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Three of the most difficult questions

News, Q&A: GMA Techline | June 1, 2016 | By:

From the GMA Techline Q-and-A

1. Mold inside drainage layer?

Q: Recently there has been a greater demand for sustainable projects in Southern California and I was glad to see published in Geosynthetics magazine “Geosynthetics: The science that makes green roofs work” (June/July 2014, Vol. 32, No. 3).

I am curious if there are any reports or case studies of mold growing inside the green roof’s drainage layer and how designers could mitigate this situation. Given the right combination of moisture and fertilizers, even in dry arid climates, I imagine mold could be problematic.

A: Neat question and, yes, various organic materials have grown within geotextile filters and even clogged the drainage pipes that they enclose. A major issue is “ochre,” which grows when the drainage system is in a cyclic wet-then-dry location. High iron oxide content soils also encourage growth. We have six recorded geotextile filter failures in this regard. The remedy is high-pressure flushing and/or chemical (biocide) cleaning.

I suspect that mold is a subset of ochre but we do not have specific case histories.

Hope this helps,

Bob Koerner | GMA Techline

2. Reservoir geomembrane liner?

Q: What about a reservoir geomembrane liner over a secondary hot mix asphalt (HMA) liner with adequate anchorage below the crest (fill embankment) and at the bottom (minimum water depth)?
The secondary HMA liner is intended to be impermeable and should therefore limit wind-induced uplift due to the increased volume and corresponding decreased air pressure beneath the geomembrane when uplifting begins.

Since Giroud, et al. (1995) assumed a permeable medium beneath the geomembrane, it seems that a submembrane resisting (suction) component applied to their general approach would be appropriate. Has this been considered?

A: The HMA liner directly beneath the geomembrane seems to make your situation quite different from a permeable medium that is beneath the geomembrane for the reason you suggest. I would go a step further and use a textured geomembrane with the added hope that some bonding might occur depending, in part, on what type of texturing you use.

Bob Koerner | GMA Techline

3. Static electricity?

Q: We have a site covered with an exposed geomembrane cover (EGMC) used as long-term intermediate cover. There was recently a shallow fire that was attributed to static electricity from the EGMC. Workers actually got shocked from coming into contact with the EGMC.
I have never heard of this phenomenon before, but expect you have. Any experience with this? In a sense, I suppose it’s like the static electricity you get from rubbing an inflated balloon against your hair.

A: Indeed, static electricity has been felt by field personnel and it likely propagates via the carbon black in the formulation. The resins themselves are insulating materials as in electrical wiring coatings. That said, I have never heard of it causing a fire before.

Bob Koerner | GMA Techline

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