TOPIC:    Geomembranes/Regulations for Stormwater Containment

Each month Tim Stark introduces a new technical topic for discussion and possible action. This month’s topic is: “Geomembranes/Regulation for Stormwater Containment”.This topic generated significant discussion with the main “take-aways” listed below:

Concerns/Suggestions for Geomembranes/Regulations for Stormwater Containment

- Terry Sheridan of Geo-Storage described Villanova Conference on Stormwater Containment and presentation by Philadelphia Water Department (PWD)
- PWD is issuing a 2025 regulation requiring a minimum 60 mil thick geomembrane for stormwater chambers
- Terry Sheridan has designed10 to 20 chambers and 2/3 of them are lined with 30 mil PVC GM and two cushion geotextiles, i.e., above and below the GM
- Terry says design needs a flexible GM to resist uplift due to groundwater rise
- Angular aggregate is placed on top of the GM so a thicker cushion geotextile is needed on top
- Contaminated sites but water is not leachate so chemical resistance is not a GM issue
- 60 mil will be difficult to form and weld in below ground chambers
- 30 mil PVC GM can be factory fabricated, which facilitates construction
- William Nichols at PWD is handling the new GM regulations
- 90 to 95% of chambers have the GM placed on the side and bottom of the chamber
- 5 to 10% of chambers have the GM wrapped around the top of the chamber and along the sides and bottom of the chamber

- Application/Function =retention (usually unlined) or detention (GM lined)
- Stormwater Management Trend= store stormwater underground
- Chambers are 3 to 5 ft deep and vary in size from small to large
- Need highly flexible GM materials– PVC, flexible LLDPE, and polypropylene are recommended due to geometry changes, groundwater uplift, and rough subgrade
- A lot of detail work, e.g., pipe penetrations, so a flexible GM is preferred
- Require 30 mil flexible membrane liner (FML) to William Nichols at PWD
- 30 mil PVC geomembranes are readily available around Philadelphia so cost effective for chambers
- Two local installers install the factory fabricated PVC GM panels
- Tim Rafter at ALCO uses 40 mil PVC GM – include in Department Meeting
- Mark Ramsden at EGI – works with
- GM Requirements – no concerns about UV exposure or chemical resistance so flexibility is the key attribute
- Corner of chambers at 90-degreeangles so high-density materials could stress crack
- Need to quantify flexibility using FGI flexibility testing

- Field CQA requirements = ?Fill system with water and monitor rate of leakage, which must be less than 1inch/hour – Volume = ?
- Air lance and vacuum box testing the installed GM

- PWD Discussion Topics: (1)GM flexibility, (2) GM thickness and (3) allowable leakage rate
- Straw Poll of FGI Members that have used 60 mil GM in this application?
- 60 mil PVC and unsupported EIA GM –requested by engineer
- 45 mil reinforced polypropylene = thickest used by Cooley
- If require 60 mil, GM would have to be PVC, EIA, or polypropylene GM
- Thickness increasing to protect against puncture so include thicker cushion geotextile
- Create Technical Note on Geomembrane Flexibility – Function and Importance