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<Nathaniel Sperry>
posted
Hi,
I'm working on several projects where clients want to establish 6" concrete slab foundations with minimal excavation depth (6 inches) within the critical rootzone of several healthy mature Oregon White oak (Quercus garryanna) and California Black oak (Quercus kellogii)trees. I have proposed an aeration system similar to the one illustrated in Harris's Arboriculture text using geotextile fabric, gravel and perforated pipe at the base of the excavation. Excellent compaction reduction requirements are required beyond the limits of excavation and arborist oversight of excavation and root pruning is required.

What feedback do you have to offer about this proposal? Where have you seen the process fail? I am looking for good examples of sucessfully installed aeration systems- everything from specifications to graphics to seat of the pants wisdom gathered during similar installations. Web sites, e-mails, texts, report excerpts etc. are welcome.

Timely responses would be appreciated as I have a hearing to attend next week!

Nathaniel Sperry
nsperry@uswest.net
605 Howard Ave.
Eugene, OR 97404
541-461-1737
 
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<James Causton>
posted
Reply to post by Nathaniel Sperry, on March 30, 2000 at 00:18:49:

I have installed 8 aeration systems since '93. The first one I did involved raising grade within 4ft of a 32"DBH Western Redcedar by 15 inches and pouring concrete for a garage and driveway. After 7 yrs there is no dieback in the crown and no visible evidence of any stress imposed on the tree. I have the files from that project stored in my old computer, I will dig them out over the weekend and see what is in there that may be of use to you. I also have photographic documentation of the installation, which I could possibly share with you.

James Causton. (WA State)
 
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<Russ Carlson>
posted
Reply to post by Nathaniel Sperry, on March 30, 2000 at 00:18:49:

Nate, I can offer anecdotal information on several projects I worked on. I don't have the reports or drawings, since it was done witha former employer.

Designers wanted to keep a 38" white ash (F. americana) in front of a new building in a corporate park. The surrounding grade was to be raised 4 to 6 feet, and paved. They managed to move things around enough to keep a 25' x 30' area around the tree, so it ended up in the middle of a drive circle. The aeration bed worked, and the tree has survived for about 12 years now, with little signs of stress.

The basic design followed the old method, described in Harris (and others). We used 4" perforated Schedule 80 pipe, a 6" gravel bed with geotextile over the top. The pipe layout made long rectangular loops, 6' apart, extending 30 feet under the fill, and interconnected at the far ends. We installed 4' tall risers within the circle, with 90 degree caps. These helped catch the wind and forced air through the system. Also allowed watering and periodic fertilization.

IMO, these systems should really be considered temporary. Although they may keep the old root system alive for a time, I think their purpose should be seen more as a crutch to keep the tree functioning until the tree can adapt with new root development. Part of the problem is that the roots will take a long time to invade the gravel, since that is a poor medium for growth. Wouldn't this be a great use for Cornel mix? Also, the geotex will prevent the roots from reaching upward. Be sure you give the tree as much room around the base as possible, and encourage the growth of new roots.
 
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<Scott Cullen>
posted
Reply to post by Russ Carlson, on March 30, 2000 at 00:18:49:

Russ,

Geotextiles have a number of functional uses. Two include load spreading and "infiltration barriers" (IB). IB means if you lay gravel or aggregate on top of soil or material of a different texture the two materials will infiltrate each other. Same thing happens if you put finer materal on top of coarse.... if your intent is for the gravel to be a drainage layer it silts up and gets plugged.... if your intent is to have the aggregate carry load it settles into the softer stuff below and the load also settles. Engineers typically like to avoid settling by compacting before laying the aggregate layer... they like it but root don't! So the idea of the geotextile is to both spread the load and prevent infiltration... hopefully satisfying the engineering requirements for stability and avoiding the compaction of native soil, i.e. root area. One of the "newer" techniques is to create a "sandwich" of geotextile, coarse aggregate, and geotextile." The angular, coarse stuff locks together and can't infiltrate material above or below.

Now, one of the other functions of geotextile is to allow water movement... it's called either permeability or permitivity depending on which manufacturer or theorist you read. The horticultural world has discovered that relatively light, thin geotextiles can block weed growth because the shoots can't get through the pores. Structural grades are much heavier. Your root growth observation is very important here. I researched geotextiles several years ago for a TCI article and nobody has any data about the horticultural implications of permeability - permitivity... how water movement is affected by the textile characteristics and how that affects plant performance, if at all. ( I talked to the manufacturers, the two leading proponents who were teaching for NADF, Dr. Dick Harris, and Dr. Bonnie Appleton at VPI who has done all the work on weed barriers... nobody had a clue.) Now add to the puzzle whether a fabric can be load bearing and infiltration barring enough but have pores that might allow root growth.
On another topic... your post says Schedule 80 pipe. That's the next heavier grade after Schedule 40? Are there guidelines for the schedule needed to avoid crushing given the depth of fill above? BTW if there is any risk that perforated pipe might silt up geotextiles are also used as "filters" and can be installed over or around the pipe.
 
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<Ed Milhous>
posted
Reply to post by Russ Carlson, on March 30, 2000 at 00:18:49:

Two comments:
This past summer, I had about 2 cu yds of unsifted river rock (various sizes w/sand in it) dumped on my gravel driveway, under a Norway maple. After several months of drought, I was (finally) relocating the remainder of the rock, and was surprised to find that the maple had grown roots throughout the mixture, 6-8 inches above the gravel.
This year I recommended using the Cornell mix on a project, which the developer agreed to do. But, when the time came to do it, the cost turned out to be around $125 per cu yd. (Seminars I had attended led me to expect a cost of about $50 per yard.) The trees were not worth it. I can't think of many places such a cost could be justified.
What are other people paying for this product?
 
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<Scott>
posted
Reply to post by Ed Milhous, on April 02, 2000 at 00:17:48:

Ed, there was a field demonstration at the ISA conference last Summer of Dr. Chris Starbuck's "Missouri Gravel Bed" method of holding bare root stock. The root growth into the gravel in only 2 months was startling.
 
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<Russ Carlson>
posted
Reply to post by Scott Cullen, on April 02, 2000 at 00:17:48:

The geotex materials in my experience do not allow much water flow. This is simply based on observation and a simple experiment. Take a piece of geotex, form a 'saucer' and fill with water. See how quickly it drips through. Do the same with a piece that has been buried for a few years to see if there is any difference. I didn't find a difference, and I thought the water permeability was rather slow.

The root permeability is the main issue. If you put those fabrics down, the roots will not penetrate easily (or at all). So the tree can't form a new, higher root system. Keep the old roots alive beneath may work for a time, possibly indefinitely for some trees. But the tree needs to expand its soil resources somehow or it will decline.

We didn't worry about the pipe system silting up, as the whole system was drained on one end into a culvert. We ran it into a drop basin with the outflow invert level with the ground line at the low end of the undisturbed area. That area was also separately drained off. So water inserted in the pipe system didn't just flow into the drain, but held until it infiltrated the soil. The well around the tree was drained so there was no standing water there.
 
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<Scott>
posted
Reply to post by Russ Carlson, on April 02, 2000 at 07:02:43:

The real point is that geotextiles come in a wide range of weights, thicknesses and pore sizes. One size does not fit all applications. One of the specifications in the materials charts is "permeability" (or "permitivity" depending on manufacturer). The civil engineer usually specs the fabric... most likely for load bearing and non-tear strength. We just don't know enough to add horticultural requirements to the spec.

We don't know enough to know if there is some permeability spec that will allow root penetration.... say into a Cornell type structural mix that will retain it's structural characteristics and not be distrurbed by root growth.

The situation you describe is most applicable to a "full" root cover. In many situations the need is to bridge over large woody roots so they are not damaged and continue their support and transport functions... the absorbing roots may have a more functional soil area beyond the "bridge."
 
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<Wulkowicz>
posted
Reply to post by Nathaniel Sperry, on March 30, 2000 at 00:18:49:


Here's a quick, awkward drawing:

I don't use preforated plastic pipe (with or without a geotextile sock since the perforation area is usually around 16%) as it is expensive and no longer appropriate. A simple geotextile strip drain about 6" high is perfectly adequate and has an aeration area of about 80%. Stop thinking of it as a drain and consider it an aeration device that comes in 100' rolls.

Whatever size slab laid has the perimeter fitted with the geo-strips and the roll continued off to a protected area where the the strip is pointed upwards and sleeved over a 2" plastic pipe. (Note that a DIFFERENCE in height between the pipe connected to one end of the strip AND the other pipe end is what creates the air flow forces (ask any prarie dog or Steve Vogel). Two 4' tall pipes at each end are relatively ineffective.

The idea is that the venting of the strip vents the gravel area beneath the slab--which I assume where the area of concern is located.

I've tried to show a couple of construction details, but typing the explanations is painful tonight, so if someone wants to give a call, we can talk it thru the details. This system requires only rolls of strip drain as dictated by the slab dimensions, duct tape, and can be installed right on the form face. After the forms are stripped, but before backfilling, the roll, cut to length can be led off to the desired location for the permanent pipe vents. (4" and 12" with strainer caps to prevent someone from filling them up are quite adeqaute.

_______

Someone should shoot geotextile salesmen with a tranq. dart gun for all their claims. The sad truth is that when a geotextile fabric works, it often renders itself useless for its intended application. I've seen many failures by well-intentioned engineers who listened to the sales pitches and didn't bother to think just a bit further.

Here, I can maintain the aeration from the inside face into the gravel well beyond the life of the building. Aerating the soil outside is a different matter that depends on soil type, compaction, etc.

There are many little tricks and details in this design, but I'm happy to share them.


bob
 
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<Nina Bassuk>
posted
Reply to post by Ed Milhous, on April 02, 2000 at 00:17:48:

I too have heard about ridiculous prices for the soil that we developed here at Cornell. The City of Ithaca makes it for about $20.per cubic yard. The City of Palo Alto, CA got it made for about $35. per cubic yard. I believe it is being sold in the Chicago area for $50.cubic yard. There is no reason that it should cost more than that. Hopefully with more people becoming licensed to make it the cost should come down.
 
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<Scott>
posted
Reply to post by Wulkowicz, on March 30, 2000 at 00:18:49:

Bob, most of the installation specs I've seen (all I guess and that's not too many) for something like Enkamat or Enkadrain show installation under a slab or against a wall to be backfilled. That seems similar to your plan.

What is the crush resistance of these materials compared to schedule 40 or 80 PVC? Say you wanted to do the typical installation at existing grade to be covered with extensive fill. What loading or depth of fill will the materials tolerate?
 
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<Wulkowicz>
posted
Reply to post by Scott, on April 05, 2000 at 00:50:50:


Enkadrain was the first supplier I spec'ed when I began the designs. I then used LunDrain, but they may be out of business or absorbed by another.

There is a myopia about this material as a drain. In fact, when I contacted the manufacturer about bursting strengths, they had no idea why I should ask that. I have a 3 acre park in Chicago that uses the strips in 18" heights for drainage, aeration, and irrigation--and doing it happily for 7 years now. Yes, it's very similiar to drainage systems and can still do that, but here Nate asked for something to aerate beneath the slabs I assumed. If the strip is used with a form, I wrap a length of duct tape with a tab that extends top and bottom from the strip into the gravel top and bottom. When the concrete is formed and the forms removed, the strip is held in place along the lenght and width of the new slab. Any kind of backfilling can be done with a little care and the strips are no disturbed.

The important thing is the remainder of the strip after the pour which is dug or trenched out to a desired location where it turns vertical and becomes the vent pipe. You can even leave it horizontal and slide it over a pipe with an elbow and a riser. (More physically stable that way before backfilling.

As I said, thinking of the product as only one use keeps one from seing the other uses. This design is about as cheap as it can be made, materials and labor, and I'd give it a life expectancy of more than 60 years (if someone bothers to ask about the little details and not just barging ahead without understanding the design intentions.

_________

I usually design to AASHTO ratings of H-20 (32,000 lb/axle) in anticipation of excaviting equipment and trucks later wandering around, so I'm fairly sensitive to issues of loading. Since the strips are installed vertically and have a minimal width, they are quite restistive to even unusual loads. In the configuration I've shown, there are no loads or threats to worry about (except digging up--and I use warning tapes just above the geotextiles.) You can carry the top of the strip to the surface, although I wouldn't know why) and they would be pretty much unaffected by loads. Being puleed out of the soil like a long band-aid zip tab is another matter.

As I said, Nate asked for aeration; as a specific answer, this is cheap, easily handled and installed, and very long lived. The pipe route is now passe' as far as I'm concerned. Interestingly, I had more trouble with engineers who said they only learned circular cross-sections and that's all they had in their tables (drainage). So much for flexible thinking.

The photo is the 3 acre formal park with the bottom half excavated 6' to see the patterns that echo the landscape designs above. It has no traditional irrigation; it keeps all storm water; and I control the water levels to an 1/8 of an inch over the entire park. It provides aeration in the spaces between events and I've only found it necessary twice to intervene and provide water during two summer droughts. My time was about 2 hours in each instance to refill the park. This is a 3x3 use (irrigation, drainage and aeration). Nate has a 1x use with the same materials so it will be extremely effective.


Bob

Picture is Copyright 1993, Bob Wulkowicz
 
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<Wulkowicz>
posted
Reply to post by Wulkowicz, on April 05, 2000 at 15:13:30:


Here's a shot of the typical product. (I don't use the fittings.)

And a list of some suppliers.

If I can, I'll make more drawings.

b
 
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<Wulkowicz>
posted
Reply to post by Wulkowicz, on April 05, 2000 at 15:13:30:


Here's a shot of the typical product. Just substitute the word "air" for "water."(I don't use the fittings.)

And a list of some suppliers.

If I can, I'll make more drawings.

If the image doesn't print, try: http://www.thiessenteam.com/terradrain-prefabricated-strip-drain.html

I don't know why it's garbled

b
 
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<Ed Milhous>
posted
Reply to post by Nina Bassuk, on April 02, 2000 at 11:12:13:

Thanks for the input!
This product offers such promise, but if the cost is too high to justify, it will be very difficult to get a builder/developer even to consider it. The world revolves around economics to them.
 
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<Julian Dunster>
posted
Reply to post by Nathaniel Sperry, on March 30, 2000 at 00:18:49:

I think the geotextile info everyone posted is good, though my experience with it mainly in road building wher it works very well on soft ground.

When you come to investigate which roots to cut back I highly recommend the Air-Spade as a means of seeking out the roots and then deciding which ones to cut. I admit to bias, since I also distribute the unit in the PNW. Get one and check it out. It is absolutely an indisposable tool for root work - you can very quickly expose the root systems without any cambial damage at all, and then you will know where to make the cut with most efficiency for the new design and the tree. I have never ever seen a better way or more efficient way of undertaking this type of work. It will save you time and money.

Having spent all yesterday morning working with a group to dig trenches around trees in preparation for transplanting, I am "hot to trot" on the system. The system works very effectively in compacted soils, such as you typically find under sidewalks, and with experience you can quite effectively "lift" the compacted soils and fracture them to assist in aeration.

Cheers

Julian
 
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<John Blessington>
posted
Reply to post by Nathaniel Sperry, on March 30, 2000 at 00:18:49:

It would be interesting to know if Nathaniel's system has now been installed, what he chose (and why?) and how the trees are doing so far.
There was much interesting discussion, though not too much comment on the effects of a 6" excavation. In UK 'NO DIG' is very much the recommendation within stipulated protection zones.
If pipe systems require vent pipes to produce a draught to aerate the roots how does a stip drain effectively aerate the stone layer (given as an alternative)? There has been concern expressed (elsewhere)that buried pipes/sytems can become filled with soil CO2 or even methane in organic rich soils. Any experience or information?. How does water penetrate these piped/vented systems other than by active irrigation (hoping no one forgets).
There appears to be a body of experience and anecdotal knowledge about but precious few well researched papers.(Does anyone know different?).
In most examples discussed levels were raised substantially. How was this material retained and edged so as not to carry out substantial additional root severence or compaction?
If we value these trees enough to spend so much time to consider them and money to retain them should we really be carrying out such works other than on a limited experimental basis until the science is properly done and published (rather than relying on sales pitches) or we risk encouraging potentially damaging operations- when our advice should be 'back off'.
 
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