Pre-construction

The five-step construction process for sub-surface and ground-level LIDs.

Overview

The construction guidance in this section applies to all sub-surface and ground-level LID practices: bioretention gardens, bioswales, rain gardens, exfiltration trenches, enhanced swales, permeable pavements, infiltration trenches, infiltration chambers, and soakaways. It was developed for project managers, engineers, and contractors on the pre-construction steps required for successful construction of sub-surface and ground-level LID practices.

Design verification and site walk-through

When the design is 90% complete, the project manager (PM) should conduct a walk-through of the site to verify that the designs match site conditions. This site walk-through is a critical step in the transition from design to construction and helps the project manager plan for future pre-bid or pre-construction meetings. Think carefully about how a contractor will be able to build the LID practice. At this stage, determine if the design drawings and construction notes address specific LID construction requirements, and determine whether they contain inaccuracies or are missing detail. The table below identifies key points and describes considerations for the site walk-through.

Design verification points and key considerations

Verification point	Key considerations
Adjacent land uses	Have land uses changed while the design was in progress? Are there any planned land-use changes on adjacent properties and is their drainage connected to the site? Will adjacent land uses create dirty runoff that may flow onto the construction site or into the LID feature? Is additional erosion and sediment control (ESC) measures required beyond what is marked in the ESC plan?
Contributing and receiving drainage areas	Where is water coming onto the site and where is water leaving the site? Are any environmentally sensitive areas downstream? Are there any apparent changes while the design was in progress? If so, should these changes motivate modifications to the design, particular ESC measures, etc.?
Seasonal maintenance operations	Will winter snow storage and salt or sand application impact the LID feature?
Location of LID features	Do the proposed LID locations match the site? Does the footprint of the LID fit within the site's constraints?
Existing and planned infrastructure/utilities	Confirm accuracy of design drawings. Are there any unmarked utilities not detailed within the drawing set?
Property boundaries and building foundations	Confirm accuracy of design drawings. Do fence lines align with property boundaries?
Natural heritage features and existing vegetation	Do the designs accurately capture existing natural heritage features? Which areas on the site should be protected during construction, and will these exclusion areas make the LID facility more difficult to construct? Where are the drip lines of the site's trees? Are there any invasive species on the site which could colonize the LID practice?
Access, sequencing, staging, and constructability	Where will the contractor access the site? What is the ideal construction sequence, and where could materials and supplies be stored during construction? Do site characteristics constrain standard construction practices? Will it be a difficult job? Do constructability issues require a field change or amendment to the design or project tender?

LID construction notes

LID design drawings need detailed construction notes within the design drawings. These notes should include:

• sequencing and staging areas
• material specifications
• testing requirements
• installation procedures
• elevations and invert details for inlet, outlet, and overflow features
• grading details for conveyance features
• erosion and sediment control requirements
• Inspections and maintenance responsibilities during construction and prior to assumption

The last two bullets require further comment. LID facilities are most vulnerable to sedimentation and clogging during their own construction and construction of adjacent and connected lands. LID construction contracts should specify a sufficient combination of ESC measures and inspection and maintenance of the LID facility during the construction and assumption phases to ensure that it will function without remediation. Contracts for larger-scale projects, e.g., where the LID facility in question is part of a road reconstruction project or subdivision development, should contain clear terms covering responsibilities for keeping sediment from entering newly constructed LID features and removing it if necessary.

Tendering and contract

The project contract and tendered drawings, including details, notes, ESC requirements, material specifications, and testing requirements should align. Given that most standard contract templates do not include wording specific to LID construction, considerations for LID can be placed under special provisions within the contract.

Details within the drawings and contract should also be accurately reflected in the tender bid form. A detailed bid form that reflects the entirety of the work allows contractors to deliver accurate bids, lessens the likelihood of project shortcuts, and helps to ensure transparency amongst bids. Ultimately, clear bid pricing reduces the potential for misunderstandings and conflicts and attracts experienced contractors.

Minimum contractor requirements

Many contractors are unfamiliar with building LID practices. Consider mandating contractor training, attendance at pre-bid meetings, LID project references, and bid bond insurance as prerequisites.

Communication, inspection plan, and utilities coordination

Be sure to verify all design changes

A simple departure from the design—installing a T-junction (top) in place of a Y-junction (below) would have made it extremely difficult or impossible to inspect this underdrain and flush it out. (Image sources: CVC)

After securing a contractor, a pre-construction meeting between the project manager, project engineer, and contractor will facilitate an efficient job site and reduce the potential for miscommunication. These meetings should cover:

• LID design details and construction notes
• material specifications, inspections, and chain-of-custody
• access routes and storage areas
• protection of LID practices: phasing, ESC, and perimeter fencing
• equipment requirements and recommendations
• verification of field changes
• plant establishment and warranty-period maintenance
• ownership and assumption protocols
• project inspection plan
• verification processes for field changes to the design and material substitutions

The table below expands upon the bullets above and provides helpful guidance on communication topics at the pre-construction meeting. Given the lead-up time needed for testing requirements of specified materials, these discussions should occur soon after the contract is awarded, ideally 2 months prior to project construction.

Pre-construction communication topics

Communication topic	Guidance
LID design details and construction notes	Provide overview of LID design, ideally through a design brief. Discuss LID design details and construction notes, and ensure details and notes are understood by all parties involved. Discuss project phasing and general timeliness.
Material specifications, inspections, and chain-of-custody	Material substitutions must be confirmed by the project engineer. Material testing requirements and their timelines must be understood by all parties, including who is responsible for testing. Material acceptance protocols, with chain of custody, should be understood by all parties, including who is responsible if the material fails. Biomedia manufacturing and testing should start prior to construction, ideally 2 months in advance.
Access routes and storage areas	Discuss access to the site along with material and equipment storage with the contractor. Material storage should be downstream of LID features. Heavy equipment routes should be agreed on to minimize impact to the LID feature and protected areas.
Protection of LID practices during construction: phasing, ESC, and perimeter controls	Review and discuss the ESC plan. Discuss frequency of ESC inspection, protocols for repair and replacement, and parties responsible. Discuss communication protocols for sediment contamination. Discuss rehabilitation protocols for sediment contamination and parties responsible.
Equipment requirements and recommendations	Wide track, low-ground pressure vehicles are recommended to minimize compaction. Excavation should occur from outside of LID feature whenever possible. See Excavation. Gravel and biomedia should be installed by a slinger truck outside of practice whenever possible. Avoid heavy machinery within LID footprint whenever possible.
Verification of field changes	The project engineer must verify all field changes to materials and design. In the design engineer’s absence, the project manager should verify field changes.
Plant establishment and warranty-period maintenance	Discuss planting timelines at the pre-construction meeting. Avoid planting in the summer months (June, July and August). Fall planting is the ideal scenario. Plants will require regular maintenance (irrigation and weeding) to ensure establishment, especially over the first year. Ensure that plant maintenance and its frequency is understood by responsible parties. Plant deficiencies should be addressed prior to warranty period initiating.
Ownership and assumption protocols	Discuss how testing will confirm LID practice functionality according to assumption protocols.
Project inspection plan	Project inspection plan should be detailed and understood by all parties. Sign off by all parties involved on photos, video, and social media post may be required.

Inspection plan

Communication about inspection points between the owner, the engineer, and the contractor are critical for a successful project (adapted from TRCA, 2016).

The inspection and maintenance page gives detailed guidance on how to conduct construction inspections for LID practices. Generally, construction inspections should be continuous as the work progresses. If this isn’t possible, critical inspection points are:

• site preparation
• excavation
• installation of pipes, granular, and biomedia
• finishing grades
• surface treatment installation: pavers, plants, porous concrete, etc.
• after rain events (see ESC inspection and maintenance)
• whenever sub-contractors or utilities begin work (hand-off points)

The last element—inspection of sub-contractors—requires further comment. For example, if a sub-contractor is installing the curbing, they need to understand the purpose of the curbing and curb cuts, i.e., that they are meant to direct water into the LID facility, not to the closest catch basin. Without this knowledge, the sub-contractor may mistakenly build the curbs as they usually do: to direct water into a catchbasin.

Utilities coordination

Similarly, if the LID practice is in close proximity to sub-surface utilities, the utility owner may use their own contractors to perform related work. If this is the case, these sub-contractors must also be made aware of the LID practice’s purpose, how compaction affects performance, keeping the LID practice clear of excavated dirt and generally how their work could impede the construction and performance of the LID practice. In general, communication between the contractors, project managers, and project engineers should be frequent and open.

Erosion and sediment control measures

Grooves perpendicular to the grade help prevent the development of rills. (Photo source: CVC, 2022)

STEP's Erosion and Sediment Control Guide for Urban Construction contains detailed guidance on the design, installation, inspection, maintenance, and decommissioning of ESC measures. This section gives practical advice on structural and non-structural ESC for contractors, inspectors, and those who are new to construction and ESC practices.

Establish required perimeter controls, including erosion and sediment control (ESC) measures, prior to construction and adjust accordingly during construction. ESC measures come in two basic types: structural ESC, which slows and holds runoff from the disturbed area, dissipating erosive forces, promoting ponding, and allowing suspended particles to settle out; and non-structural ESC, which primarily minimizes the exposure time of stripped soils.

For non-structural ESC measures, be sure to:

• Stage clearing and grubbing to minimize exposure of stripped soils.
• Watch the weather and adjust plans accordingly.
• Manage and demarcate heavy equipment routes away from LID practices.
• Use simple, common-sense methods, such as scarifying exposed soils perpendicular to the grade to prevent the development of rills.
• Create and use a pollution prevention plan for fuels or solvents.

Generally, all exposed soils that are not being actively worked on must have temporary erosion protection or permanent cover within 7 days for slopes 3:1 or greater and 14 days for slopes 3:1 or lesser. This should apply to all exposed soil areas year-round and until the site is stabilized.

The table below gives guidance on common mistakes made when installing structural ESC measures.

Best practices and common errors when installing structural erosion and sediment control measures

Structural ESC measure	Do	Don't	(Pass) Photo Example	(Fail) Photo Example
Sediment fencing	Stake it in, support it, and double it up adjacent to natural heritage features.	Leave vulnerable points with insufficient supports.	This sediment fencing has been doubled up and staked in. As a result, it kept a large amount of sediment from entering the adjacent natural heritage feature. It is in need of immediate maintenance, however. (Photo source: CVC)	This sediment fencing was not staked in at the likely point of failure, resulting in the release of sediment into an adjacent natural heritage feature. (Photo Source: CISEC)
Inlet controls	Support and stake in inlet controls such as sediment socks. Use catch basin inserts as a final line of protection.	Leave inlets to the stormwater management system unprotected or unsupported	This filter stock has been staked in and supported. (Source: CVC)	This catch basin was left unprotected, resulting in the release of sediment into the stormwater management system. (Photo source: CISEC)
Stockpiles	Sling soil and aggregates into the facility when possible. Install soil and aggregates upon arrival to the site when possible.	Store materials within the stormwater management system, e.g., in the roadway. Leave stockpiles unprotected when wet weather is expected.	(Photo source: CVC)	If it were to rain, fines from this stockpile would go straight into the downstream catchbasin. (Photo source: CVC)
Protecting features during construction, including using sacrificial filter fabric	Use sacrificial filter fabric when necessary to protect features, and leave ESC measures in place until all construction in the LID's drainage area is complete.	Remove ESC measures early or allow sediment to enter the LID footprint.		ESC measures at this bioretention feature were removed before construction in its drainage was complete, allowing sediment into it.(Photo Source: CVC)

Erosion and sediment control inspection and maintenance

ESC measures require ongoing attention to ensure integrity and performance. ESC inspections should take place, at minimum, during installation, after rainfall events, and at the critical inspection points discussed in the Pre-construction: Communication, inspection plan, and utilities coordination

At these inspections, keep an eye out for accumulated sediment, worn materials, compliance with the project's staging plan, and potential failure points.

Ponding in front of this catchbasin indicates that this ESC inlet control is working. Ponding water allows for suspended sediments to settle. (Photo source: CISEC)

This inlet control has worked well, but the sediment should be removed as soon as possible. (Photo source: CISEC)

Mobilization, access, staging, and perimeter controls

In tandem with installation of ESC measures, setting up access and staging areas is one of the first steps in any construction project. Perimeter controls such as construction fencing are often combined with silt fencing. Safety fencing may be needed to restrict public access to the site, especially if the site sees lots of pedestrian traffic. Together, construction fencing and structural ESC measures should demarcate:

• limits of construction
• heavy equipment routes
• equipment and material storage areas
• LID practice footprints
• natural heritage features
• access areas

Construction steps

1. Install perimeter fencing/sediment fence in alignment with design drawings and ESC plan.
2. Demarcate staging and access points with fencing or surface markings in alignment with design drawings.

Key inspection points

• Ensure perimeter fencing and access/staging areas are in alignment with design drawings.

Mistakes to avoid

• Departing from the plan to stage and limit access points, material storage areas, and equipment storage areas
• Installing inadequate perimeter controls, e.g. inadequate sediment fencing or construction fencing

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.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

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.

Mineral particles which are smaller than 2 mm, and which are free of appreciable quantities of clay and silt. Coarse sand usually designates sand grains with particle size between 0.2 and 0.02 mm.

Deposition of material of varying size, both mineral and organic away from its site of origin by the action of water, wind, gravity or ice.

Settling-out or deposition of particulate matter suspended in runoff.

A perforated pipe used to assist the draining of soils.

Credit Valley Conservation

Human application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47

Toronto and Region Conservation Authority

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.

Ground depression acting as a flow control and water treatment structure, that is normally dry.

Box like underground concrete structure with openings in curb and gutter designed to collect runoff from streets and pavement.

Sustainable Technologies Evaluation Program

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.

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.

Soil particles with a diameter less than 0.050 mm.

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

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.

Soil or media particles smaller than sand and larger than clay (3 to 60 m)

Low impact development is a stormwater management and land development strategy applied at the parcel and subdivision scale that emphasizes conservation and use of on-site natural features integrated with engineered, small scale hydrologic controls to more closely mimic pre-development hydrologic functions.

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.