Posted: 29/02/2016 at 18:55
Just a couple of things to think about. Firstly soakaways on a clay soil very rarely work. The water will take a long time to disperse through the clay that surrounds it and secondly you need to think about where the water will disperse to. It usually heads to the lowest point which may be your neighbours garden depending on your topology and geology. What type of soakaway is your builder suggesting? Is it a rainwater harvesting crate based system or more likely a ton of gravel in a membrane? If it is the gravel option it will eventually clog up so you will need access to it. Putting it under your turf may not be the best option. Is he also installing leaky pipe to collect water from the rest of the garden? Has he calculated the size of soakaway that you need? It will have to cope with the water that garden usually soaks up PLUS the run off from the patio. An arbitrary ton of gravel is not the way forward. I would image the 5m regs would apply to your case. Take a look at this great website for advice.- www.pavingexpert.com
Do you know why you are waterlogged? Is the soil compacted? We corrected the drainage issue on a clay paddock by breaking up the compaction which effectively moved the water further underground. It has been 5 years and it has not waterlogged since. Remember your turf will be laid on 75mm of compacted MOT so the water will be below this level. I suggest that you get in a proper drainage company involved before the turf goes down. The 'We can through in a soakaway' is usually the refuge of the 'We haven't got a clue'.
Secondly, I am not surprised that you have found soft clay under the patio area. Builders and developers prefer to bury rubble instead of doing their job and getting rid of it. I have never lifted an area that has rubble beneath and found it to be dry. There is always a lot of water. You need to remove the rubble ad dig down until it is solid. Backfill in layers of 75mm of Type 1 MOT and compact it until you get back to the level you need. If you backfill in one go it will settle over time and your patio will move. Remember your patio needs to be 150mm below the DPC.
Hope this helps.