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Designelements

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GeomembranesDesign elements

2008 edition


2/12Geomembrane Installation Guide | Design elements 1/3 | 03-082

Contents

1. Geometry of the works 3

1.1. Form of the basins 3

1.2. Slopes 3

1.3. Accessibility to the works 4

1.4. Access ramp 51.5. Drainage and maintenance 5

2. Criteria for selecting a Geomembrane Waterproofing System 6

2.1. Temperature 6

2.2. Ultraviolet radiation 6

2.3. Wind 6

2.4. Hypobaric pressures (underpressures) 7

2.5. Vegetation above the geomembrane 8

2.6. Settling and swelling of soils 9

2.7. Ice 9

2.8. Liquid flow-off (canal, basin feed zone, aerated lagoon, etc.) 9

2.9. Waves and wakes 10

2.10. Floating bodies 11

2.11. Leak rate 11

2.12. Chemical aggressions 11

2.13. Vandalism 11


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1.1. Form of the basins

Prefer simple geometrical forms for the works

so as to:

Jfacilitate the installation of theGeomembrane Waterproofing System;

Jlimit complex assemblies;Javoid the formation of wrinkles.

1. Geometry o the works

1.2. Slopes

The gradient of the slopes is related to the quality

of the materials on the site. It shall be determined

taking into account the rules of soil mechanics. The

slope shall be stable by itself and the Geomembrane

Waterproofing System is there only to waterproof

the structures.

The following points need to be taken into account:

J the geometry of the works (gradients, lengths of

inclines, etc.);

J the nature of the substrate materials and of the

protection layer;

J the mechanical characteristics of the various

elements of the Geomembrane Waterproofing

System;

J the friction coefficients at the interfaces of the

various layers among themselves;

J the nature of the products stored;

J the works-operating mode (presence or absence

of water, variation of water level, possibility of

fast draining, etc.).

Al th ou gh th e in st al lat io n of the Ge om em br an e

Waterproof ing System is possible on certa in

vertical or sub-vertical substrates (rocky, concreted

or masonry slopes), jobsite constraints are such

that, as a general rule, it is preferable to position

them on gradients from 3H / 2V to 2H / 1V .

2H / 1V

3H / 2V maxi

These low gradients are intended:

J to facilitate traffic both of personnel and of whee-

led rigs;

J to facilitate the assembly work on the site;

J to facilitate the installation of protection on the

waterproofing;

J to limit the strains exerted on the waterproo-

fing.

It should be noted that the composite bituminous

geomembranes have high friction coefficients at


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3m mini

Geomembrane Installation Guide | Design elements 1/3 | 03-084

the geotextile topsoil interface, for example. So

they allow for sizing with higher gradient slopes

than those of Waterproofing Systems, including a

synthetic geomembrane.

1.3. Accessibility to the works

Access and storage of materials

A storage area needs to be prov ided that is fl at ,

clean and with sufficient load bearing capacity to

enable lorries to come in and wheeled rigs to roll.

Dur ing loading and unloading operat ions, a l l

precautions need to be taken to prevent damaging

the first turns of the rolls.

The rolls must be stored flat where it i s dry, three

rolls high maximum, in their original packing.

Preparation at the crest of the slopes

It is recommended that a provisional roadway be

provided around the storage structures to enable:

J construction of the anchorage trench of the

geomembrane;

J transfer of the rolls to the jobsite from the sto-

rage area;

J execution of the works on the jobsite without risk

of damaging the waterproofing by jobsite wheeled

rig traffic (prohibited on synthetic geomembranes,

possible on bituminous geomembranes, if certainprecautions, described further on, are taken).

This provisional or permanent roadway around the

works needs to be planned as soon as the design

of the works begins. The recommended width is

about a minimum of 3m to take into account the

anchorage trench and enough space for a wheeled

rig to roll.


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Gradient of 2% to 5%

Gradient of 2% to 3%

Geomembrane Installation Guide | Design elements 1/3 | 03-08 5

1.4. Access ramp

An access ramp faci li ta tes the traf fi c of wheeled

rigs while the works are going on and for the main-

tenance operations and for the basin scouring.

The following recommendations apply:

J a gradient of about 15% maximum is recom-

mended;

J the pavement shall consists of water binding

clayey sand materials (concrete or embedded

aggregate);

J in the case of bituminous geomembrane water-

proofing, the roadway can be directly built on

the geomembrane, referring to the installation

paragraph;

J in the case of a synthetic geomembrane, it must

be mechanically fixed to the concrete slab or, an

intermediate layer of materials is to be added on

between the geomembrane and the pavement.

1.5. Drainage and maintenance

A subgrade gradient is recommended in order to:

J drain all the water at the end of rainfalls;

J avoid retention zones;

J facilitate removal of gases;

J facilitate cleaning.

Standard gradient diagram


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Anchor ing

Ballasting

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The selection of a Geomembrane Waterproofing

System for a given pro ject depends on many

parameters.

Each project is specific and the initial step of

sizing is to rank these criteria so as to be able

to determine the nature of the geomembrane to

be used.

2.1. Temperature

Temperature variations cause shrinkage and ex-

pansion of the geomembrane and can enta i l

constraints, either permanent or not, of wrinkling

at the assemblies, of tensile stress at the ancho-

rings.

These phenomena are particularly annoying for

certain types of geomembranes if they occur with

considerable amplitude during installation, which is

the case with Geonap type High Density Polyethylene

geomembranes. This may lead to working only

certain times during the day.

So it is advisable to verify the value of the thermal

expansion coefficient indicated in the geomembranetechnical data sheet.

Finally, low temperatures may make certain geo-

membranes brittle. For this, protection measures

need to be planned. The brittle temperature is given

by the cold bending tests.

2.2. Ultraviolet radiation

The UVs are one of the main ageing factors of

geomembranes. Installing a protection structure

considerably increases these materials service life.

The behaviour of geomembranes under ultraviolet ra-diation is checked by the accelerated ageing tests.

2.3. Wind

If no precaution is taken when the wind is blowing,

and while the installation is in service, certain parts

of the geomembrane may be subjected to suffi-

cient negative wind load (underpressure) so as to

lift them.

A number of so lu ti ons are poss ib le acco rd ing to

the state of progress of the works:

J

installing anchorings;J partial ballasting (bags of sand, windrows of non-

aggressive materials, ballasted tyres, water, etc.),

taking into account, as the case may be, the

weight of the geomembrane;

J total ballasting by a general protection;

J suction systems by the draining network of gases

or of balancing pressures by ventilation apertures

(air-wind); however, this latter solution requires

taking the necessary precautions to make sure

that the system is not operating under hyperbaric

pressure (overpressure).

To the degree possible, and if the form of the struc-

tures makes this possible, the geomembrane is

unrolled, beginning the installation by the crest of

the slopes, continuing along the line of the largest

2. Criteria or selecting a Geomembrane WaterproofngSystem


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Waterinfiltrations

Gas compression

Leaks intoa basincontainingorganic matter

Upward penetrationof the water table

Decomposition of organic matter

Water table levelabove the levelof the bottomof the basin

Direction of the installation of the membrane

Prevailing winds

Geomembrane Installation Guide | Design elements 1/3 | 03-08 7

grade and taking into account the direction of the prevailing winds, according to the diagram below:

2.4. Hypobaric pressures (underpressures)

The water and gases accumulated under the geomembrane exerts hypobaric pressures on it, which tend

to lift it.

Hydraulic hypobaric pressures

The hydraulic hypobaric pressures can be related

to three types of events:

J an upward penetration of the water table that

may become flush with the level of the bottom

of the basin and lift the geomembrane;

J the weather related water or the water from non-

captured sources, which can cause slipping or

wash-out under the geomembrane in the slope;

J the liquids contained in the basin in case no

specific outlet has been built or in case the

waterproofing is damaged.

The installation of a drainage system under the

waterproofing makes it possible to better distri-

bute the hypobaric pressures, to remove the gases

toward the slopes and, as the case may be, to take

action to limit the effects due to these hypobaric

pressures.

In cases where the risks of upward penetration of

the water table are well identified, the installation

of a protection structure, balancing the predictable

hypobaric pressures, is necessary.

The hypobaric pressures related to gases

All the basins may wel l undergo damage related to

upward penetration of gas.

Causes of hypobaric pressures



The hypobaric pressures related to gases can appear

in the following configurations:

J Fermentable soils

The gas is produced by the decomposition of or-

ganic matter. It is recommended that the substrate

be clear of these materials before installing the

waterproofing.

J Water tables

Any upward penetrat ion of the water table chases

the air contained in the soil out, thus creatinghypobaric pressures under the waterproofing.

Main pathology linked to the absence of

drainage of gases

If the building of a subgrade with gradient in the

structure is not planned and if there is no drainage

system, the gas is trapped under the waterproo-

fing without any possibility of being removed. In

that case a pocket is created under the geomem-

brane.

These pockets, sometimes referred to as hippo-

potamuses, often tend to grow as shown in the

example below.

They can generate major pathologies.

The solution

J The gas drainage system under the geomem-

brane;

J The building of improved subgrades with gra-

dient.

2.5. Vegetation above the geomembrane

It is possible to plant vegetation over a Geomembrane

Waterproofing System as long as shrub species

are selected, species whose root systems can be

accommodated in the depth of the earth to be

overlaid in the project.

The root system stabilises the materials on the slo-

pes; a good choice of plant species, well adapted

to the soils, as well as the earth thickness, makes it

possible to improve the operation of the project.

It is important to prepare a basin servicing plan from

its very conception.

Safety on the banks and in the basins

Several arrangements need to be made:

J building a fence around the basin;

J installing ladders, ropes with knots to enable a

person or an animal to get out of the basin fol-lowing an accidental fall.



Special arrangements need to be made when the project is designed to be accessible to the public.

2.6. Settling and swelling of soils

Generally speaking, there is always a risk of

circumscribed settling of the substrate and it is

recommended that waterproofing be installed, if

possible, with a flexible material that can fit against

the deformations of the substrate.

Nevertheless, it is necessary to build a support

structure, limiting these differential movements,and this, given the permissible deformations of the

geomembranes.

The following steps may be carried out:

J local elimination of compressible or swelling ma-

terials;

J increasing the thickness of the subgrade, capping

layer and/or base layer);

J compacting the subgrade.

2.7. Ice

The presence of ice in contact with the Geomem-brane Waterproofing System can generate strains

leading to troubles such as:

J lateral thrust when ice is forming;

J strains during water level variations;

J aggressions by floating blocks.

It may be necessary for the waterproofing to install

a heavy mineral protection in the water level rise

and fall range.

2.8. Liquid flow-off (canal, basin feed zone,

aerated lagoon, etc.)

Water coming into a basin without part icular

arrangements can generate pathologies such as

wash-out under the waterproofing, deformation of

the geomembrane, tensions at the fixing elements.

The liquid temperature may increase the risk of

degradation.

It is recommended that an outflow channel, of

concrete or of another material, be installed where

the water comes in.

Water currents in a basin or in a canal a lso

generate strains against the waterproofing on the

slopes and this increases with the flow speed, inparticular at details and connections, gradient or

section changes, pronounced curve, etc.



A protect ion structure f orming the bal last ing sha ll

always be carefully considered:

J in zones of high turbulence;

J at details and connections;

J within sections where the flow rate is greater

than 1.5m/s (guide value).

In cases where adapting to a flow rate l imit or

to a flow section limit precludes ballasting, the

geomembrane can be jo ined to i ts substrate

by welding or full-surface bonding, in bands or

at specific points, or by circumscribed or linearmechanical fixing.

A fi xi ng syst em is to be de te rmined based upon

the expected speed and turbulence on the geo-

membrane.

2.9. Waves and wakes

The waves or wakes, created by the passing of

boats or by the wind, against the bank, generate a

series of alternating hydrodynamic strains.

The support structure must be correctly sized to

be able to withstand the strains. According to the

amplitude of the phenomenon, the geomembrane

is to be covered by a protection structure or fixed

at the locations involved.

A rock-f il l or a protection of concre te can reduce

these phenomena. In these cases, it is recommen-ded to use a bituminous geomembrane because

its surface has no expansion wrinkles so a protec-

tion layer can be installed under good conditions,

closely gauging its thickness.

Protection structure where liquids come in



It is also important to provide a strong enough

surge barrier to prevent the waves from surging

over the banks.

2.10. Floating bodies

The presence of floating bodies, including ice, by

bumping or rubbing, can cause circumscribed tears

in the geomembrane.

A protection st ructure is to be provided or any ways

and means aiming either to reduce the presence

of these floating bodies or to prevent their contact

with the geomembrane. This risk increases with the

amplitude of the waves and therefore with the

extent of the surface area of the basin.

2.11. Leak rate

The structure of the Geomembrane Waterproo-

fing System depends on the permissible leak rate

during operation.

The permissible leak rate depends on economic

and hydraulic considerations, but also on the risks

undergone in case of leak (pollution or destabili-

sation of the foundation soil).

In cases where these risks are considerable, a

double waterproofing can be provided, or even a

composite system, associating layers of soils of

low permeability and geomembranes. The same

principle can apply to control the leak flow on

permeable ground.

2.12. Chemical aggressions

The liquid retained or carried along shall in no case

chemically aggress the Geomembrane Waterproo-

fing System.The conditions for operating the structures, in terms

of the materials contained, shall be defined from

the beginning and shall be respected over time so

as to define the geomembrane.

2.13. Vandalism

Vandalism, difficult to quantify, is a parameter that

can lead to applying particular protection devices:

fence, total or partial protection structure, etc.

Bituminous geomembranes, by their thickness, their

mechanical strength and their ease of repair, are

certainly more capable of responding to this type

of problem.

Our Technical Department

is at your disposal to study

and analyse the feasibility

of any project plan.


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92184 Antony Cedex - France

Tel: +33 1 40 96 35 00

Fax: +33 1 46 66 24 85

www.siplast-international.com

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