I have to say I tend to take claims from the manufacturers of products with a grain of salt, especially when they are comparing their product to one they do not produce or have financial interest in :) However, my expectation would be that increased growth is probably not so much related to the nutrient pool as it is to the root exploration and thus access to a more resilient pool of resources (water). I have done quite a lot of research with compacted soils and the interaction with roots is fairly well understood. Any soil compaction, even slight compaction, decreases tree growth, although the magnitude of the response depends upon the exact species/soil combination. So I think you would certainly expect that an uncompacted soil would maximize tree growth. On the other hand, that is not generally the question at hand in situations where you would use structural soil, i.e. you are not making a choice between good uncompacted soil and structural soil--at least one hopes not! There is also some interesting research going on with pervious pavements and various pavement sections that is quite interesting and I think we will have a better understanding of the interaction of pavements and water supply to roots in the near future.

Steve Findley Question To: Also, the CU structural soils have been cricized (by Jim Urban) as not providing sufficient organic matreial and water to support the growth. Have you tested other amended soils?

A: Yes, we tested the Carolina Stalite structural soil. This one has the advantage of not needing a hydrogel to prevent segregation and the disadvantage of higher cost if you are not reasonably near the company. Also, some are concerned with embedded energy--although I have not seen any real analysis of the energy costs associated with any of these. They all work roughly the same way. The key to success is to have the correct mix and correct compaction so that it works as a moisture-holding matrix. We find all the mixes are droughtier than our native soils (which are clay loams, loams, and silt loams). There is a new paper out (in Arboriculture and Urban Forestry) that calculates moisture to be like a loamy sand, which fits fairly well with our experience. As far as organic matter, I haven't heard that as being an issue or a concern. In any sealed system (under pavement) whatever OM is in there to start will likely decompose (unless it is within stable soil aggregates), so input over time is important. The most significant input source would likely be root turnover--i.e. roots are born, grow, and die. Fine roots often liver for very short periods of time and this can provide significant OM input. So if roots grow in it, there will be organic matter input.

Jackson Bird Asked: Thoughts on Silva Cells?

Hi Jack! It's good to hear from you. Still plugging away at answering these questions, but not sure there are any readers, so may quit soon. I don't have any experience with Silva Cells, but my understanding is that they are essentially modular vaulted sidewalks. I don't know how they compare cost-wise, but presumably they have been evaluated to be load bearing. I would think they would function like a vaulted sidewalk. In terms of stormwater, if you had a one-to-one reservoir to mitigated surface, as was tested in our work, I would think you would have some similarities and some of the evaluations of tree survival might be similar. The main differences would be the amount of storage--that would depend on the storage capacity of the soil used in the cells. You might also have a perched water table and so a wetter zone near the bottom--this would depend on the soil again. In addition, you would not have rapid lateral movement, so the methods you would use to direct water below the pavement would have to be altered.

Stan Kamys Question: Are there calculations to determine what size of cu area will treat the otherwise pervious parking lot for 25yr/100yr storwater calculations? Has the data been used to address stormwater pond configurations with a local water management district and the EPA/DEP rules.

A. Yes, the reservoir conservatively holds 30% (typically closer to 35%) by volume of water. Therefore the recommended 24" bed of structural soil would handle a 4.5 to 5" rain -so whatever storm that is for you. This allows an overflow drain to be placed at 14" up from the subgrade. Note that the system is NOT designed to handle a lot of off-site water, just to be zero runoff.

Shelley Allen-Czepiel Question: What makes the tree roots from growing up into the pavement?

SD replies: I am assuming this question is in regard to porous pavement? I am not an expert in porous pavement. However, porous pavement is rigid just like other pavement and I had never really considered a root growing through it. Nonetheless, it probably is not a consideration unless the pavement is buried. Roots follow the path of least resistance and proliferate where there is water and air and nutrients. In a conventional pavement section, this area is often the base course under the pavement. With structural soils, or uncompacted base soils (as are allowed for some uses in some countries), this path of least resistance can move lower in the profile (away from pavement). I hope that is the question you were asking.

Dion Doepker Asked: Regarding Structural Soil: Is there a requirement to the type and shape of the rock used for the aggregate

SD replies: Yes, the size and angularity of the rock are quite important, as is the distribution of sizes. Here in Virginia we used #357 (I think, if I am remembering correctly). What is critical is that the stone be large enough (at least 3/4 of an inch and up to about 2") and that there not be a lot of smaller stones mixed in. An angular stone will make better gaps. I have seem some installations of supposed "structural soil" that were more akin to crusher run than structural soil. When constructed according to specifications, the percolation rate and other measurements are made. Water should enter structural soil extremely rapidly. The best way to ensure quality control is to employ someone licensed by Amereq to install CUSoil, or work directly with the manufacturer of something like Stalite on establishing specs and working with the contractor. There are some contractors who say they are licensed to install CUSoil where we are, but after calling Amereq, I learned that they were not licensed. So make sure specs are met. It's not that difficult to meet the specs, but just mixing gravel and soil does not make a structural soil.

Mike Fleischhauer Question To: Has consideration been given to the possible issues created by tree roots continuously penetrating the geotextile (eventually the structural soil settling as the geotextile is destroyed, causing settling of pavement)? Also, has there been study of the longevity of the porous asphalt surfaces under normal use considerations (Ex. Pores pluging as vehichles on surface deposit dirt and oils)?

SD replies: I don't know that the tree roots damage the geotextile too much. Tree roots will penetrate it, but it still creates a barrier of sorts. Our main interest was to see if tree roots would penetrate it enough to puncture the compacted zone and allow greater infiltration. There have been a number of studies about the longevity of porous asphalt that others have done. I think the book by Ferguson might describe some of them, but there are many case studies as well. There has been a lot of interest in this at Villanova university (Robert Travers I think?).

Karen Engel Question To: I was disconnected & may have missed explanation, but....why do you need to use the geotextile fabric at all? What happens if you don't?

SD replies: see answer to Phil Rodbell above. Most structural soil installations do not use geotextile. However, if the subgrade may become saturated it is an added precaution to insure stability.

Phillip Rodbell Asked: So roots do grow through the geotextile? Only value is separation of soil medium from gravel?

SD answers: Yes, they grow through it, but it will restrict large roots unless it is torn/damaged. But some tearing is normal--and in the case of trees, I suppose desirable. In this case it is a "separation geotextile" so that is its only function. If it were not being used as a stormwater reservoir, then the geotextile would be unnecessary as the stones would not likely imbed in the subgrade. However, it is a precautionary measure.

Johanna Bell Question To: Could you describe the stabilizing "hydro-gel" used to stabilize the structural soils? What is it made from? How does it bind the organics, and allow the organics to be accessed by the plants at the same time

follow up... It is a polyacrylamide. As mentioned above, it doesn't stabilize the mix and doesn't bind organics.