In the process of doing the groundworks and laying trenches for water/power connections we have dug up quite a few areas of our field with some interesting results. We first noticed the interesting alluvial layers when we dug the 200m long services trench; we found we a layer of clay on top but under that it is mainly a one meter layer of ballast made up of small mainly chalk pebbles, sand, grit and some clay. Below that we hit the thick blue clay 'Kimmeridge' that made this area so attractive to Thames Water for a reservior (they would have had a natural waterproof base). We also hit spots of more dense clay and sometimes sandy stuff. Clear signs of the ancient riverbed that is now the Thames valley. This picture shows the layers in the hole we dug for the sump.
You can just see grass at the top then 30cm of light clay, then pale ballast, the dark clay below. The red mark in the centre is where we have cut through a nineteenth century land drain which indicates they has issues with the site too.
This study has been made even more interesting by trying to work out how porous - or not- the topsoil layer is and to predict how we can manage rainwater or risk of rising water table. Our entire village is in a flood plain area and this field anecdotally suffers from standing water as times during the winter.
There are two theories about the cause of this, each would require a different strategy to manage:
1, the light clay layer just below the surface absorbs so little water that rain sits in puddles till it evaporates
2, the ballast layer underneath gets saturated above heavy Kimmeridge clay and has nowhere to go so is forced to the surface
Next to us, a housing developer is trying to work this out as part of thier planning process. They have employed geologists and all the neighbours have developed theories so its become a hot and contentious subject. Establishibng the facts has been made more difficult by the near-drought conditions this year which have resulted in there being very little ground water to observe.
We favour theory 1 as it seems to reflect what we witness; when it rains the water just sits there; where we have dug down into the Kimmerigde clay, water trickles slowly into the hole from the base of the ballast layer. On this basis, to reduce standing water on our turning circle we have drilled large holes through the surface clay and back-filled with ballast to act as a series of small soak-aways.
For the polytunnel foundation we need to a, avoid floodwater coming in from the surrounding field and b, make sure that irrigation water has somewhere to go. So, having removed only a very little topsoil to achieve a level surface we have added a 20cm layer of more ballast and will add drainage channels down each side.
We really have no way of knowing if this will be sufficient and will just have to wait for the rain. Who knew that so much geology would be required for horticulture.