Monument MPM1777 - Features relating to construction of Dry Dock No.3

Features relating to the construction of Dry Dock No. 3 between 1799 and 1803 were revealed during a watching brief in 2010. These consisted of the construction cut for the dock, a section of the outer face of dock wall, two stone chutes, two stone staircases, and a stone foundation.

Features relating to the construction of Dry Dock No. 3 between 1799 and 1803 were revealed during a watching brief in Portsmouth Historic Dockyard in 2010. These consisted of the construction cut for the dock, a section of the outer face of dock wall, two stone chutes, two stone staircases, and a stone foundation. All of the dock features appeared to have been waterproofed using a narrow band of very fine plastic pinkish red clay which was observed abutting the lower section of dock wall, and lower parts of the stone chutes and staircases.

Construction Cut
What was interpreted as the construction cut for the dock was observed as a linear feature on an east-west alignment. Although only partially exposed, this feature was thought to continue around the full extent of the dock. The fill, of which only the top surface was investigated, consisted of stiff reddish brown clayey silt with frequent inclusions of ceramic building material, whole pieces and fragments of brick (dated 1750-1850), fragments of limestone and lenses of mortar.


Dock Wall
A section of the outer face of the dock wall was exposed. The wall had been built using two different construction types, one for the lower and one for the upper section. The lower part of the wall was made up of regularly coursed limestone blocks (300mm x 200mm) laid vertically and bonded with lime and sand mortar. This was observed between 2.10m and at least 2.70m below ground level. The upper part was observed 0.60m below ground level, and formed a convex overhang projecting 0.9m to the north. It had a rubble-like appearance and had been constructed using roughly rectangular shaped limestone blocks c200mm x 100mm laid in irregular courses and bonded with the same lime mortar as the lower section.


Timber Chutes and Staircases
Stone chutes for the movement of timber and a stone staircase survived above ground on the north and south of the dock. These structures formed part of the dock, and the chutes possibly acted as supporting buttresses. The chutes were removed to a depth of 3.50m below ground, but the majority of the staircase structures remain in situ.

The chute on the north side had been constructed using rectangular granite headstones and limestone blocks. The granite headstones formed the top of the chute, which was at a c50° angle into the dock. The stones had a dressed finish and were tightly laid together without mortar. The top of the chute measured 1.60m north-south x 3.00m east-west. The chute could be closed when it was not in operation by the use of iron slats which fitted into groves cut into the stones. The rest of the structure had been constructed using irregularly sized limestone blocks bonded with white hard Portland mortar (see below), of which four layers formed the chute. The first three layers measured approximately 2.00m north-south x 3.00m east-west at the top, and the fourth layer 1.00m north-south x 3.10m east-west. A total of 11 courses were removed to a depth of 3.50m below ground, but the structure continued beyond this point.

The chute on the south of the dock was of a similar construction to that on the north but the stones were more irregular in shape and of a less consistent thickness. Squared timber chocks had been used to support the slide stones at the 3rd and 5th courses. 11 courses of the chute were removed to a depth of 3.50m below ground level, although the structure continued beyond this point.

A circular post hole containing a vertical timber observed at the 9th course of both chutes was thought to have related to the construction process.

The Portland mortar used to bond the limestone is normally dated to 1830-1950, but the dock is known to have been completed by 1803. This demonstrates the advances in naval engineering at the time
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Stone staircases were present adjacent to the chutes on the north and south of the dock. These were curved and had been constructed using regular limestone blocks. The stairs were enclosed and exited parallel with the dock at its surface, turning 90° down into the dock. The staircase on the north of the dock appeared to end 3.20m below ground level, but a solid obstruction beneath may have represented a stone sub-structure. This feature was not explored further.

The staircase on the south of the dock had been built into a timber platform. This had been constructed using timber planks fixed to timber joists with large square-headed handmade iron nails. The joists were positioned at intervals and had been packed with irregular shaped limestone blocks. The planks may have been reused ships timbers, but this was not possible to prove. A series of rebates, mortises, slots and fittings were observed on the joists. The timber platform was 3.37m below ground and filled the gap between the staircase and the chute, abutting the eastern edge of the latter. It was not fully exposed, but extended beneath the staircase and beyond its southern side.


Stone Foundation
A stone foundation abutting the west side of the chute on the south of the dock had been crudely constructed from limestone blocks bonded with a hard white mortar dated to 1780-1850. The featured appeared to be of an earlier date to a similar feature which it mirrored on the north side of the dock. It may have been a foundation to a capstan base, although its exact function was unclear.

Watson, Stuart, 2011, Mary Rose New Museum Portsmouth Historic Dockyard Portsmouth Hampshire: Assessment of an Archaeological Watching Brief (Unpublished document). SPM397.