Underdrainage of Plant Materials
Many of the sites for new schools have had the top several feet of soil removed and replaced with crushed aggregate on all or part of the property, as part of an environmental remediation program. Early attempts to plant new materials on sites that have been capped as described above met with poor results due to lack of adequate drainage.
- Sites may have poor drainage if they are underlain with impervious clay as is typical throughout much of the City. The Landscape Ordinance requires that percolation tests be conducted and drainage pipes installed for parkway plantings. For these reasons, the general assumption for all schools is that an impervious layer underlies each site and that plant material will have to be underdrained.
- Underdrainage may take the form of either “French drains,” or drain tiles.
- Provision must be made to connect the drains to a nearby storm sewer, detention basin, or dry well constructed for drainage purposes. The Department of Sewers is often reluctant to allow the connection of underdrain pipes to storm sewers, because of the concern that soil will find its way into the sewers. Connecting drain lines to “Dry wells or to stone filled sumps may be a cost effective solution to providing drainage, but they are of use only in situations where there is percolation. Auguring a drainage sump beneath individual tree pits may also be a suitable method of providing drainage in circumstances where there is at least a small amount of percolation.
- Underdrainage may be eliminated for school sites that have well drained soils.
CU-Structural Soil
- The Landscape Ordinance requires the use of CU-Structural Soil™ within parkway areas under certain conditions.
- CU-Structural Soil is a specially prepared mix of crushed aggregate, topsoil, and wetting agents. It is designed to be able to be compacted to 95% of Proctor so it can serve as a base for pavement and still contain sufficient pore space to support root growth and provide for drainage.
- Quality control is important in achieving a mix that will meet objectives. Contractors shall not be permitted to attempt mixing this material.
Refer to the Cornell University Urban Horticulture Institute website for additional information about CU-Structural Soil.
Aeration Pipes
Tree roots often grow only within the top 12 inches of compacted urban soils due to the lack of oxygen at lower soil levels. Research has shown that making oxygen available to roots at lower depths encourages roots to grow deeper. The aeration pipe is intended to allow the tree roots to use the entire depth of growing medium available.
- A dual-purpose pipe shall be installed within parkway planting trenches.
- The aeration pipe is intended to supply oxygen to roots in addition to collecting excess water.
- Rigid plastic pipe appears to be the most cost effective material for this purpose.
- As with plastic underdrain pipes, a direct connection to city sewers may not be permitted.
Refer to the City of Chicago Landscape Ordinance.