Ground Heave.

Local industrial estate is block paved and there are bumps all over the place, mainly near the drains.

It was built on marshy ground and apparently the whole lot has sunk slightly except where the drains were - which had extra piles to support them.

Could it be something like that?

Thanks for your prompt reply Dave. But no. This definitely coming UP. The bit over the drain is still nice and flat. Baz

yes.

and how long can I expect this continue?

Until the soil that used to be sucked dry by the forsythia, gets as soggy as everywhere else.

Classic case of 'kill tree, ground swells'

It is heave and as TNP says it won't stop until the ground has sorted itself out WRT moisture levels etc.

You can stop it now, but that means removing the ground...IE dig out a fecking big hole 2m by 2m and at least a metre deep and infill with MOT, which will have to be whacked in layers of 150mm.

A 4 yard skip, at least 5 tonne of MOT, lots of hard work and a weeks whacker hire is what's required to do the job properly, anything else is just cosmetic

Maybe the drains are leaking, clay expands when wet, shrinks when dry.Movement of the clay may have damaged the drain pipes. In dry weather the ground will sink except for the wet bits which may be around the leaking drains? Just a thought. Don

Whacker plates can damage drains if old & weak (their impact does not go far into the ground).

By "old" I mean white porcelin or brick, rather sad nearby in that every time a house got sold they brought in the block paving crowd and whacker plates, then a few weeks later a minor drain repair, then pump- out truck, then another pump-out truck, then the whole lot up again to replace the drains. Soil is clay, houses pre 1930. House By House, magpies & pidgeons come from miles around to watch placing bets on how long.

Conventional wisdom (i.e., the NHBC) has it at 2-3 years. If the forsythia was removed 5 years ago, it should have settled down by now.

But not widely known about by small builders, it seems. Typical conversation: BCO: "You'll need to dig your footings another x00mm because of that tree there". SB: "We'll cut it down, then". BCO: "Whatever. You'll still need to dig your footings another x00mm, or leave it to settle for three years". (SB: Grumble, grumble, "...I've been doing this for nn years...next-door Council never ask for this...etc.")

Or just wait till it settles, and then re-lay.

Pretty much what mine said. I listened to the BCO. he was very rational.

I dealt with lots of claims for subsidence and a few for heave.

Clay shrinks when it dries out and swells when it gets wet. Regs have increased depths for house foundations over the years to reduce the problem for buildings.

A plant or tree getting larger may well cause a reduction in moisture content of the soil resulting in subsidence.

Cutting down may result in heave.

Situation may result from a drain leak or from forsythia being cut down. There may have been an old leak the forsythia was drinking.

Unfortunately if there is no reason for the soil to dry out again there is no reason to believe it will shrink again.

If you cannot live with it it could be expensive. If the forsythia was taken out 5 years ago I would have thought the land should have stabilised by now. Is the drain to a sewer or to a soakaway?

It is a Sewer Drain. 3 or 4 meters deep.

Baz

well if it ain't broke, don't fix it.

At that depth you should be in permanent water table anyway more or less.

I don't think its the drain. Its more likely to be simple clay movement as a result of changing its drainage. And hence average moisture content.

So that's why I say, wait till it stabilises, or dig all the clay out, and put in something else..

IME that can take years, and in the meantime he's got to put up with a drive that looks like a ploughed field!