Inspection and Maintenance: Underground Infiltration Systems

Inspection & Maintenance Guidance of underground infiltration systems' best management practices that refers to LIDs that capture and temporarily store rainwater and snowmelt from hard surfaces (e.g., roofs and pavements) below ground in geotextile filter fabric lined excavations filled with washed gravel or other void space forming structures (TRCA, 2018)

Overview

Underground infiltration systems is a general term that refers to best management practices (BMPs) that capture and temporarily store stormwater from hard surfaces. These systems treat stormwater by detaining it to allow suspended sediments to settle out and soaking it into the ground where it is filtered and cleansed by interaction with soil. Runoff water is delivered to the practice through pipes connected to catchbasins, hydrodynamic (i.e., oil and grit) separators, filters, manholes, sub-drains of other features or roof downspouts. They are installed below the local maximum frost penetration depth to ensure they continue to drain year-round. Water that is in excess of the storage capacity overflows to an adjacent drainage system (e.g., municipal storm sewer or other BMP), typically via pipe or manhole containing a control structure (e.g., weir wall), to safely convey flows during flood events. Depending on the permeability of the underlying soil, such practices may be designed without a sub-drain for full infiltration or with a sub-drain for partial infiltration. The sub-drain pipe may feature a flow restrictor (e.g., orifice cap, valve) for gradually releasing detained water and optimizing the amount drained by infiltration.

General plan and profile (cross-section) views showcasing key components and sections of a typical infiltration trench.

Key components of Underground Infiltration Systems to pay close attention to are the:

• Inlets
• Sub-Drains
• Monitoring Wells
• Overflow outlets

Trash, debris and sediment builds up at these locations and can prevent water from flowing into or out of the practice.

Associated Practices

• Soakaways: Typically service individual lots and receive only roof and walkway runoff but can also be designed to receive overflows from other BMPs (e.g., rain barrels or cisterns, rain gardens, green roofs. Also known as infiltration galleries, French drains, dry wells or soakaway pits.
• Infiltration trenches: Linear oriented soakaways designed to fit into narrow strips of land between structures or properties, or along road rights-of-way; can also receive road runoff with adequate pretreatment devices upstream of inlets.
• Infiltration chambers: Include a range of proprietary modular structures installed underground that create large void spaces for temporary storage of stormwater while providing sufficient load bearing capacity to allow construction of structures on top of them. Applications are similar to infiltration trenches. Also known as infiltration tanks or vaults.
• Perforated pipe storm sewer systems or Exfiltration trenches: Linear-oriented infiltration trenches installed parallel with conventional storm sewer pipes and catchbasins that receive stormwater from them. May include manholes with perforated risers. Also known as exfiltration storm sewer, percolation drainage, or clean water collector systems.

Inspection and Testing Framework

An inlet to an infiltration trench during the fall in need of maintenance (TRCA, 2018).

Visual Indicators Framework - Underground Infiltration Systems

Component	Indicators	Construction Inspection	Assumption Inspection	Routine Operation Inspection	Verification Inspection
Contributing Drainage Area
	CDA condition	x	x	x	x
Inlet
	Inlet/Flow Spreader Structural Integrity		x	x	x
	Inlet/Flow Spreader Structural Integrity	x	x	x	x
Perimeter
	BMP dimensions	x	x		x
Filter Bed
	Filter bed sediment accumulation		x	x	x
Underdrain & Monitoring Well
	Monitoring well condition	x	x	x	x
	Sub-drain/Perforated pipe obstruction		x		x
Outlets
	Overflow outlet obstruction	x	x	x	x
Control Structure
	Control structure condition	x	x	x	x
	Control structure sediment accumulation	x	x	x	x

An example of a soakaway under construction with a newly installed monitoring well. To ensure performance assessment and infiltration testing can be conducted efficiently over the life cycle of this LID feature, the well condition must stay in good condition and be repaired immediately upon notice of significant damage (TRCA, 2018).

Testing Indicators Framework - Underground Infiltration Systems

Component	Indicators	Construction Inspection	Assumption Inspection	Routine Operation Inspection	Verification Inspection
Testing Indicators
	Sediment accumulation testing	x	x	x	x
	Natural or simulated storm event testing		x		(x)
	Continuous monitoring		x		(x)
Note: (x) denotes indicators to be used for Performance Verification inspections only (i.e., not for Maintenance Verification inspections)

Construction Inspection Tasks

Construction inspections take place during several points in the construction sequence, specific to the type of LID BMP, but at a minimum should be done weekly and include the following:

1. During site preparation, prior to BMP excavation and grading to ensure the CDA is stabilized and/or flow diversion devices are in place and confirm that construction materials meet design specifications
2. At completion of excavation and grading, prior to backfilling and installation of pipes to ensure depths, slopes and elevations are acceptable
3. At completion of installation of geotextile/pipes, prior to completion of backfilling to ensure slopes and elevations are acceptable
4. After final grading, prior to surface course installation to ensure depths, slopes and elevations are acceptable
5. Prior to hand-off points in the construction sequence when the contractor responsible for the work changes (i.e., hand-offs between the storm sewer servicing, paving, building and landscaping contractors)
6. After every large storm event (e.g., 15 mm rainfall depth or greater) to ensure ESCs and pretreatment or flow diversion devices are functioning and adequately maintained. View the table below, which describes critical points during the construction sequence when inspections should be performed prior to proceeding further. You can also download and print the table here
   

A pretreatment device (catchbasin insert) upstream of an underground infiltration system inlet in need of cleaning out. These devices are designed to limit the amount of trash, debris, sediment, oil and grease entering the infiltration system, which helps to prevent clogging and maintain its drainage performance (Source: TRCA, 2018).

Underground Infiltration Systems: Construction Inspections

Construction Sequence Step & Timing	Inspection Item	Observations*
Site Preparation - after site clearing and grading, prior to BMP excavation and grading, prior to BMP excavation and grading	Natural heritage system and tree protection areas remain fenced off
	ESCs protecting BMP layout area are installed properly
	CDA is stabilized or runoff is diverted around BMP layout area
	BMP layout area has been cleared and is staked/delineated
	Benchmark elevation(s) are established nearby
	Construction materials have been confirmed to meet design specifications
BMP Excavation and Grading - prior to backfilling and installation of geotextile/pipes	Excavated soil is stockpiled outside the CDA
	Excavation location, footprint, depth and slope are acceptable
	Compaction of subsoil where load-bearing portions of the system will be installed is acceptable
	Excavation bottom and sides roughened to reduce smearing and compaction
BMP Installation – after installation of geotextile/ pipes/structures, prior to completion of backfilling	Installation of structural components (e.g., control manhole, maintenance hatches) is acceptable
	Sub-drain trench dams installed correctly (location, elevation)
	Installations of sub-drain pipes (e.g., locations, elevations, slopes) & maintenance access hatches are acceptable

Routine Maintenance - Key Components and I&M Tasks

Regular inspections (twice annually, at a minimum) done as part of routine maintenance tasks over the operating phase of the BMP life cycle to determine if maintenance task frequencies are adequate and determine when rehabilitation or further investigations into BMP function are warranted.

Table below describes routine maintenance tasks for underground infiltration systems (infiltration chambers, infiltration trenches, bioswales, etc.) organized by BMP component, along with recommended minimum frequencies. It also suggests higher frequencies for certain tasks that may be warranted for BMPs located in highly visible locations or those receiving flow from high traffic (vehicle or pedestrian) areas. Tasks involving removal of trash, debris and sediment and weeding/trimming of vegetation for BMPs in such contexts may need to be done more frequently (i.e., higher standards may be warranted).

Underground Infiltration Systems: Key Components, Descriptions and Routine I&M Requirements

Component	Description	Inspection & Maintenance Tasks	(Pass) Photo Example	(Fail) Photo Example
Contributing Drainage Area (CDA)	Area(s) from which runoff directed to the BMP originates; includes both impervious (i.e., roofs and pavements) and pervious (i.e., landscaped) areas.	Remove trash, debris and sediment from pavements (biannually to quarterly) and eavestroughs (annually); Replant or seed bare soil areas as needed.	CDA has not changed in size or land cover. Sediment, trash or debris is not accumulating and point sources of contaminants are not visible.	Point sources of contamination are present (i.e. accumulated sediment and debris from melted snow piles)
Pretreatment	Devices or features that retain trash, debris and sediment, oil and grease; help to prevent obstructions and clogging of the infiltration area. Examples are eavestrough screens, catch basin inserts (screens, baffles, filters) and sumps, oil and grit separators, geotextile-lined inlets, filters, isolator rows, and water quality forebay units.	Remove trash, debris and sediment annually to biannually or when the device sump is half full; Measure sediment depth or volume during each cleaning, or annually to estimate accumulation rate and optimize frequency of maintenance.	Geotextile-lined stone inlet is free of sediment, trash and debris. Stones to slow down and spread out inflowing water remain in place.	Accumulated sediment, trash and debris in the hydrodynamic (i.e., oil and grit) separator is occupying greater than 50% of its storage capacity. (Photo Source: SWC Canada, 2020)
Inlets	Structures that deliver water to the BMP such as curb-cuts and geotextile-lined inlets or pipes connected to catchbasins, hydrodynamic (oil and grit) separators (HDS), filters, manholes, sub-drains of other features or roof downspouts.	Keep free of obstructions; Remove trash, sediment and debris biannually (spring and late fall) to quarterly (spring, summer, early fall, late fall/early winter); Make note of sediment depth or volume during each cleaning or annually to estimate accumulation rate and optimize frequency of maintenance.	There are no obstructions at the inlet and stormwater can freely flow into the BMP.	Sediment has accumulated in the inlet pipe to the infiltration trench and is fully obstructing flow of stormwater into the BMP.
Overflow Outlet	Structures (e.g., sub-drain/underdrain pipe connected to a manhole, conventional storm sewer pipe, or control manhole with weir wall) that convey flow exceeding the storage capacity to another drainage system or BMP.	Keep free of obstructions; Remove trash, sediment, debris, oil and grease biannually (spring and late fall) to quarterly (spring, summer, early fall, late fall/early winter).	The overflow outlet weir wall and storm sewer pipe in the control manhole of this infiltration chamber system is free of damage and obstruction and functions as designed to safely convey excess water from the BMP.	Sediment and debris has accumulated in the overflow outlet pipe which impairs its function to convey excess water from the BMP.
Sub-drain	Comprised of perforated pipe(s) surrounded by gravel and may be wrapped in geotextile filter fabric; installed in the gravel water storage layer or chamber unit openings to collect and convey treated water to an adjacent drainage system or other BMP; may also include a flow restrictor.	Keep perforated pipes and flow restrictors free of obstructions; Inspect flow restrictor and flush with a garden hose or pressure washer regularly (biannually to quarterly).	The sub-drain outlet pipe from the infiltration chamber system is not obstructed by sediment, debris or roots and shows no signs of damage.	A perforated pipe in an exfiltration storm sewer system is clogged by sediment and debris which inhibits its drainage function.
Monitoring well	Perforated standpipe that extends from the bottom of the excavation to just below the pavement surface and contains perforations or slots to allow measurement of subsurface water level; used to track drainage performance over the operating life cycle of the BMP.	Standpipes should be securely capped on both ends and protected from damage by vehicular traffic or foot traffic by a lockable casing.	The well is undamaged and accessible and the cap is in place and secured to prevent unauthorized access	The well was buried during landscaping and found covered only by filter cloth (i.e. cap missing).

Tips to Preserve Basic BMP Function

• Prohibit stockpiling of soil, sand, compost or unwashed gravel within the CDA and inlets to prevent clogging with sediment.
• For BMPs with sub-drains/underdrains and flow restrictors, pretreatment devices that prevent floating trash and debris from entering the practice should be used to prevent obstruction of the sub-drain pipe/flow restrictor.
• For BMPs equipped with pretreatment that detains floating contaminants (e.g., oil and grease, trash and debris), such as hydrodynamic separators/oil and grit separators or forebays with baffle walls, remove trash and debris first using a bucket strainer. Floating oils and grease should then be removed off the top of the water using a vacuum truck.
• Provide a means of draining infiltration chambers by gravity (e.g., pipe and valve through the control structure weir wall) to make inspection and maintenance work that requires drainage of the BMP (e.g., filter bed and control structure inspection and sediment removal, repairs to control structure) easier to perform.
• To remove accumulated sediment from underdrain pipes and chamber system units, a hydrovac truck equipped with a JetVac nozzle should be employed that uses pressured jets of water to propel itself through the structure while scouring and directing suspending sediments to a collection point. As the nozzle is retrieved, the sediment is flushed into the manhole or catchbasin sump for removal by vacuuming. Selecting an appropriate JetVac nozzle will depend on the structure being cleaned. For chamber system units, fixed nozzles designed for culverts or large diameter pipe cleaning with rear-facing jets are preferable (consult product manufacturer for further guidance).

Rehabilitation & Repair

Table below provides guidance on rehabilitation and repair work specific to underground infiltration systems organized according to BMP component.

The manhole sump is becoming filled with sediment, trash and excess and debris, which is beginning to impair flow of stormwater into the perforated pipe of the exfiltration storm sewer system.

Underground Infiltration Systems: Key Components, Typical Issues and Rehabilitation Requirements

Component	Problem	Rehabilitation Tasks
Sub-drain/Underdrain	Sub-drain perforated pipe is obstructed by trash, debris, sediment or roots	Schedule hydro-vac truck or drain-snaking service to remove the obstruction.
	Perforated pipe caps are missing or damaged	Replace missing or damaged caps.
Isolator Row or Water Quality Forebay Unit	Sump is ≥ half full or average sediment depth is ≥ 8 cm	Schedule maintenance work; First remove floating trash and debris by bucket strainer, then oil and grease by petroleum absorbing sponge. Then fully drain the row or unit by gravity, pumping or vacuuming, and remove accumulated sediment through use of a hydro-vac truck equipped with JetVac nozzle.
Manhole control structure	Structure or pipe connection is leaking and impairing the water storage capacity or function of the BMP	Schedule work to repair cracks or seal leaking components. The BMP may need to be fully drained to make such repairs.

State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include Source, Conveyance and End-Of-Pipe Controls.

Toronto and Region Conservation Authority

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.

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.

A system flow of gully inlets, pipes, overland flow paths, open channels, culverts and detention basins used to convey runoff to its receiving waters. City of Toronto 45 Wet Weather Flow Management November 2006

Best management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls.

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground 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.

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.

An underground water storage reservoir into which stormwater is directed and allowed to percolate into the underlying native subsoil.

An excavated area lined with geotextile filter cloth and filled with clean granular stone or other void forming material, that receives runoff and allow it to infiltrate into the native soil; can also be referred to as infiltration galleries, French drains, dry wells or soakaway pits.

Initial capturing and removal of unwanted contaminants, such as debris, sediment, leaves and pollutants, from stormwater before reaching a best management practice; Examples include, settling forebays, vegetated filter strips and gravel diaphragms.

The downward movement of water through the soil, the downward flow of runoff from the bottom of an infiltration BMP into the soil.

Loss of water from a drainage system as a result of percolation or absorption into the surrounding medium (e.g., the infiltration of water into the native soil through a perforated pipe wall as it is conveyed).

Natural or artificial means of intercepting and removing surface or subsurface water (usually by gravity).

Structure to control the volume or rate of flow of water through or over it.

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.

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.

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.

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

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

Structure to control the volume or rate of flow of water through or over it.

State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include Source, Conveyance and End-Of-Pipe Controls.