4.2.5 Construction

Protecting and enhancing site soils requires planning (Chapter 2: Planning for LID) and sequencing of construction activities to reduce impacts. The following recommended steps are adapted from the Low Impact Development Technical Guidance Manual for Puget Sound (WSU-PSP, 2012) and the Building Soil – A Foundation for Success website (www.buildingsoil.org). These steps begin with land clearing and grading and continue through end of construction (prior to planting) and after planting is complete:

Land clearing and grading phase
  • Fence all vegetation and soil protection areas prior to first disturbance, and communicate those areas to clearing and grading operators. The root zones of trees that may extend into the grading zone should be protected or cut rather than ripped during grading.
  • Chip land-clearing debris on-site and reuse as erosion-control cover or stockpile for reuse as mulch at end of project.
  • Stockpile topsoil to be reused with a breathable cover, such as wood chips or landscape fabric.
  • If amended, topsoils will be placed at end of project. Grade 8 to 12 inches below finish grade to allow for placing the topsoil.

Construction phase
  • Ensure erosion and sediment control BMPs are in place before and modified after grading to protect construction activities. Compost-based BMPs (compost “blankets” for surface, and compost berms or socks for perimeter controls) give a “two-for-one” benefit because the compost can be reused as soil amendment at the end of the project.
  • Lay out roads and driveways immediately after grading and place rock bases for them as soon as possible. Keep as much construction traffic as possible on the road base, and off open soils. This will improve erosion compliance, reduce soil compaction, and increase site safety by keeping rolling equipment on a firm base.
  • Protect amended/restored soils from equipment-caused compaction by using steel plates or other BMPs if equipment access is unavoidable across amended soils.
  • Maintain vegetation and soil protection area barriers and temporary tree root zone protection BMPs throughout construction and ensure that all contractors understand their importance.

End of construction, soil prep before planting
  • Ensure vegetation and soil protection barriers are maintained through the end of construction.
  • Disturbed or graded soil areas that have received vehicle traffic will need to be de-compacted to a minimum 12-inch depth. This can be done with a cat-mounted ripper or with bucket-mounted ripping teeth.
  • Amend all disturbed areas with compost or other specified amendments at least 8 inches deep by tilling, ripping, or mixing with a bucket loader. Alternatively, place amended stockpiled topsoil or import an amended topsoil. It is good practice to scarify or mix amended soils several inches into the underlying sub-soil to enhance infiltration and root penetration. Compost from erosion BMPs (compost blankets, berms, or socks) can be reused as appropriate if immediately followed by planting and mulching so there is no lapse in erosion control.
  • Amended topsoil can be placed as soon as building exterior work is complete. During this step, vehicles should stay on roads and driveway pads. Compost, soil blends provide good ongoing erosion protection.
  • Avoid tilling through tree roots – instead use shallow amendment and mulching.
  • Final preparation for turf areas should include raking rocks, rolling, and possibly placing 1 to 2 inches of sandy loam topsoil before seeding or sodding.
  • Plan for amended soil to settle by placing amended soil slightly higher than desired final grade, or retain or import a smaller amount of amended topsoil to meet final grades adjacent to hardscape such as sidewalks.
  • Keep compost, topsoil, and mulch delivery tickets so inspector can verify that quantities and products used match those calculated for design (See Design section above).

After planting and end of project phase
  • Remove protection area barriers, including sediment fences, filter socks, and curb and stormwater inlet barriers. Evaluate trees for stress and need for treatment, such as pruning, root- feeding, mulching etc. Plan to have an arborist on-site, as appropriate.
  • Mulch all planting beds where soil has been amended and re-planted with 2 to 3 inches of arborist wood chip or other specified mulch.
  • Communicate a landscape management plan to property owners that includes: on-site reuse of organics (e.g., mulch leaves, mulch-mow grass clippings) to maintain soil health; avoiding pesticide use; and minimal organic-based fertilization.

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