Excavation StrategyPosted under: about >> Methodology Tags: deposits, ditch, drain, evidence, feature, fill, foundations, georgian, layer, medieval, objects, pit, post-medieval, post hole, prescot street, quarry pit, roman, south tenter street, strategy
The detailed excavation strategy is set out in our WSI. However as a guide to people interested this article gives a summary of the way we intend to dig the site:
Due to the size of the site, we are going to excavate in three distinct blocks. These blocks will be defined by breaks in the stratigraphic sequence such as significant truncations), or areas without archaeological deposits. Where excavation is not taking place, a thick deposit of overburden will be retained as a protection to the archaeological deposits to be left in situ.
Removal of Modern Soil or Overburden
The overburden on site is around 2 metres thick and contains the remains of Post Medieval buildings. This material will be removed under watching brief conditions by mechanical excavator.
This reduction will be undertaken in spits across the entire area of the zone to be excavated. The final spit will reveal archaeological deposits and care will be taken to ensure that sufficient material is left in this spit to allow the machine into the area without damage to underlying deposits. The final machining of this spit will be to the top of archaeological deposits. This will require care to ensure that the archaeological deposits are sufficiently cleaned of overburden, and that deposits are not removed without adequate recording.
Once excavated, the area will be surveyed by L – P : Archaeology’s survey specialist, Andy Dufton and the edge of the zone will be clearly defined and fenced to protect it from trampling by foot or machine.
Any archaeological deposits not yet ready to be excavated will remain protected under a thick layer of soil.
Recording and Removal of Post Medieval Buildings
From the results of the evaluation, we know that the 18th and 19th century buildings on site have sealed the earlier archaeological deposits beneath their foundations. In order to get to these earlier deposits, the Post Medieval remains must be recorded and removed.
Evidence of structures on site, specifically those dating to the 18th and 19th century and earlier, will be recorded and form part of the archaeological record. Typically, these will be wall and floors.
In areas with 19th century basements, the basement slab will be removed with a machine. Deeper foundations which extend to the underlying natural sands and gravels may be left in place to prevent any disturbance to the surrounding archaeology. Smaller foundations will be removed in stratigraphic sequence with the archaeological excavation. The breaking and removal of these will be by hand (where possible) under supervision of the archaeological team. As these areas are highly likely to overlie earlier stratigraphy, recording and removing these will be done with extra care.
If foundations are large deeply buried objects such as pad foundations, these will not be lifted whole out of the ground until all surrounding archaeological deposits have been removed.
All modern intrusions will be removed under archaeological supervision. Foundations will not be removed in a way that will damage archaeological deposits.
In order for the archaeological dig to begin, a secant pile wall will be constructed. This will involve the insertion of a concrete pile wall around the entire excavation area.
The secant foundations will provide the external retaining wall of the new building as well as the principle foundations of the building. They will also provide a retaining wall which allows the open area excavation of the entire site.
We will be excavating the site in stratigraphic sequence by context. This means that we will start by removing the later deposits first, and working through to the earlier deposits. Each archaeological context will be removed as a whole.
Excavation will be by hand. Care will be taken to clean and define the deposits before we begin excavation. Large homogenous deposits such as the possible cultivation soil will be sampled and may be excavated by machine if it is thought they are a single homogenous deposit.
All of the remains will be recorded according to the standard London archaeological recording method which is published by the Museum of London. This ensures that our records are compatible with the records produced by all the other digging units in London.
The method for finds collection and for environmental sampling are discussed in more detail in the WSI.