I agree, the manufacturers terminology can be a bit misleading and the
terms geotextile and geomembrane are often used interchangeably (causing
much confusion!). The safest way to be sure that the material meets with
requirements of the job is to make sure its performance parameters are
fully understood before it is selected.
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: Steve Wilson [mailto:[log in to unmask]]
Sent: 02 August 2012 08:34
To: Ben Potts; [log in to unmask]
Subject: RE: Help with Geotextile Membrane guidance
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
HP4 2SQ
01442-872968
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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