Chapter 1.7
Urban Forestry and Vegetation

• ALL
• Remove Code Barriers
• Create Incentives
• Fill Regulatory Gaps

Reduce Soil Compaction during Construction

INTRODUCTION

Undisturbed soil contains open space among individual granules, known as pores, which account for approximately fifty percent of soil volume.[1] Soil may be compacted at construction sites due to “excavation, mixing, stockpiling, equipment storage, and equipment traffic.”[2] Large construction equipment operating on the soil reduces the pore size and can cause soil volume to reduce anywhere from 12-20 inches.[3] Compacted soil can lead to many adverse circumstances on the surrounding land, including compacted soil acts like an inhospitable impervious surface, potentially causing problems such as impenetrable soil, standing water, increased runoff, and poor vegetation health.[4] Wet soils are particularly susceptible to deep compaction,[5] and ordinary landscaping tillage is typically insufficient to mitigate deep compaction.[6]

Communities of varying types and sizes have adopted ordinances regarding soil compaction. Such ordinances often use terms such as “land disturbance” or “site alteration” and appear in either stormwater management or tree preservation provisions. Ordinances set forth criteria such as what type of equipment may be used at a site and where that equipment may be used. In addition, these ordinances can address the storage of heavy building materials on site. Some ordinances provide for rehabilitation measures that must be taken in the event that compaction occurs or is unavoidable. These ordinances may also call for a post-construction soil test to be performed by a private engineer, an engineer sponsored by or associated with the governing body, or both.

EFFECTS

Soil compaction poses a significant threat to the well-being of trees, vegetation, and wildlife, particularly in urban settings.[7] Ideal conditions for trees to flourish include aerated, moist, and uncompacted soil.[8] Trees rooted in compacted soil are unable to penetrate to the depth necessary to extract the nutrients, oxygen, and water needed to sustain their health.[9] Shallow root systems in compact soil have reduced access to mycorrhizae, a symbiotic fungus crucial to tree health.[10] Many tree roots combine with mycorrhizae in the top 18-24 inches of soil, allowing for greater access to water and oxygen, while also strengthening the ability of a tree to fight pathogens.[11]

Compacted soils can lead to crown dieback, a process where poor nutrient intake causes the death of branches on the outer perimeter of trees.[12] In some circumstances, these stresses result in the death of the entire tree.[13] Malnourished root systems may also destabilize a tree, threatening structures in its immediate surroundings.[14] Tree roots serve as an anchor for the entire tree; soil compaction that negatively impacts a root system weakens the anchoring ability of the roots, making the tree susceptible to prostration during storms or high winds (for a description of tree mitigation ordinances see Tree Canopy Cover).[15] Similar to trees, other vegetation struggles to establish root systems in compacted soil, which restricts a plant’s ability to cycle nutrients through diffusion.[16]

Uncompacted soil can drastically improve a community’s ability to address the concerns associated with stormwater management.[17] Soil, when undisturbed, can help absorb stormwater, cycle nutrients, mitigate erosion, and trap pollutants and sediment.[18] Stormwater flow experiences little impediment by the tightly configured pores of compact soil, and produces runoff that contributes to flooding, decreased water quality, and damage to stream corridors.[19] Unmitigated runoff can also erode the valuable layer of topsoil on sites.[20] Compact soil is also prohibitive to the installation of some stormwater management measures, such as rain gardens, because the vegetation required to establish a rain garden is difficult to grow on compacted soil (for a description of the benefits of rain gardens see Rain Gardens).[21]

Preventing or limiting the amount of compact soil on a construction site has been shown to be one of the most effective ways of avoiding the maleffects of compacted soil because post-compaction remedies have been shown to be less sufficient.[22] In addition, post-compaction rehabilitation consumes both time and money.[23] Though various methods exist to restore soil, owners and developers have to consider the out of pocket costs of items such as compost, tilling, aeration, seed, and fertilizer.[24] Non-monetary investments include the testing and monitoring of the soil, as well as any personal labor individuals engage in.[25] Costs vary by needs and location. In Virginia’s Chesapeake region circa 2013, the cost of restoring soil to a two inch depth was estimated to run at $7,000 per acre.[26]

EXAMPLES

Bass River Township, NJ

Under the section “Design, construction, and safety standards for structural stormwater management measures,” Bass River Township sets the requirements to achieve effective stormwater management in the Pinelands Area.[27] The requirements ensure that construction does not adversely affect the ability to redirect water back into the ground.[28] The Township prioritizes avoiding the compaction of soil around sites by not allowing “heavy equipment such as backhoes, dump trucks or bulldozers” within a certain radius of specific areas.[29] Ground excavation must be performed with equipment placed outside of the area where soil compaction could occur.[30] Where developers are unable to complete a stormwater management plan without the use of heavy equipment, the code requires excavated soil to be tilled and renovated post-construction.[31] All effects of any compaction are to be reversed.[32] A post-development soil permeability test must be performed.[33] If the soil fails to meet the standards of the test, the site must be renovated and retested until the soil conforms to the code’s standards.[34] Permeability tests are to be performed by a certified professional engineer.[35] After the engineer performs the test, the results must be forwarded to the municipal engineer.[36]

To view the provision see Bass River Township, NJ Code of Ordinances § 13.08.260 (2007).

Markham, Ontario

Markham defines activities that impact the quality of soil.[37] Markham prohibits such activities unless the developer obtains a permit or is operating under one of the exemptions set forth in the by-law.[38] Compaction of soil or acts that result in the creation of impervious surfaces are included in the definition of prohibited activities.[39] In areas where natural gradients and swales do not provide drainage, site alteration must account for the effects of surface and stormwater drainage.[40] Markham holds the owner, the permit holder, and the offender liable for violations of the by-law, and all are held responsible to restore the land either to its original state or to a condition deemed satisfactory by the Director of Engineering.[41] A partial list of effects and circumstances that result in Markham denying site alteration includes: soil erosion, blockage of a drainage system or watercourse, “siltation or pollution in a watercourse” and flooding.[42]

The by-law creates exceptions to the permit.[43] Properties under one acre are exempt so long as the land is not situated next to a body of water, protected area, or hazard land.[44] Any site alteration that impacts the drainage on neighboring properties will likewise not be exempted from the permit process.[45] In addition, when 300mm (approximately one foot) of material or less is being added to a site “for the purpose of lawn dressing, landscaping, adding to flower beds or vegetable gardens,” the by-law does not apply, so long as certain conditions are met.[46] The material added may not change the elevation of the ground within 600mm of adjacent properties.[47] Additionally, the material added may neither alter nor change the nature of watercourses, swales, open channels, or ditches that have the effect of draining the land.[48] Sediment runoff as a result of added material is forbidden.[49] The by-law provides additional exemptions geared toward working in concert with existing regulatory measures.[50]

To obtain a site-alteration permit, developers must provide the Town with a site alteration plan.[51] The plan must include a detailed map of the property, along with information such as the use of the property, the use of structures on the lot, as well as other buildings abutting the property.[52] The placement of “lakes, streams, wetlands, channels, ditches, other water courses, other water bodies and environmental protection areas” that are located on or within 30 meters of the property must be included.[53] Site alteration plans must also include items such as the principal soil type, existing vegetative cover, proposed grade elevations at the project’s completion, and staging areas for equipment.[54]

To view the provision see Town of Markham, Ontario By-Law 2011-232 (2012).

ADDITIONAL EXAMPLES

Albert Lea, MN Code of Ordinances § 74-2027(2)(h) (2015) (banning equipment from driving over areas designated as a future site for runoff flows in order to prevent soil compaction).

New Castle County, DE Code of Ordinances § 40.27.320(G) (2011) (requiring soil compacted by construction vehicles and equipment to be loosened to a depth of 12 inches).

Onalaska, WI § 13-8-62(c)(J) (2009) (requiring stakes to be placed around trees before construction grading commences. Compaction of the soil is prohibited around the root zone of trees).