Monument MPM2034 - Limestone walls and steel rail, Dry Dock No 3, Portsmouth Historic Dockyard

Layers and features revealed in an observation pit located towards the western end of the northern wall of Dry Dock No 3 during archaeological monitoring in 2008. Sections of limestone stone wall were thought to have probably been part of the dock wall. A steel rail belonging to the dockyside crane railway which ran around the perimeter of the dock was also recorded.

Layers and features revealed in an observation pit during archaeological monitoring of geotechnical works in 2008. The pit was located towards the western end of the northern dock wall of Dry Dock No. 3 in Portsmouth Historic Dockyard. Dry Dock No 3 was constructed between 1799 and 1803 and extended in 1859. Sections of limestone stone wall observed in the pit were thought to have probably been part of the dock wall. Also excavated was a section of a steel rail that belonged to the dockyside crane railway which ran around the perimeter of the dock.

A layer of sandy silty clay [3] 3.75m thick with very occasional CBM (ceramic building material) fragments was recorded throughout the observation pit to it's full depth of 4.25m below the ground surface. This was thought to be redeposited natural material which formed the backfill of the construction cut to the dock, although the construction cut was not observed. A feature interpreted as the construction cut for the dock was recorded in a second observation pit to the east (see MPM2035).

A large masonry wall constructed using roughly shaped rectangular limestone blocks was revealed in the north facing section of the observation pit. The blocks varied in size but on average measured 300mm x 200mm x 100mm. They had been bonded with an off-white lime and sand mortar and had been placed in an irregular fashion with some coursing observed at higher levels.

A vertical face to the wall was observed from 3.11mOD to 2.46mOD at which point the face sloped to the south. The pit was extended to follow the wall to a depth of 1.96mOD and 0.50m back from the vertical face at which point it sloped further to the south to a depth of 1.66mOD and 1.65m back from the vertical face.

It was not possible to ascertain the exact function of the wall due to the limited size of the observation pit, but it was thought most likely that it formed part of the dock wall.

A very decayed timber beam observed as a stain within the soil was recorded under the wall at 1.66mOD. The beam ran east-west and measured approximately 0.90m east-west x 0.40m north-south and was 0.30m thick. Two iron spikes associated with the beam were observed running horizontally north-south at 1.36mOD below ground level. It has been suggested that the beam may have been part of the original timber shoring for the construction of the dock.

The fact that the silty clay backfill [3] (see above) was recorded under the timber suggested that the stone wall probably continued beyond and down to the south.

A small section of limestone wall was observed within the west facing section at the southern extent of the pit, returning north at 1.73mOD. Only 0.40m of the wall, which was 0.35m high, was visible within the pit running north-south, the rest of the wall being beyond the limits of the excavated area. It was not possible to ascertain the exact function of the wall but it was suggested that it may have represented a portion of a supporting buttress to the dock wall.

A layer of black cinder 0.15m thick at 3.26mOD overlay the backfill [3] and had been used as a consolidation layer for a 0.20m thick layer of made ground at 3.46mOD. Above the made ground was a 0.10m thick layer of brick rubble above which was a 0.10m thick concrete layer. A steel rail 60mm wide by 120mm high was set into the brick rubble and concrete layers (0.4m below the surface of the concrete) and was observed running east-west across the observation pit continuing beyond the limits of the excavation in both directions. The rail formed part of the dockside crane railway which continues around the perimeter of the dock and is connected to the dock rail system.

Sayer, Kathelen, 2009, Archaeological Monitoring of Geotechnical Works at the Portsmouth Historic Dockyard, Portsmouth, Hampshire (Unpublished document). SPM612.