10.3 BMP 720 - Infiltration Trenches
Infiltration trenches are generally at least 24 inches wide, and are backfilled with a coarse stone aggregate, allowing for temporary storage of stormwater in the voids of the aggregate material. Stored runoff gradually infiltrates into the surrounding soil. The surface of the trench can be covered with grating and/or consist of stone, gabion, sand, or a grassed covered area with a surface inlet. Perforated rigid pipe of at least 8-inch diameter can also be used to distribute the stormwater in a stone trench.
10.3.2 Design Criteria Specific to Infiltration Trenches
Comply with all criteria and standards in Modeling Your Best Management Practices, Design Criteria for All Stormwater Treatment and Flow Control BMPs, Constructing Your Best Management Practices and Accessing and Maintaining Your Best Management Practices as applicable to the project in addition to criteria within each BMP. Where criteria or standards conflict, utilize the criteria and standards contained within the specific BMP.
Due to accessibility and maintenance limitations, infiltration trenches must be carefully designed and constructed.
Infiltration trenches may be placed in fill material if the fill is placed and compacted under the direct supervision of a Washington State Licensed Professional Engineer, and if the measured infiltration rate is at least 8 inches per hour.
The base of the infiltration facility shall be level.
10.3.2.1 Cleanouts/Access Ports
Install cleanouts and/or access ports to allow cleaning and inspection of the system.
The aggregate material for the infiltration trench shall consist of a clean aggregate with a maximum diameter of 3 inches and a minimum diameter of 1.5 inches. Void space for these aggregates shall be in the range of 30 to 40 percent.
10.3.2.3 Geotextile Fabric Liner
The aggregate fill material shall be completely encased in an engineering geotextile material. Geotextile should surround all of the aggregate fill material except for the top one-foot, which is placed over the geotextile. Geotextile fabric shall be selected to avoid plugging and shall be non-woven. The non-woven geotextile shall conform to A2000 - Geotextile Specifications, Geotextile for Underground Drainage - Low Survivability, Class C.
The bottom sand or geotextile fabric is optional.
Refer to the Federal Highway Administration Manual “Geosynthetic Design and Construction Guidelines,” Publication No. FHWA HI-95-038, May 1995 for design guidance on geotextiles in drainage applications. Refer to the NCHRP Report 367, “Long-Term Performance of Geosynthetics in Drainage Applications,” 1994, for long-term performance data and background on the potential for geotextiles to clog, blind, or to allow piping to occur and how to design for these issues.
Because an infiltration trench is generally used for small contributing areas, an emergency spillway is not necessary. However, a non-erosive overflow channel leading to a stabilized watercourse or other Environmental Services/Site Development Group, per Joint Administrative Policy and Procedure Directive No. 2021-02-001, approved location should be provided.
An infiltration trench can be placed under a porous or impervious surface cover to conserve space.
An observation well should be installed at the lower end of the infiltration trench to check water levels, drawdown time, sediment accumulation, and conduct water quality monitoring. See City of Tacoma Standard Detail GSI-02. It should consist of a perforated PVC pipe which is 4 to 6 inches in diameter and it should be constructed flush with the ground elevation. For larger trenches, a 12-36 inch diameter well can be installed to facilitate maintenance operations such as pumping out the sediment. The top of the well should be capped to discourage vandalism and tampering.
Distribution pipe (if used) shall be level and shall be either perforated or slotted pipe.
Perforated pipe shall conform to WSDOT Standard Specification 9-05.2.
Slotted pipe shall have slots cut perpendicular to the long axis of the pipe and be 0.04 to 0.069 inches by 1 inch long and be spaced 0.25 inches apart longitudinally.
Distribution pipe shall be a minimum of 8 inches in diameter but shall be sized based upon contributing area.
Pipe length shall not be so long that flow cannot be even distributed throughout the facility. Pipes shall be a maximum 100 feet in length unless calculations are provided showing distribution can be even with longer pipe length.
Trench Preparation –Place excavated materials away from the trench sides to enhance trench wall stability. Keep excavated material away from slopes, neighboring property, sidewalks and streets.
Stone Aggregate Placement and Compaction – Place the stone aggregate in lifts and compact using plate compactors. As a rule of thumb, a maximum loose lift thickness of 12 inches is recommended. The compaction process ensures geotextile conformity to the excavation sides, thereby reducing potential piping and geotextile clogging, and settlement problems.
Potential Contamination - Prevent natural or fill soils from intermixing with the stone aggregate. Remove all contaminated stone aggregate and replace with uncontaminated stone aggregate.
Overlapping and Covering - Following the stone aggregate placement, fold the geotextile over the stone aggregate to form a 12 inch minimum longitudinal overlap. When overlaps are required between rolls, overlap the upstream roll a minimum of 2 feet over the downstream roll in order to provide a shingled effect.
Voids behind Geotextile – Avoid voids between the geotextile and excavation sides. Remove boulders or other obstacles from the trench walls. Place natural soils in voids at the most convenient time during construction to ensure geotextile conformity to the excavation sides. Soil piping, geotextile clogging, and possible surface subsidence will be avoided by this remedial process.
Unstable Excavation Sites - Vertically excavated walls may be difficult to maintain in areas where the soil moisture is high or where soft or cohesionless soils predominate. Trapezoidal, rather than rectangular, cross-sections may be needed.