David
I have not seen any research on long term performance in this context. I
did a simple test a few years ago on a piece taken out of a permeable
pavement that was about 5 years old. It still had a break through head of
about 50mm, but I did not measure it as it was just to show the principle to
someone.
Steve Wilson, Technical Director
on behalf of EPG
Tel 07971 277869
www.epg-ltd.co.uk
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-----Original Message-----
From: Contaminated Land Management Discussion List
[mailto:[log in to unmask]] On Behalf Of David
Lerner
Sent: 02 August 2012 08:45
To: [log in to unmask]
Subject: Re: Help with Geotextile Membrane guidance
Steve
The breakthough head depends on the contact angle of water with the
material. We have shown (in another context) that weathering of both the
material and the liquid changes this contact angle and so can lead to much
lower breakthough heads. I havent researched whether long term tests have
been done on contact angles for geotextiles, but would be surprised if they
retained their virgin properties.
David
---------------------------------------
David Lerner +44 114 222 5743 Mob: 07879 668641 Catchment Science Centre:
www.shef.ac.uk/csc URSULA urban rivers project: www.ursula.ac.uk Groundwater
protection and Restoration Group www.shef.ac.uk/gprg
On 02/08/2012 8:34 AM, Steve Wilson wrote:
> Ben
>
> Geotextiles are always permeable (not withstanding the comment below).
> They are not only woven but can be non-woven needlepunched or
> non-woven thermally bonded.
>
> Geomembranes are impermeable.
>
> The term geotextile membrane causes confusion and should be avoided.
>
> One important aspect of geotextiles is breakthrough head. This is the
> head of water that needs to develop before water will actually flow
through it.
> For example Terram 1000 has a breakthrough head of 50mm (from memory)
> and so that means it needs 50mm of water on it before water flows
> through it. If it is placed below subsoil this will stop all but the
> most intense rainfall passing through it and most of it will end up
evaporating.
>
> Steve Wilson, Technical Director
> on behalf of EPG
>
> Tel 07971 277869
> www.epg-ltd.co.uk
>
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> -----Original Message-----
> From: Contaminated Land Management Discussion List
> [mailto:[log in to unmask]] On Behalf Of Ben
> Potts
> Sent: 01 August 2012 12:17
> To: [log in to unmask]
> Subject: Re: Help with Geotextile Membrane guidance
>
> Capillary action works against gravity and is caused by the surface
> tension of the water in small spaces. This can be seen on the meniscus
> of water in a test tube as it creeps up the sides of the glass. The
> action is caused by the orientation of hydrogen bonds at the waters
> surface which have a residual charge due to the unequal "sharing" of
> elections between the oxygen and hydrogen atoms. These hydrogen bonds
> are also the reason why water is liquid at room temperature rather
> than a gas, which is what it's molecular weight would suggest.
> Downward migration of water is driven by gravity which is a more powerful
force that that created by "surface tension".
>
> Geotextile can be permeable to water if they are for use in preventing
> the mixing of granular materials in a sub-base, however they can also
> be impermeable. The impermeable membranes tend to be extruded rather
> than woven but they can be a mixture of both depending on the
> performance requirements of the material.
>
> Kind Regard
>
>
> Ben Potts Msc DIC MIEnvSc
> Environmental Scientist
> Environmental Scientifics Group
>
> M: +44 (0) 7718 786966
> T: +44 (0) 1622 632184
> F: +44 (0) 1622 739620
>
> -----Original Message-----
> From: Contaminated Land Management Discussion List
> [mailto:[log in to unmask]] On Behalf Of
> Michael Smith
> Sent: 26 July 2012 17:52
> To: [log in to unmask]
> Subject: Re: Help with Geotextile Membrane guidance
>
> Muttakir,
>
> What you propose is not a "break layer" as described in CIRIA and
> other guidance (see also CIRIA SP106) - simply a physical barrier to
> soil disturbance.
>
> A "break layer" prevents upward migration of moisture (and dissolved
> contaminants) due to capillary action and typically consists of coarse
> granular material. Such a granular layer will require protection to
> prevent infiltration of fine soil - typically a geotextile would be
> used for this both below and above the coarse material.
>
> Geotextiles will not impede upward (or downward) migration of moisture.
> When the surface of unsaturated soil is moistened by rain, water is
> drawn downwards into the dryer underlying soil by capillary action.
>
> A near-surface break layer can cause flooding because downward flow is
> inhibited.
>
> A break layer can probably be formed by a plastic egg-box structure
> sandwiched geotextile layers.
>
>
> Regards,
>
>
> Mike Smith
>
>
> +++++++++++++++++++++++++
> Mike Smith
> M A Smith Environmental Consultancy
> Farthing Hill
> Browns Springs
> Potten End
> BERKHAMSTED
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>
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> 07961-312790
>
> [log in to unmask]
>
> www.michael.a.smith.btinternet.co.uk
>
>
>
>
>
>
> -----Original Message-----
> From: Contaminated Land Management Discussion List
> [mailto:[log in to unmask]] On Behalf Of
> Muttakir Mohammed
> Sent: 26 July 2012 12:15
> To: [log in to unmask]
> Subject: Re: Help with Geotextile Membrane guidance
>
> Hi All,
>
> In my email I meant to say the membrane will be used as a break layer
> between an inert cover and underlying contamination. Sorry for the
> confusion and also sorry for double posting, I didn't realise I posted
> the same email twice.
>
> Muttakir Mohammed
> Pollution Control Technician
> Neath Port Talbot County Borough Council
>
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