The proposal to recreate a Roman Villa on an English Heritage site at Wroxeter, near Shrewsbury, for Channel 4’s January 20, 2011 programme ‘Rome Wasn’t Built in a Day’ , was made possible by constructing it on a reinforced concrete raft foundation over a rock mattress mechanically stabilised with Tensar’s TriAx geogrid.
Evans Wolfenden Partnership (EWP) Structural Engineer’s director Graham Wolfenden explains: “English Heritage was adamant that permanent damage should not be made to archeological remains under the soil so that ruled out building traditional foundations or imposing excessive loading. The construction had to be placed directly on to the existing grass and topsoil and we could not have done it without advice from Tensar.”
Due to constraints laid down by English Heritage and the sensitive nature of the sub-soils, restrictions were placed on any form of intrusive investigation. EWP undertook the design of the foundations assuming low bearing pressures (which would be kept off the historical fill material) immediately beneath the concrete raft foundation, so they reduced the pressures at the interface between the rock mattress and original sub-soils.
Following discussions between Shropshire County Council – Building Control and EWP, it was agreed that given that the building was classified as a “temporary” structure, it would be acceptable to erect the building off the rock mattress/ concrete foundation taking the above design parameters into account.
A 450 mm thickness of rock mattress consisting of sub-base aggregate, stabilised with TriAx geogrid layers, was able to comfortably support the loadings from the new building and concrete floor slab and distribute them so as not to overstress the subsoil layer.
A layer of TriAx TX160G geocomposite, consisting of a stiff polymer grid laminated to a geotextile, was first laid over the existing grassed surface. This provided the dual functions of mechanically stabilising the aggregate and also protecting the subsoil by separating fill the particles from the soil. The footprint of the 450mm deep stabilised mattress was extended two metres outside the perimeter of the new slab giving a mattress plan area of 23 metres by 23 metres.
“Without the mechanical stabilisation of the TriAx , it would be very difficult to say how much extra imported aggregate would have been needed to act as a stable base,” adds Graham. “As well as aiding the project’s sustainability, this ensured that the villa could be speedily and cost effectively recreated as a ‘temporary’ educational feature of the site, and mitigate the possibility of any differential settlement.”
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