Stablising spongy road stone - How do i stabilise spongy road stone?

[martinwilki]

I am in the process of preparing and laying a new foot traffic patio. The existing shingle has been removed, drains laid and the top surface prepared with 100mm road stone and compressed with a whacker plate. It is intended to lay a sharp sand / cement mix on top of this and then the final slabs.

The problem I have is that an area of approx 3sq meters is very sodden and remains spongy despite considerable effort with the whacker plate. As the plate was being used it was noticable that the top surface was becoming wetter and the plate was difficult to move across the surface.

The soil is clay, but this is the only area where the water is accumulating. There are no water mains on this side of the house and being in the country we are unaffected by any other property. Whilst we haven't had much rain lately I'm still assuming that this may be due to the water table as the house & patio are lower than all the surrounding land.

I am concerned that if I continue as intended that the ground will remain spongy and there will be the inevitable movement on the slabs. Can somebody please provide some guidance and advice on how to progress from here?

Would it be possible to relay the road stone with cement added to make a mix that would set hard. Should I be doing something underneath the Road Stone. Should I lay a complete concrete base across the whole patio area? Your comments greatly appreciated.

[lutonlagerlout]

martin
I guess you are in mid beds with that white sticky clay
the spongy area needs to be removed and the subgrade dug down till you hit firm ground
then add new stone and wack in 75mm layers
did you use terram or similar under the stone?
the spongy situation can occur when the type 1 gets absolutely soaked so best to cover what you have wacked as it is pouring in luton as i type
cheers LLL

[martinwilki]

Hi Lutonlagerlout, thanks for providing your comments. You were close its South Beds actually, Studham area so just around the corner from you.

This has been a bit of a DIY project following the advice of many websites including this one which I feel has been absolutely imense. I have just googled "Terram" which has led me to a type fabric material. The project was started back in June when you may recall we had all the heavy rain for weeks. The drains had just been laid and it was decided to get some stone down as quick as possible to stop all the clay and muck being walked into the house and to give us a half decent surface to walk on in the interim period. I had never noticed any real areas of wet previously but then not all the area was whacked down.

What is the benefit of Terram in this situation? Would you advise that I use under the effected area or does it need to be all over. It was evident that when we dug the drains the clay went down some considerable distance so probably not practical to dig back to solid ground and then build back up.

[lutonlagerlout]

looks like you were unlucky there
i dug a grave for my mates mums dog a few years back in studham and it was pure chalk==> rock hard


when it goes spongy like that its holding water
the terram basically stops the type 1 and the subgrade (in your case clay from migrating over a period of time and causing the patio to fail
I would cover the affected area for a few days and see if it dries out
then wack again

as its only foor traffic it may sound like overkill but best to do it right ,once
also you need to lay your flags/slabs onto the sand /cement as you go
cheers LLL

[martinwilki]

Thanks again LLL, I'll follow your suggestions and see if I can get it to dry out. Assuming I can compress the stone into a firm base would you see any possible reoccurence of it becoming spongy again?

Also would you advise providing a stronger Sand/Cement mix over the affected area?

[lutonlagerlout]

the sponginess normally goes after a few days of dryness
sometimes it almost sets like concrete
good luck with the weather
LLL