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<Mike M>
posted
As an ISA Certified Arborist, I find myself trying to practice what I preach. I have to put in an informal driveway winding through mature oaks in my new home. I thought of using all my woodchips but I think I would be making a mess that would be a nightmare to correct. I am told there is a fabric that can be used under stone, that will prevent the stone from mixing with soil and really compacting the soil. First question: Anyone know the name of this heavy duty, long lasting fabric barrier that acts like a landscape fabric? And next; anyone have any feedback on the best way to construct a driveway, stone or whatever, in a way that has the least amount of impact on root systems?
Thanks all.
M
 
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<Russ Carlson>
posted
Reply to post by Mike M, on April 06, 2002 at 19:18:16:

First, the fabric is called (generically0 geotextile fabric. You can probably get it at any contractor supply, or buddy up to a local contractor to get just as much as you need. Place this on existing grade, then cover with an adequate amount of stone. Depending on the weight bearing capacity needed, you will probably want at least 6 inches of stone, maybe 8. You can also sandwich with multiple layers of fabric. The fabric distributes the weight, so it doesn't compress all in one area.

Second, build it somewhere else! [Smile] There is no way to install a driveway without impact. You can minimize it, though. Best way would be to build a suspended driveway, using reinforced concrete supported on piers. This has been done successfully (personal report, Australia). Very expensive, I suspect.

Next best is to build over existing grade, as described above, with fabric layers. You could top with pavement if necessary. If the trees are mature with wide spreading root systems, the transport roots probably extend past the driveway width. While the pavement will cause the death of fine roots, deeper transport roots will likely survive. Be sure to avoid compaction, excavation and other root disturbances.
 
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<Scott>
posted
Reply to post by Russ Carlson, on April 06, 2002 at 19:18:16:

I can add a little to Russ's first point.

The geotextile you would use in a situation like this will be significantly heavier than the "landscape" fabric used to suppress weed growth or keep a gravel path from infiltrating into soil below. Some years ago a tried to get specs for what was actually being used in these "sandwich" tree root zone installations and nobody seemed to know. When I did my research there was no "web." The manufacturers were Exxon, Amoco and Reemay. Dupont was in the game and may still own some of the trademarks, but I think licensed to business to somone else... maybe Reemay. Do a search on "geotextile" and look for those names or combine them in a search.

The material I've seen published is generally descriptive and it was collected by American Forets in their now defuct "ReLeaf for New Communities" project literature. The program may still be running under NAHB and NADF auspices. I think Steve Clark promoted the idea.


The bottom loayer of fabric does two things. First it prevents infiltration of the stone into the soil and root area below. This minimizes physical damage to roots and maintains porosity in the stone layer. Second it does somewhat distribute the weight.

To really distribute weight the sandwich is used. As I understand it the stone must be an angular aggregate (with minimal "fines" if you want it also to be a porous, breathing layer). The concept is that the trapped aggregate is all locked together by the angular intersections and it effectively spreads load. Size of aggregate probably depends on loading. You can appreciate that the fabric needs to be pretty tough to not have the angular material punch through under load.

Of course there needs to be a driving surface on top on the sandwich, again type and depth depending on load. If you want the sandwich to be a breathing layer either the top surface must be porous or the sandwich must be positively vented out to the sides.

As Russ said, expensive. Probably less than a helicopter, or the Luke Skywalker speeder or the Geogrge Jetsonmobile. But not cheap. You could park at the road and walk in.... no Mom's not going to like that idea with three kids and groceries.

Sometimes I think you may just have to plan on an evolving landscape... even rural landscape. Do the best job of mitigation you can. Site the drive to keep the trees you really don't want to use. And plan on some gradual dieback or decline of affected trees, with eventual removal. Plant or groom replacement trees to fill in the holes as the existing ones decline.

Russ is right. Short of builing an elevated highway, there will be impact.

Good luck.
 
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<JPS>
posted
Reply to post by Scott, on April 07, 2002 at 21:24:38:

The sharp, locking gravel I've worked with is referd to as "traffic-bond" you lay it down, compact it and drive right on it.
 
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<James Causton>
posted
Reply to post by Mike M, on April 06, 2002 at 19:18:16:

Hi Mike, Somewhere, I have specs and pictures of a project I undertook about 9 or 10 yrs. ago. The project required building up a foot to 18 inches of fill over the root system of an established Thuja plicata and then pouring a concrete driveway over it. The design involved a geotextile membrane on the parent soil, perforated 1 1/2 inch pvc pipe in pea gravel, another layer of gtm over that, and then sand and the cement on top. over the 9 or 10 yrs. since this was done, there has been NO dieback in the crown of the tree; no problems with the driveway either. If you give me a couple of days I can dig out the files and upload them for you. The basic method was outlined by Steve Clark in his workshops for NADF "Building With Trees". When it comes to computer stuff, I am neither good nor fast, hopefully I get it in the end!!! Please be patient,

James.
 
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<Russ Carlson>
posted
Reply to post by Scott, on April 07, 2002 at 21:24:38:

Hmmm, if we had Luke Skywalker speeders, maybe we'd just need landing pads, not driveways.
 
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<Guy Meilleur>
posted
Reply to post by Russ Carlson, on April 08, 2002 at 07:09:21:

Also:

Before fabric is laid grade must be fairly even, or fabric will stretch/pucker/rip/fail. Skim off the high spots, fill in the dips. Pine stumps will quickly become holes so best to get them out.

I've helped put in driveways over root systems a bit down here; the fabric does work, sandwiched works better.

A woman from VA Tech Urban Forestry Dept.(Sharon Day?) did a good study on this--go straight there or reply if you want the citation.

Also, re NADF's Building WITH Trees workshop--if they are coming to your area, do NOT let them include an open-ended gasfest on Firewise communities. That killed an hour of the workshop here and screwed the whole thing up, imho.
 
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<Scott>
posted
Reply to post by Guy Meilleur, on April 09, 2002 at 21:29:56:

Guy, If you get a chance can you post that VA Tech citation. 8 or so years ago when I was researching this technique I was referred to Bonnie Lee Appleton at V.T., since she'd done a lot of work with landscape fabrics and at that point they did not seem to have a lot of material on structural use of fabrics.

SC
 
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<Scott>
posted
Reply to post by Russ Carlson, on April 08, 2002 at 07:09:21:

Or just a tethering post... it never touches ground. But I suppose when you step out you'd want a dry, firm walkway.

Come to think of it, where did George Jetson park his commuter vehicle (to the Spacely Sprocket Company)? You remember George, his boy Elroy, daughter Judy, Jane (his wife). But what was the dog's name?
 
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<Guy>
posted
Reply to post by Scott, on April 10, 2002 at 15:28:38:

Scott, it's not at hand, but I know I found it by going through the Va. Tech urban forestry dept faculty description, and I'm pretty sure the author was sharon day.

I was just there to see how far Va. Tech is lightyears ahead of NC in uf instruction.

O and the dog was Astro wasn't it?

Now, re valuation, the field guide refers to effect on neighbors as a factor in Location Rating. what about that, guys? I know the Jetsons are more fun and all but there's brisk business in appraisal of trespass-cut boundary trees here in the 'hoods.

I call it the Happy Saw Syndrome
 
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<Wulkowicz>
posted
Reply to post by Scott, on April 07, 2002 at 21:24:38:


I can add a little to Scott's point.

The functions of geotextiles are as broad as a salesman's imagination. They can be one of the most oversold items around and I find them in specs and drawings for a surprising number of landscape uses.

They had humble beginnings in not having a warp or a woof, which meant they were not easily tearable in their weakest direction like most fabrics. Then came the magic. And by the time it came to roadway construction, you'd couldn't find a project that didn't have geotextiles somewhere--soemhow.

I was asked to troubleshoot a project where the dying new transplants had actually drowned because the filter fabric quickly worked as they were designed and filtered, and collected fines from the soils above and then formed an impenetrable barrier at the bottom of the tree pit. The drawings looked fine; very rational, quite convincing, The practical result: a very, very expensive failure,,,

As to structural weight distribution, the "best" way to distribute weight is a sheet of something like plywood The usefullness of weight issues in geotextiles is both narrow and subtle--not at all like the salesmen's claims. One thing is true, if you've seen one magic bullet, you haven't seen them all--yet.


Bob Wulkowicz
 
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<JPS>
posted
Reply to post by Scott, on April 09, 2002 at 21:29:56:

Astro?

But we will know soon. I think a live actor remake is coming out soon, ala Rosie O. in the Flintstones.
 
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<JPS>
posted
Reply to post by Wulkowicz, on April 08, 2002 at 07:09:21:

Hmmm, sounds like a good temporary driveway, 3/4 inch marine plywood with 6-8 inches of course chip.
 
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<Scott>
posted
Reply to post by Wulkowicz, on April 08, 2002 at 07:09:21:

One of the things I was looking for in my research several years ago was a horticultural specification for geotextile to go along with the structural specification. The fabris are indeed rated for tear strength but also for permeability (some manufacturers call it permitivity). I wondered if the ability to allow air and water flow was important. I could have added silt up to the list. Nobody had any clue. Including the experts in weed suppression using landscape fabric.

As an aside I think the main function of a single layer is reinforcement or to prevent infiltration. Load spreading comes mostly from the aggregate sandwich between two layers of fabric.
 
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<Guy>
posted
Reply to post by JPS, on April 11, 2002 at 22:29:24:

JPS, the way I've read the research, More chips are more useful than plywood, which also has a nasty habit of breaking if surface underneath is not flat and firm.

The "sandwiching" method is nicely illustrated in NADF's Brochure #20, which I find an excellent info/mktg tool for anyone selling preservation consults. Sure, a little knowledge can be a dangerous thing when tree owners get some info from you and decide they can do without you, but that's a good risk, imo.

O and that research was done by SUSAN not sharon Day--hope to have citation next wk.
 
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<Mike M>
posted
Reply to post by Guy, on April 11, 2002 at 06:43:54:

First of all, I want to thank everyone for the responses to my inquiry re: how best to build a non-permanent driveway over my new oak tree roots. I haven't been a part of the conversations because we just finished moving in to our new house here in Parksburg, PA. And lots of changes to computers, phone lines and cable so I haven't had access to the internet. Wow, I did't like the feeling of isolation! Now, thanks for all the help on sandwiches and bridging. We found out here that wood chips are a mistake. They get messy and slippery. And if you want to change the strata, you have to remove...yep, remove the chips. So for now, we are going to barely remove any soil where the drive is going, and then covering with geotex fabric, the heaviest we can get. We like the idea of marine plywood, but there is the economic issue. $$$I would rather buy groceries. And a thin layer of course stone on top of geotex...as thin a layer as possible. Since we aren't planning on topping with concrete or asphalt, the 2" stone is at the top of grade. I will be using a stone topping with filler or dust so we get some consistant levelness, but keeping the amount of stone as thin as we can. Thanks everyone! Have a safe spring.
 
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<JPS>
posted
Reply to post by MIke M, on April 11, 2002 at 08:13:44:

I think you should go with several inches of traffic bond. The way it interlocks and distributs loading will be better then a minimal layer of stone.
 
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<Chris>
posted
I just bought a house that needs a driveway poured over the roots of a tree. What would be my best chance of not damaging this very mature tree
 
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RCA #354
BCMA #PD0008b
Administrator
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The first step is to get advice from a qualified arborist. Try to find someone with some experience in dealing with trees on construction sites.

Avoid any cutting of the soil around the tree, and limit any addition of fill material to only the area of the driveway. Do not compact that soil any more than necessary. If you are careful, the roots that extend below and beyond the area of the driveway will survive and still carry nutrients from soil on the other side of the driveway.

Check for an arborist at the American Society of Consulting Arborists web site at ASCA, or find a Certified Arborist or Board Certified Master Arborist at the International Society of Arboriculture site at ISA

Don’t take chances with construction near the tree, if you want it to remain healthy and strong.


--------------------
--
Russ Carlson, RCA, BCMA
 
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