21.1 A100 - Liners

21.1.1 Purpose

Liners are intended to reduce the likelihood that pollutants in stormwater will reach groundwater. Where necessary, a liner is incorporated into the base of the treatment facility as the facility is constructed. In addition to groundwater protection considerations, some facility types require permanent water for proper functioning. An example is the first cell of a wetpond.

21.1.2 Design Criteria for All Liner Types

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.

Check each BMP to determine if a liner is required or recommended and if there are specific liner requirements for a given BMP.

Table 4 - 27: Lining Types Recommended for Stormwater Treatment Facilities shows recommendations for the type of liner generally best suited for use with various stormwater treatment facilities.

Liners shall be evenly placed over the bottom and/or sides of the treatment area of the facility as indicated in Table 4 - 27: Lining Types Recommended for Stormwater Treatment Facilities. Areas above the treatment volume that are required to pass flows greater than the water quality design flowrate (or volume) need not be lined. However, the lining must be extended to the top of the interior side slope and anchored if it cannot be permanently secured by other means.

For low permeability liners, the following criteria apply:

Where the seasonal high groundwater elevation is likely to contact a low permeability liner, liner buoyancy evaluation and recommendation for use and possible anchoring by a Washington State Licensed Professional Engineer is required.

Where grass must be planted over a low permeability liner per the facility design, a minimum of 6 inches of good topsoil or compost-amended native soil (2 inches compost tilled into 6 inches of native till soil) must be placed over the liner in the area to be planted. Twelve inches of cover is preferred. Compost shall comply with A900 - Compost.

If a treatment liner will be below the seasonal high water level, the pollutant removal performance of the liner must be evaluated by a geotechnical or groundwater specialist and found to be as protective as if the liner were above the level of the groundwater.

Table 4 - 27: Lining Types Recommended for Stormwater Treatment Facilities

WQ Facility

Area to be Lined

Type of Liner Recommended

BMP T610 - Presettling Basin

Bottom and sides

Low permeability liner or Treatment liner (If the basin will intercept the seasonal high groundwater table, a treatment liner is recommended.)

BMP T1110 - Wetponds - Basic and Large

First cell: bottom and sides to WQ design water surface

Low permeability liner or Treatment liner (If the wet pond will intercept the seasonal high groundwater table, a treatment liner is recommended.)

Second cell: bottom and sides to WQ design water surface

Treatment liner

BMP T1140 - Combined Detention and Wetpool Facilities

First cell: bottom and sides to WQ design water surface

Low permeability liner or treatment liner (If the facility will intercept the seasonal high groundwater table a treatment liner is recommended.)

Second cell: bottom and sides to WQ design water surface

Treatment liner

BMP T1130 - Stormwater Treatment Wetlands

Bottom and sides, both cells

Low permeability liner (If the facility will intercept the seasonal high groundwater table, a treatment liner is recommended.)

BMP T808 - Basic Sand Filter and BMP T809 - Large Sand Filter

Basin sides only

Treatment liner

BMP T810 - Sand Filter Vault

Not applicable

No liner needed

BMP T820 - Linear Sand Filter

Not applicable if in vault

Bottom and sides of presettling cell if not in vault

No liner needed

Low permeability or treatment liner



21.1.3 Design Criteria for Low Permeability Liner Options

This section presents the design criteria for each of the following four low permeability liner options: compacted till liners, clay liners, geomembrane liners, and concrete liners.

21.1.3.1 Compacted Till Liners

Liner thickness shall be 18 inches after compaction.

Soil shall be compacted to 95% minimum dry density, modified proctor method (ASTM D-1557).

Soil shall be placed in 6-inch lifts.

Reference Table 4 - 28: Acceptable Gradation for Compacted Till Liners.

Table 4 - 28: Acceptable Gradation for Compacted Till Liners

Sieve Size

Percent Passing

6-inch

100

4-inch

90

#4

70 - 100

#200

20



Other combinations of thickness and density sufficient to retard the infiltration rate to 2.4 x 10-5 inches per minute may be used instead of the above criteria. Environmental Services/Site Development Group will require a soils report or other documentation to verify that the proposed depth and compaction of the media is sufficient.

21.1.3.2 Clay Liners

Liner thickness shall be 12 inches.

Clay shall be compacted to 95% minimum dry density, modified proctor method (ASTM D-1557).

Other combinations of thickness and density sufficient to retard the infiltration rate to 2.4 x 10-5 inches per minute may be used instead of the above criteria for compacted till liners. Environmental Services/Site Development Group will require a soils report or other documentation to verify that the proposed depth and compaction of the media is sufficient.

The slope of clay liners must be restricted to 3H: IV for all areas requiring soil cover; otherwise, the soil layer must be stabilized by another method so that soil slippage into the facility does not occur. Any alternative soil stabilization method must take maintenance access into consideration.

Where clay liners form the sides of ponds, the interior side slope shall not be steeper than 3H:1V, irrespective of fencing.

21.1.3.3 Geomembrane Liners

Geomembrane liners shall have a minimum thickness of 30 mils. A thickness of 40 mils shall be used in areas of maintenance access or where heavy machinery must be operated over the membrane.

Geomembranes shall be bedded according to the manufacturer's recommendations.

Liners shall be installed so that they can be covered with 12 inches of top dressing forming the bottom and sides of the stormwater treatment facility, except for linear sand filters. Top dressing shall consist of 6 inches of crushed rock covered with 6 inches of native soil. The rock layer is to mark the location of the liner for future maintenance operations. As an alternative to crushed rock, 12 inches of native soil may be used if orange plastic “safety fencing” or another highly-visible, continuous marker is embedded 6 inches above the membrane.

If possible, liners should be of a contrasting color so that maintenance workers are aware of any areas where a liner may have become exposed when maintaining the facility.

Geomembrane liners shall not be used on slopes steeper than 5H:1V to prevent the top dressing material from slipping. Textured liners may be used on slopes up to 3H:1V upon recommendation by a Washington State Licensed Professional Engineer that the top dressing will be stable for all site conditions, including maintenance.

21.1.3.4 Concrete Liners

Portland cement liners are allowed irrespective of facility size, and shotcrete may be used on slopes. However, specifications must be developed by a Washington State Licensed Professional Engineer who certifies the liner against cracking or losing water retention ability under expected conditions of operation, including facility maintenance operations. Weight of maintenance equipment can be up to 80,000 pounds when fully loaded.

Asphalt concrete may not be used for liners due to its permeability to many organic pollutants.

If grass is to be grown over a concrete liner, slopes must be no steeper than 5H:1V to prevent the top dressing material from slipping.

21.1.4 Treatment Liners

The following criteria apply for treatment liners:

Depth of liner shall be 2 feet.

Organic content shall be a minimum of 1%. Organic content shall be measured on a dry weight basis using ASTM D2974.

Cation exchange capacity shall be a minimum of 5 millequivalents/100 grams. Cation exchange shall be tested using EPA lab method 9081.

The treatment liner shall have an infiltration of 2.4 inches per hour or less.

If native soils are proposed to be used for the treatment liner, one sample per 1,000 square feet of facility area shall be tested. Each sample shall be a composite of subsamples taken throughout the depth of the soil log for the proposed treatment layer. Testing shall be performed by a professional Washington State Licensed Professional Engineer or Washington State Licensed Professional Geologist.

Certification by a soils testing laboratory that imported soil meets the organic content and CEC criteria is required.