The Tufa Field Geology
The City of Bath lies in a valley surrounded by seven hills. These are largely limestone, the hill of Combe Down being quarried extensively to provide stone for the Georgian buildings which now confer World Heritage status on the City. Other smaller quarry sites are evident on the hill of Odd Down. Quarried stone has been used at least since Roman times for building and road construction.
The Tufa Field is approximately 0.9 hectares in area ( about 9000 square metres) , with a roughly trapezoidal perimeter, approximately 150m at it’s longest boundary, with an average width of 66m .
Our site contains two major deposits, split along it’s length. The southern half of the site extending about halfway to the North is a layer of Fuller’s Earth mudstone while the northern half is Fuller’s Earth Rock Member. Fuller’s Earth is a well known feature of the Bath area, being commercially mined until the late 1970’s. It is highly water-retentive and forms an effective aquifer making for complex hydro-geology at the site.
It is probable that this layer ‘powers’ the springs lower down the hill, at Moorlands Schools, the Moorlands Park, the Oval green space and at various other points along the boundary layer with the more impermeable limestone material in the adjacent slope.
The ability to effectively store large amounts of water provides a significant buffer to mitigate flooding in the main Avon valley.
Surface water flooding is a particular concern as more green areas are paved over. Green fields, especially uncultivated ones, provide considerable surface water buffering, by absorbing sudden downpours.
The increased flood risk to Moorlands Schools when the Tufa Field is developed is highlighted by this map,.
Information from the British Geological Survey http://mapapps.bgs.ac.uk/geologyofbritain/home.html
For existing residents in the surrounding area, there is always concern for land movement. The sub soil structure to a significant depth contains large quantities of yellow clay ( mudstone). This is subject to shrinkage and expansion, according to water content, and , in extreme circumstances, slippage. As the clay dries in summer, shrinkage takes place, leading to subsidence. In winter, when runoff water from Odd Down is absorbed by the mudstone layer, the clay expands, causing ground heave.
Building on such land, especially where there are discontinuous layers, has to be designed very carefully, and may require stabilising raft foundations to prevent land movement damage to buildings over time.
Artificial changes to the water table can have significant impact on existing structures, effectively changing settled foundations.
External links : Factors Controlling the Growth of Tufa