater Road South, Dublin Port. Capital dredging works comprising the removal of approximately 250,000 tonnes of spoil, most of which will be dumped at sea. It is proposed to retain approximately 10,000 tonnes of the silt arising out of the works within the confines of the proposed new berth by burying it in a deep reception pit and capping it with a layer of gravel. The purpose of this method statement is to identify how these works will be carried out. The requirement for the silt removal is to minimise release of silt to the marine environment during the works, to place the silt in a prepared reception pit within an adjacent section of the works area, and to cap the silt with a layer of minimum 2m of as-dredged gravel also to be obtained from within the project boundaries. The depth of the final capped area is to be at or below the design dredge depth for the works. In order to minimise the volume of silt to be removed the slopes of the berth pocket will be dredged by box cut. This will be executed by removing material to the -11m LAT design depth well into the slope, creating a void into which the higher material can slump thereby forming a natural angle of repose whilst maintaining the required -11 LAT depth within the berth pocket.

2.

Method Statement 2.1

Silt reception pit

The initial operation will be the preparation of the reception pit in an area adjacent and to the east of the silt removal area (see sketch no. 1). The area will be dredged conventionally using the backhoe dredger and standard bucket, loading the material into the split barges for disposal at sea. As part of the same operation the area between the reception pit and the new quay wall will also be dredged to the required -11mCD depth in order to maximise the distance between the quay wall and the top of slope from the reception pit. The pit will be dredged to a depth of -15mCD which will allow placement of a 2m layer of contaminated silt followed by the 2m gravel cap which will be finished at the design depth of -11mCD. The pit base dimensions will be 28 x 43m which will allow sufficient space for all the silt. As can be seen from the sketches, the reception pit will extend beneath the slopes of the berth pocket design both to the south and east thereby enabling the pit to be located at a safe distance away from the quay wall and from the _____________________________________________________________________________ Malone O’Regan Consulting Engineers

Dublin Port Company Ltd. Berth 50A Development – Method Statement for Dredging and Retention of Silt _____________________________________________________________________________

area from which silt is to be removed. Once the reception pit has been satisfactorily created, the silt can be placed within the pit. 2.2

Silt removal.

The key to the successful removal of the silt is to be able to clear all the silt to the agreed level with the minimum release of material into the water column. An Environmental Profiling Grab (EPG) will be used for this purpose. Details of the EPG are appended to this method statement. The grab has an extra wide opening capacity and a horizontal closing action which enables the grab to remove thin layers of material leaving a flat surface following each pass of the grab. The grab has an enclosed design with venting valves which avoids spillage of material during the removal operation. The whole process is fully controlled and indicators show when the grab is fully closed. The EPG will be used by the backhoe dredger to load material directly into a split hopper barge. Once a full load is achieved, both the barge and dredger will relocate to the reception pit where the material will be removed from the barge again by using the EPG. The EPG will then be lowered to the reception pit and only when the grab is within the pit will it be opened to release the material. This method causes the minimum disturbance to the material during the dredging and placement process which results in little or no release of silt to the water column. The silt will be removed from the agreed area boundaries and level. Frequent hydrographic surveys will be executed in order to monitor and supervise the operations and to maintain the backhoe dredgers systems with the most up-todate information.

2.3

Gravel Cap Placement

The capping material will be dredged from the area underlying the contaminated silt.

_____________________________________________________________________________ Malone O’Regan Consulting Engineers

Dublin Port Company Ltd. Berth 50A Development – Method Statement for Dredging and Retention of Silt _____________________________________________________________________________

When all the silt is in place, the gravel will be dredged using the standard backhoe bucket and placed into the split hopper barge. Once again the backhoe dredger and barge will relocate to the reception pit and the first approximately 1m layer will be placed by excavating material from the barge by the backhoe dredger and releasing the gravel onto the silt from just above the silt level. Frequent monitoring surveys will track the progress of the placement and following the completion of the first 1m layer, the subsequent 1m layer can be placed by bottom dumping material directly from the split barge. Any trimming of the area to the -11mCD design depth will be carried out by the backhoe and the material deposited at sea as normal.

3.

Clarifications 3.1

Quantities

Quantities have been calculated assuming a berth pocket width of 30m and a silt layer of 1.04m.

3.2

Slopes

As described in the method statement the slopes will be formed by box cut and allowed to achieve a natural angle of repose. Whereas the berth pocket will be cleared to the design depth, the final slope may not be the 1:10 shown on the design even though sufficient material will have been removed to allow for that slope to form.

_____________________________________________________________________________ Malone O’Regan Consulting Engineers

Dublin Port Company Ltd. Berth 50A Development – Method Statement for Dredging and Retention of Silt _____________________________________________________________________________

_____________________________________________________________________________ Malone O’Regan Consulting Engineers