2011:424 - MARSH ROAD, DROGHEDA, Louth

Monitoring was carried out at Marsh Road in Drogheda in response to the construction of a car park for Irish Rail. The work was carried out for Courtney Deery Heritage Consultancy. The monitoring revealed that the site was used extensively during the construction of the Boyne Viaduct between 1851 and 1855.

A series of engineering test pits were excavated to assess the soils and locate potential services. Seventeen test pits were excavated at regular intervals throughout the site. Four trenches were also excavated to expose existing services. The test pits measured approximately 1.5m in length, 0.5m in width and were generally 0.9m in depth. A soil profile across the site, based on the test pits, shows a varying amount of modern dumped construction material, e.g. concrete, stone and redeposited clays, spread over the site, up to 0.9m in depth at Marsh Road but thinning out to the south of the Boyne Viaduct embankment.

The lower car park area

The ground level in the lower car park was reduced by 0.7–0.8m from the existing ground level (c. 6.45m OD). Excavation revealed the existing surface, consisting of a modern deposit (late 20th-century) of hardcore (crushed limestone c. 0.1m in diameter) mixed with modern rubbish up to 0.3m deep. This covered a variety of deposits excavated to a depth of 0.4m deep. The most extensive deposits were located along the Marsh Road frontage and were composed of fragments of black limestone, including occasional red bricks (c. 1%). The deposit was contained within a matrix of fine, grey, silty dust and covered an area measuring 40m x 40m. This was interpreted as a deposit of waste stone chipping and dust derived from the working of the masonry on site for the construction of the Boyne Viaduct. A large pond is located immediately to the east of the viaduct, adjacent to the car park. The pond was a quarry pit from where some of the stone used in the construction of the bridge was derived. The deposit of stone chippings originates from the rough dressing of the stone to construct the bridge piers, which appears to have occurred on site. No 19th-century tools or machinery were identified.

Burnt deposits consisting of cinders, oyster shells, burnt limestone, slate and occasional fragments of red brick were identified c. 30–40m from the Marsh Road street frontage. This suggests that limestone was being burnt at the site to produce lime mixed with crushed oyster shell. Lime is essential in order to manufacture mortar. This is again interpreted as evidence for 19th-century activity related to the construction of the viaduct.

There were also deposits of modern concrete, crushed red brick, mortar, timber and steel wire pressed into the natural deposits of silty brown clay that underlay the ‘viaduct deposits’. The topsoil and sod that existed at the site prior to the construction of the viaduct was absent and is likely to have been removed in the mid-19th century as part of the railway viaduct’s construction.

The earthen embankment at the southern end of the site

The excavation for the car park at the base of the large embankment at the southern end of the site illustrates the depth of the most recent dumped material (19th-century). This revealed that up to 8m and more of deposits had accumulated over the existing natural slope of the ridge that runs parallel to the River Boyne. The embankment is constructed from dark bands of broken fragmented limestone deposited on the level old floodplain and up against the existing sloping valley edge. These embankment deposits were constructed at the site, forming the connection between the stone viaduct and the edge of the valley. The engineering works for the new pedestrian stairs have been designed to support and leave this embanked material in situ in order to minimise the potential to disturb this loose material that supports the junction of the viaduct and the solid ground. The stone used in the construction of the embankment is the residue of quarrying and masonry work on site from the construction of the Drogheda/Boyne Viaduct.

Bedrock was located at the base of a service trench at the bottom of the embankment close to the perimeter wall at the north-eastern end of the site. It was not visible in any of the other trenches. The bedrock consisted of limestone calp, similar to the broken stone found over the entire site. The natural clay above the bedrock consisted of an upper stratum of mid-brown sticky clay with frequent angular stone inclusions and a lower stratum of black clay, also with frequent angular inclusions. The area of the site is still subject to flooding at high tides and would have been subject to scouring and deposition over the millennia. The natural clays are the results of this process. The embankment in which the viaduct is set is a man-made structure that was built abutting the southern end of the viaduct and is constructed from clay and the quarried rock described above.

Edmond O’Donovan & Associates, 77 Fairyhill, Bray, Co. Wicklow