Subsidence in Bristol

[saveasteading]

5 hours ago, Russell griffiths said:

i'm no structural engineer

 

Perhaps there is time?

 

I will add this. Some egg sucking info to be sure, as other people may be reading..

 

Trees drink water from the ground. if that ground is sand you just get dry sand and it is still strong.

But clay is thousands of horizontal layers of silt from ancient lake beds, and there is water between the layers. So when it dries the volume reduces and the layers draw closer together, usually causing vertical shrinkage.

 

A building with foundations on it will settle. If it is uniform then nobody really notices.

Then come winter it gets wet and rises again. No harm done.

 

The surface dries first of course because of direct heat from the sun, and because trees have tiny roots there.

The hole we dug in clay last week is 1.5m deep and utterly dry. Last winter it was like plasticine.

 

But if there is  a thirsty tree or a pond on one side then it will affect the nearby wall more than others.

Again it will probably recover to level but it may take longer. If someone has meantime filled any cracks or underpinned, it can cause stress elsewhere.

 

I think the insurance industry has come to realise that they shouldn't rush the decision and it needs a proper SE to advise.

 

 

 

[saveasteading]

For anyone interested. Freshly dug 2.2m hole this morning. 

it is very dry throughout, with a touch of dampness in the bottom layers. That is ground unaffected by trees but it will have shrunk throughout.

In the winter this surface was sopping wet and any wheel ruts sat full of water.

 You can see horizontal banding which is different eras of clay and thin bands of weak sandstone.

[saveasteading]

Digester. An efficient 2 days got this done. The groundworker is very good as well as experienced. He said it would take 3 days but I insisted that with our complementary skills it would be easy in 2.

And so it was.  Wouldn't it be good if we could save 1/3 cost all the time.

 

We disagreed on hole size and concrete mix but I insisted on mean -ness / efficiency. Only 200mm ish extra all round, 1:8 mix, using 2 bags of ballast and 12 bags cement,  mixed by the digger bucket. Strapped down using 2 x retired ratchet straps.

All is good. See pic.

Other good news was coming across ancient land drains at 4m or so centres: as we had anticipated but not yet seen. They've collapsed but will still help tge drainage.

 

1.7 days for this,  allowed some other work about site too.

[Alan Ambrose]

I’m curious - how do you mix concrete with a digger bucket?