New one to me - Anyone seen this before

[nando]

block paved a hundred metre drive in january this year, dug out and prepared base to a high standard in dry conditions yet was asked to go back and have a look by customer today, never seen anything like it before, the main lay (the herringbone) seemed to have lifted 20 mil higher than the edges in a 15 to 20 metre area, like a sheet- the blocks still tied in together moving like a spring board 20 mil above sand i dont mean its a bit spongey its actually lifted above the screed like a raft of block paving, the screed is still in perfect tact. I'll obviously re lay area but really would like to know the cause. Been laying blocks for 14 years and im baffled. I cant sleep appreciate any anwers ???

[digerjones]

i feel for you mate it is not a nice feeling when you have done the best possible job and it goes wrong. was it layed in any frosty conditions, has a pipe burst what were the ground like after you ad dug out. you will get some anwers on here off better experienced people than me.
good look diggerjones

[GB_Groundworks]

clay heave?

paste:

Heave is often said to be the reverse of subsidence and this is probably a fairly simple way to introduce it. Heave, as known with domestic structures, arises when a clay is able to absorb more water than it had hitherto and thereby expand. There is a different type of heave than mentioned above that arises when one excavates an over-consolidated clay such a London Clay.

Here the reduction in pressure on London Clay gives rise to a heave as the clay tries to expand vertically to be restored back to what it was prior to being compressed in the past.

http://www.heritagetesting.co.uk/Factsheet9.1SoilHeave.html

[williams]

Got to be heave.

I had a patio a few years ago, got called back to my surprise as i have had iirc 2 call backs in 8 years.

Got there and it looked like the pikeys had done it, paving was 3 or 4" higher in places :O levels had completly changed and it looked like they had someome rip up what i did and completey mess it up!

Anyway the manhole which was on conrete obviously hadnt moved or if it had it was very little, i proved to them what had happened and they were understandable it want my fault.

There was also a lot of oak trees nearby....

This is one reason i use a decent membrane now as i believe that it will help if the clay does start moving- i see the membrane as sort of a hammock which supports the base as one big thing instead of localised areas wanting to move it will now want to move a huge load which is much harder.
Maybe i,m wrong but it hasnt happened since and i pretty much only work on clay.

[lutonlagerlout]

must be the ground,very unusual but if it was wrong it would have sunk
the mark of a decent contractor is someone who goes back and puts stuff like this right without a big fuss
feel for you
LLL

[Rich H]

I don't think that's what he's saying - the screed hasn't move but an area of blocks has 'risen' and is arched over the screed with air between the two. Baffled.

[lutonlagerlout]

ahhh wait a minute
elementary my dear watson
the clay has heaved and pushed against the edges,the paving having no where to move has to take the easiest route and springs up
I have seen this before with oak flooring laid on damp screed,the oak took on water expanded against the walls (50mm if IIRC) and the centre of the floor stuck up like a kids trampoline

or maybe it heaved and then shrank back ,but the interlock is holding the blocks up
its very xfiles

or lastly what about helicopters or such? i know luton airport had problems a few years back with the draft from planes pulling up blocks on the taxi area?

LLL

[nando]

thanks everyone for comments, like rich h says its a full area lifted the screed is still perfect it has'nt dropped, its like superman came over night and lifted an area as one then left it floating 20 mil above the screed. Went back today and its dropped slightly but still a bit springy, i thought it could have something to do with the warm air but then it would be a common problem and its not. Id have thought aswell if it was movement underneath, the blocks would have dropped seperately and not lifted all perfectly flat as one.(not sayin it is'nt movemant just tryin to give it an explanation) Anyway gonna re whacker and screed sand relay blocks and give area a good whacker and prey it solves it, would re do base but with it lookin like its not moved cant see the point. If anyone knows of a better soloution let me know im doin it wednesday. cheers lads