I have enjoyed reading the comments, but as a late comer to this web series, I am wondering if the video is going to be posted on the site.

If trees in an urban area respond the same way as the ones in the greenhouse study, it would be expected that any techniques that can provide uncompacted loam type soil under pavements would grow larger trees than those in an equal volume of CU structural soil.

A study by Tom Smiley has been comparing trees in an urban plaza setting in 7 different treatments: compacted soil, gravel/soil mix, Stalite, Stalite/soil mix, and suspended pavement over non-compacted soil. Suspended pavement over non-compacted soils (analogous to Silva Cells) are providing the greatest amount of tree growth and health (Smiley et al, Comparison of Structural and Non-Compacted Soils for Trees Surrounded by Pavement, 2006 International Society of Arboriculture; Tom Smiley personal communication).

Another study was just started in Minneapolis and St Paul comparing trees grown in compacted soils, vs CU structural soils, vs. Silva Cells, vs Swedish Structural soils. Stay tuned for results at http://www.deeproot.com/ in the next few years!

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.

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.

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.

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.

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.