Slipway Ground Investigation

Description

The site is located in the small town of Pembroke Dock,in west Wales, about 4 km north west of Pembroke. The area to be investigated is part of Pembroke Port and is located just east of Carr Jetty, approximately 1.3 km west of Pembroke Dock railway station. The area to be investigated is comprised by two slipways (Slipways 1 and 2), surrounding quay walls, the intertidal zone immediately north of the slipways and adjacent to Carr Jetty, and the seabed north of this. Lot 1: The western slipway (SW 1) comprises a series of concrete slabs that run down to the quay wall. The slipway is currently used in combination with a mobile boat lift. Two pairs of concrete beams are present on the eastern slipway (SW 2). Steel rails are set on the western set of beams and are occasionally used to move vessels to and from the estuary via a winch. As part of the port masterplan for Pembroke Port, MHPA proposes to construct a new, wider slipway in the location currently occupied by Slipways 1 and 2 and the quay between them and the winch house located at the top of the slipway. The eastern set of beams are in significant disrepair and are incomplete over the lower length of the slipway. Both sets of beams runnorth, beyond the existing quay wall by between 5 and 10 metres. The quay walls consist of masonry (limestone blocks topped with a granite cope) often with concrete facing. The whole of the slipways and surrounding quay walls are Grade II listed structures. The middle pier between SW 1 and SW 2 sits at around 6 m OD and is covered by concrete slabs. The eastern side of SW 2 is similarly covered by concrete slabs. A car park, a building of unknown use,and an electrical sub-station are located along the western flank of SW 1. Exploratory holes on land will need to be broken out using power tools. It is not envisaged that diamond coring will be required to penetrate the concrete or granite prior to commencement of boreholes. The currently proposed slipway will be approximately 67m wide and approximately 180 m long, extending around 40 m beyond the existing quay wall. The end of the slipway will terminate at −7.98 m OD. This will require the level of the existing slipways to be lowered to achieve the desired gradient. The middle pier will also be removed as part of the work. It is intended that the material removed during the slipway construction be placed in a disused pond about 200m south of the site, within the port area. The existing walls of the slipway will need to be maintained and therefore options will need to be considered to allow lowering of the slipway level whilst maintaining the existing flank walls. Beyond the end of the existing quay wall the slipway will require excavation into the sediment/rock to reach the design level. Knowledge of the level of the top of rock and properties of the overlying material are currently insufficient to enable a suitable solution to be designed. It is currently unclear whether retaining structures will be needed at the edges of the slipway in the offshore area due to the uncertainty of the ground conditions. In addition, the existing Carr Jetty (also Grade II listed) is located approximately 15m from the proposed edge of the slipway in the offshore area. The founding material and level of Carr Jetty are unknown. Previous site investigations have been carried out in the area, including overwater boreholes, shallow land boreholes and window sample holes, and an overwater geophysical survey.