Excavation
Date 21 November 2011 - 24 April 2012
Event ID 993498
Category Recording
Type Excavation
Permalink http://canmore.org.uk/event/993498
HU 4752 4152 Following test trenching in 2011 (DES 2011, 172), to examine structural cracks and voids that had been noted within the N and W walls of the bastion, a further programme of test trenches and standing building recording was completed over four main stages from November 2011 to April 2012 while consolidation of the walls was completed.
Previous excavations in the E bastion showed that shrinkage of the 17th-century earthwork and the resultant void between it and the 18th-century masonry walls had caused structural instability, severe cracking and partial collapse of the wall. The test trenches in the W bastion showed that although a partial, but largely negligible, void existed in places between the masonry wall and the earthwork, the present structural issues originated in the later addition of poorly bonded masonry. While the W bastion is being compressed, due to the failure of parts of the masonry structure, the buried earthwork seems to provide it with a reasonably solid structural backing.
The problems of structural cracking at the fort stem from two main sources. Firstly, the creation of a thickened cope-topped cap around the fort walls, which may have been added for safety or perhaps to improve the security of the fort. Evidence in the W bastion indicates that this process started from the S and moved N. To the S, the added face bonds into the battered rear face of the scarp wall and is seated on bedrock. Secondly, the bedrock falls away rapidly to the N, leading to a decision to simply dig into the peat bank and apply a foundation, onto which the added wall face could sit, rather than take the added wall face down to bedrock. Whether it was a question of speed, cost, time or availability of materials, it resulted in the thickened wall top being carried on a wall which was voided to the rear and was in turn carried by a bonded foundation based on a truncated peat bank. The slippage of the foundations and the lack of a bond to the existing battered rear wall led to the structural failures.
Archive: RCAHMS (intended)
Funder: Historic Scotland
Paul Fox, Kirkdale Archaeology 2012
OASIS Id: kirkdale1-310855
