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Healthy Trees- Sustainable Landscape ASLA National Conference 2008 - page 36 / 51





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Root Aeration System Design Guideline for Preserving Trees in Construction Grade Fills

This handout was designed to assist site design professionals or agency reviewing staff with basic understanding of objectives, limitations, and parameters to consider in tree preservation dealing with issues of proposed grade fills.

The purpose of the Root Aeration Mat (RAM) is two-fold: first, to provide necessary air/gas exchange between the atmosphere and the critical root zone that will be covered by a proposed grade fill situation; and second, to spread this loading and provide a stable base in conjunction with other engineered materials to support the proposed structural or nonstructural fills.

Roots of trees for the most part require porous soils of 80% compaction as found in forest soils. Even fills of a few inches over the existing grade of roots can bring about suffocation and toxic build up in a short time. Root Aeration Systems have been employed successfully for many years in a wide variety of construction scenarios, including retaining walls, parking lots, state highways, and toe of slope conditions.

Each site and application varies thus materials and design will vary. The Care of Trees, Inc. Foresters familiar with tree root structure and functioning in relation to species, soils, and moisture regime impacted by construction situations will review the design scenario and site, prescribe specifications, layout and supervise installation. Our latest methods and materials are beyond those found in state or local manuals.

Caution: Only licensed professionals with much experience in root system investigations and impact from design and construction impacts should attempt to specify and prescribe root engaging technologies. To meet structural concerns an experienced geotechnical professional should be engaged in product and materials selections under proposed paving or curbs.

The following are step by step guidelines that apply to a wide variety of situations:

1. The Project Forester or ISA Certified Arborist will determine the health and condition of trees in consideration. Will the tree withstand added construction stress? Are they worthy of consideration for special preservation measures? What is past maintenance history or prior impacts from site development?

2. Based upon accurate tree survey location including grades at base of tree, determine the appropriate Critical Root Zone for the tree(s. Investigative digging may be needed since roots vary in width and depth with species, soil type, moisture level, and urban infrastructure involved. (Refer to CRZ Handout sheets)

3. Project Forester will review grading and utility plans with the appropriate design team. How and where will cut and fill impact the CRZ? How will proposed utilities impact CRZ? Are there options for utility placement?

4. Determine how many square feet of Root Aeration Mat are needed?

5. No soil should ever contact that part of the tree normally above ground. How is soil to be kept from the trunk or root flare? If a retaining wall is needed what design and material will fit the tree root system based upon the investigative dig?

6. The project forester will design the appropriate venting system to fit the site and objectives. How will the RAM be vented to atmospheric conditions for the air/ gas exchange to the soil surface? Will a retaining wall, curb, or toe of slope be used?

7. For installation under structural fills the review of a geotechnical professional is required. We will not install structural systems unless a geotechnical has reviewed and approved. If none are available that are  familiar with this system we can provide geotechnical analysis and answer design loading questions.

8. For structural fills typically 24” is minimum needed between existing grade and proposed finish grade of pavement. Geogrids and other stabilization methods can be prescribed when less than 24” is available.

9. Resolve additional pertinent design / construction issues:

10. What will be the appropriate sequence of events to facilitate construction but not compromise protection of the tree root system?

b. Will the est. impact to the trees require related tree protection items such as root pruning and silt/tree protection fence? How will those be sequenced?

c. Will additional arboriculture specifications be prescribed for the impacted trees? These may include mulching; fertilization/ soil conditioners;; crown pruning; cabling/lightning protection; and IPM needs.

d. How will selective removal of saplings, brush, and vines within the protected CRZ be prescribed?

11. All work within the CRZ shall be laid out by the project forester or arborist and reviewed by the landscape architect/ civil and site superintendent. Installation should always be by ISA Certified Arborist with experience in this type of work.

Budget number for the root aeration mat systems is usually between $ 2.00- 3.00/ square foot installed depending upon degree of difficulty and economies of scale.

For a site / tree assessment and plan review contact one of our local offices.

For an in- office educational power point presentation regarding these and other state of the are tree preservation planning, design, or construction measures, call the below office.

The Care of Trees, Inc., the leader in “root engaging technologies”.

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