Construction

Overview

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

Construction stages and LID types

The five-step construction process for ground-level LIDs.

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 and surface layer installation: vegetated LIDs and finishing grades and surface layer installation: permeable pavements finishing grades and surface layer installation: sub-surface LIDs
5. Post-construction

Most LID practices work at the sub-surface and ground-level by routing stormwater flows from impervious surfaces into excavated or natural depressions or by allowing stormwater to pass through a pervious 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 sub-surface and ground-level LID types. To illustrate, STEP's recommended processes for excavation do not differ between LID practice types. 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 depending on whether the LID practice's surface is vegetated or permeable pavement. Some sub-tasks in stage 3 will also vary depending on the LID type. 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 LID construction stage.

Green roofs, blue roofs, stormwater planters, 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

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

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

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)
  • infiltration chamber installation
• stone choker layer
• curbing
• pre-treatment and inlet

Finishing grades and surface layer installation

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	bedding layer
finish grading	placement and finishing
large stone and riprap	paver installation
plant verification and installation	tamping
mulch placement	joint cutting
stabilizing contributing drainage area and planting adjacent vegetation	joint aggregate
--	curbing
--	stabilizing contributing drainage area

Post-construction

Post-construction tasks ensure that the LID practice was built to specs and that any outstanding issues with it are resolved before assumption. The post-construction page give guidance on:

• Addressing deficiencies
• Final certification

Sustainable Technologies Evaluation Program

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

A hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil.

The accounting of inflow and outflow of water in a system according to the components of the hydrologic cycle.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

An alternative practice to traditional impervious pavement, prevents the generation of runoff by allowing precipitation falling on the surface to infiltrate through the surface course into an underlying stone reservoir and, where suitable conditions exist, into the native soil.

The practice of intercepting, conveying and storing rainwater for future use. Captured rainwater is typically used for outdoor non-potable water uses such as irrigation, or in the building to flush toilets.

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

Open space floodway channels, road reserves, pavement expanses and other flow paths that convey flows typically in excess of the capacity of the Minor Drainage System.

Gravel, or crushed stone of various size gradations (i.e., diameter), used in construction; void forming material used as bedding and runoff storage reservoirs and underdrains in stormwater infiltration practices.

Filter fabric that is installed to separate dissimilar soils and provide runoff filtration and contaminant removal benefits while maintaining a suitable rate of flow; may be used to prevent fine-textured soil from entering a coarse granular bed, or to prevent coarse granular from being compressed into underlying finer-textured soils.

A perforated pipe used to assist the draining of soils.

An underlying bed filled with aggregate or other void-forming fill material that temporarily stores stormwater before infiltrating into the native soil or being conveyed by an underdrain pipe.

A mixture of mineral aggregates bound with bituminous materials, used in the construction and maintenance of paved surfaces.

a top dressing over vegetation beds that provides suppresses weeds and helps retain soil moisture in bioretention cells, stormwater planters and dry swales.

The total surface area upstream of a point on a stream that drains toward that point. Not to be confused with watershed. The drainage area may include one or more watersheds.

A broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations.