CU-Soil™ |
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| CU-Structural Soil® Frequently Asked Questions |
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| What volume of CU-Structural Soil® is needed for a given tree? |
| What is the recommended depth for CU-Structural Soil®? |
| Is CU-Structural Soil® compatible with underground utilities? |
| What is the recommended length and width for CU-Structural Soil® installation? |
| Won't the soil migrate down through a CU-Structural Soil® profile after installation? |
| Does hydrogel break down over time? |
| What happens when roots expand in CU-Structural Soil®? |
| Is CU-Structural Soil® susceptible to frost heave? |
| Can you add normal soil in the tree pit and CU-Structural Soil® under the pavement? |
| What type of trees should be grown in CU-Structural Soil®? |
| Can you use balled-and-burlapped, bare root, containerized or boxed trees in CU-Structural Soil®? |
| Can you store large quantities of CU-Structural Soil®? |
| Can CU-Structural Soil® be utilized around existing trees? |
| Has CU-Structural Soil® been used to mitigate storm water runoff? |
| Can turf be grown on CU-Structural Soil®? |
| A point to Ponder! |
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What volume of CU-Structural Soil® is needed for a given tree?
The Urban Horticulture Institute at Cornell has found that approximately 1 cubic meter of soil is needed for 2 square meter of crown projection (the anticipated area under the drip line of the tree at expected maturity). Trees growing in CU-Structural Soil® in areas that normally use irrigation to grow trees should also provide low volume drip irrigation in CU-Structural Soil® installations. |
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What is the recommended depth for CU-Structural Soil®?
We suggest a minimum of 600mm, but 900mm - 1 meter is preferred. Roots will grow to the full depth of CU-Structural Soil®. A base course of gravel is not needed on top of CU-Structural Soil® below the pavement, because it was designed to be as strong as a base course. Properly compacted to 95-100% Proctor Density or Modified Proctor Density, it has a CBR (California Bearing Ratio) of 50 or greater. |
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Is CU-Structural Soil® compatible with underground utilities?
If underground utilities are encountered within the proposed rooting zone, CU-Structural Soil® can conform to the backfill needed around utilities and can be easily installed. |
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What is the recommended length and width for CU-Structural Soil® installation?
There is no established minimum. However, CU-Structural Soil® was designed to go under the entire pavement area. This homogeneity would ensure uniform engineering characteristics below the pavement, particularly in regard to frost heaving and drainage. Ideally, the installation should focus on a whole footpath section from building face to edge of the kerb, potentially for the whole length of the street. If it is impossible to use the entire footpath area using CU-Structural Soil®, it can be placed in a 1.5 to 3 meter wide trench parallel to the kerb.
Should CU-Structural Soil® be used in a tree pit if it is not placed below the surrounding pavement? This is the wrong use of this material. The success that we've seen with CU-Structural Soil® is due the large soil volume that the roots can grow into. CU-Structural Soil® was designed to be used where soil compaction is required, such as under footpaths, car parks, medians, plazas, and low-access roads. Where soils are not required to be compacted, a good, well draining soil should be used. |
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Won't the soil migrate down through a CU-Structural Soil® profile after installation?
The excavation of a seven-year-old installation did not show any aggregate migration. The pores between stones in CU-Structural Soil® are mostly filled with soil so there are few empty spaces for soil to migrate to. |
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Does hydrogel break down over time?
Over a long period of time, the soluble salts from which the hydrogel was produced, i.e. potassium (from potassium hydroxide) and ammoniacal nitrogen (from acrylamide) are released. The inert hydrogel becomes a very minimum part of the soil system. Beyond that, we believe that colonizing roots and other organisms will, over time, replace the spatial and tackifying roles of the hydrogel. Research on this subject is ongoing. |
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What happens when roots expand in CU-Structural Soil®?
There will come a time when larger buttress roots will likely displace the stone, but if the roots were, as we have observed, deep down in the profile, the pressure they generate during expansion would be spread over a larger surface area. We have seen roots move around the stone and actually surround and encapsulate some stones in older installations, rather than displace the stones. |
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Is CU-Structural Soil® susceptible to frost heave?
This topic has not been rigorously tested, but we have not observed frost heave damage in the Ithaca , New York installations. Based on drainage testing and swell data on this extremely porous system, CU-Structural Soil® appears quite stable. |
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Can you add normal soil in the tree pit and CU-Structural Soil® under the pavement?
If the tree pit is sufficiently large, greater than 1.5 meter x 1.5 meter, and the opening unpaved, a conventional soil should be used in the open tree pit surrounding the root ball with CU-Structural Soil® extending under the pavement. The available water in CU-Structural Soil® is approximately 7%. Using a good sandy loam or well-structured loam in the tree pit opening will provide more moisture. |
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What type of trees should be grown in CU-Structural Soil®?
Depending on the stone type and subsequent soil pH, moderate to highly drought tolerant and alkaline soil tolerant trees should be used. In Ithaca , New York where we make CU-Structural Soil® using limestone as the crushed gravel in the mix, we have had success with numerous trees. For more information see www.hort.cornell.edu/uhi/outreach/csc/index.html |
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Can you use balled-and-burlapped, bare root, containerized or boxed trees in CU-Structural Soil?
Trees from any production system can and have been used. It is important to water the newly planted tree as would be expected in any soil. |
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Can you store large quantities of CU-Structural Soil®?
CU-Structural Soil® is produced by licensed producers and is preferably not stockpiled. It is mixed as necessary and should be delivered and installed in a timely manner. If any stockpiling is required, protection from rain and contamination should be provided. |
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Can CU-Structural Soil® be utilized around existing trees?
There are several instances where CU-Structural Soil® was utilized adjacent to existing trees. It appears that if few tree roots are damaged during the installation, the trees continue to grow well. Research is currently underway to investigate this issue. |
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Has CU-Structural Soil® been used to mitigate storm water runoff?
Through the efforts of Ted Haffner, graduate student at Cornell, and Ithaca City Forester Andy Hillman, we created a parking lot in Ithaca that combined porous asphalt and CU-Structural Soil®. This parking lot reduces or eliminates surface run-off and allows trees to grow within it. The porosity of CU-Structural Soil® after compaction is about 26%. Moreover, of that porosity, about 31% are large pores that determine water infiltration. The infiltration rate of water through porous asphalt into CU-Structural Soil® is greater than 24" per hour. Conventional loamy soil compacted to engineers' specifications for pavement installation has only about 2% large pores and 0.5% water infiltration per hour. A 600mm base of CU-Structural Soil® under a porous asphalt parking lot can accommodate 150mm (6") of rainfall in 24 hours within its pores. The water will then seep back into the ground water over time. |
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Can turf be grown on CU-Structural Soil®?
Trials at Cornell University have shown that tall fescue grows well on compacted CU-Structural Soil® and is wear tolerant. Sod installation or seed with cellulose mulch works well when laid directly on the compacted CU-Structural Soil®. Irrigation needs to be applied in both installations to establish the turf and can be subsequently eliminated where turf can be grown without irrigation. We are continuing to work on CU-Structural Soil®. Given the need for trees within heavily urbanized paved areas and the requirements for compacting the soil under pavement, CU-Structural Soil® is a proven, viable option to help green our cities. |
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A point to Ponder!
There are other structural soils; however, only CU-Structural Soil® has over a decade of research and hundreds of installations. There are now 71 licensed producers of CU-Structural Soil® in the U.S. and Canada and over 500 installations, from Quebec to Puerto Rico to California . CU-Structural Soil® has been used in many different climates and is compatible with irrigation when that is necessary. As with any new technology, we're learning more about it as we continue to do research on its uses. |
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