dealing with subsidence

[Anonymous]

Anyone got much experience dealing with subsidence? Can't make my mind up if I'm worrying too much.

I'm tring to weight up how much movement i need to see before I really start stamping my feet at the insurance company. I'm getting about 0.25mm per day- 1.5mm ish per week but also movement in other areas from the main crack.

We had a bore hole dug and they have put the problem down to trees. I have had a 50ft conifer, an 80ft poplar and a few blackthorne and cherrys removed but they are now saying I need more out and they will get on with it. I don't know if to start shouting or drop the trees my self or just sit back and hope the house is still up right by the time they get round to it.

so what do you think is 1.5 mm a week to much or not. Over the last week our bedroom door no longer shuts [about 25ft from the crack] and a tile has cracked in the bathroom [about 30 ft away]

 

[Dibs-h]

I remember reading ages ago - that removing whole trees close to a house, could cause issues with heave (or something).

1.5mm per week - I'd look to do something or get it done soon'ish than later.

HIH

Dibs

 

[Anonymous]

heave! is that when you have had to many beers the night before

just done a bit of digging you could well be right dibs. Printed it off as a bit of back up, going to get onto the engineer straight away

 

[henton49er]

Andyoaks,

Depending upon your soil type, removing the trees can make the problem worse not better. Many clay based soils swell on increased moisture content causing heave in the surrounding ground. Removing trees without taking other measures at the same time will tend to increase the moisture content in the ground as they are no longer absorbing ground water to thrive.

Assuming that this is not the case, how long have you been getting 1.5mm settlement (or subsidence) per day? This equates to 10.5mm per week and approaching 50mm per month. This is a serious problem if it continues at this rate for any length of time.

Do you have subsidence (which is a lowering of the ground and building foundations often caused by lower lying problems such as collapsed drains, old mine workings etc) or a change in ground level (up or down) caused by, for example, a change in moisture levels in the ground?

Remedies for subsidence include underpinning, grout injections etc depending upon severity and soil type. Changes in moisture content can often be dealt with by cut-off drains which can take ground water away and provide a stable moisture level and hence stable ground conditions.

My advice would be for you to get either a geotechnical engineer or a specialist underpinning contractor to look at the problem and to come up with a solution that they are prepared to warrant will work (or you can claim against their Professional Indemnity insurance).

Unfortunately neither will be cheap - hopefully your house insurance should cover this. It is better to have your insurance company in at the beginning as they may consider that what has been done already is detrimental and refuse to cover the full cost of resolving the problem as a result.

Mike

 

[Anonymous]

Mike thanks for your input.

It is the insurance that is on with things and it is them that has/is instructing on the removale of trees. Yes we are on clay and the movement seams to be at about 45 degrees rather than straight down. As we are in lincolnshire and there is't a hill within miles it's a bit worrying as to why we have this angled movment.

I'm just waiting on the insurance companys engineer ringing me back, I think I am going to insist he comes back out

 

[RogerS]

Hmmph...******* insurance companies. They'll drag you through **** and back before sticking their hand in their pocket to stump up and do what they should be doing in the first place. Ba*tards. Hate 'em more than *ankers.

 

[Dibs-h]

Considering the only thing you've really had done is remove very large\marge trees (I'm ignoring the borehole as that will have been lined) - and clay soil - my money would be on heave. And the movement probably started shortly after having the trees removed.

Subsidence (proper) would be unlikely - IIRC your house is very old. I found this to be a good explanantion,

What is Heave?
Heave is the expansion of the ground beneath part or all of the building. This is normally taken to mean an upward movement of the site caused by expansion or swelling of the subsoil (or the opposite to subsidence). The most common cause of heave is removal of trees and other vegetation, causing the clay subsoil to re-wet and swell.

If a tree pre-dates the house, then removing the tree may cause the soil to uplift to a greater volume than when the property was built. A tree that postdates the age of the property may possibly be removed and allow the ground to revert to equilibrium without causing damage.

Heave or long-term recovery can only occur if a persistent soil moisture deficit exists. This is where the soil remains in a partially desiccated condition following the process of re wetting during winter months. This is more likely to occur if the removed tree was mature and had a high water demand, such as an oak tree.

From - http://www.rics.org/site/scripts/docume ... mentID=818

HIH

Dibs

 

[powertools]

I purchased my house in 1982 in a derilict state due to the drout of 1976 and the fact that there were 3 large trees close to the house that made matters worse it had subsided to the point that you could put your arm through the upstsirs wall. There was no insurance company involved as I had purchased the house in this state but the cost of underpinning was £12,000 which was a lot of money in those days.
If I can help with any question you may have I will

 

[OLD]

My understanding is that the trees dry out the land so it sags after they are removed it takes quite a while for the land to re hydrate and stabilise then you can do remedial repairs.

 

[jasonB]

As Old says it will take some time after the trees have been felled to see the ground return to its original level if the problem was just the trees taking moisture.

I would also be looking at the drains as the trees in their quest for moisture may well have damaged drain runs and you could be washing soil out from under the foundations.

Trees would have caused a gradual subsidance over a period of time, whats the total amount of movement you have had and over what timescale, also when were the trees felled.

J

 

[Russ]

Nasty business. We had a problem with subsidence with our old house in London. The insurance company and appointed structural surveyors were both a complete waste of time and money. Took 2 years for them to do sod all while the house was moving left right and center, cracks appearing all over the place etc.. Was a complete nightmare and many sleepless nights.

Bit the bullet and paid for the work to be done, cost a fortune but at least we could move on.

Insurance is the biggest con in the world, anyone that works in insurance will tell you that.

Hope you get it sorted!

Russ

 

[Anonymous]

J
drains is out of the question, they run in a different dircetion as does the water main.

In total we have had 25mm since it started about 12 months ago. Doesn't sound much but we had quite a time where it didn't move and the insurance company were getting ready to repair. It seams to come in fits and starts proberly down to rain rall. However we are in a moveing period and have had about 7-8 mm over the last 30 days.

Since posting I have been having a further look around the house and I think it has gone down and is now on its way back up but as its coming up its pushing apart. So even though the movment is upwards the crack size is still increasing. Still i'm not 100%, just going to have to run it past the engineer and see what he thinks. One of the new cracks on an out side wall seams to be pushing the lower few courses of bricks out wards from under the rest of the building without much of a gap from the ones above [does that make sense]

 

[Dibs-h]

Andy

I wouldn't have any more work done to any more trees - unless the trees are in a dangerous state, or you plan to excavate where it stood and build something.

Unfortunately I don't think anymore can be done until things settle and this could take some time. Not the ideal answer.

Dibs

 

[SimonB]

My parents suffered from subsidence under their extension. I seem to remember the movement measure attached to the wall for quite some time, could well have been a year or so. Presumably that was to determine once any movement had stabilised. The room was then underpinned. The insurance company paid up but then refused to insure the property, and I believe my parents then had real problems finding someone to insure them.

 

[treeturner123]

Andy

The real question is, what changed 12 months ago? If there was no movement before then, it is indicative that some external force has occurred approx 12 months ago. Personally from my experience as a surveyor (not a structural one mind you), sudden movement of this nature is the result of a distinct change. Someone in this thread mentioned the 1970s when we had a significant drought. It may be that in linacs last year there were similar conditions which the trees have exacerbated. In the 1970s, a good deal of underpinning was done which subsequently added to some heave when the sub soil returned to its 'normal' state.

My advice would be to get your own Structural Engineer, preferably a local person with a long memory and see what he/she feels is the issue.

If it is trees, then the best approach is often to trim back in stages to avoid heave and insert a root barrier close to the house.

You do not say what age the house is so there is no means of knowing what depth your foundations may be, but more modern houses tend to have deeper foundations than those built pre 1970.

Finally I would say that in general terms conifers present less of a problem than trees such as willow, poplar and alder.

Phil