Filtration soil is a type of engineered soil that is designed to help absorb and filter stormwater runoff. It is often used as part of green infrastructure efforts to manage stormwater and prevent flooding and pollution problems. Filtration soil helps to mimic natural hydrologic processes by allowing water to infiltrate back into the ground, reducing runoff volume and filtering out pollutants.
What is Filtration Soil?
Filtration soil, also known as engineered soil, structural soil, or amended soil, is a specially designed planting medium that allows rainfall and runoff to pass through it while filtering out contaminants. It typically consists of:
- Sand – forms pores that allow water to drain through
- Topsoil – provides nutrients for plants
- Clay – helps absorb and retain some water
- Organic matter like compost – supports microbes and retains moisture
The sand, topsoil, clay and organic matter are mixed together in specific ratios to create a lightweight and porous soil. The pore spaces between the soil particles provide channels for stormwater to infiltrate into the ground. The filtration process helps to trap sediment, nutrients, metals, bacteria and other pollutants as the water filters through the soil profile.
Filtration soils are engineered to have an infiltration rate of at least 0.5 inches per hour according to standards, which allows them to absorb runoff more quickly than regular soils. They are also designed to retain enough moisture for healthy plant growth.
Where is Filtration Soil Used?
Filtration soils are a key component of many green infrastructure applications focused on more sustainable stormwater management. Common uses include:
Bioretention Areas and Rain Gardens
Filtration soil mix is used as the planting bed in rain gardens and bioretention cells. These shallow landscaped depressions collect and absorb runoff from nearby paved surfaces. The filtration soil helps drain water into the underlying native soil while filtering pollutants. Plants help uptake excess water and provide aesthetic value as well.
Green Roofs
Lightweight filtration soils can be used as the growing media in green roofs. The shallow depths and drainage characteristics allow green roof systems to be installed on rooftops without requiring major structural reinforcement. Filtration soils help drain excess water and prevent ponding while supporting plants.
Permeable Pavements
Pervious concrete, porous asphalt, and permeable pavers allow stormwater to drain through gaps in the pavement surface into an underlying stone reservoir. Filtration soils are often used to fill the bottom portion of the reservoir to provide additional filtration and promote infiltration into the native soils.
Stormwater Curb Extensions and Planters
Filtration soils are used in specialized tree box filters and stormwater curb extensions or planters built into the sidewalk area. These small landscaped areas intercept road and sidewalk runoff allowing it to infiltrate into the ground.
Bioswales and Vegetated Swales
In open channels like bioswales and roadside vegetated swales, filtration soils are layered underneath the channel lining to slow down water flow and allow infiltration. Check dams are also useful in bioswales to pool water and promote infiltration.
Benefits of Using Filtration Soils
There are many advantages to using engineered filtration soils as part of stormwater management plans:
Increased Stormwater Infiltration
Filtration soils have a high void ratio and infiltration rates that exceed minimum standards. This allows more stormwater to be absorbed into the ground, reducing harmful runoff.
Enhanced Pollutant Removal
As stormwater filters through the soil profile, sediment and other contaminants are trapped in the small pore spaces. This helps improve downstream water quality.
Groundwater Recharge
Water infiltrating into filtration soils can recharge local groundwater supplies and provide baseflow to nearby streams and water bodies during dry periods.
Flood Mitigation
By providing greater infiltration and retention capacity, filtration soils can reduce the frequency of flood events that result from large stormwater discharges.
Improved Soil Health
Filtration soil mixes incorporate organic matter, microbes, and nutrients to help support diverse microbial communities and healthy soil structure.
Urban Greening Benefits
Filtration soils allow green infrastructure tools like rain gardens and green roofs to be installed in urban areas, helping expand urban greenspace.
Composition of Filtration Soils
Filtration soils contain a blend of mineral and organic materials that provide an optimized balance between infiltration, filtration, retention and plant growth capacity. Typical components include:
Sand
Sand forms the main structure of filtration soils and creates large void spaces for water flow and retention. Concrete sand or bank-run sand are common choices. The sand grain size distribution and percentages can be customized to achieve the desired infiltration rate.
Topsoil
Topsoil provides organic matter and nutrients to support biological processes and plant growth. Sandy loam topsoils are preferable to achieve proper drainage.
Clay
Clay particles, when mixed with sand at 5-10% by volume, help bind the mixture and provide surface area to assist in filtering out pollutants and retaining some moisture.
Compost
Compost offers an organic component to support soil microbes and vegetation. Well-aged compost that meets specifications should be used to avoid leaching nutrients.
Additional Amendments
Some specialized filtration soil mixes also incorporate amendments like biochar, vermiculite, perlite, or zeolites. These can help retain minerals and enhance microbial communities.
The proportions of each component are engineered based on the site conditions and desired performance goals. Most filtration soil blends have 40-60% sand content with the remainder organic matter and fines.
How Does Filtration Soil Work?
Filtration soils provide stormwater management through these key mechanisms:
Infiltration
The highly porous structure creates void spaces that allow rapid percolation of water through the soil profile. The infiltration rate meets minimum standards for water to quickly drain before it runs off the surface.
Filtration
As water drains through, sediment and contaminants are physically strained out by the sand grains and organic matter, improving water quality. Dissolved pollutants are also retained through absorption.
Storage
Some amount of rainfall is retained in the pore spaces for a period of time before it slowly infiltrates deeper into the native soil. This storage helps reduce peak discharge rates.
Biological Processes
Microbes in the soil degrade organic pollutants and cycle nutrients. Plants take up excess water and pollution through their root systems.
Evapotranspiration
Plants also release moisture to the air through transpiration from their leaves. This evaporative process helps reduce the volume of water that becomes runoff.
Conveyance
In larger storm events, the excess water filters through the soil to an underdrain system or outlet that safely conveys filtered water back to the storm sewers or receiving waters.
Getting the Right Filtration Rate
One of the most important characteristics of filtration soils is the infiltration rate. Adequate infiltration capacity is needed so the soil can absorb potential runoff volumes without excessive ponding. Minimum infiltration rate standards typically specify 10-30 inches per hour but can vary by region.
The infiltration rate is determined by the:
- Sand grain size distribution and percent fines
- Compaction level during installation
- Hydraulic head pressure of ponded water above it
- Accumulation of sediment over time
Ideally the filtration soil and overall bioretention system must be engineered for the site’s specific rainfall patterns, contributing drainage area, and runoff volume. Soil specifications can then be customized to achieve the target infiltration rates.
Proper construction techniques that avoid over-compaction are also crucial to maintain the soil’s designed infiltration capacity. After construction, testing using a double-ring infiltrometer helps verify the infiltration rate meets requirements.
How to Install Filtration Soils
Filtration soils require proper construction and installation practices to function as engineered:
Excavation
The bioretention area is excavated to the design depth with allowance for mulch, ponded water, and soil layers. The native soils at the bottom should be scarified or tilled to improve infiltration.
Underdrains
Slotted underdrain piping is installed on the bottom with a slight slope to help collect filtered runoff and convey it out of the system. Clean stone layers are placed around the underdrains.
Layering
Filtration soil is placed in lifts of 6-12 inches, lightly compacted between layers. Amendments can be added as needed to meet specifications. A top mulch layer helps retain moisture.
Grading
The area is graded to achieve the desired base depth for ponding above the soil bed. Smooth slopes promote sheet flow across the entire surface.
Vegetation
Native plants adapted to the local climate and soil moisture levels are planted at appropriate densities across the entire area.
Oversight
Proper engineering controls should be in place during construction to ensure the filtration soil meets particle size distribution, compaction, and infiltration rate requirements.
Maintaining Filtration Soils
Like other stormwater systems, filtration soils require regular maintenance to continue functioning properly. Some key tasks include:
Inspections
Inspect bioretention areas after major storms to check for standing water. Measure infiltration rates periodically to identify loss of capacity.
Sediment Removal
Remove accumulated sediment, litter and debris from pavement and soil surface to prevent clogging. Avoid compacting the soil bed during cleaning.
Vegetation Care
Water plants, remove weeds, prune woody plants and replace dead vegetation to maintain optimal soil moisture and nutrient uptake.
Soil Amendments
Add compost to improve organic content. Rototill or cultivate several inches into the soil to maintain porosity.
Underdrain Flushing
Periodically flush underdrains to remove mineral deposits or soil particles that can clog the perforations and reduce drainage capacity.
Major Renovation
Severely degraded filtration soils may need removal and replacement if they no longer meet infiltration rate standards after other remedies are attempted.
Common Problems with Filtration Soils
Some potential issues that may arise with filtration soils include:
Compaction
Heavy machinery or excessive foot traffic during construction can overly compact the soil, reducing the infiltration rate.
Fine Sediment Accumulation
Clay, silt, and fine sand particles can migrate into the voids over time, causing clogging and reduced infiltration.
Nutrient Leaching
Excessive nutrients from composts or fertilizers can leach out of the soil into groundwater if not properly controlled.
Prolonged Saturation
Slow drainage can lead to saturation, ponding issues, and unhealthy soil conditions for plants. Pipe obstructions or clogging may be the cause.
Preferential Flow Pathways
Water flowing directly down soil cracks or root holes can bypass treatment rather than filtering through the entire soil matrix.
Plant Decline or Mortality
Plants not suited for the conditions or maintenance issues can result in sparse or dead vegetation, which reduces soil benefits.
How to Improve Infiltration in Compacted or Clogged Soils
There are several methods that can potentially be used to restore and improve the infiltration rate in filtration soils that have become overly compacted or clogged over time:
Aeration or Spiking
- Aerate by punching holes 12 inches apart with an aeration fork, spade or soil spike.
Roto-Tilling
- Use a rototiller or cultivator to churn soil 8-12 inches deep to loosen compacted layers.
Sand Incorporation
- Till in a 1-2 inch layer of coarse sand to help open up the soil structure.
Compost Incorporation
- Till 2-4 inches of mature compost into the soil to increase organic content.
Vertical Mulching
- Bore holes and fill with a mix of gravel, sand and compost to create vertical flow paths.
Restoration Planting
- Plant deep-rooted grasses and herbaceous plants to help break up compacted layers.
Underground Soil Lifting
- Inject high-pressure air into the soil through probes to fracture and expand layers.
New Soil Installation
- Excavate and replace severely compacted or clogged soil with freshly engineered filtration soil meeting the design specifications.
Pretreatment Measures
- Install forebays, filter strips or sediment traps upstream to prevent excessive fine particles from accumulating in the soil.
Alternative Soil Mixes for Filtration
While standardized filtration soil specifications are commonly used, some variations in soil components and proportions may be considered:
Higher Sand Content
Increasing the sand content above 50% creates more void space for water flow. However, irrigation needs also increase to support vegetation.
Expanded Clay or Shale
Calcined clay aggregates provide surface area for adsorption while creating pores for permeability. Expanded shale works similarly.
Biochar
Added as 5-10% of the mix, biochar helps retain nutrients and provides microbial habitat.
Crushed Recycled Glass
As a substitute for sand, crushed glass effectively filters heavy metals and helps drainage while reusing waste material.
Zeolites
These naturally occurring minerals have high cation exchange capacity to bind metals and positively charged contaminants as water filters through.
Granular Activated Carbon
Derived from carbon-rich sources, granulated activated carbon has a high capacity to adsorb organic pollutants, chemicals and excess nutrients from infiltration.
Coconut Coir
Coir fiber from coconut husks increases moisture retention while also separating and supporting sand particles to maintain porous structure.
When to Use Alternative Subgrade Options
While properly engineered filtration soils are suitable for most applications, some challenging site conditions may call for alternative subgrade options underneath permeable pavements or bioretention areas:
Low-Permeability Subsoils
Where native soils have very low permeability, an underdrain system intercepting at the bottom of the filtration soil layer is recommended to collect and convey excess filtered water.
Shallow Water Table or Bedrock
An impermeable liner with underdrains may be needed if the water table or bedrock is too close to the bottom of the filtration soil layer to allow proper infiltration.
Karst Terrain
In areas prone to sinkholes where stormwater infiltration could contaminate groundwater aquifers, impermeable containment with filtration and a sealed outlet may be required.
Hotspots or Contaminated Sites
At brownfields or industrial sites, an impermeable liner helps prevent mobilization of subgrade contaminants into the filtration soil media.
Compacted Urban Sites
On highly compacted urban redevelopment sites, engineered soil lifts or structural cell systems that support the pavement while bridging over poor soils may be useful.
Cold Climates
Insulating the underdrain and subgrade soils can help prevent frost heave and maintain infiltration capacity in freezing conditions.
Filtration Soils vs. Rain Gardens: Key Differences
While rain gardens also absorb and filter runoff, there are some key differences between these systems and bioretention areas using filtration soils:
- Location – Rain gardens are often retrofitted into existing lawns using the native soil. Filtration soils are mostly used in streetscapes, parking lots and other redevelopment sites.
- Drainage – Rain gardens drain into surrounding soils. Filtration soil beds contain underdrains to collect excess filtered water.
- Scale – Rain gardens are relatively small-scale for residential lots. Filtration soil systems manage larger scale runoff volumes from pavements and roofs.
- Soil Specs – There are no set soil specifications for rain gardens. Filtration soils must meet customized gradation, infiltration rate and other engineering criteria.
- Ponding Depth – Rain gardens have shallow surface ponding before filtering through soil. Filtration soils are designed to handle greater ponding depths.
- Installation – Rain gardens can often be installed by homeowners or volunteers. Filtration soil systems require professional design and engineered construction.
- Size – Rain gardens may cover just 100-300 sq ft. Filtration soils are used for much larger bioretention systems managing runoff from one or more acres.
While both can provide stormwater filtration, carefully engineered filtration soils are suited for larger-scale applications where specific hydrologic performance goals are needed. Home rain gardens offer a simpler option.
Getting Rid of Filtration Soil in Bioretention Areas
In some cases, existing filtration soils may need complete removal and replacement to restore proper function. This may be considered if the soil is:
- Severely compacted with minimal response to restoration efforts
- Contaminated by chemicals or toxic materials
- Clogged despite pretreatment and preventive maintenance
- Experiencing prolonged saturation or ponding issues
- Causing slope stability issues or sinkholes
- Composed of poorly engineered mixes that do not meet specifications
Removing and replacing failed filtration soils includes the following steps:
Excavation
Fully excavate the bioretention area using backhoes, front end loaders or other equipment to access the filtration soil layer.
Underdrain Inspection
Inspect underdrains for obstructions or damage. Flush or repair as needed. Geotextile fabric around the underdrain may need replacement.
Filtration Soil Removal
Entirely remove the filtration soil layers for disposal or reuse on site for general landscaping areas. Scrape off any caked layers.
Subgrade Preparation
After excavating down through the filtration soil, scarify the underlying native soil to improve infiltration. Compact moderately.
New Filtration Soil
Install fresh filtration soil in lifts according to the specifications. Tamp lightly and water between lifts to reach the desired compaction and grade.
Vegetation & Mulch
Replant vegetation and replace mulch layer based on the original design or improved planting plan.
With the filtration capacity restored, the bioretention system can continue providing sustainable stormwater management on site.
Conclusion
Proper stormwater management is crucial for both flood mitigation and water quality in the built environment. Filtration soils provide an important tool as part of low impact development and green infrastructure strategies. By mimicking