The Brew Cabin

I am currently involved in a development where a building and car parks are to be placed within a mature pine wood. In order to protect the retained trees roots my current research suggests a no dig approach for the carparks and access roads with the use of a geogrid (e.g. Tensar SS30) with DTp1 to 100m thick on top.
Is this sound advice? Can you suggest any other methods (bearing in mind the order of the day is to minimise damage to roots whilst allowing them to breathe and receive water)? Is this method also suitable for laying the concrete slab foundations for the building? How much does this geogrid stuff cost?
May I just add that I was blown away by this fantastic site! So very informative!!

The plan to use a geo-grid for the car park area seems fine to me, although that 100mm depth of sub-base is probably a touch on the shallow side for a multi-vehicle area.

What sort of surface will be used for the car park - have you considered a permeable surface?

Not too sure about the procedure for building foundations as it very much depends on the building type and dimensions. You really need specific on-site advice from a Structural Engineer for such a project.

Yes the surface layer must be gravel with no fines so that the roots can "breathe".
I presume hardcore is similar to DTp1 but with no guarantee of the granular content? If this is so, could hardcore be used with the fines screened (seived) out?
I suppose my question is how do you make up DTp1 with the guaranteed content?

"Hardcore" is a term I dislike because it means nothing definite. Old flags and broken bricks can be hardcore, as can road planings, quarry waste or 'scalpings'. There's no guarantee of particle size, no grading envelope, no quality control, no hardness requirement, no refusal of organic material ... it's whatever anyone wants it to be. That's why I never use the term in any of my descriptions or advice.

If it's a DTp1 material that's required, then ask for DTp1. similarly with 50mm crusher run or whatever other aggregate is required.

It's not easy to "make up" a DTp1 compliant material unless you have access to a quarry or a huge stockpile of aggreagtes with a laboratory and specific testing equipment. You can't really start with, say, a 100mm crusher run and turn it into a DTp1 without more work than it's worth - leave it to the quarrymen or recyclers.

For a no-dig gravel surface, I think you're going to have to forget DTp1, which has too many fines, and instead opt for a "no-fines or low-fines granular sub-base material suitable for SUDS projects" - eventually the industry will come up with a catchier name than that, but for now, it's the best we have.

Thanks
The research I have suggests the use of DTp1 because it was low in fines!......Is this wrong then?
Sorry for my ignorance but what's a SUDS project?
Since my initial posting I have heard that the use of Geoweb is preffered due to the low tensile strength of Geogrid. But this Geoweb stuff looks pretty pricey and I am trying to minimise costs where possible. Do you agree with the Geoweb school? or do you think the Geogrid would be sufficient for a small car park (up to 30 cars)?
You also suggested a subgrade greater than 100mm, what thickness would you suggest and what thickness of gravel on top?

Many thanks for all your help.

Presume trees are not protected by TPO's or other classifed area restrictions, if so could be in difficultly with compaction above root area.

Also another consideration is the DTP1 altering acidity of soil, we have in past had to use a gritstone base ( not as structurely sound ) rather than limestone, around a special conifer tree, in order to retain soil ph level.

I agree with Ability that assuming trees are not TPOd or in a conservion area then you have nowt to lose legally as you can cut them down if you want, as for the PH level 99%i(ncluding al coniferous species) of tree will grow better in a PH nuetral to a PH aidic surrounding ie; avoid limestone at all costs as this wil turn surroundings to alcaline far further than you might think. As for the geoweb we recntly priced a job using this to create 21 raised borders in a private garden it came out at 51 grand does that answer your question that it may be a bit pricey. neadless to say we or any other of the contractors got the goahead. Alan Ditchfield (Fully qualified tree surgeon and Horticulturalist).

Thanks guys
No problem with TPO's but the idea here is to retain as many trees as possible (planning conditions apply - so we cant just cut them down, neither would we want to). The main question is how to form a carparking area in a cost effective way which minimises damage to tree roots and retains the trees which will be directly adjacent to it. I am aware of the pH issues (The aggregate is most likely to be sourced from the estates own quarry and will therefore be similar to the glacial material the trees are already growing in). If anyone has any details of establishing car parks or roads in a similar situation I would love to here about their methods and relative success.

have a look on WWWdotCOOPERCLARKEdotCOdotUK this is who we used to price the job mentioned earlier they were v helpfull.

Cell-Web is a product from GeoSynthetics Ltd, and is generally used to stabilise granular sub-base material. This, or a similar geo-mesh would probably be a better product for this type of project because it lies at the interface of the sub-grade and the sub-base and massively increases the performance of the sub-base without having to dig to find a stiffer sub-grade, use a capping layer, and/or use increased thicknesses of sub-base. On previous projects we've used products such as SS2, ParaGrid, amnd TeleGrid.

I'll see if I can persuade one of the geo-grid boffins to intervene and offer some pertinent advice for this project.

Going back to your other questions, DTp1 doescontain a significant proportion of fines and might not be the best material to use for this particular project. SUDS is Sustainable Urban Drainage Schemes (or Systems), and is a developing series of methodologies that aims to return as much surface water to the ground as is possible and thereby minimise loading our rivers and streams with run-off. Permeable paving systems are a part of 'SUDS' and the permeable pavements rely on no/low fines sub-base materials that allow surface water to pass through freely, either to an underground containinment or attenuation system, or to a natural aquifer.

Most aggregates suppliers will have a in-house sub-base material that is suitable for use on such a pavement. At the moment there is no 'off-the-shelf' descriptive term, such as DTp1, and we may see the use of proprietary brand names for these sub-base materials over the shorter term, before a nationally agreed spec is finalised.

I'll speak with a couple of specialists in this field tomorrow and ask them to chip in with their two penn'orth.

Thanks Tony
As allways, very informative.
I look forward to reading the boffins views.

Is this cellweb a geotextile? if so and Tony says this is adequate then i would definately go with his advice as it would probably be in the region of 20-30 times cheaper than using Geoweb, Tensar or similar grid systems.