Shrinking mortar - Shrinking of resin between sandstones

[Scott6471]

Hi,
First post!
A year ago a friend (ex bricklayer of some 20 years ago) helped me lay a new patio. We made sure to try and do everything the right way, a new retaining wall was built and all areas filled with MOT to raise the level overall. All compacted with a whacked plate and then the Indian sandstone slabs laid on a bed of mortar (slabs were also brushed with a proprietary cement mix on the back for better adhesion). Gaps were set to about 10mm between slabs. To point we used a modern resin compound (weatherpoint 365 by Marshall’s) which is brushed in to the joints after a thorough soaking all over. The finished job looked fantastic.
About 6 months ago I noticed a crack in the retaining wall along the cement mortar line in a zigzag pattern along one edge, which I put down to some ground settling (the walls were laid on foundations 6 inches deep). Since the hot weather we’ve had now I’m noticing some serious shrinkage in the mortar joints between the slabs, literally running the length or width of a slab (I’d add an image if I knew how). The majority of the shrinkage seems to be where the most MOT was used to backfill (we extended one section, only about 3 sq mts, but had quite a bit put in to fill it).
My question is this, is it likely that the shrinkage is just caused by the moisture being drawn out of the mortar below and the weathershield 365, or would it suggest a bigger problem?
Also, is it possible to just fill the gaps with more 365, I have half a tub left over still

Sorry for the long post and thanks in advance,
Scott

[lutonlagerlout]

its tough without pics scott
but sounds like you might be on clay
the recent dry weather has caused a lot of subsidence as the ground dries out it shrinks,and when it gets went again it heaves

did you compact the type 1?

you can add more 365 but need to rake out the affected joints
cheers LLL

[exoticpete]

We have seen lots of subsidence due to the hot dry summer bit of a nightmare really if you are on clay there is not a lot you can do if the subsoil starts moving about, worst affected areas are on new builds heavy clay with imported 'screened topsoil' eg crushed clay ....

We are in Berkshire, heavy London clay in most areas

[Scott6471]

Thanks guys.
Yes all type 1 was compactedThe clay makes sense. We are very heavy clay soil. To the side of my property (extended about 18 months ago), there is a gap formed of about 2 inches where the soil met the foundations, the dry weather has clearly shrunk that. The movement in the ground due to the clay also makes sense about the cracks in the retaining wall.
Is it best to leave it for a while more to allow for any further settling or should I make good straight away?