5.6.10 Water permeability normal to the geotextile plane without load

Permeability through a medium (geotextile is this case) is the amount of (water) flow per time, commonly referred to as Hydraulic Conductivity (Koerner 1994). When permeability is divided by the thickness of the layer through which flow occurs this is called permittivity. Permittivity is a better measure of flow characteristics for comparison amongst different materials than permeability because it expresses flow characteristics per unit of thick- ness (or head) and per unit area.

Permeability The pure water permeability of a geotextile cannot always be determined at laminar flow conditions and should therefore be described by the general quadratic flow equation (from the Forchheimer equation):

H = av + bv2 Eq. 67

where, H v a b

represents the head loss across the geotextile in m; the filter velocity in m/s ; product-depending resistance coefficient in s; and product-depending resistance coeficient in s2/m.

The second term of Eq. 67 is negligible at lower flow velocities and the laminar flow equation results in:

H = a v Eq. 68

while the first term is negligible at higher flow velocities resulting in the turbulent flow equation:

H = bv2 Eq. 69

The relationship between the head loss H and the filter velocity v can also be expressed by the equation:

H = CV^ Eq. 70

is the resistance coefficient (sn m1 -"I; and the exponent varying between 1 and 2 depending on the flow type: laminar flow: n = 1 turbulent flow: n = 2 transition zone: 1 < n < 2.

where, c n

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Eq. 70 can also be written as:

i = ci vn

with,

Eq. 71

Eq. 72

where, i T, the geotextile thickness in m; and i

ci

is the hydraulic gradient across the geotextile;

resistance coefficient in sn m-n.

For laminar flow is n = 1 and Eq. 71 can be written as (see also section 5.1.3):

v = & i Eq. 73

where, K, the permeability coefficient or the filter velocity for a hydraulic gradient

of 1.

Permittivity Also Eq. 68 can be written in that way. Only in the case of laminar flow can the &-value be determined and from that the permittivity q, being the permeability per unit of thickness or the filter velocity per unit of hydraulic head loss:

Eq. 74

where, W is the permittivity in s-'

For most coarse-textured geotextiles laminar flow is not obtained. In these cases the water permeability of geotextiles can be characterised in two ways:

the hydraulic head loss at a given filter velocity ; and the filter velocity or discharge rate at a given head loss.

The unknown quantities a and b of Eq. 67 and c and n of Eq. 70 can be deter- mined from the best-fit curve through the paired experimental data v and H. Values of H calculated for a given v or inversely of v calculated for a given H according to both equations do not differ significantly.

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As the viscosity of a liquid influences its flow, a temperature correction will be necessary if experiments are carried out at temperatures T different from the reference temperature R. For laminar flow a linear relationship exists between the velocity at reference temperature, vR, and the velocity at the measured temperature, vT, namely:

VR = h V T Eq. 75

in which Rr = qT/rlR is the temperature correction factor given by the ratio of the dynamic viscosity of water at the measured temperature, qT, and at ref- erence temperature, qR. The dynamic viscosity qT of water at a temperature T ("C) is, according to Poiseuille, given by:

1.779

1 + 0.03368 T + 0.00022099 T2 q T = Eq. 76

where, qT dynamic viscosity at temperature T in mPa s

Not considering the flow regime, the temperature correction for Eq. 67 results in:

2 a

H = - VR + VR Rr

Eq. 77

and for Eq. 70 in

H = c R $ ' v ~ Eq. 78

This temperature correction does not apply to Eq. 67 and Eq. 70 (Bezuijen et al. 1994, Dierickx and Leyman, 1994), because the viscosity does not exert an important influence on the water flow under turbulent flow conditions and should not be considered for turbulent flow. Since Eq. 67 consists of a fictive laminar and turbulent part, temperature correction of the laminar part may suffice for a correct temperature correction of the water permeability param- eter (Dierickx and Leyman, 1994). Hence the quadratic equation, after taking into account a certain reference temperature, can be written as:

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Eq. 79

For Eq. 70 the splitting into a fictive laminar and turbulent part is not SO

evident but knowing that the temperature correction for laminar flow is R, and that for turbulent flow no correction is required, the following exponen- tial flow formula is applicable according to Dierickx and Leyman (1994):

H = c ~ - ~ v $ Eq. 80

Permeability determination is executed by one of two methods: constant head or falling head. These are briefly described in the next sections.

Constant head method. A unidirectional flow of water normal to the plane of a single layer of geotex- tile, without load, is applied under a range of constant heads. A geotextile specimen is installed between two flanges in a tube. A manometer at both sides allows the determination of the head loss across the geotextile at vari- ous discharges. The determination of the permeability of geotextiles according to the constant head principle is described in EN IS0 11058 (1999) and ASTM D 449132. The required parameters can be calculated from the head loss, dis- charge and temperature measurements.

i Falling head method. ~

A unidirectional flow of water normal to the plane of a single layer of geotex- tile or related product, without load, is applied according to the falling head procedure. The principle, possible registration methods and data analysis are described in EN IS0 11058 (1999) and ASTM D 449132.

Concluding remarks As temperature correction for flow velocity is only applicable at laminar flow, and flow in the transitory zone is found mostly between laminar and turbu- lent, it is advisable to carry out permeability measurements within a narrow range of 18 - 22" C close to the reference temperature of 20" C, to minimise the error introduced by applying the temperature correction, regardless of the flow regime.

Typical values of permittivity are 0.02 - 2.2 s-l and corresponding permeabili- ty values (permittivity multiplied with thickness Tg) K, = 0.0008 - 0.23 c d s (0.0016 - 0.46 flimin). Conventional soil permeameters cannot be used to deter- mine the permittivity, as the flow coming through would be too large (normal- ly the soil will control the flow). Few US states have permittivity requirements

I (Koerner 1994). Those that do specified values ranging from 0.1 - 0.7 s-'.

32 If dIo > 0.78 mm (US Sieve No 200) it recommended to use ASTM 2434-68 (1994) for tests on granular material.

2 13

However, all of them had requirements for the permeability such as: &>&, and &>lo& for critical (flow) conditions where mono-filament fabrics are used (POA > 4, and AOS < No. 100 sieve). Some required absolute values: &> 0.01 c d s , or 0.1.

5.6.11 Water permeability normal to the geotextile plane with load

When a geotextiles is under pressure it is expected that the flow characteris- tics will change. Determinations of the permittivity or permeability across the plane under load have been developed, but no conclusive results seem to have been reported to date such that definite guidelines can be given. Koerner (1994) reports the following trends:

for woven mono-filament geotextiles there is no change to a slight increase when under load (it would seem that pressure stretches the weave; POA increases); for woven slit film geotextiles there was too much scatter in the data to report trends; for non-woven heat bonded geotextiles there is no to slight decrease when under load; and for non-woven, needle-punched geotextiles there is a slight to moderate decrease depending on the magnitude of the load.

ASTM D5493 describes testing under load and normal to the plane of the geo- textile. ASTM D 4491 only deals with permittivity without load. Similarly EN IS0 11058 (1999) only deals with unloaded geotextiles, however, there is work planned in the CEN/TC 189 programme to test geotextiles under load and flow perpendicular to the plane (Dierickx personal communication). Only French guidelines specifically mention the effect of loading (Santvoort 1994): they suggest that permittivity determined in the laboratory (no load) is at least 3 times higher than with load. It is assumed that the specification val- ues mentioned in the previous section, permittivity without load, are actually valid for conditions under load, although this is not mentioned specifically. The typical value range for permittivity under load (Koerner 1994) is 0.01 - 3.0 sC1, which is rather close to the values without load (see Section 5.6.10).

5.6.12 Water flow capacity in-plane

The water flow capacity in the plane of a geotextile or related product is deter- mined at a range of constant heads. Because the flow of water within the plane will be influenced by the applied load, it is necessary to specify the load at which the water flow capacity is determined (ASTM D 4716 suggests 10, 25, 50, 100, 250 and 500 kPa). Normally the water flow capacity is measured

214

under varying compressive stresses, typical hydraulic gradients and defined contact surfaces. Existing test equipment can be classified in parallel (ASTM) and radial flow models (Figure 63). Parallel models are preferred when the fabrics have preferential flow directions. Radial models give an average as flow occurs in all directions. For both flow models the relation between the head loss H and the filter velocity v is given by Eq. 63 or Eq. 70, the parame- ters a and b or c and n depend on the flow regime.

Linear regression between H and v exits in the case of laminar flow, hence Darcy’s law is applicable.

For parallel flow, taking into account that the flow section A is given by:

A = Tgw Eq. 81

where, w the width; and

T, the thickness of the geotextile specimen.

The permeability coefficient in the plane Kp can be calculated as follows:

V L Q L K p = - - % = - - Rr

T,wt H Tgw H Eq. 82

The transmissivity, O, being the discharge rate in the plane of the geotextile per unit hydraulic gradient and per unit width can be obtained from:

e = K,,T, Eq. 83

For radial flow, the permeability in the plane is obtained by applying the radial flow formula:

Eq. 84

with R the outer and R, the inner radius of the geotextile specimen. Hence,

Q 2 n H T g

KP= Eq. 85

from which the transmissivity can be calculated according to Eq. 83.

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PARALLEL MODEL

I

water supply

RADIAL MODEL

water supply

Figure 63 Parallel and radial flow model for permeability determination plane.

of a geotextile in its

To determine the flow regime under specific test conditions at least 5 hydraulic gradients are required for each applied normal load. The flow regime is considered laminar if a linear relation between head loss and discharge exists. The best-fit curve passing through the origin:

H = a iQR, Eq. 86

allows determination of the regression coefficient al and, consequently, the permeability in the plane and the transmissivity q for each applied normal load.

For parallel flow it follows from Eq. 82 that:

L al =

%TgW

hence,

Eq. 87

216

For radial flow it follows from Eq. 84 that:

al = ln($)

hence,

Eq. 89

Eq. 90

Typical examples of the influence of the normal load on the transmissivity of geotextiles are given in Figure 64.

Based on Figure 64B Koerner (1994) concluded that most geotextiles reach a constant value for transmissivity at loads of 24 kPa and greater. The yarn structure is then tight and dense to hold the load and maintain hydraulic properties. However, Figure 64A does not show this phenomenon.

In the European standard (EN IS0 12958, 1999) parallel flow is accepted and from the simple figure in ASTM D 4716 parallel flow is also intimated. The geotextile or related product should be installed between two foam rubbers fixed on parallel plates and loaded with a constant normal stress. The supply side is connected to a constant head reservoir of which the water level can be adjusted; the outlet has a constant overflow level. Two manometers, one at the supply and one at the outlet allow determination of the head loss within the geotextile. To make temperature corrections, the discharge has to be deter- mined and the temperature measured. Water flow capacity should be deter- mined at 4 normal loads (ISO, ASTM recommends 3 loads selected from the values given above) and for each load at two hydraulic gradients. To obtain representative results at least 3 replicates are required. Typical values for transmissivity are (Koerner 1994): 0.01 - 2.0 X m3min-'m-'. None of the US states are reported to require transmissivity values.

Since measurements are not always done under laminar flow conditions, espe- cially for geotextile-related products with a central core, only the in-plane flow capacities per unit width at defined loads and hydraulic gradients are deter- mined. Transmissivity can only be used in case of laminar flow, and since this is not always the case with geotextiles or geotextile related products, the EN IS0 standard does not use transmissivity but water flow in the plane.

217

transmissivity in m2/s x 105

5

4

14

6

4

2 ~I 1 O0

6--'<

needle punched, non-woven materials

J ',: ~ _ - 509 g/m2 PETlPPstaple - Y ',

542 g/m2 PETcontinuous

~ _ _ _ _ 542 g/m2 Ppcontinuous ~~ 509 g/m2 PPstaple

407 g/m2 PP staple

, \", \,> ",

1 o1 102 103 load in kPa

, P

B Transmissivity response versus applied normal stress for various needle punched non-woven polyester (PET) and polypropolene (PP) geotextiles (&er Koerner 1994).

5.6.13 Water penetration resistance

In France in 1981, problems that occurred with drainage systems that had geotextile envelopes were attributed to resistance to wetting of geotextiles (Lennoz-Gratin 1987). The phenomena seemed typical for fine textured non-

218

woven geotextiles. Since then tests have been developed to assess the phe- nomena (Lennoz-Gratin 1992, Dierickx 1994, 1996b). The test on water pene- tration resistance determines the hydrostatic head supported by a geotextile which is a measure of the opposition to the passage of water through the geo- textile. This is done by subjecting a geotextile specimen to a steadily increas- ing water head on the face - under standard conditions - until water pene- trates at three places, and noting the corresponding head. The apparatus for determining the water penetration resistance is shown in Figure 65. The com- plete test procedure is described in IS0 811 (1981, and equivalent BS 2823- 82) and a new EN standard (prEN 13562,19991, possibly the same with minor modifications, is under preparation.

Laboratory tests on wettability problems showed that the initial heads varied between 5 and 30 mm (Dierickx, 1996131, which seems hardly enough to be of practical value.

inflow

_raL.

geo tex t ile

7 sealing

‘supporting grid I I

mirror

Figure 65 Apparatus for determining the water penetration resistance of geotextiles.

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5.6.14 Degradation and durability properties

There are a number of processes that have a bearing on how long a geotextile lasts. They are (Koerner 1994): temperature, oxidation, hydrolysis, chemical, biological, and sunlight (Uv) degradation. Ageing, which is the alteration of physical, chemical, and mechanical properties caused by the combined effects of environmental conditions over time, is another term used to describe degra- dation. Ageing manifests itself in numerous ways (ASTM D 5819): blistering, chalking, changes in chemical resistance, in puncture, burst and tear resist- ance and other index properties, discoloration, embrittlement, permeability changes, stiffness changes, and tensile or compressive changes.

McKyes et al. (1992) describes various geotextiles tested in Canada, mention- ing that only the fibreglass fabrics more commonly used in the past for enve- lope material seemed to become brittle over time and collapsed when exhumed. Several general types of guidelines to assess the degradation and durability of geotextiles have been issued or are under preparation: ASTM D 5819, and ENV 12226 (1996). To obtain exhumed specimens the following guidelines are issued: ASTM D 5818, and EN IS0 13437 (1998).

TemDerature. Higher temperatures will cause polymer degradation systems to occur at a higher rate, while low temperatures will cause brittleness which impacts strength on the material. ASTM D 4594 describes a test to determine the effect of temperature on the stability of the geotextile. The stability is expressed as a percentage change in tensile strength or in elongation as meas- ured at specific temperature and compared to values obtained at standard conditions for testing geotextiles (i.e. Section 5.6.4). ASTM D 746-95, which deals with the effects of cold temperatures on plastics, also addresses brittle- ness.

Oxidation. All types of polymers are susceptible to some form of oxidation, but polypropylene and polyethylene (PP and PE) more so than others (Hsuan et al. 1993). The European Committee for Standardisation (CEN) has drafted a pre-standard for testing resistance to oxidation (EN IS0 13438, 1998). ASTM has a recommended practice for high temperature oxidation testing for plastics (ASTM D 794-93). With drain envelope applications little difficulty, if any, is expected of oxidation processes.

Hvdrolysis. Hydrolysis is the chemical reaction between a specific chemical group within a polymer and absorbed water causing scission and reduction in molecular weight. This can affect yarns internally or externally (Hsuan et al. 1993). Polyester geotextiles are chiefly affected when exposed to high alkalin- ity values. High pH affects some polyesters, while low pH is more harsh on polyamides. The European Committee for Standardisation (CEN) is working

220

I

I on resistance to liquids. Resistance of geotextiles to hydrolysis can be deter- ~

mined according to ENV 12447 (1997).

Chemical. Manufacturers have tested a wide range of materials such as acetate, nylon and rayon, with chemicals such as sulphuric acid, hydrochloric acid, organic acids, bleaching agents, and salt solutions. ASTM Committee D35 is also working on chemical degradation assessment of geotextiles after either laboratory (D 5322) or field immersion (D 5496) procedures. CEN has drafted a pre-standard: ENV IS0 12960 (1998) to determine the chemical resistance of geotextiles. These types of tests will be particularly useful when the materials are used with landfills to assess the effect of leachates.

Biological. Bacteria and fungi can deteriorate polymers if they can attach themselves to the yarn and use the polymer as feedstock. This however is unlikely for the commonly used polymeric resins. In Europe a pre-standard ENV 12225 (1996) is currently available as a prospective standard for provi- sional application, which describes measurement of micro-biological resist- ance by a soil burial test. ASTM prescribes no methods, except the biological clogging (ASTM D 1987) which is more a mechanic problem rather than bio- logical one. Ionescu et al. (1982) described test results on four PP, one PE and a composite geotextile when subjected for 5 - 17 months to distilled water (the control test), sea water, compost, a particular soil, iron bacteria, laven-syn- thesizing bacteria, desulfovibrio bacteria and a liquid mineral. Results showed no measurable effect of permeability (although some roots were grow- ing through the geotextile), only small variations in tensile strength, and no structural changes visible on infrared spectroscopy.

Sunlight (UV). Degradation by sunlight or ultra violet rays (W) is of impor- tance because of storage of drain envelope material onsite, just before con- struction, and when improperly stored (e.g. in direct sunlight). The UV region of light is subdivided in UV-A (wavelength 400 - 315 nm), which causes some polymer damage, UV-B (315 - 280 nm), which causes severe damage to poly- mers (Cazuffi et al. 19941, and UV-C (280 - 100 nm). The last-mentioned is only found in outer space. According to Santvoort (1994) PE is most sensitive to 300 nm, PE to 325 nm, and PP to 370 nm UV wavelengths. Obviously geo- graphic location, temperature, cloud cover, hours sunlight per day, wind and moisture content, all play a role in UV degradation. Draft European stan- dards on testing of pipes note that material changes be expected only after a total radiation of 3.5 GJ/m2 has taken place (document 155N677, or standard ENV 12224-96). In the Netherlands average annual radiation is 3.4 GJ/m2, while in Egypt it is 6.86 GJ/m2 (Omara and Abdel-Hady, 1997). ASTM pro- vides guidelines on how to test for UV degradation in G23 (Carbon arc), D 4355 (Xenon arc), G53 (UV Fluorescent), D 5208 (UV Fluorescent), and D 1435 (Outdoor weathering of plastics). A number of authors have recently

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