BS5837 Adjusted RPA

[Gary Prentice]

I'm sure I was told of 10+m piles on a site, but hesitated to post due to uncertainty in my memory. It would be interesting to find out at what depth of pile the costs equalise against a mass fill strip/trench foundation. At some average depth of footing the concrete costs, excavation, spoil disposal and underground blockwork must make piling economically the cheaper option.

 

Sorry for the derail.

[daltontrees]

I'm sure I was told of 10+m piles on a site, but hesitated to post due to uncertainty in my memory. It would be interesting to find out at what depth of pile the costs equalise against a mass fill strip/trench foundation. At some average depth of footing the concrete costs, excavation, spoil disposal and underground blockwork must make piling economically the cheaper option.

 

Sorry for the derail.

 

Not really a derail. In my case the piling was the only option due to depth and site contamination. But it raises a fair point, could piling be used as a legiyimate way of minimising the likelihood of root damage? Clearlyt hrere would be no point in doing this if a building is going to be put up, but ground beams between piles could work in some situations where only a wall is to be built. I've done something akin to it on a very small scale, basically stopping a foundation on either side of a root, then building up brickwork and building in a RC lintel at ground level and then building the wall as normal on top of it.

 

And back tot eh original posting, I xpect there's no benefit in this sort of way of thinking there because (a) there'll be sod all beneficial rooting in teh car park and (b) there's much more likely and better rooting to be protected on the other side.

[Ruskins Trees]

Use an Air Spade to see if there are any roots. If no roots helps argue vs root protection areas. If roots are found development could be compromised.

[daltontrees]

Air spade sounds OK occsionally, but if it's just a 5837 survey and TCP the onus surely isn't on the developer or surveyor to prove where the roots are? Isnt it just necessary to have sound arb reasons to expect rooting to be non-circular and to portray the RPA accordingly? I don't have the BS handy to give the exact wording, but I'm sure you don't have to prove it.

 

Air spade was (along with ground-penetrating radar) what I was thinking of as being the luxury of a lottery-winner.

[Island Lescure]

How expensive would it be to use a ground penetrating radar to figure out where the roots are? At least on the parking side.

[daltontrees]

How expensive would it be to use a ground penetrating radar to figure out where the roots are? At least on the parking side.

 

I was thinking more of showing that there are plenty of roots on the other side. Back to the OP, I think the objective is to demonstrate that an asymmetric RPA is appropriate. Can GPR even be used on steep AND level ground?

[Jon Heuch]

Can GPR even be used on steep AND level ground?

 

Not quite sure what your question is:

 

Can it be used on level ground - yes.

Can it be used on steep ground - yes. It becomes more difficult, just as it becomes more difficult to push a child in a pushchair on steep ground.

 

It would also be possible to take the radar antenna out of its buggy and search for the main roots coming from the base of each tree - it's a bit laborious but quicker than airspading; the results can be truthed by some simple poling (not in the car park or on hard surfaces!) or even excavation as visual confirmation removes any uncertainty.

 

But neither of these factors, are deciding factors. Based on the photo that started this train off:

 

It would be possible to put a series of scans across the grassed area below the trees. It would be possible to put a series of scans over the car park, provided it was car free! It would then be possible to compare them for rooting activity.

 

As for cost, it would depend on the location and the service provider.

[daltontrees]

Not quite sure what your question is:

 

Can it be used on level ground - yes.

Can it be used on steep ground - yes. It becomes more difficult, just as it becomes more difficult to push a child in a pushchair on steep ground.

 

 

I mean can it be used across a strong break in slope like that surely at some point it will not be scanning directly perpendicular to the ground, it will be covering a wedge of ground. See attached. Area A is OK and so is B, but what information can you get for area C, D and E?

[daltontrees]

There's a threat to the trees, both from the development, and from the occupiers of whatever is developed. Just because the trees are off-site doesn't mean that they are not material considerations.

 

Ed

 

Please remember I was responding to a specific question namely "So, if the neighbour wants to keep the tree, the development proposal can't be approved?". The thread was started by someone asking about plotting RPAs but as these things go it has drifted in to a general discussion about whether the development should proceed. I was just saying to Kevin that development needn't be prohibited just because a neighbur wants to keep a tree.

 

What would a developer do then? If the tree is not already protected, he could sever the roots to the boundary. Then apply for consent.

 

But say he didn't and the Council refused consent because it valued the tree and thought it would be lost if development was authorised. So What would a developer do then? Sever the roots to the boundary, and then re-apply? This was part of the rationale for me saying "Council'd probably have to TPO the trees."

 

Back to that survey, then. If the OP reported that the volume of soil required for the ongoing vitality of the trees on the embankment had no significant element under and beyond the retaining wall, would a Council question that? Would they prod, scan or airspade the site to disprove the surveyor?

 

Rhetorical questions, but for what it's worth personally based on the photographs I'd be suggesting to the developer that the subsurface constraints on the car park side are minimal.

 

But you've raised an interesting point about off-site conditions (an age-old issue in Planning) but when I think about it trees don't quite behave like other structures and in view of the position on abatement of encroachment I can't see how they can be protected by off-site conditions alone. The consent could be granted with conditions to protect the trees then their owner could just chop them down because the conditions are not binding on him.

[Jon Heuch]

I mean can it be used across a strong break in slope like that surely at some point it will not be scanning directly perpendicular to the ground, it will be covering a wedge of ground. See attached. Area A is OK and so is B, but what information can you get for area C, D and E?

 

C: It's best to avoid an air gap between soil and antenna so running the radar along a specific gap like this should be avoided. it's not that it won't work, it will just be difficult (not impossible) to compare with other scans either side. In practice ground is rarely dead flat and interruptions are common and it's just a fact of life.

 

D: The reflection from the base of the wall should be sufficient to mask all signs of roots. I have been in discussions with people as to whether, if the antenna was angled at 45 degrees, you might be able to ascertain the depth of building foundations.....possibly, but I don't think it's a starter but it might be an interesting project.

 

E: on the whole the radar antenna is too big to fit into the gap between tree and wall (as seen in the photo). The antenna could be placed out of it buggy in or near this area but it would be difficult to provide a plan of what was found. It might be useful if you backed up any findings with some excavation to ground truth root presence......but if you are going to do that might you not just start with some excavation....?