More root questions

[Gary Prentice]

I met someone today who has a imprinted concrete surfacing across the width of the back garden. It's started to crack from one fence line to the other. The crack is widest at the end nearest an offsite tree, a semi-mature hornbeam (definitely a hornbeam Stubby), dbh of @600mm situated about 1500mm from the start of the crack. The slab is about 5 years old, damage became noticeable towards the end of last summer.

 

The tree is owned by a LAMO, a housing association, who are demanding a structural engineers, or similar, report and evidence that the tree is the cause. 

 

Okay, a couple of questions;

The slab that has cracked is approximately 5 x 5 m. There's a couple of adjoining slabs which are much smaller with expansion/construction joints separating them. The cracked slab seems quite large to me and I wonder if the crack is nothing to do with the tree but due to contraction/expansion of the slab. A bit of research online gives widely different guides relating to sizes and joints. Thoughts/experiences anyone?

 

If we start to do some digging and come across live roots going under the slab, what sort of size would they have to be? - to be claiming 'on the balance of probability' that they're the cause. 

 

The garden's covered in concrete apart from a narrow border to the bottom, furthest from the house. At around the centre point of the edge a second crack is also starting which, if it continues, will go on to meet the larger crack about midway. On the other side of that fence is a sycamore of a similar size to the hornbeam.

[Gary Prentice]

Sorry Edward, I missed your reply.

 

The clients are elderly and probably can't afford a lot of technical/professional input.

 

What I was trying to ascertain was some views on the situation. I'm going to do a couple of trial digs over the weekend to see what the slab depth is and what's under it. If there's a whacking great root at the start of the crack we can go from there, but what happens if there is a 50mm diameter root under a 150mm thick slab? 

 

I've done a fair few TPO apps involving light structure damage/actionable damage ( all the way to appeal in a case both the LA and I hoped would answer some 'grey area' nuisance acceptions) but nothing involving this type of construction.

 

Speaking to them this evening I've said again that I can't stray outside my field because it would be wrong to do so, but it doesn't hurt to learn more about the associated issues.

 

[Khriss]

In the subsidence claims I handle - I do get tired of them always blaming the tree - irrespective of other possible causes . Relative youth of slab would concern me -am no structural engineer but 150mm on a 5m x 5m seems a bit skimped ? K

[Rough Hewn]

Is the slab concave-subsidence or convex- root expansion or just shlt concrete?

[Gary Prentice]

The furthest edge from the house may have subsided, the cracked edge appears to be raised by a couple of mm. so it looks like the slab has cracked in half and the distal half has tilted. 

 

I couldn't find any roots adjacent to the damage and wonder if this settlement.

The properties on made ground, a former mill site. The garden was lawned before the slab was laid.

 

geological maps are devensian tills over pennine coal measures 

[Rough Hewn]

Many years ago I worked in heavy industry, making huge concrete slabs for pre floors. 2.5m x 1-9m long
5cm - 9cm thick!
Each slab contained 60-160 high tensile steel wires 6mm thick.
Concrete slabs without rebar or wire will probably bend and crack at some point. If not professionally mixed concrete, stress lines will help crack it.

Can you dig it up all up and re-lay with steels in?
Or is there height to whack another 6" over the top with steel?