foundations-free draining soil

[munchingB]

im going to pour a foundation trench and my soil is very free draining sand onto chalk. its been quiet warm and sunny here recently and im wondering if anyone thinks that there might be a case for lining the trench with polythene or tarps to hold some of the water? cheers

 

[noseall]

Volume is usually the saving grace here as well as being below ground level.

It is likely to shrink a bit but unlikely to cause any structural issues.