Embed Size (px)
Citation preview
The Properties andPerformance
of Tensar BiaxialGeogrids
The essential guide to theproperties and performance of
Tensar Biaxial Geogridswhen used in constructing:
Road pavementsTrafficked areas
FoundationsLoad transfer platforms
Figure 1: Simple practical demonstration of the benefits ofTensar biaxial geogrid.
2
Introduction to this guideThis is your essential guide to theproperties and performance ofTensar biaxial geogrids whenreinforcing unbound aggregates.The major topics of importance are:interlock, load spread and pavementperformance. Each topic is
discussed, and informationpresented from actual testing ortrials. The important features arehighlighted. At the end of the guideis a comprehensive list of propertiesof the Tensar SS biaxial geogrids.
Brief history of Tensar Biaxial GeogridsIn the 1970s Netlon extrudedmeshes were successfully introducedinto civil engineering as a techniquefor stabilising soils. In the 1980sTensar biaxial geogrids weredeveloped from these early ideas,specifically for reinforcing unboundaggregates. They have been usedextensively in the construction ofroad pavements, trafficked areas,foundations and load transferplatforms. During the last 20 years ahuge number of projects have beencompleted successfully using Tensar
The performance of Tensar BiaxialGeogrid in granular material
The simplest way to see how wellTensar biaxial geogrid reinforcesgranular material is to use it over awet, soft subgrade, where previousattempts with unreinforced materialhave resulted in deep rutting andfailure, as shown in Figure 1. Apractical demonstration of this isshown in Figure 2, overleaf, whichsummarises the results from a trialcarried out to investigate the benefitof Tensar geogrid in a workingplatform for very heavy cranes overa soft clay subgrade.
The section using Tensar SS2 gavesatisfactory results, even after manypasses of the 300t test crane.Initially, the crane passed along thesame track path, then moved fromside to side along multiple paths.The ruts were then filled in, and thetrial continued along a single trackpath. In the comparative section, awoven polypropylene geotextile wasused, and the settlement on the firstpass was 350mm. On the basis ofthis trial Tensar SS2 was chosen forthe platform.
SS geogrids, in a wide variety ofconditions and climates.
FEATURES
• High quality durable polymers
• Unique interlock mechanismbetween geogrid and aggregate
• High angle of load spread throughreinforced granular layers
• Improved pavement performance
• Confidence from extensive thirdparty trials and records ofperformance
It works!
Tensar. The valueengineeredsolutionIn Tensar you’ll find a partnerwith the experience andflexibility to respond to yourproject requirements. Fromdesign to completion, we’llmake sure you always benefitfrom a practical, cost-effective solution to yourspecific need.
Figure 2a: Crane used in trial.
3
Figure 2b: Comparative trafficking trial of heavy crane over reinforced granular platform.
Tensar biaxial geogrids work, asdemonstrated above. This is becausethey interlock very efficiently withgranular materials. When granularmaterial is compacted over thesegrids, it partially penetrates andprojects through the apertures tocreate a strong and positiveinterlock. The interlockingmechanism is similar to the effect ofa snooker ball rack.
The snooker ball rack confines theballs above due to its high stiffnessand the strength at the corners(junctions). Also, to confine thesnooker balls effectively, the rack hashigh, flat sides. If cyclic load isapplied to the top ball, there will benegligible settlement. However, ifthe rack is very flexible, or thecorners are weak, then cyclic loadwill cause the stack of balls to settle.A further important feature of thisanalogy is that the rack stabilises thesnooker balls above without relyingon support from neighbouring racks.Thus interlock is localised.
The apertures of Tensar biaxialgeogrids are very much like thesnooker rack. The Tensarmanufacturing process produces aunique grid structure, consisting offull strength junctions and stiff ribs,which present a square, thickleading edge to the aggregate foreffective mechanical interlock.Interlock helps prevent dilation ofaggregate particles, so that a veryhigh effective angle of shearingresistance is mobilised. Vertical loadapplied through aggregate particlesabove the grid can generate tensileresistance in the ribs with very smalldeflection. The combination of thesefeatures ensures that, in Tensargeogrid reinforced granular layers:
• Tensile load in the grid isgenerated at very small deflectionsof an applied vertical load
• Reinforcement benefit can begenerated within the loaded area
These features are demonstratedbelow using a number of practicaltests and trials.
Figure 3: The snooker ball analogy.
Figure 4a: The importance of the shape of Tensar biaxialgeogrid ribs.
Figure 4b: The unique cross sectional shape of Tensar ribs provides bearing points for fill particlesunlike other grid types with thinner or more rounded profiles.
After filling rut and several passes
Tensar SS2Woven PP geotextile
Soft clay subgrade
Number of passes
230t crane carrying 70t load
Reinforcement layers
1.0m thickgranular
layer
Measuringpoint
Single pathSingle path
First pass
Sett
lem
ent
(mm
)
Multiple path
Interlock
0
0
100
200
300
400
10 20 30 40
Figure 5: Confinement versus membrane effect.
Comparison with geotextilesProvided that they are sufficientlyrobust to resist damage, both wovenand non-woven geotextiles canimprove pavement performance byproviding a separation function.They can prevent contamination ofthe granular fill by intermixing withthe subgrade soil. The onlymechanism which allows geotextilesto offer a structural contribution toa road pavement or trafficked area isas a tensioned membrane under thewheel paths. For this mechanism towork effectively, the geotextile mustbe anchored outside the wheel pathand then deform sufficiently so thatit can carry tension.
For the tensioned membranemechanism to develop adequately,the following should occur:
• Relatively deep ruts should form topermit the membrane to develop
• The geotextile should be anchoredoutside the rutted area and loadtransferred by friction
• The ruts should be maintained,implying that fixed wheel pathsmust be followed
• Formation of the ruts will deformand remould the subgrade soils
• The ruts can act as invisible sumps,providing a water source to softenthe subgrade
• Performance above the ruts willdiffer from performance betweenthe ruts
Based on these points, the only typesof application likely to benefit fromthe tensioned membrane approachwill be roads where fixed wheelpaths are followed, and large rutdepths are acceptable, for examplenarrow unsurfaced haul roads. It isunlikely that the required conditionscan be met in the construction ofpermanent pavements.
As shown on Figure 5, the interlockmechanism of Tensar geogrids isdistinctly different to the tensionedmembrane. By interlocking with theparticles, Tensar geogrids confine theaggregate layer and prevent lateraldisplacement. Load is distributedfrom the wheel to the subgradewithin the loaded area. Unless theformation and maintenance of deepruts is acceptable, geotextiles canonly act as a separator. The twomaterials are not directlyinterchangeable without designreview and amendment.
As part of a literature review of theuse of geosynthetics in pavements,Webster (1) described a pavementtrafficking trial, which comparedfour geotextiles and a geogrid witha control section. The results aresummarised on Figure 6, whichshows rut depth versus the numberof passes of a 5t military truck overan unsurfaced granular pavementconsisting of six different sections as shown.
Confinement effect Tensioned membrane effect
Tensar geogrids Geotextile
4
Figure 7: Geotextiles and Tensar geogrids perform differently.
These results show that all fourgeotextiles perform either worsethan or similar to the control,whereas the geogrid is significantlybetter. The histogram shows thenumber of passes to form a 50mmrut, and also indicates the strengthof each product tested. It isimportant to note that the strongestgeotextile (strength is reported as agrab strength of 4450N which isequivalent to a tensile strength ofabout 90kN/m) gave the poorestperformance. This probably occurredbecause the geotextile created asliding surface, encouraging theaggregate to displace laterally. Thegeogrid, (tensile performance datareported as 8.4kN/m at 5% strain issimilar to the longitudinal behaviourof Tensar SS1 which had a strengthof 12.5 kN/m), is able to interlock
with the aggregate and confine it.This greatly decreases lateral spreadof the aggregate, thereby reducingrut depth.
Webster (1) presented a literaturereview of 104 papers andpublications, as part of thepreparation for a major aircraftpavement trafficking trial (describedlater in this document) to be carriedout by the US Army Corps ofEngineers (USACE). One of theconclusions from this review was:
If geotextiles are included in thestructure no structural supportshould be attributed to thegeotextiles.
On the basis of this study, and themany trials and tests reviewed,geotextiles were omitted from theUSACE aircraft pavement trial.
Tensar biaxial geogrids interlock efficiently with aggregates – geotextiles cannot.
Number of passes
GeotextileTensar geogrid reinforement
50mm
Control
Tensar SS grids
G5
G2
G4
G6
Ru
t d
epth
(m
m)
pas
ses
for
50m
m r
ut
Trial sectionControl G2 G4 G5 G6 Tensar SS
1100N
2100N4450N
580N
8.4 kN/m at 5% strain
5t military truck
Control G2 G4 G5 G6 SS grid
G2, G4, G5 and G6 geotextiles, value is grab strength
0 1000 2000 3000 4000 5000 6000
6000
5000
4000
3000
2000
1000
0
0
10
20
30
40
50
60
70
80
90
100
5
Figure 6: Comparison of geotextile and geogrid in USACE trafficking trial.
Figure 8: The University of Oxford, UK model footingexperiments, without (top) and with (bottom)reinforcement at similar loads showing differentbehaviour.
Twenty years of research, tests and trialsOver the last 20 years, a largenumber of tests and trials have beencarried out by independentorganisations, to investigate theperformance of aggregate layersreinforced with Tensar biaxialgeogrids. These tests and trialsprovide a huge body of high qualitydata which gives the basis formethods to design aggregate layersreinforced with Tensar geogrids, andis unrivalled by other geosynthetic
materials. They can be divided intothe following main categories:
• Static load tests
• Cyclic load tests
• Trafficking trials
• Other tests
Some of these tests and trials, andtheir results, are described in thefollowing sections.
Oxford University tests (early 1980s) – improvingbearing capacity and load spreadModel footing experiments werecarried out by the University ofOxford, UK (2), to investigate thebenefit of reinforcing a granularlayer over soft clay. Two of theexperiments are shown on Figure 8,and some of the load versussettlement graphs are shown onFigure 9.
The experiments consistentlydemonstrated that an improvementin bearing capacity of around 40%was achieved in the reinforced cases.Reinforcement was also found to
change the failure mechanism. Thegravel layer was confined byinterlocking with the reinforcement,which then resisted tensile strains atits base. This prevented gravelparticles from moving laterally awayfrom the loaded area, which can beseen in the unreinforced test onFigure 8 (upper) as a reduction ingravel thickness below thefoundation. In addition, failureplanes were driven deeper into thesoft clay in the reinforced tests.
Static load tests
Load (kPa)
Dotted lineindicatestest withbiaxial grid
Su = 6 kPa
Su = 10 kPa
Su = 16 kPa
Sett
lem
ent
(mm
) Load75mm widestrip footing
Gravel
Tensar biaxial geogrid
Soft clay Su = undrained shear strength
50mm
0 40 80 120 160
0
10
20
30
40
50
60
6
Figure 9: Some results from the Oxford University model footing experiments.
Figure 11: Model footing experiments reported by Guido et al (4).
Figure 10: Load spread improvement.
Earlier work for Oxford Universityhad provided some of the earliestinsights into geogrid performance(3). The effect on angle of loadspread was evaluated and dataindicated a mean angle increasingfrom 38˚ in the unreinforced caseto more than 50˚ with grid. Thissimple approach indicates thatgranular layer thickness may bereduced by around 50% to give asimilar stress on the subgrade, seeFigure 10.
In 1987, Guido et al (4) reported theresults of larger model footingexperiments, intended mainly to lookat the effect of multi-layers of Tensargeogrid beneath foundations. In thiscase the test medium was sand, andno soft layer was present. Theparameters varied in the tests areshown in Figure 11, and were:geogrid width (b), vertical geogridspacing (∆z), depth to the top layer(u) and number of layers (N). Figure11 also shows the effect of varyingthe geogrid width (b). It can be seenthat there is only a small increase inbearing capacity for widths greaterthan 2.5B. This provides justificationfor one of the important
observations given above, namely,that reinforcement benefit byinterlock is generated within theloaded area. It is not necessary toanchor grid well beyond the loadedarea to get maximum benefit.
Further tests by Guido et aldemonstrated that maximumreinforcing benefit is achieved when:
• The depth to the upper geogridlayer is less than 0.25B
• Vertical spacing of geogrid layersis 0.25B or less
• 2 or 3 geogrid layers are used(but more than this does not givefurther improvement)
Guido model foundation tests (1987) - optimisinggeogrid layout
Grid width (b/B)
Tensar SS1
Tensar SS2
Bea
rin
g c
apac
ity
rati
o (
BC
R)
Effect of grid width (b/B) u/B = 0.5, z/B =0.25, N= 3
BCR = q (with grid)/q(without grid)at failure
sand
Tensarbiaxial
geogrid
B = 305mm
squareplate
u
b
z
z
q
2
1.8
1.6
1.4
1.2
1
0 1 2 3 4
7
Figure 12: Full scale foundation tests carried out by FHWA.
Full scale foundation tests by FHWA (1997) -confirming Guido work
(Test TL146) and the much widergeogrid layer (Test TL186) give thesame performance. This conclusionis almost identical to that fromGuido et al, again supporting theobservation that the reinforcementeffect provided by the interlockmechanism is localised. Otherconclusions concerning the optimumdepth to the upper geogrid layer,the spacing and number of geogridlayers are all similar to those ofGuido et al.
Cyclic load testsThe static loading tests describedabove provide useful insight into themechanisms and benefits whenreinforcing granular layers withTensar biaxial geogrids. Theconclusions have been used todevelop design recommendationsfor applications such as reinforcedfoundations and load transferplatforms. However, for pavement
design, loads are generally wellbelow static failure load, but theyare repeated many times. In order tomodel better the effect of trafficloading on a reinforced granularlayer, cyclic loading tests have beencarried out in which a relatively lowintensity load is repeated manytimes.
Design recommendations for foundations and load transfer platforms have beendeveloped from static load test results.
More recently, full scale foundationtests have been carried out by FHWAin the USA. These are reported byAdams and Collin (5). Squarefoundations up to 0.91m wide weretested using sand as the subsoil.Figure 12 shows the results fromthree of the tests, looking at theeffect of geogrid width. It can beseen that a single layer ofreinforcement gives around 50%increase in bearing capacity, but thatthe narrow geogrid layer
Load (kPa) 600mm
300mm
900mm
150mm
Tensar SS35
Tensar SS35Test TL146
Test TL186
Control
Tests carried out usingmedium sand
Sett
lem
ent
(mm
)
Control
Test TL146
Test TL186
60
70
00 100 200 300 400
10
20
30
40
50
80
90
100
8
Figure 13: Cyclic plate tests carried out by University of Waterloo.
Research carried out at theUniversity of Waterloo, Canada, inthe mid 1980s, was reported by Haaset al (6). This consisted of a series oflaboratory cyclic loading tests on fullscale pavement sections. The pavement sections wereconstructed with an asphalt layerover a granular base layer. Thesubgrade strength (in terms of %CBR) was varied in the numerousexperiments carried out. Each set oftests was referred to as a “loop”. A test load of 40 kN was appliedthrough a circular 300mm diametersteel plate, representing one side ofa standard 80 kN design axle.
The results from Loop 2 are shownin Figure 13. In this case CBR =3.5%, and the graph shows thesettlement of the plate versus thenumber of load cycles. Comparisonof the three sections shows that:
• Reinforcement of the 200mm basehas increased the number of loadapplications by a factor of three toreach a given settlement
• 100mm of reinforced base givesthe same performance as the200mm control
A total of six loops were tested withCBR ranging from 0.5 to 8%,together with a range of pavementthicknesses and reinforcementlayouts. From this research it wasconcluded that:
The introduction of a Tensar geogridallowed a three times increase in thenumber of load applications.
The work carried out at theUniversity of Waterloo was used todevelop a design method forgranular road base reinforced withTensar biaxial geogrid, based on theAASHTO pavement design manual.The pavement is designed by theconventional AASHTO procedure,then the granular base thickness isreduced by 33% to give the samedesign life. This design method(often referred to as the “one-thirdrule”) has been used since the mid1980s, and has been extended tosub-base design as well as otherforms of trafficked area. It isapplicable over a wide range of CBRvalues. Large numbers of roadpavements have been designed inthis way, and measured performancein two cases is reported later in thisdocument.
University of Waterloo cyclic load tests (mid 1980s) -improving pavement performance
Cyc
les
for
20m
m s
ettl
emen
t
Control200mm base
Reinforced200mm base
Reinforced100mm base
Subgrade CBR = 3.5%
Section
Test Loop 2Test load applied by 300mmcircular plate loaded to 40kNAll sections have 75mm asphalt
200mmbasecontrol
200mmbaseSS1
100mmbaseSS1
40000
400000 80000 120000 160000
50
40
30
20
10
0
30000
20000
10000
0
Number of load cycles
200mm base control
200mm base reinforced
100mm base reinforced
20mm
sett
lem
ent
(mm
)
9
Figure 14: Trafficking trials at TRRL.
Trafficking trialsCyclic load tests provide usefulinformation on the performance ofTensar geogrid reinforcedpavements. However, the nature ofthe load does not correctly modelthe effect of a wheel passing overthe pavement surface. This can onlybe done using trafficking trials, andseveral such trials have been carriedout to investigate the performanceof Tensar biaxial geogrids in full
scale pavements. Trafficking trialsare all carried out in a similarfashion. A pavement is constructed,generally with several differentsections representing the conditionsto be investigated, including acontrol section. A wheel of knownload is then run over the section,and the development of the rut andother deformations are observedand recorded.
Figure 14 shows some of the resultsfrom trafficking trials carried out byUK’s Transport and Road ResearchLaboratory (TRRL - now known asTRL). The trials were carried out bothin the test facility and in the field.The test facility work again identifiedthe reinforcing effect of Tensarbiaxial geogrids, and the report (7)stated:
When geogrid reinforcement is used,a given sub-base thickness can carryabout 3.5 times more traffic.
The graph on Figure 14 shows across section of one of the testsections after 800 axle passes. Theunreinforced control section has adeep rut at the surface, and a rutcan also be seen at the top ofsubgrade. On the reinforced section, the surface rut is about half thedepth, and there is negligible ruttingat the top of subgrade. Similarbehaviour was observed for CBR’s of0.4% and 1.6%. These results
substantiate some of the earlierobservations concerning granular layers reinforced withTensar biaxial geogrids:
• Interaction by interlock ismobilised with minimaldeformation of the geogrid
• Tensile strains and deformation inthe subgrade are minimised
• Interlock confines the aggregateand minimises lateral displacement
• Rut depth for similar pavement lifeis reduced
This is quite different to thetensioned membrane mechanism,which requires large deformationsboth of the geosynthetic and at thesubgrade surface. Furthermore, thetensioned membrane does notconfine the aggregate, and can helpto encourage lateral displacement ofaggregate particles.
Webster (8) reported the results of amajor trafficking trial carried out inthe early 1990s, aimed specifically atlight aircraft pavements. The testload consisted of a single 130 kNwheel, and the test pavement wasfinished with 50mm of asphalt.Different base thicknesses wereinvestigated, and the subgradeconsisted of clay with CBR of 3%and 8%. The results from onesection of this trial are summarisedin Figure 15.
One purpose of the USACE trial wasto compare the performance ofdifferent forms of geogrid and mesh(see Figure 15). It can be seen thatthe various geogrids and meshestested give greatly differentperformance. A study of theseresults and the products tested,identified grid properties whichaffect the reinforcement mechanism(Table 1). These included the rib’sshape, thickness and stiffness, and
USACE pavement trials (early 1990s) - comparinggeogrid types
TRL pavement trials (mid 1980s) - investigatingrut profiles
Hei
gh
t ab
ove
dat
um
(m
)
Transverse position (m)
Top of subgrade
After 800 passesBefore trafficking
0.4
0.3
0.2
0.1
0
-0.1
-2.1 -1.5 -0.9 -0.3 0.3 0.9 1.5 2.1
Top of sub-base
TensarSS1
80kNstandardaxle
Subgrade CBR = 4.8%
10
Figure 15: Pavement trial carried out by USACE comparing different grids.
Table 1 Geogrid properties affecting base reinforcementGeogrid item Property Judgement
Rib Thickness Thicker is better.
Rib Stiffness Stiffer is better. Need test to measure stiffness.
Rib Shape Square or rectangular are better than rounded or curved shapes.
Aperture Size Related to base aggregate size. Optimum size not known. 0.75 to 1.5 inches (20-40mm) probably good target range.
Aperture Shape Round or square is better.
Aperture Rigidity Stiffer is better.
Junction Strength Need some minimum strength. All geogrids tested were adequate.
Grid Secant Modulus Need minimum secant modulus value. Optimum not known. (ASTM D 4595) Should use that of SS2 as minimum.
Grid Stability The “Grid Aperture Stability by In-Plane Rotation” test developed by Dr Thomas Kinney shows good potential for traffic performancerelationship. A minimum secant aperture stability modulus at a specifiedtorque may be a good index test requirement.
the aperture’s size, shape, rigidityand stability. It should be noted thatgrid tensile strength (ie. ruptureload at large strain) was not foundto be relevant to a grid’sperformance. This property is notused when designing with Tensarbiaxial grids. The study alsodetermined that the prevailingmode of failure of the pavementwas lateral movement of the baseaggregate away from the appliedwheel loads. This movement wasprevented by the Tensar grids.
The report stated:
By interlocking with the base layeraggregate, geogrids reducepermanent lateral displacement,which accumulate with trafficpasses.
The grids also effectively separatedthe aggregate base from thesubgrade, in spite of their relativelylarge apertures, without the use ofa separation fabric. The majorconclusion of the work was that:
The performance of the variousgeogrid products tested rangedfrom no improvement up to 40percent reduction in total pavementthickness requirement. The relativelyrigid sheet-type geogrid (Tensar SS2)performed the best of all productstested. The lighter weight version ofthis product performed second best.However, one other sheet-typeproduct and one woven-typeproduct with good strengthproperties failed to provide anymeasurable performanceimprovement. The remainingwoven-type products providedmarginal performanceimprovement.
Number of passes
0
10
0 500 1000 1500 2000 2500
20
30
40
50
60
70
80
90
100
25mm
Control
Tensar SS2
Grid PET2
Tensar SS1
Grid PET1
Grid PP
130kN singlewheel mm asphalt
350mm granularbase
Control PP PET2 SS1 SS2
CBR = 3%
Pass
es f
or
25m
m r
ut
Control PP PET2PET1
PET1
SS1 SS2
Trial section
Ru
t d
epth
(m
m)
100
0
200
300
400
500
600
11
Figure 16: Apparatus for in-plane torsional rigidity test.
Figure 18: The Newcastle University trafficking trials.
Newcastle pavement trials (1996) - furthercomparative testing A more recent trafficking trial hasbeen carried out at the NewcastleUniversity, UK (10). In this case, thepavement was unsurfaced. Theresults on Figure 18 show similartrends to the previous trials andtests. After 52,000 wheel passes, the
sections with other forms of geogrid(coated woven polyester andextruded PP) have similar rut depthto the unreinforced control. Rutdepth for Tensar SS2 is about halfthat of the control, which is similarto the behaviour observed in boththe TRRL and USACE trials. This trialincluded Tensar SS30, which hassuperseded SS2, and it can be seenthat both Tensar grids give a similarperformance.
Figure 17: Traffic improvement factors for Tensar SS2from USACE trial.
For the USACE trial, TIF for a rutdepth of 25mm for Tensar SS2 isplotted against base thickness inFigure 17. This shows that for basethickness less than 400mm, TIF isaround 5, but as the base becomesthicker TIF reduces.
The USACE report introduces theconcept of Traffic ImprovementFactors (TIF). For any specificpavement, TIF is defined as:
Number of passes with geogridTIF =
Number of passes without geogrid
where the number of passes isdefined for a specific failure orserviceability criterion.
Mechanical properties of geogridconsidered relevant to basereinforcement established in theUSACE trial (8) listed in Table 1,include aperture stability. This ismeasured using a torsional testcarried out in the plane of thegeogrid, which was developed byKinney & Xiaolin (9). The apparatusis shown in Figure 16, and themoment applied to the testspecimen imitates the torsionalloading applied to a section of gridin a pavement due to a passing
wheel. Testing has established that aproduct required to provide thegeosynthetic function ofreinforcement in groundstabilisation should have high in-plane stiffness, in addition to theability to interlock effectively withaggregate particles. To quote fromKinney & Xiaolin:
the aperture rigidity modulus is ameasure of material property whichis significant to the geogridperformance in base reinforcementapplications.
Grid sample
Clamp
Pulley arrangementto apply torsionalload to a grid
Multiple wheel pathsSingle wheel pathTIF = 1(no improvement)
TIF
Ru
t d
epth
aft
er 5
2000
pas
ses
(mm
)
Base thickness (mm)
Trial section
Control
64 kN wheel over 400mm sub-base
PPPET
TensarSS2
TensarSS30
10
9
8
7
6
5
4
3
2
1
0
0
50
100
150
100 200 300 400 500
12
Figure 20: Concept of pavement design with Tensar biaxialgeogrid - reduced pavement thickness for similar performance.
TRL pavement trials (2000) - relating performance inpavements to geosynthetic stiffness
Product TIF Load at 2% strain (kN/m) Comments
Woven PP geotexile 1.5 14.0 must rely on tensioned membrane
Reinforced geotexile 2.1 26.0 must rely on tensioned membrane
Welded grid 3.2 15.0 limited interlock possible
Tensar SS40 13.5 14.0 efficient interlock
Design recommendations for road
pavements and trafficked areas
have been developed from cyclic
load tests and trafficking trials.
Table 2: Summary of TIF for 40 kN/m products in TRL trial related to stiffness
The woven geotextile has similarstiffness to Tensar SS40, yet providesnegligible reinforcing benefit. Thereinforced geotextile (a compositeconsisting of a non-woven geotextilereinforced with high modulusaramid fibres) has twice the stiffnessof Tensar SS40, but provides verylittle improvement in performancecompared to the control section. Thelower part of Figure 19 shows rutprofiles measured in this section,compared with Tensar SS40. After5000 passes, not only is there deeprutting and heave in the sub-basesurface, but also in the subgradesurface below. For Tensar SS40 after10,000 passes, there is a smaller rut,
negligible heave and littledeformation of the subgradesurface. The welded grid consists ofvery thin polyester strips welded toform a grid shape, with similarstiffness to Tensar SS40. The thinstrips do not interlock effectivelywith the aggregate and theimprovement in pavementperformance is less than 25% of thatprovided by Tensar SS40. Thiscomparison has similar conclusionsto many others, and againemphasises that the most importantfeature of a geogrid to reinforce aroad pavement effectively is itsability to interlock with theaggregate particles.
rut depth versus number of passes forfive of the sections tested. Four ofthese sections include a 40 kN/mbiaxial geosynthetic product at thesubgrade level. Table 2 summarises TIF(for a 40mm rut depth) for these fourproducts, and also gives their stiffness(in terms of load at 2% strain fromtensile tests).
In 2000, TRL carried out a furtherpavement trial, incorporating a varietyof geosynthetic materials (11). Thepavement consisted of 320mm of sub-base over a clay subgrade with CBR =1.5%. The pavement was trafficked bya 40 kN double tyred wheel along afixed path, representing one end of astandard design axle. Figure 19 shows
Number of passes
40mm rut
All products have qualitycontrol (QC) strength of40 kN/m
ControlReinf geotexTensar SS40Woven geotexWelded grid
Ru
t d
epth
(m
m)
Tensar Geogrid
00
20
40
60
80
100
120
140
160
2000 4000 6000 8000 10000
13
Top of sub-base
Dep
th b
elo
w e
dg
e o
f p
it (
m)
Tensar SS400.2
0
0.1
0
-0.1
-0.2
-0.3
-0.40.6 1.2 1.8 2.4
Distance from edge of pit (m)
Before trafficking
After 10000 passesTop of subgrade
Top of sub-base
Dep
th b
elo
w e
dg
e o
f p
it (
m)
Reinforced geotextile0.2
0
0.1
0
-0.1
-0.2
-0.3
-0.40.6 1.2 1.8 2.4
Distance from edge of pit (m)
Before trafficking
After 5000 passesTop of subgrade
Figure 19: Comparative pavement trial carried out by TRL.
Clay, CBR = 1.5%
40 kN doubletyred wheel
Each plotted point is themean of 15 rut depthmeasurements
320 mm sub-base layer
Control
No
Pas
ses
for
40m
m r
ut
Wovengeotex
TensarSS40
Control Wovengeotex
Reinfgeotex
Weldedgrid
TensarSS40
Trial section
Reinfgeotex
Weldedgrid
0
1000
2000
3000
4000
Plate tests reported by Vanggaard (1999) -investigating layer modulus
Plate loading tests
The results from plate loading testsare commonly used as inputparameters for pavement design.Vanggaard (12) reports the results ofplate loading tests carried out toinvestigate pavement modulus at a
number of sites in Denmark. In eachcase, the subgrade modulus (Em) ismeasured, then the modulus on topof the granular sub-base (Ev2). Thetest arrangement and results aresummarised on Figure 21. Therelationship between Em and Ev2 is ameasure of the increase in verticalstiffness created by the sub-base.Figure 21 shows the results for the control sections (without anygeosynthetic), and sectionsreinforced with Tensar SS30. Thevertical difference between the twolines is a measure of theimprovement in vertical stiffness ofthe sub-base by reinforcing it with Tensar SS30. Figure 21 includessome results from woven coatedpolyester grids, which showconsiderably less improvement.
Plate loading tests reported by Seiler (1995) -investigating layer modulusDuring upgrading of a section of theBerlin to Munich railway line inGermany, plate load tests werecarried out to determine the benefitof reinforcing the sub-base withTensar biaxial geogrid (13). In thetest section the subgrade was veryweak with CBR around 0.5 to 1.0%(modulus 7 to 15 MPa). Figure 22summarises the results of plate
loading tests on the sub-base. Sub-base thicknesses of 400 and 600mmwere tested. Using a single layer ofTensar SS2 at the base of the layerresulted in approximately 100%increase in modulus. These resultsare similar to those reported byVanggaard, and are importantbecause many pavement designmethods use modulus as theprincipal design parameter of thegranular layers. This testingdemonstrates that including Tensargeogrid in a granular layereffectively increases its modulus by a factor of 2 or more.
Figure 22: Effect of using Tensar SS2 on sub-base modulus.
ControlWoven PETTensar SS30
E v2 (M
Pa)
Em (MPa)
Circular steel plate
Sub-base
Subgrade soil
Subgrade soil
Em measued onsubgrade
Ev2 measued on sub-base
Geosynthetic
UnreinforcedReinforced with Tensar SS2
E v2
mo
du
lus
(MPa
)
Sub-base thickness
160
120
80
40
00 20 40 60 80
400mm
40
20
80
60
0
120
100
600mm
14
Figure 21: The arrangement and results from plate tests to establish pavement modulus.
Figure 24: Void trial carried out by The University of Wales.
Figure 23: Views of the void trial before (top) removing supportand inside void.
Void trial - The ultimate demonstrationof interlock
The University of Wales (14) carriedout a special trial to investigateusing Tensar biaxial geogridreinforced pavements to span acrossvoids. The aim of this trial was tosee if pavements reinforced in thisway could provide an early warningsystem of a void appearingunexpectedly beneath a road, forexample, in areas of old mineworkings. The requirement was thatthe void would create a depressionin the road sufficiently deep to bedetected easily, but able to survivelong enough for remedial measuresto be taken safely.
The arrangement of the trial isshown on Figure 24. A 3m diametervoid was formed in betweenblockwork walls, then filled withsand. A pavement consisting of0.6m of granular sub-base wasplaced above the sand-filled void,reinforced with 2 layers of TensarSS35 geogrid. Two layers ofkerbstones were placed on top ofthe sub-base to give 5 kPasurcharge (Figure 23). The sand fillwas then removed to create thevoid. The underside of the lowergeogrid layer after formation of thevoid can be seen on Figure 23(lower). The geogrid was monitoredwith strain gauges at variousdistances from the centre of thevoid and the results for the lowestlayer are shown in Figure 24. It isimportant to note that the geogridwas not fixed or anchored to the
top of the blockwork wall – it wasjust resting on top.
The results from this trial areremarkable for a number of reasons:
• Strain reached a maximum of 4%after completion of voidformation, and increased verylittle until 72 hours were reached,when the trial was demolished
• Tensioned membrane theorypredicts that load in a membraneused in this way should be wellabove the breaking load of thegeogrid – it clearly was not
• Membrane theory and creepproperties of the PP geogridwould suggest that grid strainshould increase rapidly with time –this did not happen
• Strain (and therefore load)measured in the geogrid at thetwo points resting on top of theblockwork wall (1.75m and 2.25mon Figure 24) is zero, indicatingthat the support mechanism doesnot rely on friction between thegrid and the top of theblockwork wall
This trial shows how effectiveinterlock is in creating a stiffenedgranular mattress, and how superiorit is to a tensioned membrane. As atensioned membrane, the granularpavement should have collapsedvery quickly. However, the sub-base/geogrid composite created a0.6m thick gravel mattress spanning3m at strains well below failure.This unique composite action can beutilised in all applications wheregranular layers are reinforced withTensar biaxial geogrids.
0.6m thicksub-base
Kerbstonesurcharge
0.0m 1.5m
2 layers ofTensar SS35
3.0m diameter void
Location of straingauges measuredfrom centreoutwardsTime (hours)
Strain in lower layer of Tensar SS35
Stra
in (
%)
0.0m
0.75m
1.75m
0.25m
1.25m
2.25m
200
0
1
2
3
4
5
40 60 80
15
Figure 25: Comparison of reinforced and unreinforced pavements in Australia.
Glenlogan Park Estate, Queensland - confirming the “one-third rule”
Performance in service
A pavement was built in 1997 as partof a housing development in SouthQueensland, Australia. A section of thepavement was built using Tensar SS30placed at the subgrade level, but withthe granular layers designed using aone-third reduction in thickness. Bothsix months and two years afterconstruction, Benkleman Beam (BB)deflection tests were carried out onthe reinforced and unreinforcedsections of pavement. Both series oftests gave consistent results,demonstrating that the thinnerreinforced section of pavementdeflected consistently less than thethicker unreinforced pavement.
In November 2000 furtherperformance testing was carried out using the falling weight
deflectometer or FWD (15).
The deflection results are shown onthe upper graph of Figure 25, andthey show a similar trend to theBenkleman Beam tests, namelyconsistent results with deflection ofthe thinner reinforced pavementsignificantly less than the thickerunreinforced pavement. FWD tests canbe analysed to interpret layer modulusin the pavement. This is shown for thesub-base layer on the lower graph ofFigure 25. The results are consistent,and show that the modulus of thethinner reinforced sub-base is onaverage more than double that of thethicker unreinforced sub-base. Thisobservation is almost identical to theresults from plate loading testsdescribed earlier in this guide.
Chainage (m)
SubgradeCBR = 1.5%
Sub-base
Base170mm200mm
510mm
Ch 1540to 1665
Ch 1360to 1540
360mm
TensarSS30
Chainage (m)
Glenlogan Park Estate, Queensland
Glenlogan Park Estate, Queensland
Sub-base modulus from FWD20th November 2000
Deflection from FWD20th November 2000
Left outer
Left inner
Right outer
Right inner
Left outer
Left inner
Right outer
Right inner
Unreinforced(average = 1.31mm)
Unreinforced(average = 28 MPa)
With Tensar SS30 (average = 1.22mm)
With Tensar SS30 (average = 63MPa)
Def
lect
ion
(m
m)
Sub
-bas
e m
od
ulu
s (M
Pa)
0
0.5
1.5
1350 1400 1450 1500 1550 1600 1650
2.5
160
140
120
100
80
60
40
20
0
1
2
1350 1400 1450 1500 1550 1600 1650
16
Figure 26: Comparison of reinforced and unreinforced pavements in USA.
Huntington and Ksaibati (16)describe a pavement built in 1995 toevaluate the performance of biaxialgeogrid. A control section was builtadjacent to a section reinforced withTensar SS1 geogrid (described by itsUS designation of BX1100 in thepaper), and with a one-thirdreduction in granular base thickness.
After three years service in 1998, thesections were checked using aFalling Weight Deflectometer (FWD),
Wyoming, USA - confirming the “one third rule”and by measuring rut depth. Theresults are summarised on Figure 26,which shows that both sections havealmost identical characteristics.
Both trials described above confirmthe “one-third rule”, namely that aTensar geogrid reinforced pavementwith a 33% reduction in granularthickness gives similar or betterperformance when compared to thethicker unreinforced control section.
Manufacturing processTensar biaxial geogrids aremanufactured from carefullyselected grades of polypropylene(PP). A long service life is required inmost civil engineering applicationsand the grade of PP used in Tensargeogrids combines the optimumvalues of strength, stiffness,toughness and durability.
Biaxial geogrids are made byextruding a sheet of PP to veryprecise tolerances, punching anaccurate pattern of holes, thenstretching the sheet under controlledtemperature, firstly in thelongitudinal direction, then in thetransverse direction. This processcreates a geogrid with square oralmost square apertures, called abiaxial grid because it is stretched intwo orthogonal directions.
The polymer’s long chain moleculesare orientated in the direction ofstretching resulting in a dramaticincrease in both strength andstiffness. This orientation passesthrough both the narrower ribs andthe thicker nodes, and is unique tothe patented Tensar manufacturingprocess.
The resulting product is a monolithicgrid with square edged ribs andintegral junctions which possessboth geometrical and molecularsymmetry; critical for consistency inmanufacture and efficient loadtransfer in service. Aperture sizeshave been carefully chosen to match with typical gradings ofpavement aggregates.
Figure 27: The Tensar manufacturing process and thestretched biaxial geogrid.
Wyoming Department of Transport
With SS1Control
Measuredperformanceafter 3 years inservice
Rut depth (mm) FWD deflection (mm)
Measurement method
Designed for 310 ESA per day for 20 years
Def
lect
ion
/ru
t (m
m)
20mm friction course
100mm hot mix
granularbase
CBR = 4%
Tensar SS1
Control
420mm
280mm
Punchedsheet
Biaxial geogrid
0
1
2
3
4
17
Figure 28: Test arrangement and result for ISO 10319 tensiletest on Tensar SS30.
Tensar biaxial geogrids are extremelydurable (17). They are not affectedby hydrolysis, and are resistant toattack by aqueous solutions of salts,acids and alkalis. They have nosolvents at ambient temperature. PP is not a nutrient medium formicro-organisms and is, therefore,not affected by them. In addition,the tough monolithic form of Tensarbiaxial geogrids gives them a highdegree of resistance to installationdamage.
Ultra-violet light (UV) can damageunprotected polymers very rapidly,by breaking down the polymerchains. Tensar biaxial geogrids aremanufactured with a minimum of2% well dispersed carbon black,which gives a very high degree ofprotection by preventing UV frompenetrating beyond a thin layer atthe surface. This excellent UVresistance means that no specialwrapping or covering is requiredduring handling, and there is noneed to specify minimum durationbefore cover is established if thegrids are to be exposed duringconstruction.
Quality control testing
Durability and UV resistance
Tensar SS30 QC test
QC limit
QC test on Tensar SS30Typical result
Method: ISO 10319
Strain (%)
Load
(kN
/m)
20
15
10
5
0
0 2 4 6 8 10 12 14
25
30
35
40
18
Q05288
For Tensar biaxial geogrids, qualitycontrol (QC) tensile testing is carriedout using the method specified inInternational Standard ISO 10319.This requires a specimen width of atleast 200mm. Strain rate is 20% perminute and test temperature is20ºC.
A typical test from an ISO 10319 QCtest is shown on Figure 28. Thesetests are carried out at prescribedintervals according to the certifiedquality control procedures. Thespecified QC strength per metrewidth is the 95% lower confidencelimit determined in accordance withISO 2602-1980.
Tensar geogrids are manufacturedunder tightly controlled conditions.The quality assurance procedurescovering design and application andthe manufacturing process havebeen certified by the BritishStandards Institution as aRegistered Firm inaccordance with BS EN ISO 9001.
References
1. Webster, S L, Geogrid Reinforced Base Courses for Flexible Pavements for Light Aircraft: Literature Review and Test Section Design. Geotechnical Laboratory, Department of theArmy, Waterways Experiment Station, Corps of Engineers, Mississippi, 1991.
2. Milligan, G W E, & Love, J P, Model testing of geogrids under an aggregate layer on soft ground, Proc Symp Polymer Grid Reinforcement, Thomas Telford, London 1985.
3. Oxford University, The use of mesh products to improve the performance of granular fill on soft ground, Report 1346/81 to Netlon Limited, 1980.
4. Guido, V A, Knueppel, J D & Sweeny, M A, Plate Loading Tests on Geogrid-Reinforced Earth Slabs, Proc. Geosynthetics ’87 Conference, New Orleans, USA, pp 216-225, 1987.
5. Adams, M T & Collin, J G, Large model spread footing load tests on geosynthetic reinforced soil foundations, Journal of Geotechnical and Geoenvironmental Engineering, ASCE,p 66, January 1997.
6. Haas, R, Walls, J, & Carroll, R G, Geogrid reinforcement of granular bases in flexible pavements, Transportation Research Record 1188, 1988.
7. Chaddock, B C J, Deformation of Road Foundations with Geogrid Reinforcement, TRL Research Report 140,1988.
8. Webster, S L, Geogrid Reinforced Base Courses for Flexible Pavements for Light Aircraft: Test Section Construction, Behaviour Under Traffic, Laboratory Tests and Design Criteria,Geotechnical Laboratory, Department of the Army, Waterways Experiment Station, Corps of Engineers, Mississippi, 1992.
9. Kinney, T C, & Xiaolin, Y, Geogrid Aperture Rigidity by In-Plane Rotation. Geosynthetics ‘95, Nashville, 1995.
10. Knapton, J, & Austin, R A, Laboratory testing of reinforced unpaved roads, Proc Symp on Earth Reinforcement, A Balkema, Rotterdam, 1996.
11. Blackman, D I, Greene, M J & Watts, G R A, Tensar International Limited: Trafficking trials for sub-base reinforcement, TRL Report PR/IS/13/2001, 2001.
12. Vanggaard, M, The effect of reinforcement due to choice of geogrid, Proc Int Symp on Pre-failure deformation characteristics of geomaterial, Torino 1999.
13. Seiler, J, Trials and practical experiences with orientated and woven geogrids on the Hochstadt - Probstzella section of the Berlin - Munich railway, Geotechnik, GermanGeotechnical Society, 1995.
14. Bridle, R J, Jenner, C G & Barr, B, Novel Applications of geogrids in Areas of Shallow Mineworkings, Proc 5th Int Conf on Geotextiles, Geomembranes and Related Products,Singapore, Vol 1, pp 297-300, 1994.
15. Pavement Management Services, FWD Testing Report, St Jude Circuit, Jimboomba, Glenlogan Park Estate, Test Report, 2000.
16. Huntington, G, & Ksaibati, K, Evaluation of Geogrid Reinforced Granular Bases, Geotechnical Fabrics Report, January/February 2000.
17. Wrigley, N E, Durability and long-term performance of Tensar polymer grids for soil reinforcement, Material Science and Technology, Vol 3, pp 161-172, London, 1988.
Tensar SS geogrid specificationsProperty Units Tensar geogrid
SS20 SS30 SS40 SS2 SSLA20 SSLA30
Polymer (1) PP PP PP PP PP PP
Minimum carbon black (2) % 2 2 2 2 2 2
Roll width m 4.0 & 3.8 4.0 & 3.8 4.0 & 3.8 4.0 3.8 3.8
Roll length m 50 50 30 50 50 50
Unit weight kg/m2 0.22 0.33 0.53 0.29 0.22 0.33
Roll weight kg 46 & 44 67 & 64 65 & 62 60 43 65
Dimensions
AL mm 39 39 33 28 65 65
AT mm 39 39 33 40 65 65
WLR mm 2.2 2.3 2.2 3.0 4.0 4.0
WTR mm 2.4 2.8 2.5 3.0 4.0 4.0
tJ mm 4.1 5.0 5.8 3.8 4.4 7.0
tLR mm 1.1 2.2 2.2 1.2 0.8 1.7
tTR mm 0.8 1.3 1.4 0.9 0.8 1.5
Rib shape Rectangular with square edges
Quality Control Strength (longitudinal)
Tult(3) kN/m 20.0 30.0 40.0 17.5 20.0 30.0
Load at 2% strain (3) kN/m 7.0 10.5 14.0 7.0 7.0 11.0
Load at 5% strain (3) kN/m 14.0 21.0 28.0 14.0 14.0 22.0
Approx strain at Tult % 11.0 11.0 11.0 12.0 10.0 9.0
Quality Control Strength (transverse)
Tult (3) kN/m 20.0 30.0 40.0 31.5 20.0 30.0
Load at 2% strain (3) kN/m 7.0 10.5 14.0 12.0 8.0 12.0
Load at 5% strain (3) kN/m 14.0 21.0 28.0 23.0 15.0 25.0
Approx strain at Tult % 10.0 10.0 10.0 10.0 10.0 9.0
Junction strength as % of QC strength (4)
Minimum junction strength % 95 95 95 90 95 95
(1) PP denotes polypropylene.
(2) Carbon black inhibits attack by UV light. Determined in accordance with BS 2782:Part 4: Method 452B:1993.
(3) Determined in accordance with BS EN ISO 10319:1996 and as a lower 95% confidence limit in
accordance with ISO 2602:1980 (BS 2846:Part 2:1981).
(4) Determined in accordance with GRI GG2-87 and expressed as a percentage of the quality control strength.
(5) Tensar SS geogrids are inert to all chemicals naturally found in soils and have no solvents at ambient
temperature. They are not susceptible to hydrolysis and are resistant to aqueous solutions of salts, acids
and alkalis and are non-biodegradable.
(6) All quoted dimensions and values are typical unless stated otherwise.
Roll Length(Longitudinal)
Roll Width(Transverse)
Ribs
AL
tLR
tJ tTR
Junctions ATWLR
WTR
19
Tel: +44 (0)1254 262431
Fax: +44 (0)1254 266868
E-mail: [email protected]
www.tensar-international.com
Tensar International Limited
New Wellington Street
Blackburn BB2 4PJ
United Kingdom
Your local distributor is:
Contact Tensar International or your local distributor to receive furtherliterature covering Tensar products and applications.
Also available on request are product specifications, installation guidesand specification notes.
The complete range of Tensar literature consists of:
• Tensar Geosynthetics in Civil Engineering A guide to the productsand their applications
• Erosion Controlling erosion on soil and rock slopes
• Steep Slopes Constructing embankmentswith steep slopes
• Asphalt Pavements Reinforcing asphalt layersin roads and trafficked areas
• Ground Stabilisation Reinforcing unbound layersin roads and trafficked areas
• Retaining Walls Constructing retaining walls in reinforced soil
• Foundations over Piles Constructing over weak groundwithout settlement
• Basal Reinforcement Constructing embankments overweak ground
• Railways Reinforcing ballast under railway track
Tensar is a registered trade mark.
©Copyright Tensar International Limited
Printed January 2003 Issue 3, 79010055
The information provided verbally, or in this document or as a Free ApplicationSuggestion is of an illustrative nature and is supplied without charge. It does not formany contract or intended contract with the user. No liability in negligence will arise fromthe construction of any project based on such information or material. Finaldetermination of the suitability of any information or material for the use contemplatedand the manner of use is the sole responsibility of the user and the user must assume allrisk and liability in connection therewith.
Tensar geogrids are manufactured under tightly controlledconditions. The quality assurance procedures covering design andapplication and the manufacturing process have been certified bythe British Standards Institution as a Registered Firm inaccordance with BS EN ISO 9001:1994
Q05288
