Clay soil under large existing lawn

Hi,

Hoping someone can help. My first post and hope it’s in correct place. We’ve a lawn area, maybe about ¼ acre where some building work was carried out years back (a large old outbuilding was removed). The builder at the time has put down about a foot of soil over all the rubble of the old building but the soil used was more like modelling clay than soil. We had someone lay a lawn on it last year and they mixed in a huge volume of sharp sand – idea was to break the clay. I think fair to say it’s not worked and the soil is constantly soaking. I’m sure if we get a long dry spell it will dry up like concrete.

I’ve dug in some drainage trenches which I’ve left open to get an idea where water is going – these seem to be helping, they drain away a lot of the water. The trenches fill up with water from surrounding soil over time and it does drain off. The problem I can see is that the clay just holds onto the water for too long. We’re in western Scotland and have a lot of rain. We’ve also a hill and bits of bedrock which are not helping but I’ve established with some testing that the clay is the biggest problem.

I’ve come up with 3 options of varying effort. This follows a lot of reading on various forums and speaking to a few landscape gardeners. I’d really appreciate some advice before going down either route as I don’t feel like I made a very informed decision last time.

1. We carry on the with the drainage ditches and also add mulch on top of the soil – maybe twice a year for the next few years. Eventually this might improve the soil as worms etc. burrow down from the new “good” soil into the heavy clay. My concern is that this takes 50 years! I’ve really no gauge on how long this might take to make any difference.
2. We get a machine in and work in several tons organic matter (manure?) through the foot of clay soil. Turning it all over. Maybe use some of these clay breaker type products (gypsum?) if they work on our clay. I think I’d need to do this when soil/clay is drying up but not dried out - or it will be too hard. Seed or turf a new lawn once that’s all done. Incorporate french drains.
3. We dig off the foot of clay soil and dispose of it somewhere. Then get in loads of new topsoil and seed / turf on that. Incorporate french drains. I suspect this is going to cost around £10-£15k between access and moving all the soil around and buying in many tons of new soil.

I’m erring towards 2 or 3 but hoped for some opinions. My gardening knowledge is not great but I have read a lot on what we need our soil to be like. The problem is doing that to an existing lawn etc. Whilst I think we’d be patient to get a good lawn I don’t want to wait more than a couple of years.

Really appreciate any thoughts.