Fitzrovia (London)
The Challenges
- 4m deep Archaeological excavation on a land locked London site.
- All arising material to be screened on site prior to removal and disposal.
- Adjoining properties to be protected and supported during excavations.
The Solutions
- Phased excavations managed with a series of kingpost walls (including a 25m long, 4m wide cofferdam)
- Protection of boundary walls utilising early installation of permanent works with temporary support to prevent failure
- Retention of neighbouring ground levels; kingpost walls with spliced connections to aid safe removal post completion.
Why Kingpost Walls
One of the key functions of archaeological works is the preservation of historical (physical) data. In this case an unknown quantity (and location) of human burials spanning the majority of the 18th Century.
It was determined early on that the works, covering every inch of the project, would need to be undertaken in phases. Splitting the site into 3 ‘Zones’ would allow the Principle Contractor to manage welfare, storage, and most importantly the screening of arising material in search for human bone fragments. The screening activity alone took up almost 400 sqm of the site.
But how do you split a site into 3 without compromising the integrity of the archaeological (historical) data. Well, it’s almost impossible when you don’t know where it is, so we came up with the least intrusive method, Kingpost walling. Installing piles at 1.8m centres and inserting plunge columns to the full 10m depth, along a carefully plotted line, minimised the contact with and impact on the unknown archaeology. This then allowed the archaeologist to carefully and slowly progress the excavation with the attending contractor inserting timber planks into the webs of the columns to retain the ground at the higher level, where arising material management was taking place.
Why use the permanent works
The South Boundary wall was constructed over two centuries, the top half in the late 19th and the bottom half in the late 18th. With a basement to the adjoining owner’s side of the wall and a planned 4m excavation basement to the project side, the wall between the properties would have doubled in free standing height. It was agreed that the early installation of the secant wall to this elevation was necessary to protect this structure and the best strategy for the efficient sequence of the works. However, Historic England (who control archaeological licensing) would not allow the Principle Contractor to simply pile the secant wall, they were insistent on removing any ‘surviving’ burials first. With this in mind a TW scheme was needed to excavate to the 4m depth, whilst protecting the boundary wall structure from collapse, to clear the way for the secant wall.
To overcome this combined civil and structural challenge a Kingpost wall was provided with a series of diagonal bracing from the head of the Kingpost wall to support the boundary. Works were delivered successfully uncovering nearly 200 burials, carefully removing each one over a period of 12 weeks. Due to the careful placement of the Kingpost wall only 1 of the 200 burials was impacted by the temporary structure. With archaeology complete the area was back filled, making way for the permanent works to be installed.
Beyond secant piling and into the rest of the bulk archaeological excavations a further temporary works scheme was designed to prop the secant wall using horizontal corner bracing and raking props.
Why spliced connections on the kingpost walls
The North East corner of the site presented it’s own set of unique challenges where the Principle Contractor needed to carryout UXO probing works following a study showing bomb damage to the site during WWII. This corner was sloped, banking down to the neighbouring property. Historic England would not allow the PC to reduce the area and provide a flat surface for probing, so instead the area had to be raised. In order to facilitate this a Kingpost Wall was proposed. During the Party Wall negotiations the only stipulation from the adjoining owner was that the steel plunge columns would need to be removed to a depth of 1m below the external ground level. To enable this to be carried out safely we stipulated a splice connection detail that could be removed from within the original auger core, preventing further damage to their surface finishes
To find out more or how iTW can support you with your Temporary Works requirements contact us;
Phone – 07923904734
Email – James@itw-uk.com
Website – www.itw-uk.com
