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Chapter 4
Construction Practices for Environmental Stewardship
4.11. Soil Management in Construction

4.11.1 Earthwork and Soil Management

General environmental stewardship practices for earthwork and soil management include the following: [N] [N]

Minimize the extent of disturbance activities to minimize impacts to soil outside the project's construction limits.

Mitigate construction-related soil compaction in vegetation restoration areas.

Stockpile and reuse native soils where practical.

Minimize erosion potential and weed species invasion by establishing a healthy plant cover.

Note the topsoil's structure and function.

Mitigate construction-related soil compaction in vegetation restoration areas by ripping the soil to loosen its structure.

When stockpiling topsoil, mound soil no higher than 1.3 m (4 feet) high for less than 1 year and preferably less than 6 months. Cover to prevent soil erosion and contamination by weeds.

Use only well composted soil amendments and incorporate them as specified.

Avoid walking, operating equipment or driving vehicles on planting areas after soil preparation is complete.

Minimize erosion potential by establishing a healthy plant cover.

 

4.11.2 Contouring
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The erosive power of water flowing down a slope should be recognized during earthmoving. The steepest direction down a hillside is also the direction of greatest erosion by overland or channelized flow.

The overall topography of the graded surface should be designed to minimize the uncontrolled flow of runoff.

Channelized flow should be diverted to ditches cut into the soil that more closely follow the level contours of the land.

Dispersed sheet flow should be broken up by terraces or benches along the slope that also follow topographic contours.

On a fine scale, the ground surface can be roughened by the tracks of a bulldozer driven up and down the slope, or by a rake or harrow pulled perpendicularly to the slope. In either case, the result is a set of parallel ridges, spaced only a few inches apart, that follow the contours of the land surface and greatly reduce on-site erosion.

 

4.11.3 Subgrade Preparation and Landscape Excavation
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Proper subgrade preparation includes the clearing and grubbing of land. Subgrade preparation measures that are particularly important for restoration sites may include the following stewardship practices.

Except for muck soils, the subgrade should be free of organic debris, demolition debris, and large stones and rocks.

If no fill is required, the ground should then be smoothed and compacted.

If the area requires fill, the surface should be scarified or roughened to facilitate a bond between the original soil and the fill.

Fill should not be placed on a smooth compacted soil.

Work necessary to reshape the ground and to remove material unsuitable for planting is performed as landscape excavation. The excavated areas are often backfilled or plated with imported material. [N]

  • The Inspector should ensure that the work performed is carried to the staked limits. Irregular areas may be difficult to stake using conventional methods and adjustments may be necessary.
  • Over or under excavated areas should be corrected before the placement of topsoil, rock mulches, or landscape borrow is permitted.

The depth of topsoil, rock mulches, or landscape borrow is critical for plant growth so the outcome of the entire landscaping project may depend on the performance of this initial work. Some details that usually need to be checked during this operation are: [N]

  • Only legal loads are to be allowed; however, some projects have roadways requiring less than legal loads on large trucks.
  • It may be necessary to cover loads or take other measures to prevent spillage caused by slumping and sliding or wind.
  • When the Contractor has his own disposal area, he should furnish the Resident Engineer with evidence that he has a Use Permit and also that the owner is satisfied with the condition of the disposal site after the Contractor is finished with it.
  • When the Department furnishes disposal areas, the Resident Engineer should designate where material may be placed and will specify how the area is to be cleaned up and reshaped.

 

4.11.4 Topsoil Preservation
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Topsoil is that uppermost layer of soil capable of growing and supporting vegetation. On a typical highway project, earthwork normally starts by scraping the ground surface, then transporting the material and placing it where fill or embankments are needed. The first layer to be excavated, usually the topsoil, is often placed at the bottom of the fill. The last layer excavated is left exposed on the final grade surface. This process results in the placement of the nutrient- and organic matter-rich topsoil at depths that are inaccessible to plant roots. The exposed geological parent material is usually a suboptimal planting medium because it is low in organic matter and often has plant nutrient imbalances. [N] [N] [N]

Just 12 state DOTs (24 percent) say they stockpile desirable/uncontaminated topsoil to facilitate revegetation. Topsoil has become a commodity that contractors often strip and sell. Soils management on construction sites is an important factor in invasive species control. In addition to removing native cover that may effectively compete with invasive species and opening new pathways for infestation, the change in soil characteristics that may occur with loss of topsoil can also bolster invasives. One Florida study on roadside soils and invasive species suggested that disturbance alone may not increase the presence invasive species, but that a change in soil characteristics through the addition of clay and limerock from road construction may enhance invasion. [N]

Claassen et al. have performed a large percentage of the studies on topsoil usage and compost, with funding from FHWA and Caltrans. Among their many helpful recommendations are for DOTs to test nitrogen content in soils and ensure slow-release of amendments over three years; to use compost over use of commercial fertilizer; and to ensure that compost amendments have adequately decomposed. Where topsoil is not available other amendments can be used, the quantity and quality of the Nitrogen (N) materials applied is critical. The N release should be slow enough to keep plant-available N at modest levels, but the total amount of N amended should be high enough so that the site does not run out of N before the plant community is well established. The N amendment should be able to support three to five years' plant growth, for example. Controlled release of N is important because excessive N availability promotes weedy annual grass growth, drying out the site and crowding slow growing perennials. While the maintenance of moderate, sustained nitrogen levels may be achieved from commercial, slow release fertilizer sources, the inclusion of organic matter in the amendment is also important to improve the hard setting and poor water holding capacity of low organic content materials. [N] Biomass associated with compost has been more effective than N amendments that were evenly disturbed throughout the profile (0-30 cm) or applied deeply within the profile (20-30 cm). [N] Studies of plant communities established on "problem soils" amended with commercial fertilizers have shown vigorous initial growth, but that vegetative cover often becomes sparse or nonexistent within several years. [N] In addition to transportation related studies, those of fertilized mine reclamation spoil observed that revegetated areas tended to be highly productive for two to five years followed by a sharp decline in plant growth and nutrient availability. [N] Reapplication of topsoil to subsurface materials enhanced reestablishment of vegetation by increasing nutrient availability, water holding capacity, and microbial activity. [N] Compost can be used to replace the organic matter and nutrients and can act as a surface mulch to protect against erosion, extreme temperatures, and droughtiness. [N]

Long-term nitrogen release rates from most yard waste compost materials approached the N release rates of moderately fertile soils. Composts were shown to be able to regenerate the N availability characteristics of low-nutrient substrates that have been stripped of topsoil organic matter. Well-cured composts and co-composts (biosolids blends) approached the N release rates of highly fertile soils. Compost application provides longer N release duration compared to chemical fertilizer and also provides organic materials for improved infiltration and microbial activity. Potential compost sources and soils at the site should be analyzed before amendment, as compost products and the soils that are to be revegetated vary in fertility and water availability.

Topsoil is recommended on all disturbed sites and slopes 2H:1V or flatter that should be permanently seeded or used as a planting medium for plantings or nursery stock. Topsoil may be added to a rock mulch to enhance slope protection and provide soil for seed germination and plant growth. Topsoil can be mixed with organic material such as compost or manufactured soil amendments to improve the growing capability of seeded and planted vegetation.

Topsoil normally should not be used on slopes steeper than 2H:1V or on sandy or silty slopes steeper than 3H:1V. Additionally, topsoil should not be placed on frozen, extremely wet, or smooth slopes.

To the extent practicable, aboveground vegetation, including litter, should be mixed or otherwise incorporated into the topsoil prior to excavation. Topsoil should be excavated from the existing roadway shoulder to a depth of 150 mm (6 inches). For new alignments, topsoil should be excavated to the depth it exists and stockpiled.

The topsoil should be placed into stockpiles at locations designated on the plans. Stockpiles should be treated with temporary soil stabilization and erosion control measures as per Stormwater Pollution Prevention Plan (SWPPP). Topsoil stockpile height should not exceed three meters (10 feet).

Stockpiling topsoil will result in the disruption and loss of beneficial soil microorganisms, and if stockpiled over a length of time (six months +/-) may result in total or partial loss of soil microorganisms.

If topsoil is stockpiled prior to placement, the top one foot of the stockpile material should be mixed with the remainder of the stockpile to ensure that living organisms are distributed throughout the topsoil material at the time of final placement.

The use of micro-organism inoculates may be necessary to re-establish micro-organisms in topsoil material that has been stockpiled for over a nine (9) month period.

Apply a temporary soil stabilization and erosion control treatment to the exposed topsoiled areas to protect the topsoil prior to permanent seeding.

Soil binders may be applied to disturbed soil areas or soil stockpiles requiring short-term protection. Soil binders consist of applying and maintaining polymeric or lignin sulfonate soil stabilizers or emulsions, may be used to stabilize stockpiles. A variety of soil binders are available for use. Prior to use, the manufacturers' specifications should be reviewed and compared to the site-specific conditions. In selecting a soil binder, the following criteria should be considered: availability of product; ease of cleanup; degradability (how the product degrades and what its by-products are); length of drying time; erosion control effectiveness; longevity; mode of application and availability of application equipment; and water quality impact. [N]

Apply soil binders per manufacturer's specifications.

Soil binders should be nontoxic to plant and animal life.

Soil binders shall not be applied to frozen soil or areas with standing water.

Soil binders should not be applied during or immediately before rainfall.

Avoid over-spray onto hardscaped areas.

Check protected areas to ensure proper coverage and re-apply soil binder as needed, or implement additional BMPs.

When stockpiling topsoil, mound soil no higher than 1.3 m (4 feet) high for less than 1 year. Cover to prevent soil erosion and contamination by weeds.

Mitigate construction-related soil compaction in vegetation restoration areas by ripping the soil to loosen its structure. After final slope grading and prior to placement, cut slopes should be cross-ripped horizontal to the slope to assist in anchoring the topsoil. The spacing of the ripping shanks should be one meter (three feet) and penetration should not exceed 300 mm (12 inches) in depth. Where embankments are constructed, offsetting lifts of material to create an uneven surface prior to topsoil placement should be considered. Smooth slopes are not acceptable.

Following construction, stockpiled topsoil should be uniformly redistributed (placement) to a depth of 150 mm (six-inches). Placed topsoil should be cat tracked vertically to the slope to compact the topsoil and to create horizontal pockets (safe sites) to hold seed and water.

Leave the topsoil surface in a roughened condition to reduce erosion and facilitate establishment of permanent vegetation. The roughening establishes safe sites for seed to germinate and grow.

Where quantities of topsoil are limited, it is recommended to cover the more critically disturbed areas to the proper depth, rather than cover all areas. If necessary, the more favorable sites may be left without topsoil.

Approved compost and/or manufactured organic soil amendments can be added to the topsoil to increase the organic content of the soil and assist in rebuilding soil microorganism populations. Topsoil can be added to rock mulch for added slope protection, to reduce the potential of erosion, and to enhance vegetative growth.

Do not mix organic material such as wood bark or fiber, grass hay or grain straw in topsoil unless nitrogen fertilizer is included (organic material uses nitrogen to break down and decompose the fibers).

Compost derived from livestock or green urban waste (trees, leaves, lawn clippings) is far superior to noncomposted manure or wood fiber.

Mix compost with surface soil to reduce drying of seeds in arid climates. A literature review by Montana DOT found that compost is rarely mixed with surface soil, which can be accomplished with a compost blower. MDT explored various equipment and methods to incorporate compost and then evaluated vegetation condition and erosion condition over three years. The research indicated that compost blowers work well, and that compost applied as a blanket is more effective at excluding weeds. [N]

Periodically, and after each storm event or snow melt, inspect, repair, and reseed if necessary to control erosion and loss of topsoil.

Procedures for periodic maintenance apply to both temporary soil stabilization or permanent seeding application.

 

4.11.5 Top Soil Specification for Imported Topsoil
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The Arizona Department of Transportation developed the following guidelines and requirements for topsoil that is brought onto the site. [N]

  • When sources are furnished by the Contractor, prior to hauling any topsoil to the project site, the Contractor shall submit a written soil analysis prepared by a laboratory, for approval by the Resident Engineer.
    • Requirements to be met and attested to in the soil analysis are:
    • Soil shall be fertile.
    • Soil shall be friable.
    • Soil shall be from well-drained arable land.
    • Soil shall be from land that has been producing healthy vegetation.
    • Soil shall be non-toxic.
    • Soil shall be reasonably free from subsoil, refuse, roots, heavy clay, clods, noxious weed seeds, phytotoxic materials, coarse sand, rocks over 5 centimeters in diameter, sticks, brush, litter, and other deleterious matter.
  • The characteristics of the soil shall meet the requirements of the Standard Specifications.
  • Topsoil that has pH values that are too high can be amended with soil sulfur and/or gypsum as recommended by an accredited soil laboratory and approved by the Resident Engineer.
  • Soil obtained from more than 4 feet (1.2 meters) deep should not be considered as topsoil even if it complies with the grading, pH, soluble salts, and plasticity index requirements. [Topsoil usually is no deeper than 3 or 4 feet (1 meter). Anything below that usually has calcium carbonate chunks, poor structure, and does not have soil bacteria and microbes necessary to good plant growth.]
  • Certificates of Analysis shall be submitted to the Resident Engineer for each source of topsoil proposed for use.
  • The Resident Engineer's approval shall be obtained prior to delivery of the topsoil from the source to the project.
  • The Resident Engineer at the job site gives final approval of the material, after testing in accordance with the Standard Specifications. Six random samples shall be taken after final placement of each 20,000 cubic yard (16,000 cubic meter) lot delivered to the site.
  • Sub-grade material shall be scarified to a depth of 6 inches (150 millimeters) prior to placement of topsoil.
  • Placement shall be uniform and any compacted areas are to be broken up by cultivation.

 

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Continue to Section 4.12 »
 
Table of Contents
 
Chapter 4
Construction Practices for Environmental Stewardship
4.1 General Construction Site Stewardship Practices
4.2 Work Area
4.3 Construction Involving Historic Properties and/or Other Cultural Resources
4.4 Construction in and around Drainage Areas and Streams, Wetlands, and Other Environmentally Sensitive Areas
4.5 Erosion and Sedimentation Control
4.6 Vehicle Fluid, Fuel, and Washwater Control
4.7 Air Quality Control Practices
4.8 Noise Minimization
4.9 Materials Storage, Collection and Spill Prevention on Construction Sites
4.10 Vegetation Management in Construction
4.11 Soil Management in Construction
4.12 Establishing Vegetation at Construction Sites
   
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