On site geology is the Oxford Clay Formation. Which is described as:
“Silicate-mudstone, grey, generally smooth to slightly silty, with sporadic beds of argillaceous limestone nodules.”
BGS Borehole 398037 which is situated to the south west of site shows considerable thicknesses of Clay, extended to the base of the borehole at 44.81 mBGL.
BGS describe the Oxford clay formation as being blocky and fissile in certain areas, so there is potential the clay in to pervious (support of flow through fissures) in certain localities.
Trial pits showed varying thicknesses of made ground resting over the Oxford Clay.
Site works comprised of 2no. trial pits with soak away tests conducted in each on. The positioning of these pits were sited in anticipation of a fairly large soak away being required. Clay typically being assumed to have a low infiltration rate.
Holes were dug using an excavator, with care being taken to keep the side vertical. This is important so as to be able to deduce an accurate volume, and pit internal surface area.
The location of the pits can be viewed on the location plan in Appendix 1. Dimensions of the trial pits were chosen so as to representative of the final soak-away. Trial pit dimensions can be seen on the Trial Pit Log sheets in Appendix 3.
Calculations show that based on a 1 in 10 year rainfall event, allowing for a 20% increase in rainfall events owing to climate change, that a trench or pit soak-aways could provide adequate infiltration drainage for surface water from the proposed development. Results from Trial Pit 1 are adopted as a worst case scenario.