Construction

From LID SWM Planning and Design Guide
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Overview[edit]

  • Importance of construction inspections
  • Contractor familiarity
  • What you can expect from STEP guidance: practical advice, specific to LID, on how to ensure successful construction


Construction stages and LID types[edit]

STEP divides the construction process for LID practices into five over-arching stages:

  1. Pre-construction
  2. Excavation and grading
  3. Sub-surface components
  4. Finishing grades: inlet, outlet, and surface layer
  5. Post-construction

Most LID practices work at the ground-level by routing stormwater flows from impervious surfaces into excavated or natural depressions or by allowing stormwater to pass through a traditionally impervious surface, as is the case with permeable pavements. These depressions are designed and constructed to the meet goals of the LID practice, which may be quality control, quantity control, or water balance restoration. Bioretention gardens, bioswales, rain gardens, enhanced swales, exfiltration trenches, permeable pavements, and infiltration systems (chambers, trenches, and soakaways) all fall into this category.

For this reason, Stages 1-2 and 5 of the LID construction process are fundamentally similar for all ground-level LID types. To illustrate, STEP's recommended processes for excavation do not differ depending on the practice type. Excavation is excavation, whether it is for a bioretention garden or a permeable pavement parking lot. On the other hand, stage 4 sub-tasks will vary substantially from LID type to LID type, since the construction processes for installing the surface of an LID practice will differ depending on whether the practice is vegetated or not. Some sub-tasks in stage 3 will also vary. For example, permeable pavements often require compaction of sub-surface storage layers. The following sections give a brief description of each over-arching stage, a list of sub-tasks for each stage, and links to the page dedicated to each main sequence construction stage.

Green roofs, blue roofs, and rainwater harvesting systems have specific construction sequences that differ from the main sequence described above. One day we will write guidance for these LID practices as well.

Pre-construction[edit]

Pre-construction activities set the stage for the successful construction of an LID practice. The pre-construction page gives guidance on:

  • design verification and site walk-through
  • LID construction notes
  • tendering and contract
  • communication, inspection plan, and utilities coordination
  • erosion and sediment control measures
  • mobilization, access, staging, and perimeter controls

Excavation and grading[edit]

Excavation and grading are necessary for installing LID practices with sub-surface components, re-grading land to hold more water, and re-routing overland flow routes into an LID practice. The excavation and grading page gives guidance on:

  • clearing and grubbing
  • excavation and rough grade
  • sub-grade
  • final excavated grade and verification

Sub-surface components[edit]

Most LID practices use sub-surface features--gravel storage reservoirs, liners, underdrains, monitoring wells, etc. The construction process for sub-surface components works from the ground up. While some LID practices include all the sub-surface components listed below, most designs will not include one or more of these layers. Permeable pavements can have different construction requirements at this stage, mostly regarding compaction of sub-surface layers. Installing infiltration chambers also requires guidance specific to that LID type.

The backfill granular, utilities, and pipes / sub-surface components page gives guidance on:

  • geotextile
  • underdrain
  • impermeable liners
  • overflow
  • monitoring wells
  • storage reservoir
    • sub-base reservoir (permeable pavements)
    • base course (permeable pavements)
  • stone choker layer
  • curbing
  • pre-treatment and inlet

Finishing grades and surface layer installation[edit]

This construction stage differs between LID practice type. The finishing grades and surface layer installation: vegetated LIDs page has guidance for vegetated LIDs, and the finishing grades and surface layer installation: permeable pavements page has guidance for and non-vegetated LIDs. In many cases the surface of infiltration systems will be traditional asphalt, concrete, or pavers; STEP does not provide guidance on installing non-permeable surfaces.

Stage 4 guidance for vegetated LIDs and permeable pavements
LID practices with vegetated surfaces Permeable pavements
soil media installation and soil amendments PP
finish grading PP
large stone and riprap PP
plant verification and installation PP
mulch placement pp
stabilizing contributing drainage area and planting adjacent vegetation PP
as-built survey PP

Post-construction[edit]

If you assume the works too early, u make an ass out of me.

LID practice pages[edit]

Each construction page listed below :)

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