Geogrids in cold climate Temperature controlled tensile tests & Half-scale installation tests at different temperatures

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Geogrids in cold climate

Temperature controlled tensile tests & Half-scale installation tests at different temperatures

Björn Bonthron Christian Jonsson

Civilingenjör, Väg- och vattenbyggnad 2017

Luleå tekniska universitet

Institutionen för samhällsbyggnad och naturresurser

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Abstract

Due to the findings of extensive damage on geogrids used in a road embankment in northern Sweden, the Swedish Transport Administration (TRV) started to investigate the reason of these damages. Since the geogrids were installed at low temperature, below 0°C, it was suspected that the damages were connected the low temperature.

To analyse whether low temperatures have an influence on the extent of installation damages, both a half-scale setup and temperature controlled tensile tests have been carried out on geogrids.

In total five different types of geogrids have been tested; 3 extruded polypropylene geogrids, 1 woven PET geogrid, and 1 welded PET geogrid. All geogrids had an aperture size of approximately 35 mm and specified tensile strength of approximately 40 kN/m.

The Half-scale tests was conducted by building a small road embankment inside a freeze container, at the Luleå University of Technology (LTU). The embankment contained crushed aggregate, type 0- 70 mm, and geogrids. The purpose of the half-scale test was to simulate installation of geogrids at different temperatures and thereby investigate whether low temperatures have an influence on the rate of installation damages.

The half-scale test was done for each type of geogrid at the temperatures: +20°C, -20°C and -30°C.

First, the geogrid was covered by 150 mm of crushed aggregate. Then a vibratory plate (160 kg) was used to compact the crushed aggregate. After each installation, the crushed aggregate was removed carefully by vacuum suction. The geogrid was removed and then analysed by visual control and tensile tests conducted according to ISO 10319:2008 (wide width tensile test).

Results from the half-scale tests indicate that 2 out of 5 of the tested geogrids were affected by the testing procedure. The results indicate that:

- one of the geogrids of polyprophylene (here referred to as G2) was more damaged at lower temperatures compared to installation at +20° C.

- the geogrid of woven PET (here referred to as G5) was less damaged at lower temperatures compared to installation at +20° C.

Results for the other geogrids are either inconsistent or shows no significant variation of the measured parameters as function of temperature. Hence, these results cannot be interpreted as damage during installation.

Temperature controlled tensile tests were done by tensile testing single strands from the geogrids to failure, inside a temperature controlled chamber. The purpose of these tests was to investigate how the strength properties of the geogrids are affected by low temperature. The test was repeated 5 times for each geogrid and temperature (+20°C, 0°C, -10°C and -20°C). Force and strain was measured during the tests.

The results from the temperature controlled tensile tests show that the maximum strain decreases with lower temperature for all tested geogrids. The maximum strain decreased by 16% - 49% when the temperature dropped from +20°C to -20°C.

The results show that the tensile strength increases with lower temperature for all tested geogrids except for the welded PET geogrid (here referred to as G1). For G1 the tensile strength decreased by approximately 7% at a temperature drop from +20°C to -20°C.

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For the woven PET geogrid (G5) and the polypropylene geogrids (G2-G3) the tensile strength increased between 13%-45% at a temperature drop from +20°C to -20°C.

The E-modulus increased at lower temperature for all tested geogrids. The secant E-modulus at 2%

strain increased by 13%-71% at a temperature drop from +20°C to -20°C.

Summarized conclusions from the tests:

Strength properties changed for all tested geogrids as the temperature decreased. All tested geogrids got stiffer at lower temperatures. The magnitude of the effects is different for different geogrids.

The tensile strength increased with lower temperature for all tested geogrids except for the welded PET geogrid, which got lower tensile strength at lower temperature.

The half-scale test indicates that the amount of installation damages at geogrids can be dependent of the temperature at installation. However, these indications can only be seen at two out of five tested geogrids. The effect cannot be connected to a specific step in the installation procedure and cannot be explained by the results from the temperature controlled tensile tests.

The results from the half-scale test have a statistically low reliability since only one installation for each temperature and geogrid type was done. The compaction equipment used during the test was small, and had low compaction energy compared to a vibratory roller compactor commonly used in construction work.

With respect to the discussion above, further studies should be focusing on developing the half-scale test. It is suggested that the test is scaled up to a full-scale test in order to simulate a real installation as close as possible. The test should also be conducted several times for each geogrid at each

temperature in order to enable statistical analyses.

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Sammanfattning

På grund av att omfattande skador upptäckts på geonät som använts i en pålad vägbank i norra Sverige, började Trafikverket utreda orsaken till skadorna. Eftersom geonäten installerats då

temperaturen var under 0°C, misstänktes det att uppkomna skador kan kopplas till installation i låga temperaturer.

Två typer av försök har utförts för att analysera vilken effekt låga temperaturer har på geonät, halvskaletest och temperaturkontrollerade dragtester på enstaka ribbor.

Totalt fem olika geonätstyper har testats; 3 st tillverkade av extruderat polypropylen, ett tillverkat av vävt PET och ett tillverkat av svetsat PET. Testade geonät har en maskvidd på ca 35 mm och

specificerad draghållfasthet på ca 40 kN/m.

Halvskaletestet utfördes genom att en liten vägbank byggdes inne i en fryscontainer. Vägbanken bestod av bergkrossmaterial, av typ 0-70 mm, och geonät. Syftet med halvskaletestet var att simulera installation av geonät i låga temperaturer och därigenom undersöka om låga temperaturer har någon effekt på hur mycket installationsskador som uppkommer på näten.

Geonäten installerades var och ett för sig i vägbanken. Vid installationen täcktes geonäten med 150 mm av bergkrossmaterialet, vilket därefter packades med en 160 kg tung vibratorplatta. Efter installationen grävdes geonäten upp försiktigt genom att det packade krossmaterialet sögs upp med hjälp av en sugbil. För att utvärdera hur geonäten påverkats av installationen, utfördes både okulär besiktning och breda dragtester enligt ISO 10319:2008 på näten. Ett halvskaletest utfördes för varje geonät i temperaturerna +20°C, -20°C och -30°C.

Resultaten från halvskaletesten är att två av de fem testade näten blivit påverkade av halv- skaletesten. Resultaten indikerar att ett polypropylenenät, benämnt G2, fick ökad mängd installationsskador vid låga temperaturer. Endast detta nät fick nämnvärda synliga skador vid installationen. Dessa skador var i form av veck och uppkom över hela nätbredden på mitten av ribborna, under utrullning av nätet i låga temperaturer. Det är inte undersökt om dessa veck bidragit till minskad hållfasthet hos nätet. Resultaten från halv-skaletesten indikerade också att nätet av vävt PET fått minskat antal installationsskador vid lägre temperaturer.

Resultaten för övriga nät är för små eller otydliga för att tolka som installationsskador.

Temperaturkontrollerade dragtester utfördes genom dragtester till brott på enstaka ribbor från näten, i en temperaturkontrollerad kammare. Syftet med dessa dragtester var att undersöka hur nätens hållfasthetsegenskaper förändras då temperaturen sjunker. Testet repeterades 5 gånger för varje nät i varje temperatur (+20°C, 0°C, -10°C och -20°C). Under testen mättes kraft och töjning kontinuerligt.

Resultaten från temperaturkontrollerade dragtesterna på enstaka ribbor visar att alla testade geonät får mindre maximal töjning då temperaturen sjunker. Maximal töjning minskade med mellan 16% - 49% då temperaturen sjönk från +20°C till -20°C.

Vid kyla ökade maximal draghållfasthet för alla testade geonät, utom det svetsade PET-nätet som istället fick minskad draghållfasthet. För det vävda PET-nätet och polypropylen-näten ökade maximal draghållfasthet med 13%-45% då temperaturen sjönk från +20°C till -20°C. För det svetsade PET- nätet sjönk maximal draghållfasthet med 7% då temperaturen sjönk från +20°C till - 20°C.

E-modulen ökade för alla testade geonät då temperaturen sjönk. Ökningen för sekant-E-modulen vid 2% töjning ökade med mellan 13% och 71% då temperaturen sjönk från +20°C till -20°C.

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Sammanfattande slutsatser från utförd studie är att alla testade geonät får tydligt förändrade hållfasthetsegenskaper då temperaturen sjunker. Alla testade nät blir styvare vid lägre temperaturer.

Effekten är olika för olika nät. Vid kyla ökade maximal draghållfasthet för alla testade geonät, utom det svetsade PET-nätet som istället fick minskad draghållfasthet.

Halvskaletestet indikerade att mängden installationsskador som uppkommer på ett geonät kan vara beroende av temperaturen vid installation. Indikationer på detta har dock endast kunnat ses på två av fem testade nät. Effekten kan ej knytas till ett vist material eller installationsmoment och kan heller inte förklaras med hjälp av resultaten från temperaturkontrollerade dragtester.

Det statistiska underlaget från halv-skaletestet är mycket litet. I halvskaletestet har en lätt packningsutrustning använts i jämförelse med vad som ofta används vid verklig installation.

Med avseende på ovan, föreslås det att fortsatta studier inriktas på att noggrannare undersöka hur kyla påverkar mängden installationsskador. Förslagsvis utförs full-skaletester som bättre efterliknar verklig installation i kyla med större packningsutrustning. Fullskaletesterna bör även repeteras för att möjliggöra statistisk analys.

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Legend

CMD Abbrevation for cross mashine direction

E E-modulus (Pa)

F Force (N)

G1-G5 Geogrid ID

H Height (m)

MD Abbrevation for machine direction

N0 Number of tensile elements within one metre of geogrid

Nm Average number of tensile elements in 1 m width of the geogrid ns Number of tensile elements in test specimen

Pa Active soil pressure (Pa)

Pp Passive soil pressure (Pa)

PP Abbrevation for plastic material polypropylene

PET Abbrevation for plastic material polyethylene terephthalate

qQ Variable load (Pa)

qG Permanent load (Pa)

T Tensile force (N)

Tg Glass transition temperature (°C)

w0 Width between first and last tensile element within one meter (m)

Xd Designing material property

Xk Characteristic material property

γ Heaviness of the soil (kN/m³)

γM Partial coefficient (safety factor)

ε Elongation/Strain (%)

η1-3 Converting factors for creeping, installation damages, degradation

τ Shear stress (Pa)

ϕ_d Designing friction angle (°)

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1 Introduction

Geogrids are a type of synthetic grid mainly used for reinforcement of soil. The soil particles are locked in the openings of the grid and the soil is thereby given tensile strength, see Figure 1 and Figure 2.

Figure 1 Principle sketch of geogrid (Carter & Dixon, 1995)

Figure 2 Geogrid preventing slope from sliding, where Tds is the design force in the geogrid, Pa is the active soil pressure (Swedish Geotechnichal Society, 2004)

1.1 Background

When the Swedish Transport Administration (TrV) performed a post construction control of a road embankment in Umeå they found extensive damage on the geogrids used to stabilize the

embankment. All the damaged geogrids had cracked diagonally through the nodes. Since the embankment was constructed during cold weather (temperatures below 0°C) TrV are interested to gain information about if and how the temperature had an effect on the material properties of the geogrids. More details about the construction of the embankment in Umeå, are presented in Appendix 9.

The damages were assumed to have been initiated either during the rolling out of the geogrids or during compaction of the overlaying crushed rock. Due to these findings TrV initiated these studies to investigate whether the material properties of the geogrids changes at low temperatures and if the risk of damaging the geogrids increases during installation at low temperatures.

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1.2 Purpose

The purpose of this master thesis has been to investigate if/how the material properties of the investigated types of geogrids changes at low temperatures. In addition tests have been performed to investigate whether these changes affect the rate of installation damages when installation is done at low temperatures.

1.3 Delimitations

Since a limited amount of different types of geogrids and materials have been tested during this work, it is recommended that the results is considered as an indication on which types of geogrids that are affected by low temperature.

The mechanisms that have been of interest during this work:

- Change of tensile strength

- Strain and E-modulus of the geogrids due to low temperatures

- How the geogrids are affected when placing and compacting the overlaying material on the geogrids at low temperature.

1.4 Objective

This master thesis aims to be a preliminary study on how low temperatures affects the strength properties of geogrids and if the amount of installation damages is affected due to low temperatures.

The work also aims to contribute information as a basis for decisions regarding further studies on the subject.

To fulfil the objectives the following questions should be answered:

1. How, and if so how much, did the tensile strength change when the investigated geogrids were tested at 0°C, -10°C and -20°C compared to tests performed at +20°C?

2. How, and if so how much, was the maximum strain in the investigated geogrids affected when tests were performed at 0°C, -10°C and -20°C compared to tests performed at +20°C?

3. How, and if so how much, was the modulus of elasticity affected when tests were performed at 0°C, -10°C and -20°C compared to tests performed at +20°C?

4. Was there any indication that specific operations during the installation procedure damaged the geogrids particularly? If so, at what temperatures did this occur?

5. Was the investigated geogrids particularly damaged when placing and compacting overlaying material in low temperatures?

6. If some of the investigated geogrids was damaged: describe the amount and type of damage.

7. Did the results from the tests indicate differences between geogrids made of different materials (considering the questions mentioned above)?

8. Did the results from the tests indicate differences between geogrids made with different manufacturing methods (considering the questions mentioned above)?

9. Is it possible to see any direct connection between the results from question 1 – 3 and the result from question 4 – 8?

1.5 Method

This master thesis has been conducted by dividing the work into three phases. The first phase is the theoretical phase where background data has been collected and a literature review has been performed. The second phase is the experimental phase, when half scale tests and laboratory tests have been performed. In the last phase all the background facts, method, theory and test results have been be analysed and discussed.

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A literature review has been carried out with focus on different types of geogrid constructions related to road embankments. The theory behind the function of these is mentioned briefly.

Furthermore, different manufacturing methods and manufacturing materials for geogrids are described as well as information about how these materials behave at different temperatures.

1.5.2 Experimental phase

The experimental phase is divided into two different types of experiment, laboratory tests and half- scale tests. Five different bi axial geogrids made from different materials and different manufactures have been tested in both the laboratory test and the half-scale test.

The five different geogrids are in this report referred to as geogrid G1 – G5, see Table 1. Detailed information about the tested geogrids is presented in Appendix 8.

Table 1 Short information regarding tested geogrids

Geogrid ID Material Specifications; max tensile strength machine direction/ cross machine direction (kN/m) G1 Welded polyester terephthalate (PET) 40/40

G2 Extruded polypropylene (PP) 40/40

G5 Woven polyester terephthalate (PET) 42,4/47,9

1.5.2.1 Laboratory test

Tensile strength test has been performed on single ribs from the geogrids. The rib exposes for tensile stress in a climate chamber until failure occurs. The tensile stress and strain are measured during the test. To obtain reference values, the geogrids has first been tested in +20° C. The test is then

repeated in 0° C, -10° C and -20° C. In order to verify the results more statistically, the test is repeated five times for each geogrid and temperature.

The test results are then analysed to determine if there is temperature dependence in the material properties.

1.5.2.2 Half-scale test

The purpose of the half-scale test is to simulate the installation procedure the geogrids, the placing and compacting of overlaying material. The test has been performed in temperature-controlled conditions inside a freeze container. Each of the five different geogrids has been tested at three different temperatures, +20°C, -20°C and -30°C.

After finishing the installation procedure of the half-scale test, specimens from tested geogrids were sent to an accredited laboratory and tested according to ISO EN 10319. The results have been used to determine if low temperatures at installation influences the rate of installation damages.

1.5.3 Allocation of responsibility between the authors

Since two students have carried out this master thesis, the responsibility has been divided. Several parts of the work have been conducted by both of the students due to practical reasons. Though the responsibility of writing the main chapters in this report, has been divided as follows:

Christian Jonsson has been responsible for writing the parts of the literature review dealing with;

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• Different geogrid types, manufacturing material and technics

• General plastic theory

• Background information about the piled road embankment and occurred problems/damages

• Testing methods for geogrids and geotextiles

Christian Jonsson has also been responsible for writing about the temperature controlled tensile test regarding method and the analysis and conclusions.

Björn Bonthron has been responsible for writing the parts of the literature review, dealing with;

• How geogrids works and are used

• How to design with geogrids

• Previous studies on geogrids in low temperature

Björn Bonthron has also been responsible for writing about the half-scale tests and the corresponding analysis and conclusions.

2 Limitations

This thesis has only been studying the performance of the geogrids in low temperatures on a short- term perspective.

The temperatures used in the laboratory tests on single ribs are 20°C, 0° C -10° C and -20°C. The 20°C temperature has been chosen as a reference temperature. The -20°C temperature has been chosen as this is the coldest temperature possible to produce with the laboratory equipment.

The temperatures used in the half-scale tests are +20° C, -20° C and -30° C. The -30°C temperature has been chosen as this is seen as a lower limit to where it is reasonable that the geogrids will be installed in Swedish applications.

The amount of installation damages is depending on several factors. Examples of factors that are not covered in this report are: shape of the granular material particles, particle size distribution, weight and type of compaction equipment, different methods for placing the top layer of soil over the geogrid.

The geogrids used in the tests are all of bi-axial type. The manufactures have specified tensile strength of about 40 kN/m in both directions and an aperture size of approximately 30-40 mm.

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3 Litterature review

This chapter will describe what a geogrid is, how it works and how it is manufactured. The literature review also covers general theory about plastic materials, standardized testing methods for geogrids and previous studies on geogrids at low temperatures.

3.1 What a geogrid is

3.1.1 Basic information about geogrids

Geogrids are a type of geosynthetic mainly used for reinforcement of soil. The International

Organisation for Standardization (ISO) defines in report ISO 10318:2005, a geosynthetic as: “generic term describing a product, at least one of whose components is made from a synthetic or natural polymer, in the form of a sheet, a strip or a three-dimensional structure, used in contact with soil and/or other materials in geotechnical and civil engineering applications”.

The term geogrid is by ISO 10318:2005 defined as; “Planar, polymeric structure consisting of a regular open network of integrally connected, tensile elements, which may be linked by extrusion, bonding or interlacing, whose openings are larger than the constituents”

This means that a geogrid is a sheet like construction material made of a plastic (polymeric) material, with openings large enough to allow soil particles to pass through, see Figure 3.

Figure 3 Principle sketch of geogrid (Carter & Dixon, 1995)

3.1.2 Materials used in geogrids

The definition of a geogrid above implies that a geogrid only can be produced from a plastic polymeric material. This is not truly the case. Examples of geogrids produced from other types of materials are bitumen-coated fibreglass (Titan Environmental Containment Ltd, 2014) and polymer coated steel (Huifeng Geosynthetics, 2014). Although it is possible to have geogrids produced from non-polymeric materials, from here on only geogrids produced from plastic, polymeric, materials will

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be considered. Currently the materials most used in geogrids are polypropylene (PP), and polyester (PET), (Koerner, 2012).

3.2 General theory about plastic materials

All plastic materials are different types of high polymers. The term polymeric materials includes both synthetic polymers, like plastics, and natural polymers like bitumen, natural rubber, proteins etc.

Polymers can be defined as; a large molecule built up by repetition of small, simple chemical units (Brydson, 1999). This means that synthetic polymeric materials, like plastics, are built up by quite simple molecules, mostly different hydrocarbon molecules, which are connected into long chains.

Polymers are produced in a process called polymerisation. The polymerisation can be done in a number of different ways. In general, the essence of these processes is to make small, simple, low molecular weight molecules, monomers, bond together into long chains. These polymer chains can be either completely linear or branched. The branched polymers are often called thermosetting plastics due to the fact that some of them “sets” or changes properties when heated. These changed properties are then more or less static i.e. the plastic does not change properties if reheated. The linear polymers, often called thermoplastics, on the other hand are capable of plastic deformations and flow above a certain temperature. This is due to the fact that the thermoplastics are composed of linear polymer molecules. These linear polymer molecules are able to slide on each other. Below this flow temperature the thermoplastics become more or less a solid material.(Brydson, 1999) In addition to the degree of branching and crosslinking in a plastic, almost all polymers are more or less amorphous (Nielsen & Landel, 1994). In an amorphous material the crystallisation in the material have been halted by for example rapid cooling. The atoms in the material does, due to this rapid cooling, not have the possibility to order themselves in a crystalline pattern, which is the most effective arrangement (Nationalencyklopedin, 2014).

When talking about amorphous materials, especially amorphous polymers, a quite important term or definition to remember is the glass transition temperature (Tg), to which a lot of physical properties of the polymer are linked. One definition of the glass transition temperature according to (Brydson, 1999) is; the temperature at which molecular rotation about single bonds become restricted. This for instance has the effect that below Tg the elastic modulus increases quite a lot, in some cases up to 1000 times, (Nielsen & Landel, 1994), and the material can become much more brittle. (Brydson, 1999) explains that the Tg is not a fix value for one type of polymer, but it is influenced by a number of factors, for example: plasticisation, molecular weight, and branching. With the use of additives, like plasticiser, the Tg can be altered.

One important effect of the Tg on plastic material is the possibility to modify and process the material. The process of extruding or the orientation of polymers rely heavily on the Tg. The tensile strength and tensile stiffness of an amorphous polymer can be increased quite substantially by means of extrusion. This happens when an amorphous polymer is subjected to tensile stress at temperatures above its glass transition temperature. During this tensile stretching the molecules in the polymer aligns with the general direction of the stress. If the polymer then is cooled below Tg the molecules will be stuck frozen in this oriented state. The orientation can be done either monoaxially or biaxiallys. The tensile stress is the applied to the polymer in one respectively two directions. If the orientation is done monoaxially, the resulting polymer will have anisotropic properties. If the

orientation is done biaxially, the resulting polymer can have the same properties in both directions if it is desirable. (Brydson, 1999)

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3.3 Types of geogrids and manufacturing processes

The plastic geogrids can not only be produced from different materials, but also be produced in a number of different ways. This report will focus on three different types of geogrids and their corresponding manufacturing method; extruded, bonded and woven geogrids.

3.3.1 Extruded

Extrusion of polymers is the base of all polymeric geogrids. As described in 0 General theory about plastic materials, the tensile strength and tensile stiffness of the polymers increase substantially when the polymer is extruded. Here however the term extruded geogrid will be used to describe a type geogrid where extrusion is the only step in the manufacturing process. The manufacturing method used for extruded geogrids are sometimes called the “Tensar” method.

The “Tensar” method, starts with a blank sheet of plastic polymer into which holes are punched using needles in a grid pattern. The sheet is then stretched or drawn in a machine at a temperature above Tg (glass transition temperature), well below the melting temperature. This increases the size of the punched holes and orients the polymer molecules in the stretching direction. The resulting grid is then cooled. The result of this is a uniaxial geogrid with a tensile strength in the direction of the stretching, also called machine direction, which is much higher than that of the original sheet of polymer. If the end result is to be a biaxial geogrid the uniaxial geogrid is processed again. At this step the grid is stretched in the width direction, also called the cross machine direction. This step is done at a higher temperature than the first step, but still below the melting temperature of the polymer. The result of this second stretching is a geogrid with square or slightly rectangular apertures, and the geogrid now has the same properties in both machine and cross machine direction. (Shukla & Yin, 2006)

3.3.2 Bonded or welded

Bonded or welded geogrids starts out as individual extruded strips of polypropylene or PET. These strips are then laid out in a grid pattern where the strips cross each other perpendicularly. At each crossing point the strips are welded together using either laser or ultrasonic welding. (Shukla & Yin, 2006)

3.3.3 Woven

Woven geogrids are manufactured from thin extruded filaments of polyester. These thin filaments are then combined into thicker multi-filaments which in turn are woven into a grid. At the junctions, where the filaments cross, the filaments are woven together at several levels to ensure proper strength in the joint. The woven grid is then often covered to protect the grid from damage during installation and environmental deterioration (Shukla & Yin, 2006). The coating can consist of either acrylic, PVC, bitumen or polypropylene (Ching-Wen & Jia-Horng, 2005; Shukla & Yin, 2006)

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3.4 How a geogrid works and is used

3.4.1 Use of geogrids

The main use of geogrids is as reinforcement, and some of its applications are:

• Reinforcement of ballast in railroad construction

• As basal reinforcement

• As sheet anchors for wall facing panels

• As reinforcement in asphalt

• For use as mattress for fills over soft soil and peat

• Use as reinforcement over pile caps (Koerner, 2012)

These examples are only a few of the applications where geogrids are used. Since the main subject of this report is geogrids used as reinforcement in embankments, the following section on the theory behind the use of geogrids, and the design process regarding geogrids will also focus on this application.

3.4.2 Soil interaction (how it reinforces the soil)

A geogrid reinforces the soil by the mechanism of interlocking. When the soil around the geogrid is compacted, soil particles penetrates the geogrids apertures and thereby forms an interlock, see Figure 4, (Carter & Dixon, 1995). The particles are restrained by the ribs of the geogrid and is therefore not allowed to oscillate during transient load. The prevention of movement of the particles both reinforces the soil and separates soil layers from mixing together. The separation prevents soil layers from contaminating each other and thereby keeping the properties of different layers. The reinforcing effect from the geogrid is achieved by interlock the lowest particles of a soil layer, and thereby prevent deformation of the layer (Carter & Dixon, 1995).

Figure 4 Interlock between soil particles and geogrid (Carter & Dixon, 1995)

3.4.3 Anchor effect

The geogrids pullout resistance or anchor effect in the soil, is a result of three different mechanisms.

Shear strength is contributed along the longitudinal and the transversal ribs by the friction between the soil and the ribs. A passive resistance is built up against the front of the transversal ribs. Most of the anchor effect is contributed by the passive resistance where the soil goes in to passive state and resists the pullout by means of bearing capacity (Koerner, 2012).

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According to (Koerner, 2012), to achieve the highest efficiency of friction between the surrounding soil and the geogrid, that is, the pullout resistance, the minimum width of the geogrids apertures should be 3,5 times the average particle size of the surrounding soil.

3.5 How to design with geogrids

3.5.1 Conversion factors

When designing a construction with geogrids, the maximum allowed tensile strength of the geogrid has to be calculated.

According to (Swedish Geotechnichal Society, 2004) the tensile strength of reinforcement is dependent on type of construction, safety requirements, tension, installation procedure and surrounding environment. Due to this, the characteristic strength of reinforcement is reduced with conversion factors that accounts for creeping, installation damages and degradation (chemical and biological).

According to (Swedish Geotechnichal Society, 2004) the designing strength of reinforcement can be calculated with the partial coefficient method

𝑿

_𝒅

=

^𝜼^𝟏^𝜼^𝟐^𝜼^𝟑^𝑿^𝒌

𝜸_𝑴

Equation 3.1

where

Xd is used for design strength

η1-3 are the converting factors for creeping, installation damages and degradation Xk is the characteristic tensile strength of the reinforcement

γM is the partial coefficient for the material (safety factor).

Examples of conversion factors from (Swedish Geotechnichal Society, 2004):

• Reinforcement of polypropylene (PP) gives creep conversion factor η1 = 0.2

• Material surrounding reinforcement of crushed rock gives conversion factor for installation damage η2 = 0.67

• pH value between 4 – 9 gives conversion factor for degradation η3 = 0.91

As indicated by Equation 3.1 and examples of conversation factors above, the characteristic tensile strength differs a lot from the tensile strength used in the design. This should be noted when designing the reinforcement.

3.5.2 Reinforcement of embankments

A geogrid reinforces an embankment on soft subgrade by counteract shear stresses in the

embankment and the underground. A geogrid interlocks with the soil in the embankment and the anchor force created prevents the slopes to slide, due to the active pressure in the embankment, see Figure 5, (Swedish Geotechnichal Society, 2004).

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Figure 5 Geogrid preventing slope from sliding (Swedish Geotechnichal Society, 2004)

The magnitude of the force that the geogrid has to be able to resist, in order to prevent sliding of the slopes of the embankment, is the same as the active earth pressure and is calculated according to 𝑻_𝒅𝒔 = 𝑷_𝒂= 𝟎, 𝟓𝑲_𝒂𝒅 𝜸𝑯 + 𝟐 𝒒_𝑸+ 𝒒_𝑮 𝑯 Equation 3.2

where Kad is the active earth pressure coefficient defined as

𝑲_𝒂𝒅 = 𝒕𝒂𝒏^𝟐 𝟒𝟓 −^𝝓_𝟐^𝒅 Equation 3.3

and where Tds is the design force in the geogrid, Pa is the active earth pressure, γ is the heaviness of the soil, Φd is the friction angle used in the design, qQ and qG is the variable and permanent load on the embankment and H is the total height of the embankment. (Swedish Geotechnichal Society, 2004)

3.5.3 Geogrid used on reinforced underground

When geogrids are used in embankments built on a pile reinforced subgrade, the main purpose of the geogrid is to prevent settlements between the pile caps. The use of the geogrid reinforcement is often economical favorable since it reduces the size of the pile caps needed (Swedish Geotechnichal Society, 2004). The load from the embankment is transferred to the pile caps, by the principle of an arch created in the embankment between the pile caps. The geogrid is placed close over the pile caps and thereby prevents the soil under the arch to settle. (Swedish Geotechnichal Society, 2004). When calculating this type of design, the cross section area under the arch, which causes the load on the reinforcement, is approximated to a wedge, see Figure 6. The approximation can be made even if the height of the wedge exceeds the height of the embankment. (Swedish Geotechnichal Society, 2004).

Figure 6 Geogrids placed over pilecaps. (Swedish Geotechnichal Society, 2004)

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19 3.5.4 Force-strain relation

According to (Swedish Geotechnichal Society, 2004) the force-strain relation of geosynthetics are very important to account for. When a geosynthetic is used as reinforcement, the maximum strength of the soil and the reinforcement can’t be combined without considering the force-strain relationship for the both materials. According to (Swedish Geotechnichal Society, 2004), a brittle failure behavior in one of the materials could result in serious consequents if the maximum strength of the soil and reinforcement are combined without further considerations, see Figure 7.

Two different curves are presented in Figure 7. The upper curve shows the force-strain relationship of a geosynthetic. The lower curve shows the stress-strain relationship of a quick-clay. The left dashed line shows what the magnitude of strain is when the clay goes to failure. The right dashed line shows what the magnitude of strain is when the geosynthetic has mobilized it’s maximum strength.

When the left dashed line is studied for both curves together, it is seen, that only half of the geosynthetics strength has been mobilized when the clay goes to failure.

This shows that there is a risk of overestimating the combined strength of two materials if the strength-strain relationships of the materials are neglected.

Figure 7 Examples of compatible deformation levels. Geosynthetic material (upper curve) compared with quick clay (lower curve), (Swedish Geotechnichal Society, 2004).

Figure 8, (Swedish Geotechnichal Society, 2004), shows that the force-strain relation differs a lot between different materials.

When a geosynthetic reinforcement is used, it is important to consider how much of the maximal strength that can be accounted for at the acceptable level of deformation. It is also important to consider the elongation of the reinforcement over time at a constant load, that is to say, the creep behavior of the material.

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Figure 8 Common force- strain relationship of different material, (Swedish Geotechnichal Society, 2004)

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3.6 Previous studies on geogrids at low temperatures

There have been a lot of studies on how much damage the installation causes on the geogrid, for example (Allen & Bathurst 1994). These have resulted in reduction factors, specific for soil material, that are used together with the geogrid. (Hufenus et al., 2005) gives examples of reduction factors for different fill materials. There are also reduction factors for creeping and chemical degradation of the geogrid. Read more about this in the section 3.5.1 Conversion factors.

However no research has been done on how low temperature affects geogrids.

According to (Han & Jiang, 2013), (Allen et al., 1982, 1983) conducted tensile tests on five types of geotextiles made of polypropylene and polyester, at temperatures of 22° C and -12° C under

different conditions; dry, fresh water, saline water. (Allen et al., 1982, 1983) also conducted 50 freeze thaw cycles between 15° C and -15° C. They found (1) little difference in tensile strength at the two temperatures 22° C and -12° C; (2) geotextile stress strain characteristics were not affected by freeze- thaw cycles; (3) Short term creep strain was dominated by the method of geotextile manufacture; (4) Polypropylene geotextile creep rate decreased at lower temperature.

(Han & Jiang, 2013) states that (Henry and Durell, 2007) conducted a comprehensive study on tensile strength, tensile modulus, elongation at failure and static puncture strength of needle- punched polypropylene geotextile subjected to temperatures from 20° C to -54° C. The test showed that tensile strength decreased under dry conditions and increased under wet conditions, when the temperature dropped from 0° C to -54° C.

Note that the two above mentioned studies were conducted on geotextiles and not geogrids. Further (Han & Jiang, 2013) tells that (Wang, 2008) evaluated punched-drawn uniaxial HDPE geogrids

subjected to tensile loads at temperatures from -35° C to 20° C. The results showed that the tensile strength increased about 1.2 times when the temperature dropped from 15° C to - 35° C and elongation at failure decreased by 30 % for the same temperature difference.

The conclusions drawn by (Han & Jiang, 2013) are; “Geosynthetics at low temperatures have higher tensile strength and stiffness, lower creep rate and lower elongation at failure. The reduction of elongation at failure is unfavorable to most applications and should be considered if the design is based on higher elongation at failure.”

The term geosynthetics includes both geotextiles and geogrids. The above mentioned studies are not conducted on geogrids made from extruded PP or PET. However, the results and conclusions are interesting since the tests have been conducted on polymeric materials.

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3.7 Review of existing methods for testing of geogrids

3.7.1 Wide width tensile test (ISO EN 10319)

A much used, and well accepted test method for determining the tensile properties of a geogrid is the wide width tensile test named ISO 10319:2008 developed by International Standards

Organization (ISO). The testing method will be described in short here, for the full description of the test see (ISO, 2008). A similar standard method have been developed by American Society for Testing and Materials (ASTM) named ASTM D4595 – 11.

Both of these tests are so called wide width tensile tests. This means that a relatively wide test specimen is used in the tests. The reason to use a wide test specimen is that some geotextiles have a tendency to contract in the width direction, under tensional load (ISO, 2008).

The general principle of the both tests are the same. In both methods a test specimen is tested in a special machine, where the specimen is loaded in tension, with the load increasing by a set strain rate. The strain rate for ISO 10319:2008 is 20 ± 5 %. The load and the strain is recorded during the test.

The ISO 10319 test have been developed to test the tensile properties of most types of

geosynthetics, and the test is applicable on both wet and dry samples. This review of the testing method however will exclude the parts specific to other types of geosynthetics than geogrids, and parts specific to wet test specimens.

3.7.1.1 Test procedure

The test specimen is fastened in the machine usually by means of special clamps, which are wide enough to grip the whole width of the specimen. The specimen is loaded with a preload of approximately 1% of the expected maximum load.

During testing of the elongation (strain), the specimen is measured using an extensometer, which is able to measure the elongation of the specimen within an accuracy of ±2%.

3.7.2 NorGeoSpec 2012

NorGeospec is an organization, which has developed a system for certification and specification of geosynthetics and geosynthetics related products. This system is called NorGeoSpec 2012.

NorGeoSpec 2012 prescribes a procedure for testing how much installation damages a geogrid receives during installation in different soil types,and thereby determining the reduction factor for installation damages.

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4 Method

Two types of tests have been conducted, temperature controlled tensile tests on single strands and half-scale test. Five different types of geogrids were tested. Table 2 shows information regarding the tested geogrids.

The temperature controlled tensile test on single ribs was conducted by pulling single ribs to failure inside a temperature-controlled chamber. The pulling force and the strain of the rib was measured during the tests. The test was repeated five times for each geogrid in each temperature. The temperatures were +20°C, 0°C, -10°C and -20°C. The purpose of the test was to investigate how the strength properties of the geogrids, changes when the temperature changes. Detailed information, regarding the temperature controlled tensile test on single strands is presented in 4.1 Temperature controlled tensile tests on single strands.

The half-scale test has been conducted by building a small road embankment inside a freeze container. The embankment was built up by crushed aggregate and a geogrid. The temperature was controlled inside the freeze container during the construction of the embankment. The geogrid was excavated after the construction and the strength properties of the geogrid were then tested in a lab.

The half-scale test was repeated once for each geogrid and each temperature. The temperatures were +20° C, -20° C and -30°C. This resulted in 15 tests (5 geogrids x 3 temperatures = 15 tests).

Geogrids are expected to get damages during installation. The purpose of the half-scale test was to investigate if the temperature during installation, affects the amount of these damages.

Lab results, showing lower strength properties, have been interpreted as installation damages.

Detailed information regarding the half-scale test is presented in section 4.2 Method half-scale test.

Table 2 Short information regarding tested geogrids.

Geogrid ID Material Specifications; max tensile strength machine direction/ cross machine direction (kN/m) G1 Welded polyester terephthalate (PET) 40/40

G5 Woven polyester terephthalate (PET) 42,4/47,9

4.1 Temperature controlled tensile tests on single strands

To get an idea on how the stress strain behaviour of the geogrids changes when subjected to cold temperatures a series of laboratory tensile tests have been done.

The tests were done using a pulling apparatus at the University of Münster Figure 9. The apparatus used was fitted with a temperature controlled test compartment, so that the tests could be carried out at specified temperatures. Four different temperatures were used in the test series; 20°C, 0°C, - 10°C, -20°C. The tests at 20°C were used as reference tests, to which the other tests could be compared. All of the five provided geogrid types were tested in the laboratory tests. The specimens of the geogrids tested were of single strand/rib type. Five repeated tests were done for each geogrid type and temperature. All tests were done in the machine direction (MD) of the geogrids.

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24 4.1.1 Test procedure

To test the tensile properties of a geogrid specimen, the specimen was clamped in the jaws of the pulling apparatus see Figure 9. The specimen was then pre-stressed so that it was pulled tight. The pre-stress was always kept well below 1% of the expected breaking force. After pre-stressing, a reference line was drawn on the specimen at the clamping point. This line was used to see if any slippage of the specimen occurred between the clamp and the geogrid, and to decide whether or not the break was a clamp break. The test length of the specimen was then measured. This was done by measuring the jaw separation with a ruler with a precision of 1 mm. The test compartment was then closed and the temperature was allowed to reach its pre-set value, with an accuracy of ±1°C. The test compartment and the test specimen was then let to stabilize at the desired temperature Then the logging was started for both the strain gauge and the load cell. When the temperature stabilized the test was started and the test specimen was loaded with a constant strain rate until breaking

occurred. Tensile force and elongation were logged during the whole test. When the specimen had broken, both the pulling apparatus and the logging was stopped. The compartment opened and the specimen inspected. The inspection was done to see if any slippage in the clamps had occurred.

Additionally the break was checked to see if the break was acceptable, and no clamping break had occurred.

Figure 9 Test chamber on pulling apparatus with test sample clamped in Note also the strain gauge rig in the lower left corner

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25 4.1.2 Preparation of test specimen

A piece, approximately 2x4 m (WxL) was cut to be used as material for the tensile tests from each of the 5 provided rolls of geogrid.

Five different strands, running in the machine direction, was marked on each of the five pieces of geogrid with a number 1-5. These marked strands was spaced evenly on the piece, and well away from the edges. From these 5 marked strands all the tested specimens where cut.

The test specimens for the tensile testing were cut as a single strand. The length of the strands were approximately 25-30 cm so that after clamping the rib in the pulling apparatus approximately 20 cm clamping distance was achieved.

When cutting out the ribs for geogrid G1-G4 for testing, all strands running perpendicular to the tested direction was cut in the middle between two nodes. For the woven geogrid, G5, there was a risk of the rib unravelling prior to, or during testing if it was cut in this way. To mitigate any

unravelling the test specimens for geogrid G5 was cut so there was a total of 3 ribs per specimen running in the tested direction. To make sure that the two extra ribs would not interfere with the test results, these strands where cut between each node.

After cutting out the specimen, it was inspected visually to see if the specimen was in any way damaged. If the specimen was damaged, it was discarded.

If the specimen was not discarded, it was marked with an ID-number containing the type of geogrid, the strand it was cut from, and the temperature it was to be tested in.

The type of clamp used in the pulling apparatus had a tendency to induce breaks in the clamping area for the extruded geogrids, and the woven geogrid. Since test results from tests where the specimen broke at the clamping area needed to be discarded, the risk of breaking in the clamping area needed to be reduced.

The problem was solved by adding some padding in the clamping area of the specimen. This helped spread the load from the clamping and reduce the risk of damaging the specimen when clamping it in the pulling apparatus. The padding was done by adding some layers of duct tape in the clamping area of the specimen, se Figure 10 for an example. The padding was added to the extruded polypropylene geogrid specimens, and the woven PET geogrid specimens.

Figure 10 Geogrid padded with duct tape at clamping place.

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4.1.3 Conditioning of specimen and equipment for testing

Both the test equipment and the test specimen needed to be conditioned to assure that the tests were done at the specified temperature. This was achieved by letting the clamping equipment and the test specimen sit inside the test chamber at the specified temperature for quite some time.

The ambient temperature in the laboratory was a steady 20°C, ±1°C. This meant that for the

reference tests done in +20°C no extra pre-conditioning of the test specimens, or the equipment was necessary.

For the 0°C, -10°C, and -20°C tests both the equipment and the specimens was pre-conditioned.

The pre-conditioning of the specimen was done by storing the specimens inside the test chamber of the pulling apparatus prior to testing.

4.1.4 Machine set up and test routines

In applicable parts the test routines and machine set up used in the temperature controlled tensile tests, followed the routines and set up described in ISO 10319 Wide width tensile test.

The pulling apparatus was equipped with hydraulically operated wedge type jaws. The clamping force was set to approximately 40 bar in the hydraulic piston.

The test specimen was placed in the clamps so that a jaw separation of approximately 200 mm was achieved. The exact distance varied from each geogrid type. This was due to that the specimen was placed so that edge of the clamping area always ended approximately in the middle between two nodes.

ISO 10319 Wide width tensile test, prescribes a strain rate of (20±5) %/min during the tensile testing.

Since the specimens was approximately 200 mm long the clamps was set to travel at a speed of 40 mm/min during the testing.

To measure the elongation of the test specimen during the test, an inductive “strain gauge” with 50 mm travel was used. The strain gauge was set up to measure the vertical movement of the cross beam on which the lower clamp was fixed. The strain gauge was placed outside of the test

compartment to eliminate the temperature effects on the readings from the shifting temperatures in the test compartment.

Summary of equipment and set up used:

• Strain rate of approximately 20 %/min

• Test speed of 40 mm/min

• Jaw separation of approximately 200 mm

• Clamping force of 40-50 bar in the piston

• Rate of logging 2 Hz

• Pulling apparatus with temperature controlled test chamber

• Clamps of wedge type, hydraulically operated

• Loadcell 100 kN

• 50 mm inductive “strain gauge”

• Temperature logger type technotherm 9500

• Temperature probe cable resistive type K

• Spider 8, ½ byte logger

• Catman logging software

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4.1.5 Calculations prior to testing

The data sheets available for the geogrids express the tensile strength in force per metre in width of the geogrid, kN/m. Since only a single strand of the geogrids was to be tested an estimation of the breaking force for the rib was calculated. This was used to set up the testing equipment, and used to validate the pre stress and breaking force of the rib.

To calculate the estimated breaking force per strand. First the exact number of ribs per meter was needed. This was calculated by the following steps

• Count the whole number of strands within approximately 1 metre width of the geogrid, N0.

• Measure the exact distance (centrum to centrum) between the first, and last counted strand, W0.

• Divide the number of counted strands with the measured distance in metres. This gives the exact number of strands per m of geogrid, Nm.

To get a representative value for Nm, an average of 3 separate calculations was calculated for each of the geogrids.

The estimated breaking force per strand (kN), Fmax, could then be obtained by dividing the, by the manufacturer, specified tensile strength per metre (kN/m), Tmax, with the exact number of strands per metre, Nm, for the relevant geogrid.

These calculations were done for all the 5 different types of geogrids.

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4.2 Method half-scale test

Geogrids are expected to receive damages during installation. The purpose of the half-scale test was to investigate if the temperature during installation affects the amount of these damages.

The half-scale test was conducted by building a small road embankment inside a freeze container.

The embankment was built up by crushed aggregate and a geogrid. The temperature was controlled inside the freeze container during the construction of the embankment. The geogrids were installed in the embankment. The installation work included rolling out geogrid on crushed aggregate, placing crashed aggregate over the geogrid and compacting the surface. The geogrid was excavated after the installation and visible damages were documented. The strength properties of the geogrid were then tested in a laboratory. The analysis used was Wide width tensile test, described in ISO 10319:2008.

Results showing lower strength properties were interpreted as installation damages.

The method of the half-scale test has been chosen in order to imitate main parts of the NorGeoSpec 2012. NorGeoSpec 2012 is a testing method used to evaluate installation damages during full-scale installation. This method describes that at least 10 areas should be marked on the geogrid before the installation. This method also describes that the geogrid should be removed after installation in a way that does not damages it additionally. The pre-marked areas are then cut out and tested according to Wide width tensile test ISO 10319:2008. It is recommended that these areas should be of minimum size 300 x 400 mm.

The half-scale test was repeated once for each geogrid and temperature. The temperatures used were +20° C, -20° C and -30°C. This resulted in 15 tests (5 geogrids x 3 temperatures = 15 tests).

Figure 11 and Figure 12 shows a principle sketch of the embankment and test setup.

Figure 11 Principle sketch over the test setup used in the half-scale test, (section view).

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Figure 12 Principle sketch over the test setup used in the half-scale test, (plan view).

4.2.1 Building of the test setup

A frame of plywood and plastic foam was built inside the freeze container to protect the inside from damage during the tests, see Figure 11, Figure 11, Figure 12 and Figure 13. A 100 millimetre thick layer of crushed aggregate was then placed on the floor of the frame. The type of crushed aggregate was 0-70 mm. This type was used during the whole half-scale test. The layer was wetted, with approximately 10 litres of water per square meter. This was done by pouring water on the crushed aggregate. The wetted layer was then frozen. This was done in order to create a hard subgrade. A 300 millimetre thick layer of crushed aggregate was then placed on the frozen subgrade. 3 digital thermometers was installed in plastic tubes in the crushed aggregate, 50 millimetres under the surface, see Figure 11, Figure 12 and Figure 14. The thermometers were installed in the crushed aggregate in order to measure the temperature close to the geogrid. 3 thermometers were also installed to measure the air temperature inside the freeze container. These thermometers were placed on the walls inside the freeze container. The surface of the crushed aggregate was compacted with 6 overlapping passes using a 160 kg vibratory compactor plate.

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Figure 13 Frame made by plywood and plastic foam. Built up inside freeze container in order to protect the inside of the container.

Figure 14 Shows compaction work of lower layer of crushed aggregate and location of installed thermometers. Red dots show the position of installed thermometers.

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4.2.2 Preparation of test setup and geogrid pieces

A 2 x 2 meter piece of geogrid was cut out using a secateurs. The dimensions were chosen so that two geogrids could be tested during each repeated test cycle, see Figure 11 and Figure 11 for principle sketch of test setup.

The geogrid piece was marked for identification. 10 areas with dimension 300 x 400 millimetre, was marked on the geogrid piece, see Figure 15. To enable the “roll out installation-step”, the geogrid was rolled up on a tube. The diameter of the tube was 150 mm. The geogrid was fixated on the tube with cable ties and a 2 mm thick string, see Figure 16. The temperature inside the freeze container was set to desired test temperature. Rolled up geogrid was then stored in freeze container and the whole test setup was let to acclimatize during 12 hours.

Figure 15 Geogrid piece prepared for testing through half-scale test. Dimensions are 2 x 2 meters. Red rectangles are 10 areas with dimensions 300 x 400 mm.

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Figure 16 Geogrid rolled up on tube before testing. Geogrid fixated on the tube by 2 mm string and cable ties.

4.2.3 Test procedure

The average temperature of the 3 thermometers in the crushed aggregate and the 3 thermometers on the container walls was documented in a protocol. The geogrid was rolled out and any visual damages were documented with a camera. The geogrid was then covered with a 150 mm thick layer of crushed aggregate of type 0-70 mm. This was done by letting a wheel-loader pour the aggregate over the geogrid, see Figure 17. The aggregate was evened out with shovels.

Figure 17 Shows wheel-loader pouring out crushed aggregate in the freeze container.

All material in test setup was left to acclimatise in desired temperature in the container during 12 hours. This was done in order to get the right temperature in the top layer of crushed aggregate.

The surface was then compacted with 6 overlaping passes using a vibratory compactor plate. The weight of the compactor was 160 kg and the compacting force 32 kN. The top layer of crushed aggregate was then removed. This was done carefully by suction, in order to not damage the geogrid,

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see Figure 18. When the top layer had been removed, the geogrid was removed as well. The geogrid was then visually inspected. Any visually damages were documented with a camera.

Figure 18 Removing of the upper layer of crushed aggregate by suction.

The geogrid had been prepared with 10 marked areas before the test procedure. These areas were cut out and marked with geogrid ID and test temperature.

A wide width tensile test was conducted on each one of the 10 specimens. The wide width tensile tests were performed by a ISO-accredited laboratory.

The wide width tensile test was also conducted on 5 reference specimens from the geogrid. These specimens had not been subjected to the half-scale test. This was done in order to compare the strength of a geogrid subjected to the half-scale test, with a geogrid not subjected to the half-scale test. Thereby it was possible to determine if the half-scale had damaged the geogrid.

The test procedure was repeated once for each of the five different geogrid in each temperature +20° C, -20° C and -30° C. Since the dimensions of the test setup was 4 x 2 meters and the

dimensions for each geogrid was 2 x 2 meters, two geogrids were tested during each repeated test cycle.

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34 4.2.3.1 Temperature readings

Temperature readings from installed thermometers are presented in Table 3 (average). As seen in the table there is some deviations between desired temperatures and measured temperatures. This error had to be accepted during the half-scale test, since the temperature could not be controlled more precise with this test setup.

Table 3 Temperature readings from installed thermometer inside freeze container and desired temperatures during half- scale test.

4.2.4 Material

A summary of the material that was used during the half-scale test.

• Freeze container type Titan arctic store

• Plywood

• Plastic foam

• Digital thermometer x 6

• Plastic cable tubes

• Crushed aggregate type 0-70 mm, 6 m³

• Vibratory compactor plate, 160 kg, 32 kN compacting force, type Swepack fb 160

• 5 different geogrid types

• Cardboard tubes with 150 millimetre diameter

• Cable ties

• Masking tape

• 2 millimetre thick string

• Secateurs

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5 Results and analyses

Results and analysis from all conducted tests are presented in this chapter. In order to make this chapter more comprehendible, all graphs and tables with raw data, and some graphs and tables with analysed data are left out and instead presented in the appendix.

Result and analysis for temperature controlled tensile tests on single strands are first presented and then followed by result and analysis for the half-scale test.

Note that “reference test” is mentioned under both chapter “temperature controlled tests on single strands” and chapter “half-scale test”. These test are not the same reference tests and should therefore not be mixed up. Detailed information about the reference tests are found in chapter 4 method.

5.1 Results temperature controlled tensile tests on single strands

The climate controlled tensile tests resulted in pure stress strain data. From this data a few

interesting points and properties have been picked out and presented in diagrams below Figure 19 to Figure 43. The properties presented are; maximum tensile force, maximum strain, force at 2 % strain, force at 3 % strain and force at 5 % strain. These properties have been chosen in order to correlate the results with the wide width tensile tests from the half scale tests. Each diagram shows a measured strength property for the tested strands. The results are presented as average values together with maximum and minimum values. The thick lines with markers show average values and the thin lines show maximum/minimum values. The results are presented separately for each type of geogrid.

Full stress strain envelopes from all of the temperature controlled tensile tests are shown in Appendix 4.

Geogrids in cold climate Temperature controlled tensile tests & Half-scale installation tests at different temperatures